charging question re: parallel batteries

[themyst]

Hi,

I have a unique situation when it comes to my parallel setup.

I have successfully paralleled two 17.5Ah 60V packs 16S 5P (Sanyo GA and LG MJ1 packs) and range has been dramatically increased (could easily hit 60 miles with a little tact on my Dualtron Spider).

My question is that the scooter came with a 66.4V charger. However, the manufacturer stated that 67.2V chargers are also fine, but they chose to include a 66.4V charger for increased cycle life.

Is there any ill effect from using the 66.4V charger to charge the internal battery and 67.2V 2A charger to charge external via their input lines? I have the two batteries disconnected when charging. Given that the two batteries should be within less than 1V of each other, I do not think there would be much ill effect during the connection as it would equalize around 66.6 -66.7V. The question would be if this is really bad for the pack over the long run, and if yes, why.

Second question would be, could I simply use the 66.4V charger to charge both packs while connected together? I understand that the external would be unable to balance and protect itself from overcharge using this method, but given that the charger is below the max charge, I would think this is safe (theoretically).

Thank you in advance for any guidance you all can provide!

 

[dogman dan]

DO NOT connect them at the discharge, and then charge both at the same time. That is not safe. You need to modify the bms charge connections to do that safely.

Close enough voltage to connect up once charged seperately, IMO. There will be a surge as the higher pack charges the other, but not that much. The reason I think its ok, is that the higher charged pack will not likely hold that full charge all the good overnight anyway. Its not that much watt hours in that last volt, so the surge is very short, and very quick the difference is much less than a volt. If you connected up one volt different at mid ride though, there could be a huge difference in watt hours, and a much longer, higher amp surge. Look how quick your voltage drops in the first block of any ride. You might lose that extra volt just by touching the throttle.


My main reason for thinking its ok, is that most bms can handle as much as 5 amps input, from regen if your bike had it. Your surge when you connect will not exceed that, I think.

But if you really want to equal those voltages, it might be possible to lower the second chargers voltage. Or, simply plug the pack into a regular light bulb for a bit to lower the voltage to 66.4 v before you connect up.


If its possible to run on just the higher voltage pack to start the ride, it will only take a half a block to lower the volts, I bet.

 

[themyst]

DO NOT connect them at the discharge, and then charge both at the same time. That is not safe. You need to modify the bms charge connections to do that safely.

Close enough voltage to connect up once charged seperately, IMO. There will be a surge as the higher pack charges the other, but not that much. The reason I think its ok, is that the higher charged pack will not likely hold that full charge all the good overnight anyway. Its not that much watt hours in that last volt, so the surge is very short, and very quick the difference is much less than a volt. If you connected up one volt different at mid ride though, there could be a huge difference in watt hours, and a much longer, higher amp surge. Look how quick your voltage drops in the first block of any ride. You might lose that extra volt just by touching the throttle.


My main reason for thinking its ok, is that most bms can handle as much as 5 amps input, from regen if your bike had it. Your surge when you connect will not exceed that, I think.

But if you really want to equal those voltages, it might be possible to lower the second chargers voltage. Or, simply plug the pack into a regular light bulb for a bit to lower the voltage to 66.4 v before you connect up.


If its possible to run on just the higher voltage pack to start the ride, it will only take a half a block to lower the volts, I bet.

You're right, after I take the charger off overnight on the external, multimeter shows the voltage to be 66.8V at rest, so that makes sense. I can't imagine the current transfer from one pack to the other of 0.4V to be all that statistically significant.

Thanks for your response!

 

[dogman dan]

I definitely never worry about half a volt different connection.

 

[flippy]

i would worry more about a potentional inbalance you get from partially charging it constantly. most of the cheap non-programmable BMS dont do any balancing until a cell goes beyond the 4.22V mark. under that you can have increasingly more inbalance that is not remedied until a cell is so unbalanced it goes beyond 4.2V while others are way below that.

 

[dogman dan]

Charge each one with the charger they provided. They may have the bms set on the internal battery, to balance them at the slightly lower voltage.

If signs of imbalance show up, then maybe try the higher charger on the internal battery.

Again, I would not worry about connecting them both, with one charged one volt higher, provided you start to ride immediately.

 

[themyst]

i would worry more about a potentional inbalance you get from partially charging it constantly. most of the cheap non-programmable BMS dont do any balancing until a cell goes beyond the 4.22V mark. under that you can have increasingly more inbalance that is not remedied until a cell is so unbalanced it goes beyond 4.2V while others are way below that.

The seller indicates the BMS of the external pack will start balancing at 4.18V, not sure about the internal pack.

 

[dogman dan]

I would not worry about it much, just charge each with the charger it came with. Then connect up. the voltage is not that different. It will be fine.

Again, don't leave them connected and charge with just one. separate them, then charge them. If you want to charge the external one with the internal charger occasionally, its fine. Just like a partial charge is fine if you just need a few miles range more. But once home, give it the charge that will balance it.