"Doug Dotson" wrote in message
...
100AH is its total capacity. Normal rule of thumb is to only
discharge it down to 50%. So, you have 50AH of useful
capacity.

Doug
s/v Callista


Not quiet. Lets assume lead acid batteries and only engine charging.

Yes the 50% of discharge is a generally acceptable figure, but you are
unlikley to get them much above 80% of fully charged because we tend to use
automotive alternator regulator designs that are built down to a price and
not up to a duty (they do the job very well on a car).

This means that you can only reliably assume 30% of usable capacity. The
problem is that over the winter that 20% between 80% and 100% of fully
charged will sulphate, which further reduces the capacity of the battery.

To overcome this either charge with a mains charger that can charge to 100%
or fit an advanced regulator that may reach 98% of fully charged.

Tony Brooks


"Nigel" wrote in message
...
Stupid question time...
If I have a 100ah battery how much power can I expect to get out of
it...
is
100ah the total battery capacity or it's useable/available capacity. My
understanding is that a normal lead acid battery shouldn't be run too
flat,
so when is flat too flat? If I should only let my battery drain down to
say
a 90% charge, have I only got 10ah before I need to recharge?