ah draw?

Cool - you can use that system. To recap - the other battery(s) are ONLY connected to the main battery when the system is charging. Hence you do not parallel idle batteries. Nor do you flatten both batteries due to a heavy discharge from one battery (unless whilst charging and the load exceeds the alternator output and hence discharges BOTH batteries - that is one advantage of the voltage sensing system which would isolate the 2nd battery - but then reconnect, disconnect etc; the better solution is the charge-lamp system with a voltmeter and vigilance or alarms). If the relay is off whilst the system is charging, it functions like the normal system with no extra battery(s). By using a master-slave setup - where the charge lamp controls a 15A or 30A MASTER relay which the controls bigger slave relay(s) that would otherwise overload the charge lamp circuit, any number of slave relays can be used to isolate any number of batteries. By inserting a diode between the charge lamp circuit and the (master) relay (#86), other +12V sources can be used to manually control the relay - eg, also through diodes, a momentary push button to join the batteries during cranking, or a switch to parallel batteries after a charging failure so you get maximum range etc. (Usually those circuits are used when the charge lamp controls a fuel pump so you can manually prime, or manually bypass in case of charger failure. The aux-battery circuit is identical in all ways to the fuel pump circuit. And note the reservations about solenoid resistance. I have had no problems with 60-Ohm (200mA) relays (solenoids/coils) and I know some systems have had 2 such relays connected. Alternators should be designed to drive a "reasonable" relay (say 200mA) since many vehicles use the charge-lamp circuit to energise relays (whether fuel pump control, electric chokes, fuel-cut solenoids, etc).    That means the alternator's regulator D+ or L (charge lamp circuit) has to be able to supply at least 200mA when the system is charging. Most charge lamp circuits can sink probably 1 Amp or more when NOT charging - ie, they must be able to ground a 3W charge lamp (3W/12V = 250mA), but also any other lamps that may come on - eg, "tested lamps" like brake-fault, low fuel (EFI), oil filter (diesel) etc in typical vehicles from the 1970s to 1990s. It's not uncommon to have 2 or 3 tested lamps, so that the charge lamp plus 2 or 3 others - eg, say 4x2W or 4x3W lamps = 12W = 1A @ 12V. If the alternator uses a relay to flip the charge lamp from ground to +12V, then it should be symmetrical - ie, source and sink the same current - probably a few Amps. But some alternators use solid-state switching so their current capability may be limited and not symmetrical. I was looking at some 200A, 400A and 600A relays (solenoids) today. They had coil resistances of about 28 Ohms - hence about 0.5 Amps. I reckon most alternators should be able to drive that, but if not, I'd use an intermediate transistor or FET, else a "normal" relay - e a typical 15A or 30A automotive relay (which are usually at least 60 Ohms, hence under 200mA).

Just in case...

coolen wrote: i was just going to put a solenoid inline between the aux battery and the isolator

Do you mean the charge-lamp relay between the battery and another isolator?? (If so, no! The charge-lamp relay is the isolator.) And I wouldn't worry about leaving the solenoid/relay off until after some drive time.... (Unless you are going above 15V to force especially high current into the Optima...) The fastest way to charge the batteries (as a whole) is to have them both connected. Only if the alternator can't charge both would I consider isolation during charging, and only then if the lower charge is detrimental (which it shouldn't be - in fact fast(er) charging should be friendlier). But you'll probably find that at normal RPM, the alternator is at less than full (current) output with just one battery, so why not use its extra capacity to charge the other battery(s)? (There are arguments.... but I think I'll research certain batteries.... I know some arguments are based on certain industry information, but I have a feeling "that crap" is about to be blown out of the water....)