weak battery pack parallel to a bigger pack?

murdomeek · July 9, 2021, 8:33pm

I have a really big main battery pack 12s10p 30qs.

But I want just slightly more capacity for a forest route I want to explore to be on the safe side. So I’ll attach a backpack battery and put it in parallel with the main pack. This is a random battery pack I had laying around 12s3p chinese cells that does 30a continuous.

If I draw peaks of 150-200a during the ride, the current would mostly route from the main pack right? Is there any risk of the ESC somehow drawing too much current from the small pack?

Would doing so be unsafe/unhealthy in any other way?

Thanks!

joshoujoe · July 9, 2021, 8:56pm

The batteries would become one so if you pull 150A, in Theory it would split 50/50 between the 2 packs wich would end up damaging the weaker one.

murdomeek · July 9, 2021, 10:33pm

That’s not true as the smaller pack would have different internal resistance during high current draw situations

Gamer43 · July 9, 2021, 11:01pm

They need to have internal resistance inversely proportional to their capacity.

I.e. the smaller pack needs to have three times the internal resistance of the bigger one.

murdomeek · July 9, 2021, 11:04pm

whats the typical IR of generic chinese cells?
is 3x 30q IR’s unrealistic? (Like are those IR values too crappy to even exist? XD

glyphiks · July 9, 2021, 11:12pm

Good discussion here.

Personally i think the best way to do it is to use the shit pack as a charge and ride pack with a buck/boost converter

murdomeek · July 9, 2021, 11:21pm

thanks for the link!
yeah thats probably the best way…

Gamer43 · July 9, 2021, 11:53pm

The total combined IR of the pack.

Meaning both packs need to be made with the same type of cells ideally from the same batch.

Otherwise you’ll get excessive load balancing currents between the two and wear out one pack faster than the other.

Fosterqc · July 10, 2021, 12:58am

I think a good BMS module would be a DC current input limiter. Man the OW folks would love that. Build in a tiny boost module for USB C too! hahaha

Venom121212 · July 10, 2021, 2:20pm

Do you even need a converter if they’re both 12s packs?

Gamer43 · July 10, 2021, 10:09pm

Yes if they have significantly different states of charge.

TheBoardGarage · July 10, 2021, 10:47pm

Backpack batteries themselves invite a bit more risk of failures as you pull more and more current through longer wires. I’ve read about the water hammer like effect that can happen, and just recently one of the Floatwheel people fried his ESC after running a backpack battery for about 7 months.

It may not be the most likely of risk, but it’s something to consider anyway.

mutantbass · July 10, 2021, 10:47pm

Check out endless sphere. There is a few threads where people mix high dischagre/low capacity batteries with high capacity/low discharge batteries and all seems fine.

I have a friend with a 12s4p where every 4p is 3 x samsung 50e + 1 x samsung 30T and he’s gets the best of both worlds. 18ah with 65a discharge. Zero problems.

Gamer43 · July 10, 2021, 11:11pm

How long has he been running this? The high discharge cell is going to fail catastrophically.

Ive seen it happen with an idiot who tried to reuse drill batteries.

mutantbass · July 10, 2021, 11:18pm

Over a year. How will it fail? The 30T becomes a defacto capacitor. With its high charge/discharge capability, it works fine.

More details would be nice.

Gamer43 · July 11, 2021, 1:12am

Capacitors can be cycled thousands upon thousands of times without issues; lithium ion batteries not so much.

Idiot used near end of life cells salvaged from drill batteries and threw them at a brushed motor. Failed in less than a week.

mynamesmatt · July 11, 2021, 10:37pm

use the old 12s3p pack with a discharge bms, hook it up to a cheap buck converter and make the output 50v, then use the buck converter to charge your 12s10p pack. The output is cc/cv usually so you can set the current output and voltage (5a works well for me)