# Modify lithium battery pack balancer

This is the lithium battery pack balancers available to me: As Robojax has extensively explained in this video, this module is capable of balancing batteries with 67mA current (4.2v/62ohm) that is way too slow for my application because my charging current is 500mA and this causes over charge of batteries before the balancer gets the chance to balance them. So I thought I might be able to use a high power resistor (or array of low power resistors) to increase power of the module. For example use a R8.2-5W or 5xR43 2512. Both solutions provide 500mA current and have good power margin for sustainability. The mosfet used in this module is A2SHB that can handle up to 3.7A.

Is there anything that I'm missing here? Why the balancer modules does not come with such resistors to be able to handle more current?

• Top balancing is generally done in the CV phase of charging, where the current is generally much less than in the CC phase.
– user16324
Jun 4 at 15:32

I think you may have a slight misunderstanding of how balancing is normally done. With rare exceptions, the balancing current is always much lower than the maximum current that the charger is capable of, and that's fine. All you need is for the average charger current to match the balance current. The average charger current is reduced by turning the charger off and on.

In any case, at the end of charge, during the Constant Voltage stage, the actual charging current is less than the maximum current that the charger is capable of. The charger doesn't do that. The physics of the cells do that. It's due to the smaller and smaller difference between the charger voltage and the string voltage.

1. When any cell is full, the BMS stops the charger
2. While the charger is off, the BMS removes some charge from the cell with the highest voltage
3. After that voltage drops sufficiently, the BMS restarts the charger

That cycle is repeated many times until the string of cells is balanced.

Once the string is balanced, the charging current drops naturally asymptotically towards zero.

Source, my company's website.

• Thanks, That explains why commonly available balancers' current is under 100mA but still the same question begs! Why not using more powerful balancer and reduce the overall charging time? Jun 4 at 8:42
• @AKTanara Good quality matched cells have minimal balancing needs, around 1-2% of battery capacity. The limit of these simple balancers is the amount of heat they can dissipate. For larger balancing current, search for "active balancer" which moves the energy to other cells instead of wasting it as heat.
– jpa
Jun 4 at 11:02
• "Why not using more powerful balancer and reduce the overall charging time?" Because of the high cost for something that is rarely used. Once the battery is balanced, keeping it in balance is easy, takes little current and a short time. Why pay 10 $/ cell to balance at 10 A to use it a couple of times in the life or a battery that can be balanced for 0.5$ / cell at 50 mA and still get the same benefit? Jun 4 at 14:53
• Got the answer. Thanks for sharing your knowledge and explaining. Jun 4 at 18:39
• @DavideAndrea Would you mind taking a look at this question please? electronics.stackexchange.com/q/669748/301585 Jun 8 at 22:29