48V / 52V Dual-use battery pack feasible?

tomtom50

100 W
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Jun 18, 2018
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Would it work to make a battery pack that can be used both 13S and 14S

A dedicated 48V charge/discharge cable connected 13S, with a 13S BMS, and at the same time, wired in parallel, a dedicated 52V charge/discharge cable connected 14S, with a 14S BMS.

After being used 48V / 13S it would be charged 13S, the 14th group neither charged or discharged, because it is not connected to the 48V charge/discharge cable.

After being used 52V / 14S it would be charged 14S, all groups charged and discharged.

It would be important to charge it the way it had just been used (13S or 14S) so all parallel groups being charged are at similar charge.

Since the BMS are connected simultaneously would they interfere with each other?
 
tomtom50 said:
Since the BMS are connected simultaneously would they interfere with each other?
Depends on the BMS. They will almost certainly discharge the 14th cell at a different rate.
It would be important to charge it the way it had just been used (13S or 14S) so all parallel groups being charged are at similar charge.
This is nearly impossible without a balancing charger.

An additional problem is that the 14th cell will degrade at a different rate than the others and lead to balancing problems as the pack ages.

Not sure why you would want to do that anyway. A 3.6 volt difference isn't that much to begin with. Why not use it in 52V mode all the time? And if you have to regulate down, you can do it with a linear regulator - the time it will be dissipating power will be very small, since the drop from 4.2 to ~3.8 volts per cell happens very rapidly.
 
billvon said:
Not sure why you would want to do that anyway. A 3.6 volt difference isn't that much to begin with. Why not use it in 52V mode all the time? And if you have to regulate down, you can do it with a linear regulator - the time it will be dissipating power will be very small, since the drop from 4.2 to ~3.8 volts per cell happens very rapidly.

Thanks. Good idea on the linear regulator.

The issue is that I have a 52V pack and I don't think KT controllers sense 14S. They will sense 13S and impose 54.6V high limit. So the controller won't work until pack drops below 55V.

Recommendation for a linear regulator to use for 20A?
 
tomtom50 said:
Recommendation for a linear regulator to use for 20A?
I would use a LM431 regulator for control and a P-channel MOSFET to pass the current, so you don't need an additional supply. (Probably need an NPN transistor in there somewhere too) Set to regulate to 55 volts.
 
billvon said:
tomtom50 said:
Recommendation for a linear regulator to use for 20A?
I would use a LM431 regulator for control and a P-channel MOSFET to pass the current, so you don't need an additional supply. (Probably need an NPN transistor in there somewhere too) Set to regulate to 55 volts.

Thanks
 
I use a 14 S pack with a pair of q128 motors and S12sh controllers w/ L CD 5 meters.
The motors shut down a few times on full charge but since I use throttle only it is just a matter of releasing the throttle then re-engaging to get the power back. This may sound like a pain in the ass , but it really isn’t. As soon as the battery level drops below 56 volts the problem goes away. I have put 1000 miles on this set up and don’t think of it as a problem.
 
Hwy89 said:
I use a 14 S pack with a pair of q128 motors and S12sh controllers w/ L CD 5 meters.
The motors shut down a few times on full charge but since I use throttle only it is just a matter of releasing the throttle then re-engaging to get the power back. This may sound like a pain in the ass , but it really isn’t. As soon as the battery level drops below 56 volts the problem goes away. I have put 1000 miles on this set up and don’t think of it as a problem.

Thanks, helpful info. I think I'll try it that way and see how it works out.
 
I've not seen the HVC with 14S on my KT controllers. I use the square wave and sinewave 6 FET 20A models, and a 25A model from Luna. I've never run my S12S on 14S though. The 14S though is the only battery I charge to 90% though.

What I've seen is a minor LVC issue. It remains set at 40-41 V with 14S. If I run my 14S that low, my 14S charger won't start up. I have to bring it past 45 V with a 13S charger. Then finish with 14S, or watch the voltmeter and stop around 46V.
 
I honestly don’t know at what level my low voltage cutoff works. When the 14s battery gets down to between 47 and 48 volts the performance is so sluggish that I stop and recharge.
 
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