Hi there,
I would not waste a discharge cycle like that if i were you.
If you want to estimate capacity do so while they are charging.
If you measure the time it takes to charge and the current while
it is charging you can get an idea how much charge it took, and therefore
the approximate capacity.
This works to some degree because as the battery ages it charges up
faster and faster using the same charge current.
For example, if you charge a brand new battery at 400ma and it takes
30 hours to reach float voltage level and you charge a used battery
(with same Ah capacity and some model number) and it only takes
15 hours at 400ma to reach float voltage level then the second battery
took approximately one-half the charge that the first battery did, which
means it will only deliver (approx) one-half the Ah capacity to the load.
The trick is to measure the capacity going *in* to the battery, which
unfortunately isnt always this easy because the charge current isnt
always constant. In this case you have to periodically measure the
current and sum up the total current*time going in to the battery in
order to determine the total charge accepted by the battery. You also
have to divide by an efficiency factor, which for lead acid is approx
1.5 if i remember right (might want to look that up) but in any case
if you always use the same factor (1.5) you should get comparable results
between new and used batteries, so you can tell when one has aged to
the point where it only holds one-half the charge as when it was new
for example.
For this kind of measurement you need a current meter and you have to log
the readings with a timer or clock, taking readings say once every two
hours. You would then calculate the total charge as follows:
EXAMPLE
(This brief example only uses two readings at 10 hours and at 20 hours, but
the more readings the more accurate the result):
t=00 hours, 400ma (start the charge)
t=10 hours, 400ma (10 hours into the charging)
t=20 hours, 300ma (20 hours into charging)
and at 20 hours the float voltage reaches 2.45v per cell so the charging
is terminated.
From the above, we had 400ma flowing for 10 hours, which is 4000mAh,
and we had 350ma flowing for 10 hours, where 350ma is the average of the
last reading (400ma) and the current reading (300ma), which comes to
3500mAh. The total is 4000mAh + 3500mAh which equals 7.5Ah of charge.
We now divide by the charge efficiency factor of 1.5, and we get:
7.5/1.5=5
so this battery capacity is roughly 5 Ah.
