Marks Electric Mower

An online lawnmower forum

2011-08-17T22:00:00.003-04:00

Earlier this year I joined Lawn Mower Forum, an online forum that's all about lawn mowers. There are a few other cordless electric mower users there, but as you might expect the majority of forum members use gas-power. It would be great to grow the electric mower community there, so I encourage anybody having mower problems to seek help there -- you'll need to sign up by creating a username and password, but it is free. If you do join, feel free to say hello to me at my own welcome thread there.

I'll continue blogging here about problems and fixes to the CMM1000 and other electric mowers that cross my path. A consequence of writing this blog is that I can enjoy getting emails and comments from fellow electric mower owners. Often these are from people seeking help with their broken mowers; regretfully, more often than not I am unsuccessful in diagnosing the problem properly. What is great about a forum like Lawn Mower Forum is that you would get several heads -- rather than just mine -- thinking about how to help out with any mower issues you may want to ask about.

That's it for now. Hope to hear from some of you over at LMF.

Another new battery

2011-07-16T15:28:00.007-04:00

Table of Contents

Important: always use sealed lead acid batteries for your lawnmower, as they are spillproof.

A couple weeks ago I replaced the mower batteries, two years after their last replacement. The two-year lifetime of the model TR22-12 battery from Tempest was dissappointing, but maybe not surprising when I think about it. The higher charge capacity (22 Amp-hours, vs. 17 to 18 A-hr for the original battery), in the same overall battery volume, may have come at the expense of a shorter battery lifetime as a matter of design. A more technical discussion can be found at Physics Forums.

This time around, I ordered two new 12V, 18 A-hr batteries from amazon.com; they are model UB12180 made by Universal Battery (or maybe it's Universal Power Group? It is difficult to tell which). The total cost was $94, which included two-day shipping; I wanted them quickly, before I had to mow again. It looks like the price with standard shipping would have been $82.

One thing to know, if you order these batteries, is that the connections are made slightly differently than in the battery that came with the mower. These batteries have tabs (below, top) instead of built-in threaded holes (below, bottom):

Not a big deal, except that I had to bend the tabs back, about 45 degrees, on the connecting cables in order to reduce the stress placed on the cables and connections:

After connecting cables to the 4 battery terminals, securing the battery strap, and replacing the two foam blocks I was good to go:

2011-06-30T21:48:00.007-04:00

There are enough posts here by now that it makes sense to do this. I'm annoyed that the "Blog Archive" at right does not let one view the entire blog contents at once.

2009

Three years and three months ago ...   (8/10/09)
   Why my wife and I bought our cordless electric mower in the first place.

Getting replacement parts; customer reviews at amazon.com   (8/11/09)
   Includes the review I wrote at Amazon.

Circuit details   (8/15/09)
   Photos of inside the mower.
   Electric circuit schematic.

The Remington MPS6017A electric mower   (9/19/09)

Handy home-made tool; fixing a broken wire   (10/31/09)
   Handy tool for propping mower cover open.
   Fixing a broken wire.

Helpful links   (11/1/09)
   Links for both the CMM1000 and CMM1200:
      Instruction manuals online (.pdf format)
      Where to get replacement parts
      Product pages at Amazon

Fixing a problem (June 2009)   (11/12/09)
   Replacing the circuit breaker.

2010

New electric mower models coming from Black and Decker   (1/30/10)
   The CM1836, CM1936, and MM1800

Mower specs   (1/30/10)
   Incomplete -- voltage, current, RPM, weight, and cutting width for different B&D mowers.

The battery   (4/30/10)
   How to prolong battery life.
   Buying a replacement battery.
   More about batteries.

Common problem with CMM1200; Broken Hinge Pin on the Switch Lever   (5/30/10)
   Step by step, with photos.

Cordless electric string trimmers (a.k.a. "weed whackers")   (7/23/10)
   Choosing and using a cordless electric string trimmer

2011

Wobbly wheels and how to fix them   (5/14/11)
   Step by step, with photos.

Table of Contents   (6/30/11)

Another new battery   (7/16/11)
   Replacing the mower batteries (again).

An online lawnmower forum   (8/17/11)
    (It's called Lawn Mower Forum)
.

Wobbly wheels & how to fix them

2011-05-14T19:57:00.002-04:00

Table of Contents

Today I fixed my lawnmower wheels which seem to have gotten rather wobbly in the 5 years we have owned our mower. I'll explain how I did this shortly, but please do be sure to read the next paragraph.

The first thing you should know before reading this is that the lawnmower wheels probably should be a little wobbly and not perfectly rigid. To make them perfectly rigid means having things so tight that they will not turn easily and freely. It looks like it would be pretty difficult to loosen wheels that have been overtightened, so be careful if you try this procedure not to pound too much with the hammer.

For tools you'll need (1) a hammer and (2) a 3/4" or 19 mm socket from a socket wrench set.

