Batteries in parallel

[mc]

Ford transits from 2000-2006 had the option of paralled batteries to boost capacity, and were joined directly together with cables. The only thing fused was the main battery earth lead (which used to go pop when idiots tried starting them in gear!)

From 2006 on, Ford added a battery saver relay to the two battery option, which directly connects the batteries together as soon as you turn the ignition on, and disconnects them either when you turn the ignition of, or if the battery voltage drops below a preset level. When disconnected, all the ignition controlled stuff runs of one battery, and 'saves' the other battery for starting, hence the name battery saver relay.

I've seen both systems have one dead battery, and still function fine. You sometimes get excess gassing, but there's usually noticeable starting problems by the time the batteries get that bad.
The biggest issue with the early system, is if you get one dead/shorted cell, that battery will pull the other one down, however it's not an instant process. Yes, the dead battery will gas due to the five good cells being overcharged, but the fumes are (should be) vented to the outside. It's no worse than driving a single battery system with the same problem, other than the good battery will continue the process for a bit longer.