I would also say Sterling were not much help on their helpline but their battery to battery charger will charge the second battery (bank) to 100% of their capacity not the 70~80% that charging only from an alternator will give you. Charging from solar will also give 100% if its sunny enough.
Since fully charged is 12.7v and 50% is 12.2v (roughly), a normal 12v dash gauge isn't really accurate enough to measure that difference. A cheapo voltmeter is, or
BMN-1 Battery | Nasa Marine Instruments
will give you a good idea of how much charge is left to use.
The more often and/or deeper you discharge a battery the shorter its life will be. So choosing a system that won't mean big discharges (so a lot of spare Ah), and that heads towards 100% charged as solar, Sterling (easy to wire in I'm told), and/or a pukka staged mains charger could mean a 10 year lifespan instead of only a couple
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