Health of the Battery

I will have to study it more to understand how it works it out, I think that it measure's both ways not 100% sure. I was looking through my favorites and seen it and thought of this thread stright away. It seem very clever way of doing it.
They use a device at work on a hybrid machine that counts total amp-hours/watt-hrs in and out, like how much you have taken out and put back in I tried to get the info on how they do it but no chance there keeping there cards very close to there chest

Battery health and capacity measurement are two different things. If a lead acid has developed high resistance due to constant overcharging corroding positve plate grid it will still have full capacity. You may not be able to use it depending on load requirement dropping terminal voltage due to internal resistance. The battery still has its capacity in A-H, but battery may be unusable in application.

You also don't want to measure lead acid battery capacity by fully discharging it. That is damaging to battery. You can do a partial discharge and measure its rested open circuit voltage and/or specific gravity to see if it matches the expected % discharge. A good lead acid cell open circuit voltage is SG + 0.845 in cell voltage.

A lead acid battery that has half its plate area covered by sulfation due to continuous insufficient recharging will have half the capacity and near double the Rs. Rs is a small resistance so doubling it may not hinder its use, just will get half capacity output.

What about this from here
Silicon Chip Online - High Current, High Voltage Battery Capacity Meter

I think this is done in c which you can download from there so it should give you some better iedas how they do it, I think I have a copy of both parts some where,