Time to get back to this.
I camped over the weekend at our 4x4 site. Bright sunshine into early evening, then ran the generator for a coupe of hours, by about 4am the compressor fridge was cycling indicating the battery voltage had dropped too low for it.
I re-measured the tiny battery compartment - buried at the front of the camper under the seat and next to the water tank. Various fittings for the water tank and electrical cables mean that I can get a group 24 battery in regularly oriented, or a group 27 mounted upright.
I don't boondock that often, but I would like to be able to do it more for a night or two at a time.
My old battery was a group 24 80Ah "leisure battery" - i.e. little more than a cheap starting battery. I don't know how many cycles it could stand to 50% depth of discharge, but I suspect it would be in the tens. And 50% of 80Ah is of course only 40Ah. My compressor fridge draws 6 amps, so over 12 hours at 50% duty cycle that is 36Ah. So each night without hookup and I was pretty much dropping the battery to 50% DoD! So clearly I needs more available Ah. That could be a bigger battery if one could fit, or a battery that can be discharged further. Or both.
A data sheet on Lifeline AGM batteries shows the ability to be discharged by 80% 500 times. If that is realistic then it is maybe an answer to my problems - that would effectively give 80Ah out of an 100Ah for more boondocking nights than I am ever going to need it over 5 years. I.e. most of the time I would be discharging the battery maybe 10%, but occasionally I could go down to 80%.
There are many cheap AGM batteries these days, but I guess not all are equals.
I've seen adverts for batteries nearly a third of the price of the above Lifeline AGM, but that states a measly 180 discharges to 50% DoD.
So it sounds like Lifeline AGMs are good.
What other brands do people recommend I look at? Specifically ones known to be proper deep cycle - able to withstand hundreds of discharges to 80% DoD.
Or is that an unrealistic expectation and maybe I have failed to read something in the Lifeline small print.
Actually, here is another brand of battery whose specification sheet indicates > 500 cycles at 80% DoD:
Rolls AGM batteryI live in England, hence the above is a UK distributor and the price is in £ not $.
On another note, I realised the cheap Chinese volt / ammeter I installed was reading high. I have ordered a Victron 700 battery meter so I can more accurately monitor my replacement battery.
Steve.
