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

Table of Contents

Important note: if you have a question to ask, please ask it at Lawn Mower Forum, either in the Black & Decker area or the Electric & Battery Operated area at that website. I apologize for being unable to handle individual requests for help posted on this blog or sent by email.
 

                                                                                
 

"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

Table of Contents

Important note: if you have a question to ask, please ask it at Lawn Mower Forum, either in the Black & Decker area or the Electric & Battery Operated area at that website. I apologize for being unable to handle individual requests for help posted on this blog or sent by email.
 

                                                                                
 

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.

                                                                                

Update, 9 Sep. 2012: It is worth mentioning here that a lot of people have been successful using a 3/16" x 1-1/2" tension pin, as first commented on by Catherine in August of 2010. Also known as roll pins or spring pins, people have obtained them at True Value Hardware, Ace Hardware, and Lowe's for less than $1.00. (Presumably Home Depot, and other home centers and hardware stores, would have these as well.)

So, a big Thank You to Catherine for the idea.

The battery

Table of Contents

Important note: if you have a question to ask, please ask it at Lawn Mower Forum, either in the Black & Decker area or the Electric & Battery Operated area at that website. I apologize for being unable to handle individual requests for help posted on this blog or sent by email.
 

                                                                                
 

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

Table of Contents

Important note: if you have a question to ask, please ask it at Lawn Mower Forum, either in the Black & Decker area or the Electric & Battery Operated area at that website. I apologize for being unable to handle individual requests for help posted on this blog or sent by email.
 

                                                                                
 

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

Table of Contents

Important note: if you have a question to ask, please ask it at Lawn Mower Forum, either in the Black & Decker area or the Electric & Battery Operated area at that website. I apologize for being unable to handle individual requests for help posted on this blog or sent by email.
 

                                                                                
 

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.