DaveS wrote:This is a subject with about as many opinions as punters, but I'll stick my head above the parapet. In my view (and it will no doubt be challenged) I reckon that the ideal solution would be a high CCA starter battery dedicated to engine cranking and a high Ah deep discharge, traction type battery for domestics. The latter is what milk floats (and, I suspect, older submarines) use / used.
A method of cross-connecting in an emergency is desirable, either by using the "both" position on a 3 way + off switch, a wired-in "emergency parallel" switch (better, since a dead battery can be isolated while using the good one to feed both cranking and domestics), or even a jump lead. The latter suggestion will have doom sayers raising images of hydrogen explosions, but since hydrogen is only given off when batteries are charging - and then only once they're almost "full" - you are unlikely to be in this position and simultaneously trying to make an emergency connection to get the engine started. Unlikely, but not impossible - see later.
Charging more than one battery is another can of worms. My view is that the alternator should initially charge only the cranking battery, preferably using a smart regulator. Once that's fully charged a VSR fitted between the batteries closes, and the domestic battery starts to charge too. That way, accidents apart, the cranking battery should always be fully charged - just like a car battery in fact, reinforcing the point that for starting CCA are more important than Ah. Incidentally, I have a strong dislike of "diode splitters": they're inefficient and they cause all sorts of subtle problems, some of which can easily shorten battery life.
Other charging devices, solar, wind, towed, or whatever, should charge only the domestic battery (because the starting battery should already be nearly full, see above) via a regulator to avoid overcharging unless they're really feeble, e.g. a tiny solar array. A slight sophistication is to have your VSR of "twin sensing" type. This costs little, if any, more than the single sensing type and means that when either battery reaches 13.7V, i.e. is well charged, the VSR closes. This could have advantages on a long engine-free passage in counteracting the self discharge of the cranking battery.
It is the above circumstances which make me think of a possible - if unlikely - scenario where hydrogen could be a danger. If the starter battery died when the domestic was being well charged by wind, say, then there might be a risk in making a connection to it. In these circumstances a minute or so of good ventilation should dispose of any risk.
And what if the big domestic battery dies? Well, that's a bit harder. Using the cranking battery will get you by up to a point, but there's then a real risk of losing the ability to start the engine. There's a lot to be said for having a spare battery - which might have its own job to do in normal circumstancies. I have two: one for the windlass, one for the electric outboard; either could be used as a spare domestic in an emergency.
And I also advocate carrying long jump leads: rafting up to another boat to get a jump start isn't daft, but how many carry the wherewithal to do it?
That's gone on a bit, and if posted on "the other place" would undoubtably generate a long and heated thread, but hope it's of some use.
Reffering to your remarks about diode splitters Dave, I dont like them either as the can create considerable volt drop between alt and batt, but Driftgate do a split charge unit which is electronic and doesent contain diodes, I am using one of them and get something like 0.08 volt dorp, from the X-Split.
I would offer you an excellent (in my and others opinion) diagram for your ideal charging system Nick, but some folk elsewhere think its too complicated, but its not, believe me and it is failsafe.