Nigel wrote:

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?

batteries are USUALLY rated on a 10 hour cycle, so a brand new and fully
charged 100Ah battery with put out 10 A for 10 hours, a 60 Ah battery will
put out 6 A for 10 hours, and so on.

The vast majority of batteries, eg car / domestic type, eg NOT deep cycle
type as fitted to fork lifts and milk floats, will die VERY quickly if you
deep cycle them, losing as much as 5% of their total capacity every time
they are deep cycled, so a 2 year old 100 Ah battery that has not been deep
cycled but has been used for engine starting may only have 75 Ah capacity
left, given this I personally rate a 100 Ah battery as a 50 Ah battery to
take this into account.

Others are sure to disagree.
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