After removing the safety key, insuring that the mower will not start accidentally, you'll need to pop off the plastic hub caps from the wheels. This can be done with your fingernails, though it may be easier to use a flat blade screwdriver or knife blade.

Next, hold the socket against the wheel -- hex side in toward the axle, square side facing out. Tap the square socket with the hammer, easily at first. Check for movement of the push ring (the thing that resembles a washer) by wiggling the wheel from side to side after every couple of hits, and gradually increase the hammer force until it is enough to move the push ring inwards towards the wheel. Do not pound the push ring so far that it is pushed tight against the wheel! You need to leave some clearance so that the wheel still turns freely. The idea here is to reduce the wobble, not to eliminate it entirely.

After adjusting the push ring, replace the hub cap by pounding it with your fist until you hear it snap back into place and you're good to go.

A final note about this procedure. You will notice that the washerlike push ring has two parts, (1) an outer flat part and (2) an inner slanted, cone-shaped part. The force from the hammer blows should push on the flat face, not on the slanted part. That's why the 3/4" socket is the right size for doing this, as it pushes completely outside of the slanted faces. A 1/2" socket, which would just slip over the 1/2" axle, would push on the slanted faces of the push ring. I did try it this way at first, but found that the push ring would not budge.

Cordless electric string trimmers (a.k.a. "weed whackers")

2010-07-23T22:24:00.003-04:00

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"In my next blog I plan to discuss cordless electric string trimmers to augment the pollution-free, low-noise mowing experience."

I wrote that about 3 months ago, as I wrapped up my blog entry on mower batteries. After much procrastinating here is my experience choosing and using a cordless string trimmer.

Not using a corded string trimmer

Before getting our cordless trimmer, my wife and I did own a plug-in version for several years. It was a perfectly good trimmer from Black and Decker -- but having to get out, and later put away, the 100 foot extension cord was enough of a deterrent that it only got used 2 or 3 times per season. My routine was to avoid using it by being as diligent as possible with the mower around the lawn's edges, and just letting the grass get tall around our more delicate plantings. It finally dawned on us that we should own either a string trimmer that actually gets used, or none at all.

Choosing a string trimmer

The first step in choosing a cordless string trimmer is whether to go with a NiCd or a lithium battery model. My experience with other cordless power tools has nudged me towards lithium, and for primarily two reasons. (1) lithium batteries are newer technology, and superior to NiCd (longer life, longer run time, lighter weight, holds charge longer when not in use) and (2) chargers provided with lithium tools tend to be better than the ones provided with NiCd tools, in terms of not overcharging and damaging the battery -- at least this is true of the NiCd tools I currently own. Of course, you do have to pay more for a lithium trimmer, but it really is worth it. A friend of ours who owned a cordless NiCd string trimmer complained that it would not run very long before the battery needed recharging.

After an internet search I found 3 lithium battery models to choose from. This site has a comparison of the Worx GT WG151 and the Troy-Bilt TB57, and there is also the Black and Decker LST1018. The customer reviews of the Worx GT seemed more favorable than the others, and I liked that it was available with a fast 30-minute charger. (It's also available with a slower 3-5 hour charger, so if you buy it make sure you are getting the one you want.) So in the end I ordered the Worx fast-charging model.

By the way, I generally make a point of checking out customer reviews at amazon.com whenever spending this kind of money ($150.00 in this case) on a power tool, and recommend that you do so as well.

Using the string trimmer, pros and cons

First, and briefly, the obvious benefits: not as loud as a gas-powered trimmer (but noticeably louder than an electric mower), and no power cord to contend with. It's nice to have something I can quickly grab from the closet -- yes, that's where we keep it -- and touch up the yard after a mowing.

I was also pleased to discover that the Worx trimmer automatically shuts off when the battery charge gets low, thereby preventing battery damage from deep discharging. I don't know if the other string trimmers I looked at have this feature, but it does seem to be an all-too-common problem among cordless mowers.

Now for my main complaint. This trimmer can put a strain on your back if you let it. That was my experience the first two times I used it. And the second time, I really overdid things by using it for about 1/2 hour (that's when the battery needed recharging), and then for another 15 or 20 minutes after a quick recharge. My back was noticeably stiff the following day, though not in a debilitating way. Since then, I have learned to limit each use to about 20 minutes or less, and to switch between a left-handed and a right-handed grip every 5 minutes or so; I have had no problems since adopting this practice. As a result having the fast charger is not really a bonus, though owners of larger yards might still find that feature useful.

A minor complaint is that I apparently don't get how to use the edging feature. The head can be rotated so that the string cuts vertically, rather than horizontally, making a nice clean edge along a sidewalk for example. But the handle must be held at an awkward angle when doing this, whereas our old corded trimmer did edging much more easily.

All in all I'm glad we have a cordless trimmer and so far am quite happy with it, in spite of the two drawbacks I discuss above.

