Charging non-ballanced battery of NiMH

[pcm81]

Greetings my fellow nerds.
I need to replace cells in a 2p7s NiMH battery. The problem is that the charger that this battery is hardwired to is a non-balancing charger that basically just applies 10.5V voltage and can be current limited, but it has no means to balance the cells. I have a excess amount of cells and would like to give this battery best fighting chance by selecting batteries with matching IR to avoid damage during charge cycles. Should i be matching cells with similar IR at low SOC, at 100% SOC or by some other criteria like IR vs SOC slope? I am also thinking of sorting batteries in  the two banks in reverse order, by IR, and adding connections between cells in two banks to basically have a cell with high IR in parallel with a cell with low IR in the 2nd bank etc, hoping to have similar IR across every cell and prevent overvolting any one during charge cycle. Any suggestions?

[Andreas]

Hello,

if the 10.5V (or 1.5V / cell) are continously applied this will "cook" the battery - especially  in summer.
For floating charge I usually apply max 1.45V/cell at 20 deg C reducing by -3 mV/K/cell at higher temperatures (e.g. 1.42V @ 30 deg C).
This keeps the charging current in "full" state below the limit of C/50 .. C/100.

Sometimes I use a NTC for the derating:
https://www.eevblog.com/forum/metrology/ad587lw-10v-precision-travel-standard/msg1449488/#msg1449488

If the internal resistance is too high then the cell is most probably dead.
It may also have a high leakage current damaging the parallel cell.
So I would parallel only cells with identically IR.

With best regards

Andreas

[tunk]

You may also want to check their capacity.
Or if possible, bypass the charger and use two
li-ion cells in series, or a single cell plus a step
up circuit.

[pcm81]

Thanks all for the feedback. The 10.5v is not constantly applied, just for charging based on -dV. I am now leaning towards using measured capacity of each cell as the parameter to arrange them by rather than internal resistance. Replacing all cells with new, not trying to salvage.