Converting a 24v lawnmower to 36v?

[heynow9991]

I've done this a few times, take an old 24v lead acid cordless lawnmower and convert to lithium. I did one where I built the battery from cells I had. It worked well at 7s.

I need to make another one and I want to do it fast and easy. I found a free 24v lead acid lawnmower. What I want to do is power it with Makita packs. I have two 18v 5 ah packs that I will use. I have adapters that allow me to power whatever I want with the packs. Just two wires coming off them.

So my choice is to either run them in series or parallel. Either 20v or 40v fully charged. I would prefer 40v but I'm concerned that I will "let the smoke out" at that voltage.

Any advice?

 

[Chalo]

There are two possible pitfalls to what you suggest. There may be electronic components in the mower that are rated less than 40V, that might fail. And the no load speed of the motor will be 50% higher on 36V than on 24V, but the running speed will be higher by a smaller margin. The motor will be running at a higher power level but in a lower efficiency range, and heating could be a problem. I doubt that mechanical overspeed would be an issue.

These packs are very cheap, $10 each or $9 each by the dozen. They're good for only 15A each, but they only weigh 3 lbs each, so you can put as many in parallel as you need to. They will charge nicely on a regular 24V lead acid charger.

https://batteryhookup.com/collections/more-featured-products/products/25-6v-6ah-153-6wh-lifepo4-battery-with-bms-24v.

 

[amberwolf]

It's probably a cheap brushed motor, no BMS, no electronics, but you should investigate before deciding P or S.

If it does have electronics, you'll have to make sure they can handle the S voltage.

If it is just a motor and the switch in the handle, then you can use series, *but*:

--the motor and blade will spin proportionally faster

--if that's faster than the hardware was designed for, you could have mechanical failures, including things flying off (like commutator segments inside the brushed motor). this is unlikely at such a small overspeed, but since things are usualy designed as cheaply as possible and only right down to the minimum required for a particular application and price point, there *is* a chance it's not capable of the higher speed somewhere inside some part.

--the switch itself will be rated for a certain voltage. Above that, there is a chance of it welding shut as it closes and being unable to open (turn off) anymore, or arc damage during opening causing contact failure to prevent closing the next time.




Another consideration is the current required to start the motor. If the new packs you use can't supply the startup-under-load current, then whatever their limiting factor is will fail. If they have a fuse, that will blow, if they have a BMS that monitors current and turns off at too high a current, that will turn off, if it doesnt' monitor current but is only capable of a certain amount, it will probably blow up the FETs at some point above that limit. Etc.

A lead-acid battery may sag down very very far under a heavy load, but it will still provide a heck of a lot of current while it's doing that, with nothing to stop it. The load usually drops pretty quick once the motor is moving, and the sag lessens, so there's enough voltage to spin the motor as fast as it needs to at that point.

But a protected lithium battery (which should be all of them, becuase of what can happen to damaged ones) will protect itself from that load, and prevent normal operation, if it's not designed to handle that load. If it isn't protected, and isn't designed for it, it could be damaged by it, which could eventually lead to a fire.

 

[fechter]

Seems like running the batteries in parallel would be low risk so you could try that and see if it’s fast enough. Lead acid batteries sag a lot under load so it might be pretty close to stock.