How To Revive a “Dead” Cordless Mower – Part 2: Replacement Batteries

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by Tim Fulton on August 8, 2011

In our last article, MetroMPG troubleshot and found out that the battery was dead on his newly acquired free electric mower. At this point, most of us would take a look at the battery to get the specifications, find a suitable replacement, buy it and throw it in the mower and go. Its a perfectly good and easy way to get a free mower working for around $70 as that is the cost of a new 28Ah replacement battery.

However, we are not MetroMPG. He is one of those resourceful guys who doesn’t really like paying for things and instead finds ways of making things work another way. In this specific case, MetroMPG has a friend who works for an alarm company. This friend routinely replaces batteries out of the alarm systems that are no longer up to the task. This doesn’t necessarily mean the batteries are junk, it just means that the batteries hold less than about 80% as much as they should. This translates to shorter run times for the mower, something MetroMPG was willing to live with.


MetroMPG contacted his friend and was able to get a few batteries shown above. Seeing as they were used, he needed to test them to make sure they were good. To do this, MetroMPG charged up all the batteries, and then discharged them one at a time. He used a power inverter and a 40W light bulb. The longer the batteries last before the voltage gets to the inverter’s built in shut off, the better condition they are in. Of the two 12Ah batteries he had, one lasted 53 minutes while discharging and the other lasted 57 minutes. Of the three 7Ah batteries, two lasted about 7 minutes and one lasted 45 minutes. He also had a good 12Ah battery laying around from another project that lasted 2 hrs.


With all his batteries tested, MetroMPG decided to use the best 12Ah battery and the 7Ah battery. This theoretically gives him only 19Ah versus the 28Ah the mower originally had, add to the fact that these are used batteries and he actually has less than 19Ah.

Once the two batteries were all charged up, MetroMPG was ready for the mowing test. He was able to cut the entire lawn successfully in just over 20 minutes with plenty of charge to spare. The battery voltage was only down to about 11.9V which is good. 11.5V is considered 50% discharged, so he is in great shape.

For more information on the fix, see MetroMPG’s forum thread.


1 wogga June 7, 2012 at 9:03 am

I have that mower (or a similar one). I’d suggest you stick to one battery or at least similar batteries of equal size/rating. Charging batteries of different size together in parallel isn’t a good idea without separate charge controllers on each battery

2 Darin September 18, 2013 at 3:08 pm

I didn’t have a single battery with enough capacity to get the job done. But I’ve now been using the mower for 2 years and a bit with two smaller capacity batteries (same rating, but different ages) in parallel. I understand the implications of running imbalanced batteries in parallel, but it has worked out well, so… lucky me.

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