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How to DIY Charge-Balance 4 Li-ion 2000mAH

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SuzyC:
Any Li-Ion charge experts out there?

I want to DIY a 4S li-ion charge balancer and I have some questions:
When to begin/continue charge balancing in charge cycle ( at 10% of charge or  50% of charge or at 100% of charge?
    (0r actively charge balance the entire charge cycle and also while the battery pack is in use?)
 
 
 Assume I'm charging 4 series-connected cells to 4.2 V charge cut-off voltage at CC 500mA

I am going to use a dedicated 10-bit A/D MCU to accomplish this job, using opto-isolators as sw's to a NPN BJT to load a 10-ohm resistor across the battery being reduced in voltage for balancing?

NiHaoMike:
Just buy an active balancer module. You're not going to be able to make your own for cheaper for just 4S.

SuzyC:
I have on hand two examples of charge balancing circuits, one from a handvac  14.4V rated(4-cells) and another from an supermarket  "20-Volt" 5-cell battery pack. What surprises me is that the complexity is great, a dedicated >28pin IC is being used in both cases, 10's of discrete transistors, caps and resistors. No inductors were being used except to provide MCU power voltage..

But all the resistors are tiny!  This seems to tell me that the actual discharging to balance currents must be small, maybe in the 40-mA range..and I was  planning on using 10-ohms at 2-Watts! 

I saw the spec sheet for the super charge balancer something like a 100-pin chip made by TI, which can balance something like 17 Li cells and would be in stock sometime maybe in Feb 2023.

My first impression of charge balancing circuits is that they are over-complicated, There must be a more cheap'n'dirty way to do this without significantly sacrificing battery life or fire safety.

So, at this point, my MCU basic idea to charge balance 4 to 6 cells would be to work with lower balancing current (40mA).

But when in the charging cycle to do this..while charging? Stop charging momentarily to balance? Wait for the batteries to reach a minimum low dischage level before attempting to balance?  Repeat at 50% 75% 100% ????  Continuously?

I have given the whole idea some thought and realize what would be an allowable mismatch between cells? And I've guessed it would be around 10% or between .33V to .42V worst case, else the battery pack would need repair or replacement.

Then, if charging takes maybe up to four to five hours at 500mA CC, and the charging current is reduced by 10% shunting of charge current with overvoltage cells, then there should be sufficient time for even a 40mA difference in charging current to balance the batteries while they are being charged.

My solution would monitor each cell voltage continuously by a MCU 10-bit A/D. This could allow a cell voltage error of no more than 15 mV( because of voltage dividers needed with the top cells in the stack).  Once individual cell voltages are known, some equalizing circuit can then be developed.

SuzyC:
Another thought to resolve. When measuring charging battery voltage, the battery voltage will be higher while charging due to the  internal resistance of each cell.

Charging past a 4.2V cutoff  level can easily damage a cell, but can the actual battery charging voltage limit be higher while charging to take account the internal resistance?

Is 4.2V a limit while charging?

Psi:

--- Quote from: SuzyC on October 17, 2021, 10:29:45 pm ---There must be a more cheap'n'dirty way to do this without sacrificing battery or fire safety.

--- End quote ---

Maybe.
If you get 4 isolated supplies, (like 4x transformers or 4x DCDC isolated converters.) Then you can simply use a single cell LiPo charge IC on each cell because you have an isolated power supply for each. You are charging them individually so no balancing issues exist.

No need for complex system to charge in series until an imbalance exists, then switch to individual cell discharge mode using resistor, then back to charging in series. etc..

Additional comments about DCDC isolated bricks
They are cheap for lower currents, like $1-$2 in volume for 5V 200mA output.
They come in versions that have a few different input voltages, 5v, 12V, 24V usually all the same price. But they get expensive if you need more charging current than 400mA.
I guess you might be able to put two in parallel. Dunno

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