[Nerys]

I thought resting voltage full charged was 12.9 to 13.0 volts. Huh strange I see 12.6v all over the place so I am inclined to accept that value.

I have "know" for years 90% was 12.9volts. 12.5 volts would be 50%

I wonder where I got that from.....

while my assumptions appear to be wrong the value stands. at 12.6 volts you should not be using that battery. (12.5 resting volts is over 20% discharges 12.3 volts is 50% discharged)

unless you have a deep cycle battery than 12.3 volts regularly is "safe" without damage.

all resting voltage of course. so if you demand power is that high that you hit 12.3 volts pretty quickly you need to increase capacity IE second battery in parallel. 2x 12v deep cycle.