Power tool batteries for ebikes

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Power tool batteries for ebikes

Post by Kneerly Down »

I have a stash of DeWalt 18V batteries and a desire to convert one of my pushbikes to electric.

The 5Ah batteries power the 18V cordless strimmer, which has a 400W motor.
They do get a bit hot and last only 20 mins or so.

But, with a 36V 350W bike motor and 2 (poss 2x2) 5Ah batteries in series and with the more varied, and lower, power demand from the ebike motor I would have thought the batteries would cope.
I know people have done this, but it doesn't seem all that widespread.
Most of the riding on the bike would be unlikely to be more than 5-10 miles and if it were over that there'd be sections where I could cope with no assistance.
I'd have a voltmeter on the bike to check that the SoC isn't too low although there does appear to be at least a rudimentary BMS on the batteries.
I like the idea of not having to buy a battery pack that most of the time is going to be dormant and will have yet another charger for.
Plus, if a battery goes bad it'll be a much smaller expense to replace than a single big pack.

So, is there a good reason not to do it?
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Re: Power tool batteries for ebikes

Post by Le_Fromage_Grande »

It could catch fire and burn your house down
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Re: Power tool batteries for ebikes

Post by Mussels »

It might work but Cheesy is correct.
I looked into something similar a while back and several people have done this but I think they have always put a lot of effort into getting near identical cells. This means getting several from the same batch and testing the capacity of each to find matching ones. Otherwise when they are connected in series stronger cells will overpower weaker ones resulting in reduced life or something more catastrophic.
You also need to check the charge/discharge rate is suitable for your motor.

If you are charging each cell individually then the main fire risk is when you are out riding, I wouldn't try charging them as a pack.
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Re: Power tool batteries for ebikes

Post by Kneerly Down »

Mussels wrote: Fri Jul 09, 2021 10:59 am It might work but Cheesy is correct.
I looked into something similar a while back and several people have done this but I think they have always put a lot of effort into getting near identical cells. This means getting several from the same batch and testing the capacity of each to find matching ones. Otherwise when they are connected in series stronger cells will overpower weaker ones resulting in reduced life or something more catastrophic.
You also need to check the charge/discharge rate is suitable for your motor.

If you are charging each cell individually then the main fire risk is when you are out riding, I wouldn't try charging them as a pack.
I would be charging them as individual batteries, as they were designed to be. I have a couple of chargers already.
I'd also be monitoring them as individual batteries, to make sure neither/none of them are discharged too far (though the onboard BMS, such as it is, may already do this.

As said, the batteries already power an 18V 400W motor in the strimmer, pretty much continuously to fully discharged, so 2 in series should be fine for powering a 36V 350W motor intermittently.

I'd be more concerned at having the 2x2 arrangement as then the 'packs' would need to be balanced or managed electronically, although they could also be used sequentially instead of parallel.
I am a bit tempted by the Aldi 20/40V batteries as they are cheap and IIRC have a 3yr guarantee, but might need to split one open to see which Samsung cells they are actually using.
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Re: Power tool batteries for ebikes

Post by Le_Fromage_Grande »

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Re: Power tool batteries for ebikes

Post by Mussels »

It sounds like have done your research, as long as you can jump off quickly if they do catch fire then give it a go.
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Re: Power tool batteries for ebikes

Post by demographic »

Depends what battery platform you use for some of this.
For instance De-Walt sells some Flexvolt batteries which give out either 18 volts or 54 volts depending on what tool they're connected to.
In the 54 volt configuration the tool windings don't get so hot. More volts, less amps for the same Watts.
The 18 volts produce more amps (and heat) for an equivalent amount of watts output.
Hiitachi/Hikoki are doing more or less the same but its 18/36 volts and I think Makita also do 18/36 but as I don't have any Makita cordless stuff I'm not sure.

I think thats how it works anyway.