Update - posted 25 Dec. 2010

After about a month into owning the Worx GT WG151 string trimmer, the battery died suddenly last August. As it was under warranty, Worx replaced it for free, for which I am thankful.

I did not use the trimmer a whole lot for the rest of the season, so we will have to wait and see if the new battery holds up better than the first one did. If I had it to do over, I would have gotten the slower-charging version of this trimmer, as I wonder if the fast 30-minute charger might be too hard on the battery. I am surprised that a lithium battery failed so quickly; hopefully it was just a defective battery and not indicative of a systemic problem.

Common problem with CMM1200: Broken Hinge Pin on the Switch Lever

2010-05-30T23:11:00.004-04:00

Table of Contents

Our friend, who owns a CMM1200 mower, was unable to run her mower because of a broken hinge pin on the orange switch lever. This is the pin that allows the switch lever to pivot and turn the mower on:

From the Black & Decker CMM1200 Instruction Manual.

Before I detail how to fix this, let me just say this appears to be a common problem on the CMM1200, judging from owners' comments at both Amazon and eReplacementParts. It also looks like Black and Decker later redesigned the mower so that, in the Type 2 version, the pin is a $1 replacement part instead of a part of the larger, more expensive switch clamshell. So owners of CMM1200 Type 2 mowers may want to simply order the pin replacement here.

To fix my friend's mower (a Type 0 version, which I didn't even know existed until now), it was necessary to come up with some makeshift hinge pin, and drill a hole in the outer clamshell to accommodate it. If possible, I wanted it to:

Be made of metal, or include metal, so that it would not break again, and
Not have exposed screw threads that would rub against (and possibly damage) the orange switch handle.

If you are looking to make your own pin, you are basically looking for a 1-1/2" long, 3/16" diameter metal piece of some sort. What I finally came up with consisted of two pieces:

A 1-1/2" long, 4-40 screw, (preferably stainless steel or brass, or other non-rusting material)
A 1" long, 3/16" diameter nylon spacer

Screw and spacer

Tools I used for this job include:

A power drill
A screwdriver for size T25 Torx head screw
Drill bit for 4-40 tap - ideally size #43, but 5/64" worked for me.
A 4-40 tap and tap wrench
A hole punch
A hand file

After disassembling the switch assembly, you'll need to drill and tap a hole in the lower clamshell, to hold one end of the screw:

Drilled-out portion of lower clamshell. This is where the original plastic pin was located before it broke off.

Close up view.

I won't offer instructions on how to tap a threaded hole -- it's really better to be shown in person how to do this, as it is just too easy to do it wrong and strip the threads. If you don't have the means to tap threads yourself, consider drilling a 1/4" deep hole that you can slip the 4-40 screw into; a 1/8" drill would work for this.

The screw head was too big to fit into the socket of the upper clam shell, but not by too much. I inserted the screw in my power drill, turned on the drill, and rubbed the screw head against a hand file until it finally fit in the socket. The locked-on feature of my drill made this easy, as it freed up a hand that would otherwise have to hold the trigger in the on position.

Insert the screw far enough so that the end threads are not damaged by the drill chuck.

Next, I hand-tightened the screw and spacer into the threads in the lower clam shell. It would be easy to strip the plastic threads by overtightening with a screwdriver, since it is a metal screw and the threads are rather small. (Ideally I would have preferred larger 6-32 threads, but I couldn't think how to incorporate that screw size in an easy, satisfactory way.)

The lower plastic clam shell (which used to include the now-broken-off plastic pin) and new hinge pin.

For the final steps:

the spring and switch lever are slipped over the makeshift pin
the heavy wire switch link is fitted into the hole in the switch lever
the two outer clam shells are screwed together

Upper clam shell at left; lower clam shell with orange lever at right. At this point of the assembly process, I had no idea what the spring (to right of 4 screws) was for. Nor did I include it when taking this photo; what you see here is a sketch of the spring that I have photoshopped into the picture. In fact the spring should already be in the assembly here, not laying aside with the still-to-be-used parts.

Lower clamshell, with orange lever and new hinge pin.

Upper & lower clamshells, showing socket that accepts the screw head.
(Pssst! Don't forget the spring -- missing here, see next pic).

The switch assembly is finally together, including the spring.

After testing that the mower will start, you are ready
to reattach the switch assembly to the mower handle.

There are any number of ways to fashion your own hinge pin.While 3/16" is a suitable and easy-to-find part diameter, it really just needs to be able to fit though both the 0.227" holes in the orange handle as well as the 0.205" hole in the upper clam shell. At eReplacementParts, "Jeff DC" has suggested cutting 1-1/2" from a standard door hinge pin.

Finally, for those of you who have yet to break the plastic pin on your mower, you might consider drilling a hole through it for a 4-40 screw, which would then provide a metal reinforcement.

