Batteries in parallel to combine charge? BMS fine with it?

[Chronified]

Hey all. 

Before this gets bumped into the "non-tech related" section. This is about power draw and BMS protection. Very tech related.

 

I'm currently throwing together a fat-ebike for the winter months, my cities public transit is trash and my Razor style 48v scooter isn't going to be able to handle the snow at all.

 

I found this badboy on Amazon for $349 (yes I'm aware of the coaster brake, eHub kit comes with a new hub w/o it)

As well as numerous fat-bike eHub wheels on eBay for around the same price.

The one I've got my eye on to come back into stock (seller says mid November) is rear drive & 48v@1200w. Yes it goes faster than 32km/h, no I don't want to hear about it.

 

 

I'm wondering if I can run 2x smaller 48v/8ah battery packs like this one in parallel instead of running 1 large 48v/20ah cell. 

 

20ah cells are hard as hell to get a hold of due to airline restrictions. I'd need to build my own or wait for a 20ah to come via sea, without access to a spot welder my only option would be the latter.

 

I guess my question is: Would the BMS system made into prebuilt cells be able to balance the load of 2 cells hooked in parallel? 

 

I'd imagine it'd also increase longevity of the cells since they'd only be under 600w load each instead of 1200w load on the single 20ah pack.

I know I'll need to charge the cells separately, I'm prepared for that. I'd rather carry 2 easy to manage 8ah cells w/ handles than 1 awkward square 20ah cell, if I need to take it into work to charge. 

[Chronified]

Setup would be something like this.

[Chronified]

Bump

[Unimportant]

On 10/5/2018 at 8:37 AM, Chronified said:

Setup would be something like this.

No.

 

With both battery pack outputs connected in parallel like that, what happens depends on the internal circuitry of the battery packs. There's 2 options:

• The battery packs disable their outputs during charging. In this case charging will work fine, each pack essentially charging on it's own. However, when charging is done and the outputs are re-enabled, any small differences in battery pack voltage (which is guaranteed to be the case, no 2 packs will have *exactly* the same voltage) will result in a short circuit with very high current as the packs try to equalize quickly. Which is harmful for the cells and might damage the output switching circuitry or trigger protections.
• The battery packs do not disable their outputs during charging. In this case you're essentially putting both battery charger's outputs in parallel. What happens then depends on the charger circuitry - but it won't be good.