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Lawn Mower Starter Battery Part 2
Continued from

Hey, Charlie, sorry for the late response, but yeah, I get what you mean. It looks like these chargers have a similar algorithm to the TP4056 chargers. That the end current cutoff is around 1/10 of whatever the maximum is set to. Knowing this has made a difference in how I charge and balance my batteries.

Here's a little update on the mower battery.

Sorry the image is a bit blurry, but you can see that after about 2 years of use, it's still got a fair amount of charge left. Although it is getting imbalanced despite my attempt to put similar capacity cells together. But it still works great, considering these are old laptop cells.

Just for fun, I did a little math, and found out that theoretically the battery could make 523 attempts to start the lawn mower between charges. Unfortunately, in real live, once the battery gets down to about 12V, it just can't produce the Amps needed for the starter.

On the other hand, just three 20A cells could replace this battery and make over 13,000 attempts between charges. For $12 I think maybe I'll try that and see what happens.

By the way, the SLA direct replacement battery for this mower is $70. A home-made LiIon battery would be cheaper, smaller, lighter, and way more fun.
-Mike G
I found your first thread and discovered this one. Good to hear that it is working out!

I have the same problem, my lawnmower also uses a battery but a replacement including shipping costs will be €75,- or a bit more.
I managed to collect the cells now, it's going to be the same size as yours 3s6p.
As for a BMS, I have one that can deliver 40A, do you think that will be enough?

I'm going to spotweld the batteries, I hope to get to it this weekend, will posts some photos of the progress Big Grin
My starter takes about 22A. I measured it by using an ammeter and shunt, along with a car battery.

If your's is similar, then a 40A BMS will hopefully work. I only had a 20A BMS and it did a great job of shutting off the circuit. So unfortunately, I have no safety features built into this one. But I also watch it very carefully!

I hope your project is successful. Smile
-Mike G
Yes I understand Smile

I am building it, school is over now, only work during the day so that leaves me some free time.  Cool

With the last cell I messed up. I used 0.3 nickelstrip and my spotwelder did put in too much heat.....
The protection melted and shorted the + side with the negative side......

This was the idea 4s8p.

And with the last cell.....

The otherside:

I ordered some new nickelstrips and some protectors.
This evening I will tear down the whole thing and start over.
Replacing the damaged cells also.

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