The battery

2010-04-30T22:17:00.009-04:00

Table of Contents

This blog entry has 3 sections:

How to prolong battery life
Buying a replacement battery
More about batteries

How to prolong battery life

A major expense in maintaining cordless electric mowers comes when you need to replace the battery. This will cost around $100, give or take, so it is worthwhile to do what we can to prolong battery life.

From reading product reviews at amazon.com, I found that batteries should last at least 4 years if properly cared for. What I also get from the Amazon reviews, as well as the owner's manual, is this: it is very easy to misuse the battery and only get 1 or 2 years of use from it. So always remember the two most important things you can do to maintain battery life:

Stop mowing immediately when the battery has lost a significant amount of charge, even if you have not yet finished your lawn. This is indicated by a battery meter reading in the "red zone" of the meter on the CMM1000.
Keep the battery on the charger when you are not using it, even during the winter months.

Letting a lead-acid battery run down too much will damage the battery internally, as explained in more detail later.

Buying a replacement battery

The CMM1000 and CMM1200 mowers each use two 12-volt sealed lead acid batteries (total voltage: 24V). For a no-brainer, guaranteed-to-work purchase, you can get the set of two batteries from ereplacementparts.com:

Replacement battery set for CMM1000 or CMM1200, from ereplacementparts.com

Bargain hunters may wish to search or shop around for two of the 12V batteries from the battery manufacturer, B.B. Battery, part number BP17-12 with optional terminal type I1. This terminal type uses screws to secure the electrical cables to the battery; if you get a battery with a different terminal type, you will be on your own in terms of getting proper connectors to secure the cables.

If you are more adventuresome when it comes to do-it-yourself electronics, you might search for and install a higher capacity battery -- preferably of the same physical dimensions as the original battery. Note, there could be inherent risk in using a battery that is specified differently than the recommended one. I am not responsible if using non-standard parts starts a fire or explosion, or creates miniature black holes that engulf our planet. That being said, I have been using a 22 Amp-hour battery for the past year, it is the model TR22-12 battery from Tempest and has the same screw-on terminals. The battery dimensions are the same as for the B.B. Battery 17 Amp-hour ones that came with the mower: 6.54" high, 7.13" long, 2.99" deep. [Note, 21 June 2011: The TR22-12 battery from Tempest is dying after just 2 years of use, so I do not recommend it for the lawnmower]

Click here to buy the Tempest TR22-12 battery at Batteryspec.com. You will need two of them for your lawnmower. I paid $114 in 2009 ($42 per battery plus $30 shipping.)

More about batteries

Cordless electric mowers run on a specific type of battery, the sealed lead-acid battery. The name refers to the chemicals used in the battery to generate the electricity, as well as their being sealed to prevent spillage when tilted at an angle or upside-down. Mower batteries are similar to the lead-acid batteries used in cars; the main differences are:

Car batteries are designed to generate high electric currents (several hundred amps) for a brief time, for the purpose of starting the engine. The battery does not supply the energy that moves the car; that comes from burning gasoline in the engine.
Mower batteries are designed to run at more modest currents (12A in the case of the CMM1000) for a prolonged period of time. The battery supplies all of the energy that runs the motor.

For a simple explanation of why lead acid batteries fail, we can perhaps think of it this way: the strength of a battery is determined, in part, by the amount of internal electrode material in the battery. Running the battery causes the buildup of lead sulfate on the electrodes, reducing the available electrode surface area and therefore reducing battery strength. This is known as sulfation, and too much of it can permanently damage the electrode -- so it's important to avoid excess sulfation by not discharging the battery too much, and keeping it charged when not in use. (Here I am glossing over many details, such as what I mean by battery strength, but hopefully less technically savvy readers can get the general idea.)

Sulfation of the battery electrodes is a normal consequence of the chemical reactions that produce electricity in a lead acid battery. As I said before, running the battery down too far will produce too much lead sulfate buildup on the electrode plates. While I myslef am still learning the details of how batteries work, the problem with lead sulfate appears to be that (1) it reduces the current capacity of the battery, perhaps permanently if there is too much of it, and/or (2) the lead sulfate can flake off of the electrodes, in which case it is permanently removed from the charge/recharge cycle and thus lowering battery charge capacity.

If you would like more details, try an internet search on battery sulfation.

Well, that's about it for now. In my next blog I plan to discuss cordless electric string trimmers to augment the pollution-free, low-noise mowing experience.

-- Mark

p.s
In writing this blog entry, I found the following sources useful:

The book Practical Electronics for Inventors by Paul Scherz
This WiseGeek article on battery sulfation
The Wikipedia entry, Lead-acid battery
This online lead acid battery tutorial

Mower specs

2010-01-30T20:09:00.004-05:00

Table of Contents

I'll be adding specs to this entry as they become available. I recently measured the current of my CMM1000 and am looking for a chance to measure the motor RPM as well. Opportunities to measure specs of the other models will be rather rare, unfortunately.

