Originally Posted by ta_man
I got 3 new SMC 4000 hard case packs today. I checked the cell voltages before doing anything. All were unbalanced:
Is this typical, or should I be concerned?
Those voltages on discharged cells are normal and you should not be concerned. It's best to balance during charge and not do a passive balance (ie match voltages prior to charge).
As you discharge a 2 cell pack, the voltages will start to deviate. The more you discharge, the more it deviates. I've posted this on another thread before and I'll cut and past the response again.
I once got into a discussion about cell balancing with the Engineers at a battery manufacturer. They were saying that most of the cell balancers for the RC market do not work correctly. I didn't understand what they meant at first, but after spending months trying to understand how commercial cell balancing chips work - I think I finally get it.
Let's say you have a two cell pack where both cells are fully charged 100%. Even though the manufacturer matches the cells prior to assembly, no two cells are exactly alike. Assume you partially discharge the pack by taking out 1.0 Ah out the pack. Both cells with the same initial voltage will probably be at slightly different voltages during and after the discharge. If you charge and put 1.0 Ah back into the both cells - they both reach 100% charge and 4.20V perfectly.
Now assume the same scenario, where you start with both cells 100% charged and you once again take out 1.0 Ah. This time you attach a balancer that bleeds off current from the higher cell and now the voltages are matched. Now you remove the balancer and recharge the pack. The cell that was brought down by the balancer is now not fully charged and the cell that was at a lower voltage during balancing is now overcharged. The cell balancer actually created a cell mismatch!
The bottom line is that passively balancing cells by forcing a matched voltage of partially charged pack is actually useless.
The balancer should only be used when fully charging the pack. It's meant to bleed off current to specific cell only when any cell reaches over 4.20V/cell. This ensures each cell gets a full charge and no cell gets overcharged.
Checking for cells in/out of balance with a multimeter should only be done when the pack is fully charged.