What is the proper charging for worn-out forklift lead-acid batteries?

Question

I know the standard way to cycle charge a battery: constant current at e.g. C/10 to C/4 until 2.40 V per cell, keep that constant voltage until it drops to about C/100. The hydrometer will show about 1280.

That's standard for brand new batteries. But what if the battery's capacity has fallen to 50% or less due to irreversible sulfation or active material loss? At the factory there is a forklift in this condition, no need to change the expensive pack because its capacity is OK for the little use the vehicle gets.

The original charger now causes the cells to bubble long before reaching their usable capacity, and the hydrometer stays in the red (discharged) zone.

This is my opinion: as the active surfaces diminish, there is less surface to convert the PbSO4 into sulfuric acid, so one has to accept that the maximum achievable density naturally will be lower, and the same will be true for the voltage. So, cycle charging to 2.4 V is abuse and it should be set lower. I couldn't find info on this, even at batteryuniversity.com

Do you agree?

Answer 1

I suspect that one or more cells are shorted, so the charger is effectively charging a lower-voltage battery than it is designed to, so it is overcharging the remaining "good" cells.

Check the voltage across each individual battery or cell to locate the shorted cell. You will have to replace the faulty battery or cell.