Battery-operated models

CMM1000
Voltage: 24V, DC
Current: 12A
RPM: ?
Cutting width: 19"

CMM1200
Voltage: 24V, DC
Current: ?
RPM: ?
Weight: 76 lbs
Cutting width: 19"

CM1836
Voltage: 36V, DC
Current: ?
RPM: ?
Weight: 62 lbs
Cutting width: 18"

CM1936
Voltage: 36V, DC
Current: ?
RPM: ?
Weight: 72 lbs
Cutting width: 19"

120Vac Plug-in models

These all have a Black and Decker 12A motor, according to the product info at Black and Decker. I suspect there is a step-down transformer involved, otherwise these would consume much more power than the 12V battery-operated CMM1000. As such, the wall-outlet-current is unknown until I can find a model and measure it.

MM575
Motor current: 12A
Wall outlet current: ?
Weight: 41 lbs
Cutting width: 18"

MM675
Motor current: 12A
Wall outlet current: ?
Weight: 46 lbs
Cutting width: 18"

MM875
Motor current: 12A
Wall outlet current: ?
Weight: 52 lbs
Cutting width: 19"

MM1800
Motor current: 12A
Wall outlet current: ?
Weight: 42 lbs
Cutting width: 18"

New electric mower models coming from Black and Decker

2010-01-30T19:21:00.007-05:00

Table of Contents

I just saw that Black and Decker is coming out with three new electric mower models -- two cordless and one corded. Amazon has them available for pre-order, with a release date of March 1, 2010. The new battery powered models are the CM1836 and CM1936, which have 18" and 19" cutting widths, respectively, and use a 36V battery. Compare this to 19" and 24V for the CMM1000 and CMM1200.

The new corded model is the MM1800, with an 18" cutting width.

Here is the info from Black and Decker on these new models:
CM1836
CM1936
MM1800

I can see reasons why the CM1936 could be better than the CMM1000 and CMM1200. For one, the 36V battery is easily removable, so it would be easy to swap in a second battery and cut larger lawns. Second, the 36V battery would require less current than a 24V battery, for the same amount of power. The lower current would mean smaller guage wiring is necessary. The cutting width of 19" is the same as in previous models.

On the other hand, the CM1836 appears to take a step backward, with it's narrower 18" cutting width. The battery is not easily removable, so in that respect it is no more convenient than the CMM1000 and 1200.

Of course, I am quite attached to my trusty CMM1000 and wouldn't dream of trading it in. Yet.

Fixing a problem (June 2009)

2009-11-12T23:41:00.009-05:00

Table of Contents

The Story

Last spring, my CMM1000 mower developed a problem where it would simply stop running after 5 or 10 minutes. It would restart again after a minute, but then only run a few minutes before stopping again.

The problem turned out to be a faulty circuit breaker, which is supposed to cut the motor off when the current exceeds 40 Amps but was actually tripping at a lower current. At first I misdiagnosed the problem as either a worn out battery or bad switch contacts. At a loss as to what the cause was, an internet search led me to other people with the same problem ... and more importantly to somebody who had fixed it by replacing the breaker. And so I extend a hearty thank you to "rothompsons", whoever you are, for posting the solution at forums2.gardenweb.com.

After ordering a new breaker for $19 ($12 part + $7 shipping) from ereplacementparts.com, the mower was up and running again! In the intervening month it took to find and fix the problem, I used our trusty manual push-reel mower to cut our grass.

By the way, my original breaker lasted 3 years, and rothompsons's lasted 2-1/2. Another person did report that their mower was 6 years old when they first encountered the problem. At any rate, you can expect your breaker to fail eventually. You'll know it's the breaker if your mower stops, and can be restarted only after you hear a "click" about 30-60 seconds later. The click is the automatic reset switch inside the breaker, after it has cooled down enogh to close the circuit again.

Repair details

If you need to replace your circuit breaker, here is where you can find it on the mower's Main Circuit Board:

Main circuit board, showing location of circuit breaker.

The breaker is in series with the main switch, and when tripped breaks the connection from the battery to the motor. It is attached to the main circuit board using nylon-insert lock nuts:

Main circuit board, showing electrical path between Battery(+) and Motor(+) terminals.

A view of the brand new circuit breaker, part # 5140004-81 from ereplacementparts.com:

The brand new circuit breaker (includes nuts and lockwashers) from ereplacementparts.com

Request of readers:

And finally, here is a request of anyone reading this blog. If you can find these circuit breakers for cheaper than the $12 I paid for this one, I would love to hear about it! Please note, it is manufactured by Cooper Bussman, and is their Shortstop series 120 circuit breaker. It should be rated for:

24 V
40 A
Must be of the automatic reset, thermal cycling type, not the manual rest type.

I have found a 12V version for under $4 at www.wiringproducts.com, but so far not a 24V version. It would be nice to order a few of these to have in stock.

