Charging with load balance. Always or periodically? (li-Ion)

Question

Having an external charger with Li-Ion load balance (based in passive balance; resistors).

Is the best practice to charge a li-ion battery pack *1 always with load-balancing?

Or is better to apply "normal" charge; just positive and negative wires to battery pack extreme. And periodically check cells to detect unbalance and apply a loadbalance charge?

*1: 4xli-Ion cells in serial without a BMS circuit.

Update: DO NOT use a li-ion pack without BMS circuit, it is required for supply/discharge too, as it avoid to go below certain voltage.

solution: Add a BMS circuit (with loadbalance) and charge with a normal power supply instead of an external charger with loadbalance and visual monitoring

Answer 1

Batteries have Vr=ESR * I drop and dVc = I * dt/C for a good 10850 nominal C~10kF , ESR = 50 mohm new, while Vbat= Vc+Vr.

So as SOC drops below 10% ESR rises sharply and when mismatched aging causes C mismatch to accelerate down.

The inverse occurs from CC to CV and weakest C gets overcharged. So always keep them balanced or run in parallel.

For longest life, otherwise match all cells from the start <0.1% and prevent long time over 4V and under 3.2 or 3.0.

Trickle charge start if less than 3.2 V due to high ESR state.

Read up on Battery University and consult with OEM for best advice.

How you use your batteries depends on your priority for longest run time (short term) or most charge cycles ( up to 10x more charge cycles, long term if you have dual packs and only use SOC from 80% to 30% i.e. have the available capacity with BMS or parallel cells.

Eventually the weakest parallel cell supplies less current and the stronger cell gets weaker faster supplies more current until they match for Capacity, if not being over-stressed. But heat will stress aging faster 2x per ~10'C rise from Arrhenius Effects.

Answer 2

It is best to balance the state of charge (not "load balancing") of a string of Li-ion cells over more time because it can be done at a lower current.

For example, if you have a battery with 2 cells in series, and one self-discharges at 1 mA and the other at 1.1 mA, there is an imbalance current of 0.1 mA. To correct for that imbalance, you need to remove an average of 0.1 mA from the first cell. You can do it with:

• 0.1 mA continuously ("24/7")
• 1 mA once a day for 2.4 hours
• 10 mA once a day for 14 minutes
• 100 mA once a week for 10 minutes
• 1 A once a month for 4 minutes

So, as you can see, the limitation is the balance current of the BMS and the time available for balancing.