Friday, April 30, 2010

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, 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

   Replacement battery set for CMM1000 or CMM1200, from

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  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

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