Helpful links

2009-11-01T16:13:00.006-05:00

Table of Contents

From time to time I run across useful info about the CMM1000 and CMM1200 mowers. As this blog grows, I plan to keep adding links to this entry so they can all be found in one place.

Instruction manuals

CMM1000
At amazon.com, click here
At safemanuals.com, click here, then click on "Download the complete user guide"

CMM1200
At tylertool.com, click here, then click the "MANUALS" tab, then click on "CMM1200 Manual"

Replacement parts

Available at ereplacementparts.com
For CMM1000 parts click here.
For CMM1200 parts click here.

[Added May 2011:]
As of spring 2011, I'm finding some parts discontinued at ereplacementparts. An alternative, though incomplete, supplier is abtecparts.com
Thanks to Ajay for telling us about abtecparts.

Product pages at Amazon

Useful mainly for the customer reviews you can find there.

For the CMM1000 click here.
For the CMM1200 click here.

Handy home-made tool; fixing a broken wire

2009-10-31T22:56:00.007-04:00

Table of Contents

Handy tool for propping mower cover open

I'll start by saying that it's a real good idea to make a tool like this because it relieves stress on the wiring and connections when you have the top cover open. By using a tool like this, hopefully you won't bend and break one of the wires like I did.

When opening the mower cover, I used to let the wiring and cables support the weight of the cover. In my defense, I'll say that the wiring certainly looked sturdy enough and up for the task. AWG 10 wire is fairly hefty and strong, but I wasn't aware of the short length of solid AWG 24 that was the weak link in this system. While replacing the broken wire, I made a prop tool to support the cover as shown in the photos below.

The pictures pretty much tell you all you need to know to make one of these yourself. I'll just add that (1) about 15 inches between the two end slots seems to work pretty well, and (2) when you use one of these, use a small tool or pen as shown in the second photo to keep the cover from sliding off the mower.

Fixing a broken wire

Here is something you shouldn't have to worry about if you either:

1. Use the cover prop tool described above, or
2. Don't remove the cover from your mower in the first place.

Since I did neither of these before now, a wire broke and needed to be fixed. The break happened within the white insulated part of the wiring, between the battery "-" terminal and Main Circuit Board:

Note, this is part of a wire assembly that runs:
1. From the battery "-" terminal to the Main Circuit Board, and
2. From the battery "-" terminal to the motor "-" terminal

Since the break was not between the battery and motor, the mower would actually run. However, the broken wire was part of the braking system, so when the mower was shut off it would take 10 seconds or so to spin down. Normally the brake stops the mower within 1 second.

Here is the complete (but broken) wire assembly, removed from the mower:

An close up look at the break reveals two wires, a small AWG 24 solid (hence not too flexible) wire had been crimped into an AWG 10 stranded cable. Repeated flexing or stressing at this connection by my repeated removal of the cover was almost surely the cause.

Not sure why there is this combination of small and large wiring here, it probably has something to do with the braking action that stops the motor by shorting it through this wire assembly. Best thing is to reconnect these wires back together, but I don't have whatever tool was used to press the small wire into the strands of the larger cable. Instead, I used a soldering iron (40 W) to redo the connection:

Add about a 1/8" - 3/16" or so thick coating of hot-melt glue around the joint to prevent flexing, and we were good to go!

The Remington MPS6017A electric mower

2009-09-19T20:41:00.008-04:00

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Today I got a look at another brand of electric lawnmower, the Remington model MPS6017A, that some friends of mine own. I was called over to investigate a weird screeching noise it was making upon start-up and shut-down. We traced the problem to a loose nut atop the motor spindle, and simply tightening that nut stopped the noise.

Unfortunately, the battery terminals were badly corroded and one of them broke off. As the battery seemed to be on its way out (not holding charge as long as it did when new, somewhat over a year ago), replacing it is a good idea anyway. I'll just take this opportunity to mention that battery-powered mowers should be stopped for recharging whenever the battery charge gets noticeably low, as running down this type of battery does shorten its useful life. Fortunately, the Remington can run on household 120V in addition to the battery.

Moving on to the main reason why I'm posting today -- this was a good opportunity to see how a different brand, my friends' Remington, compares with the Black and Decker. While I didn't get a chance to actually use the Remington on a lawn, I did notice differences between the two brands that piqued my interest.

For starters, the Remington uses five small 12V batteries to generate 60Vdc, whereas the Black and Decker uses two larger-sized batteries for 24V. Presumably the higher voltage Remington uses a lower electrical current than the Black and Decker, assuming they run at comparable power levels.

Some things I like better about the Remington are:

The battery is easily removable, so that one could quickly swap in a second battery pack when doing larger lawns.
The option to use 120 V AC household power when the battery loses its charge.
The circuit breaker has a manual reset button. On the Black and Decker, you have to wait about 30 seconds for the breaker to automatically reset.
The lower electric current means that the wires and connectors are smaller, therefore easier to work with in case you need to redo any wiring.

