Oh I see what you mean.
The 4mm² cable from original split charge relay will probably protect itself by its higher resistance as you say that may be part of the design. It too is fused - 20-25A I think.
I think I'm content that by having a far better alternator charging system I'm less likely to get a severely depleted hab
battery as it will be full when I park up and the
solar should cope with consumption as we don't use a lot.
Thinking more about the whole current flow thing.
Isn't what happens with a heavily discharged LB just the same as jump starting a flat
battery?
We don't particularly worry about melting the jump leads in the 'boost' phase but do normally wait before taking mega current for the starter.
There are three issues here.
Firstly it's the high current draw. Yes, we use jump leads, but there is a good reason why you're advised not to use leisure batteries to jump start. They're not designed to give (or take) high currents.
Secondly, it's running the hab
battery down. Sorry, but your solution won't really charge the hab
battery significantly higher than the standard, so it will run down under load when parked up with no sun.
Thirdly is the core issue and itself has a few aspects:
Your engine
battery is a different types of
battery, with more, thinner plates to cope with high currents in the starter
battery and fewer, thicker plates to deliver power for a longer time in the leisure
battery.
The batteries are at a different state of charge, but the alternator is set up to recharge the starter
battery. That'll be recharged in maybe half an hour, and the alternator will switch to float voltage level long before the hab
battery is even half way through the bulk phase.
To properly charge a
battery, the charge cycle needs to have at least three phases, each of which is different according to how big the
battery is and what its state of charge is. It is simply impossible for any sort of split charge or VSR system to do this.
There are three ways to charge the hab batteries properly.
1. You can fit a second alternator to charge the hab bank. That works, but few motorhome have the space under the bonnet.
2. You can fit a mains smart charger running off an
inverter powered by the engine
battery and alternator, but that's expensive and a lot to go wrong. Bit of a bodge, really, but if you have an
inverter and a charger already, it makes some sense.
3. Most people choose the alternative of a
battery to
battery charger. Yes, they cost more, but they do the job really well. The unit properly charges the hab
battery in a multistage cycle, respecting the chemistry, size and state of charge of the hab
battery, leaving the alternator to look after the starter
battery's needs. It works well.