On Sat, 16 Sep 2017 09:41:25 +0100, (Roger Hayter)
wrote:

snip
p.s. I have various electronic measurement tools including a watt hour
meter but it's no use as a form of protection tool and it doesn't
compensate for the varying capacities at varying loads. It could give
me a 'worse case' value though (30A x the voltage x the period
measured as a percentage (50% say) of the potential at that rate).

The model of the battery as a voltage source in series with a resistance
is quite a good one I think, and the on-load voltage plus predicted
voltage drop is quite a good measure of what the off-load voltage has
fallen to.

Ok.

Which just leaves the temporary loss of capacity at high
load, which would recover if the battery was left off load.

As an aside on that I understand gel batteries aren't quite as linear
as some other LA types. ie, They are quite slow to produce power and
hence why they aren't generally used for engine starting (compared
with flooded or AGM etc).

The latter
might be a relatively small effect at the moderate current of about 10A
per battery that you are using. But as you might guess I don't know
any better than you.

Quite. ;-)

An interesting experiment would be to run the battery down to the
indicated 50% discharge depth, then, without charging, use it again the
next day and see if you get a significant amount of use before it falls
to the same voltage.

Ok, I could try that. My only reservation is that it's not 'good' to
leave LA batteries in a low charge state if possible but I could
certainly discharge it to a higher DOD (say 70%) and try it from
there?

FWIW, when I have also been monitoring the charge voltage (to test the
compatibility of chargers etc, gel once again being a bit 'odd' in
it's charging requirements amongst LA batteries in general), I've
discontinued the charge process say before going to bed and resumed it
the next day and it returns to it's previous 'level' very quickly.

If not, then your measurement system is doing
quite an accurate job of protecting the battery without wasting
capacity.

Ok Roger. I'll give that a go and let you know what I see (so thanks
again for the input). ;-)

Cheers, T i m