Here is what I like better about my Black and Decker:

It has a meter showing when the battery charge is low during mower operation, which the Remington lacks. This is important for not running the battery charge down too low and damaging the battery.
The blade width is slightly more, 19" vs. 17" for the Remington.
It looks like there is room to add larger-capacity batteries, if one were so inclined.
(added 11 Apr. 2010) The company has not gone out of business, like Remington has, and it is possible to buy replacement parts when necessary.

That's it for now.

Circuit details

2009-08-15T23:18:00.014-04:00

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Today we'll be getting fairly technical. After some photos of the mower's innards, we'll check out the circuit schematic. This is intended for people with an understanding of basic electronics; if you're not one of those people, just enjoy the cool pictures!

When we remove the black plastic cover from the mower, we find that the two batteries are secured with a strap and two styrofoam blocks:

Click on photo for full picture.

Four cables are used to connect the batteries, motor, and main circuit board to one another:

Click on photo for full picture.

Note the wiring of the two 12V batteries in series, to generate 24V for the motor.

There are two printed circuit boards inside the mower. Black and Decker says nothing about them in the owner's manual, so I have taken it upon myself to name them:

1. The Main Circuit Board (green side visible in photo below) sends power from the battery to the motor. It contains the main switch and circuit breaker.

2. The Charger Interface Circuit Board (white side visible in photo) controls the recharging of the batteries, and is (apparently) not active when the mower is in use.

Click on photo for full picture.

Here is the circuit schematic for the CMM1000, Type 5. I imagine Types 1 through 5 may all have the same circuit, and perhaps the CMM1200 does as well, but I wouldn't swear by it.

Click on figure for full picture.

Some rambling observations about the circuit:

1. The two 12V batteries are wired in series to produce 24V. The batteries that come with the mower are from B.B.Battery, model # BP17-12 (17 Amp-hours, 12 Volts). I have since replaced them with 22 Amp-hour batteries of the same physical size. I'll write more about them in a future blog.

2. The Main Circuit Board controls the power to the motor via a circuit breaker and the main switch. The main switch is controlled by a cable that runs up to the handle where you, the user, pull on it using the switch lever.

3. If the mower is running and the operator releases the handle, the main switch returns to the "Off/Brake" position. This disconnects the battery and instead shorts the motor terminals. Without going into details, the short provides a fast breaking action for the motor rather than letting it spin down gradually. When the breaker opens and the handle is not released, the main switch stays in the "On" position and the motor spins down gradually.

4. The brake wire used to short the motor is rather long, and makes 14 loops of a roughly 5" x 1" area. (See 3rd photo.) Perhaps this is to provide some small inductance, but I don't understand exactly why this would be beneficial.

5. The circuit breaker is rated for 24V and 40A. It contains what appears to be a bimetal switch. Presumably the heat generated when the current rating is exceeded bends the bimetal into an open position. I have found that about 30 seconds after opening, the breaker switch on my mower has cooled enough to close and the mower may be restarted. I also found that heating directly with a hair dryer does not cause the bimetal to open, so it must be quite hot when it does.

6. There is a 68 ohm resistor inside the circuit breaker. It would provide a current path whenever the breaker opens, and is here presumably because of the inductive load presented by the motor. I'm admittedly curious about why the resistor is there and details of what it really does. (How I got in the position of prying open the circuit breaker is a story for another day.)

7. The Charger Interface Board is a maze of components and it would take a long time to break it down into a detailed schematic, so for now it remains something of a black box. The main features (not shown here) are 4 diodes + 1 capacitor (full wave rectifier?), an LM317 regulator, and a 14-pin IC. There are also oodles of resistors and some more diodes and capacitors. All I know for certain is, as shown in the schematic, that it contains the two indicator lights and receives the charger plug.

Getting replacement parts; customer reviews at amazon.com

2009-08-11T18:30:00.013-04:00

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In the process of making up for three years of not blogging, today I'll post the mower review I wrote at amazon.com. Yet to come are photos of the mower's innards, a basic circuit diagram, and some trouble I went through in recent months to fix the mower.

Getting replacement parts

Here are extremely useful links for all CM1000 and CMM1200 owners. Replacement parts can be ordered online from ereplacementparts.com:

CMM1000 parts at www.ereplacementparts.com

CMM1200 parts at www.ereplacementparts.com

At the ereplacementparts website, you'll need to click on the appropriate type number in order to see the drawing and parts list for your mower. Type numbers are similar to software version numbers: whenever Black and Decker upgrades the mower design, it increments the type number. Type 5 was the final version of the CMM1000.

