Connecting batteries in series

[mudflap5]

Currently have a 14s8p battery pack. Would like to connect a 7s8p pack in series and run the ebike at 21s8p.

Both packs are using the same cells and both have a bms.

If these two packs are connected in series, will the both discharge at the same rate?

 

[Nobuo]

If the cells are same model, same or very similar batch, and have a similar number of cycles, both packs should have identical internal resistance, so them would work exactly as if you made a 21s8p battery.

Of course BMS should have similar discharge and charge rating.

The only thing to really take into account is that when you connect them in series, there is a chance to destroy BMS mosfets gates, so an anti-spark connector between both packs should avoid this happening

 

[dnmun]

except there is the problem of having the channel resistance of the mosfets in the second BMS inserted between #14 and #15.

so you have two sets of mosfets to eat up power and if it is the type of BMS with the opposing mosfets then you have a total of 4 sets of mosfets losing power. still do not understand why he does not just make a 21S pack with a 21S BMS. and use a decent BMS like the D131.

 

[mudflap5]

If the cells are same model, same or very similar batch, and have a similar number of cycles, both packs should have identical internal resistance, so them would work exactly as if you made a 21s8p battery.

Of course BMS should have similar discharge and charge rating.

The only thing to really take into account is that when you connect them in series, there is a chance to destroy BMS mosfets gates, so an anti-spark connector between both packs should avoid this happening

Thanks. I suspected that it would be OK to do so, just needed another opinion. Will use the XT-90S connectors.

 

[mudflap5]

except there is the problem of having the channel resistance of the mosfets in the second BMS inserted between #14 and #15.

so you have two sets of mosfets to eat up power and if it is the type of BMS with the opposing mosfets then you have a total of 4 sets of mosfets losing power. still do not understand why he does not just make a 21S pack with a 21S BMS. and use a decent BMS like the D131.

I would like to build just one pack, but for the capacity I need, it will not fit on the bike, have to split the pack. The 14S pack is already built.

 

[dnmun]

you could hack the BMS so you can bypass the output mosfets and carry the LVC signal down to the BMS with the controlled set of output mosfets to avoid the increased resistance of the mosfets at #15 and the loss of power from having to push current through a second set of mosfets.

then you can retain the HVC for each pack and figure out how to make it work with the two separate chargers.

 

[granolaboy]

You'll need to protect the BMS's from a possible reverse-current situation that would occur if one of your BMS's trips.

Just get a diode rated higher than your max Volts and Amps.

 

[dnmun]

can you explain how a reverse current would damage either BMS if one of them turns off? i cannot see how that can happen.

 

[granolaboy]

dnmun: I don't personally understand it either, other than I've seen more than a few posts from people (including Ping) saying they are necessary.

Threads like this one: https://endless-sphere.com/forums/viewtopic.php?f=2&t=35600

I don't know if I really need them or not, but I'm currently working on adding another battery in series to my scooter, and I'm going to add the diodes. They aren't going to hurt anything if they aren't needed, and I'll be glad I put them in if I do.

Perhaps someone with more EE knowledge could chime in on this...I'm only degreed in math and science

 

[dnmun]

i have asked ping to remove that since it misinforms people but since he makes money selling people the diodes he has no interest in removing it. it should be obvious there is no risk of overvolting the mosfets but everyone just blindly assumes there is a risk.

i am not an EE but i did take all the coursework for a BS in EE when i got my masters and i did go to grad school in EE for a year. but it does not take an EE degree to calculate what the voltage will be on the mosfets when the BMS shuts off.