* navti wrote, On 5/31/2007 4:12 PM:

Lets ASSUME I have a brand new 100 ah battery and its sitting at 12.6
volts.

I run it through a 3A DC lamp for 10 hours = 30ah.

According to my chart 70 per cent charged (30 per cent discharged )
is
12.3 volts,

If they were only 50 amp hour batteries then 30 ah would be a 60 per
cent discharge and my chart shows 40 per cent charged is 12 volts.

Is this the correct methodolgy ?


Yes, this is the way to do it. The problem you have is that there's a
lot of guesswork in the discharge curve. Once you've used a battery
type for a while, it becomes predictable, but the first time through
you don't know if 12.3 volts is 30%, or maybe 20% or 40%. Thus the
first time you have to take it fairly deep to have any confidence.

The good news is that the battery will possibly get a bit stronger in
the near term as its exercised because some of the sulphation is
removed. So if you're able to get 50 Ah out of it and it still over
12 volts, you probably have some useful life left in them. But if
your 30 Ah test takes it under 12V, you have a problem.

One more thing - while under the load the Voltage will show lower, and
then it will jump up a tad when the load is removed. The higher
Voltage is the "Open Circuit" measurement.