Quote:
Originally Posted by Volt_Ampere
Most BMS do not provide enough balance current to do the job in a reasonable amount of time with the packs we use. I designed and built my own and I use a 1 amp balance current. This causes a fairly large voltage drop in the sense lines so I don't sense the voltage during the time that the balance current is on. I turn it off for a couple seconds to measure the cell voltages. I used power resistors and gave them plenty of air cooling space because at 1 amp they are dissipating 4W each. This is why most BMS do not balance at a very high current - they don't want to deal with the heat generated.
|
If they are limiting their balance currents so as to not deal with the heat, then they don't really know what they're doing..
I can't imagine a routine circumstance where more than two or three cells would have to be drained concurrently.. that's 4 to 12 watts to dissipate, which isn't much.. Some lithium ion LED flashlights dissipate that much energy.
My REC BMS has an internal temp sensor that stops balancing current if the unit gets too warm.. first time I hooked it up, I had 3 cells low so it actively drained the other 11.. and ya, it got warm and tripped its internal temp limit.. it just waits a few, cools down, and starts over again.
But that was upon initially hooking the system up, since then, I don't think I've ever seen it work on more than 1 cell at a time.
All I know is that these 100mA balance currents are a joke if trying to use them on a larger pack. From experience, I figure a BMS needs about 50mA per kWh of pack capacity. So if you have a 5kWh golf cart battery, you need about 250mA of balance current..