You will find the type number printed on the back of the mower, facing you as you are pushing it:

Model CMM1000, Type 5

Reviews at amazon.com

Here are the customer reviews for the CMM1000 and CMM1200 at amazon.com:

CMM1000 reviews at amazon.com
CMM1200 reviews at amazon.com

Here is my own review at amazon.com of the CMM1000, written after I had used it for a full year:

_____

(Rated 4 stars out of 5)
Good mower, but not for large lawns
May 24, 2007
By Mark W

I've had this lawn mower for 1 year now, and am thoroughly happy with it. My wife and like the quiet motor (about as loud as a vacuum cleaner, but much quieter than a gas engine mower). It is a good mower for small lots. We have a 1/4-acre, and one battery charge can do about 3/4 of our yard. I typically mow half the yard, let the battery recharge, then finish the yard later that day or the following day.

Two important points on mower care:

(1) Keep the blade sharp. Performances degrades noticeably with a dull blade. I bought a second spare blade, and change the blade when the one on the mower becomes dull. Then I can sharpen the dull blade at my leisure, and still have a working mower in the meantime.

(2) Keep the battery charged, and stop mowing when the battery charge gets low. Letting the battery run down will shorten it's useful life. I've calculated that it only costs about $3 extra a year to keep the battery on the charger when not in use. If the battery indicator gets low while mowing, stop and recharge it rather than continuing mowing on a low battery. It might sound inconvenient, but I do prefer this to having to run out and buy gasoline for a gas mower. Also, keep the battery charger on during the winter.

Having looked inside this mower, it does look like there is room for a larger capacity battery. Somebody who is mechanically and electrically inclined could probably increase their mowing time by 50% or more by retrofitting a longer-lasting battery. (Anybody who tries this assumes any and all risks involved, of course ...) It uses two 12-Volt sealed-lead-acid batteries to generate 24 Volts.

Things I like about this mower are:

The easy wheel height adjustment, which takes about 2 or 3 seconds to do. The height adjusts in 1/3-inch increments. I like to cut grass a little shorter along sidewalks than in the main part of the yard, and it's easy to do this.

Mowing in "mulch" mode. I don't need to collect and dispose of grass clippings, though one can use the grass-collector that is included with the mower if one wishes.

Using the mower to cut up raked leaves and light brush.

Not buying gasoline.

Starting the motor is very easy.

Two things I don't like are:

Dull blades do not cut worth a darn. It seems that gas mowers can still cut grass fairly well with a dull blade.

Sharpening the blade could be easier if the blade did not have this twisty, bent shape.

UPDATE, Aug. 2009
I had to replace the circuit breaker on the mower a month or so ago; the mower is now 3 years old. It cost $19.00 to fix on my own.

The problem was that the mower would keep shutting down after running just 5-10 minutes. After waiting about 30 seconds it could be started up again, only to shut off again after a short while. It's running okay now, but it took me some time to diagnose the problem correctly.

Replacement parts are available [...]:
[...]
Search on cmm1200 at that website if you have the cmm1200 model.

_____

Note: The good people at amazon.com had edited out the replacement parts link "[...]" from my review; it is the www.ereplacementparts.com website I gave above.

Three years and three months ago . . .

2009-08-10T20:59:00.003-04:00

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I am three years behind.

My wife Pat and I bought our cordless electric mower just over three years ago. Wanting to cut down on home expenses, we bade farewell to the man we had been paying to cut our grass and decided we'd start doing it ourselves. We just had to pick out our new lawnmower.

This was the spring of 2006. While there was already a considerable movement towards using less energy, it had not really hit the mainstream. This was just before Al Gore's An Inconvenient Truth would play in theaters nationwide. Corporations were not yet bandying about their Green products with the alacrity they do today, the Toyota Prius notwithstanding. There was just our reluctance to using a gasoline powered mower, both for the noise it made and the annoyance of having to buy gasoline for it. So we went out and bought a battery powered model, the Black and Decker CMM1000*, for $450.

As I've used this mower I have learned to maintain it and not be annoyed with its shortcomings. Being fairly handy with electronics and mechanical things, it has not been difficult--for the most part. But it may be more difficult for others, and that is why I am starting this blog. Battery powered mowers have different issues than the gas mowers many of us grew up with. My main hope is that people find this blog a useful resource as they deal with the frustrations that can come with owning and using a cordless electric mower. I had wanted to start blogging about, or at least documenting the mower, the first summer we had it, but am just now getting around to it. Three years later.

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*The CMM1000 is no longer available, having been replaced by the newer CMM1200.

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Update, 6/30/2011: This blog now has a Table of Contents

.

Marks Electric Mower

An online lawnmower forum

Another new battery

Table of Contents

Wobbly wheels & how to fix them

Cordless electric string trimmers (a.k.a. "weed whackers")

Common problem with CMM1200: Broken Hinge Pin on the Switch Lever

The battery

Mower specs

New electric mower models coming from Black and Decker

Fixing a problem (June 2009)

Helpful links

Handy home-made tool; fixing a broken wire

The Remington MPS6017A electric mower

Circuit details

Getting replacement parts; customer reviews at amazon.com

Three years and three months ago . . .

Marks Electric Mower