On Aug 23, 5:38*pm, Tony V wrote:

What I would like is an effective way to measure battery capacity - that
doesn't cost more than the charger.

My experience says that's a waste of time and money. About 20 years
ago I invested a lot of time in measuring SLA discharge
characteristics in the hope I could predict the failure of the glider
battery. What I found was that most failures were caused by one cell
going bad and that it was not predictable. The battery keeps looking
good with only a slow decay in capacity until the flight where one
cell goes bad and there is very obvious fall off in performance.

Maybe measurement techniques are better now. I loaded the batteries
to sink about 1 amp and continuously sampled voltage and current using
the game port of my Beeb computer. The software stored off the values
and plotted the results in real time All the results were saved to a
data file and periodic testing and comparison of the discharge plots
showed how the battery was holding up. Despite all the data I was
never able to to predict the cell failure that rendered the battery
useless.

My solution was to buy a new glider that had room for 2 batteries.

Andy (GY)