RE: field charging lipos
The PB battery has a watt hour rating. That is the basic limiting factor. The deep cycle battery just means that you can discharge it a bit further than a conventional automotive battery.
The charger comes into play because it may be able to charge a LiPo battery to a higher voltage than the power source battery. My chargers can do this, and use standard automotive electrical systems as a power source. (~13-14 Vdc) in, and about 22vdc out (6 Lipo cell packs) to be charged.
Lets say that I have a 5000mah lipo to charge. It should not be completely discharged, so I'd guess 3000-4000mah as the needed charge.
Then there is some loss in the charger, and if a balance charge is done. So, I'd go back to 5000mah as more or less the maximum expected
energy needed from the power source battery. A small lawn tractor battery might be a 24AH battery, and I'd expect to get about four or so charges
before it needs charging.