On Wed, 25 Jul 2007 10:35:06 -0700, Pete C
wrote:

Hi,

If you're able to use a hydrometer, this FAQ tells how to relate
battery 'SG' to state of charge:

http://www.batteryfaq.org/
http://jgdarden.com/batteryfaq/carfaq4.htm
http://jgdarden.com/batteryfaq/carfaq4.htm#soc

Bear in mind 'state of charge' isn't a percentage of the battery's
*rated* capacity, as they lose capacity as they get old or sulphated.

State of charge is a percentage of the *actual* capacity of the
battery, taking into account it's age and condition.

To work out the actual capacity you need to compare the drop in state
of charge to a known amount of charge used.

So if drawing 1 amp for 10 hours (10 'amp hours') from a battery makes
it's state of charge drop by 25%, the actual capacity is about 40 amp
hours when it's fully charged.

If the above battery then reads a 75% state of charge, then
approximately 30 amp hours of charge are available until it's
completely flat. (Though of course it should be recharged when the
state of charge is around 50% or less)

If the above is a bit too then try and get someone to explain it to
you in person as it's a little difficult to do in a short posting.

hope this helps,
Pete.
Sorry Pete I cannot relate to the above, it is a far too simplistic.
Here in the UK and I feel also in the US battery capacity is normally
quoted at a 20hour rate.
Therefore in the example offered above - 1A for 10hrs giving a 25%
drop in "State of Charge" the actual original battery capacity would
have been more like 35AH given a Peukert of 1.25

Ok I realise that you used the term "about" but we are talking major %
differences here.

To put the test another way are you saying that if the load had been
100ma for 100hours = 10amp hours your conclusions would have been the
same?
I would then suggest the battery's max capacity would have been 22Ah.

--
Richard

Nb "Pound Eater" Parkend G+S
"Governments are like Nappies, they should be changed often."
(For the same reason)