Cordless lawnmower SLA > lithium upgrade advice

[mrzed]

Just moved to a new house, and now I have a 1/3 acre property with more grass than I can handle by dragging the 100' extension cord around. I do however have 116 tested makita cells from DrBass in the garage from my earlier plan on building myself an ebike battery that got long delayed (something about a toddler, a grade schooler and a full-time job I suspect). With the new big lawn, the ebike takes a back seat, because I have legs to pedal, and there's no way I'm burning gas to cut a lawn, I'd rather get a scythe.

I did find for $20 one of http://solarispowerproducts.com/index.htm. It's a 24v SLA mower with a brushless motor. I thought I was being clever, but now I'm concerned because the thing likely has a controller attached of some sort, so I'm a little concerned it may try to outsmart me if I mess with the voltage too much.

I can't find anything out about the actual specs of the thing online, but the nut of the issue is this:

- Go 7s and hope the higher voltage at full charge doesn't trip some error in the little brain attached to the motor
- Go 6s, but perhaps limit the depth of charge I can get because the LVC it uses for the SLA will be lower than what is reasonable for the makita cells

The brain is built right into the motor apparently. I'm assuming it has a circuit of some sort in there, because of the 4 wires coming out for the LED battery indicator, on/off switch, and the description on the unit of the error codes from the LED's.

 

[amberwolf]

How about go 7s and not fully charge?

 

[dogman dan]

I would try 7s fully charged, and see what happens.

Still just under 30 volts, if there is something in the controller like a 30v capacitor. It will drop to lower pretty quick in use, so I see no danger to the electronics. It could have a high voltage cutoff set to 28v, but I doubt it's really that low.

If you were to charge the cells to 4.1v, you'd only have 28.7v or less.

I run my 24v mower which lacks a controller on either 6s or 7s. When the grass is thick and tall, it takes 7s to power through without a lot of choking on it.

Cool that your mower has a controller, it will prevent the kind of insane amp spikes my mower gives my batteries because nothing stops it from sucking up 70 amps or more if I hit a thick spot in the lawn. The thick wires get really hot, and then the motor overheats and stops running till it cools and the reset clicks.

You can get a cheap wattmeter or voltmeter to know when to stop before overdischarge, but I have found that when that mower starts to sound slow, it's still in the safe zone to stop in time. So now I just go by the sound of the mower, when it's not cutting. Once it's idling slow, I stop.