I think the deep-cycle battery in the defender is dead, it's 5 years old and now does not seem able to hold a charge; voltage has dropped from 12.5 to 11.6 in 2 days without any load. Wondering what is the best way to definitively determine whether it is dead or not. It is charged via a piranha system installed by the previous owner, I hope to get time this week to check whether this is properly installed and working, but I'm reasonably confident that it is.

I am contemplating borrowing the battery from my tractor for a Kimberley trip next year, the tractor will be unused whilst we are away, and has a huge cranking battery, 880cca and 250rc, half a meter long. I've worked out I can fit it where I currently have a second winch mounted, under the rear of the Defender. What are peoples thoughts on whether I am likely to risk ruining this battery, which will no doubt be very expensive to replace if I do. I'm thinking it should have a huge amp-hour capacity so I shouldn't drain it enough to do harm by running a fridge and a couple of lights for a few days with solar input to boot, but I have never had much luck with batteries so am after other opinions. We will probably be looking at 3 day stays, maybe 4 days at the most, in any given spot.
Secondly, how many amp-hours can I reasonably expect to get out of a 140-watt solar panel on a sunny, Kimberley, July day. I'm thinking I should be able to operate my 40l Engel fridge (measured current draw whilst running is 1.8 amps) plus about 4 hours use of 2 fluro lights, each drawing 1 amp, pretty much indefinitely with a 140watt solar charger, does that sound right?

Thirdly, my wife would dearly love to be able to run our second fridge, an older 20-litre engel, as a freezer. I measured it's current draw today at 4.2 amps, and I figure that to run as a freezer would probably require 100% duty cycle. Has anyone done this successfully, or should I forget that idea?

I am a notorious killer of batteries, which has no doubt cost me a lot of money over the years, so I'm keen to kick the habit and get this setup working properly.