Battery capacity tester which can dump energy into another battery

[Freesurfer]

I have a bunch of used lead acid batteries that are of the same model/capacity rating, but they have been pulled from different environments so some variation is to be expected. I have arranged them in several series strings, and then paralleled these strings for extra capacity.

The problem is that when putting these batteries in series, the weakest battery in that string determines how much power I can consume from that string. For example one string when discharged may have 11.8 - 11.8 - 9.00 - 11.8V.
When charging, it may be the other way around - 13.8 - 13.8 - 14.9 - 13.8V. Obviously the one battery is in worse shape than the others. But trying and failing is very time consuming so I'd rather fully charge all the batteries, run a test and then arrange them into the setup accordingly.

But since I do not have mains here and need to charge from a generator, it would be nice with a battery capacity tester that would dump its discharged capacity, into another battery instead. I have seen such testers demonstrated on youtube before but can't find any appropriate one. It would be nice if it is a somewhat beefy tester. The batteries are 12V/86Ah

[NiHaoMike]

A boost converter module that supports current limiting will do what you're looking for.

[capt bullshot]

You might check these battery chargers used for model aircrafts / cars / boats. Many of them are intended to be supplied by a standard car battery. Usually they support all common battery types. Most of them should have options for charging lead-acid batteries, most of them should have a "discharge battery" function and some might have the ability to dump the discharged energy into their input (if a lead-acid battery is connected), especially if they offer high discharge currents.