"Oily" wrote in message news: snipped-for-privacy@bt.com...
Ok, my way of looking at it. Bit long-winded, bear with me please.
Let's assume our 12 volt battery 1 is fully charged (12v nominal) and is capable of delivering, say, 360A. Not unreasonable, looking at some CCA figures. It will only deliver such a current if, as you rightly say, the load is of a low enough resistance to allow/cause such a current flow. If we connect, say, a starter motor with an internal resistance of 0.1 ohm, then with our 12v battery we would draw 120A, well within the current supply rating of the battery. If the battery were capable of this theoretical 360A CCA rating, it's own internal resistance would be in the region of 0.033
0hms, yes? If we now connect an identical partially charged battery (9v?) in parallel with this one we create a circuit where the load isn't a motor but a partially charged battery, which might have an increased internal resistance of, say, 0.06 ohms? Theoretically this will give a circuit current of 3v/0.066ohms = 50A, which could be handled very simply with a straight forward 70A relay and decent cable.Now, let's look at a real-world application for a dual battery system, the reason for doing it in the first place, and the original poster's reason.
If, say, a winch is connected to battery 2 and it draws 400A at full load (my 8000lb winch is more than this, so it's a perfectly typical figure) then the winch will draw 200A from each battery (assuming a full charge in each and the split-charge relay is energised), the 200A from battery 1 passing through our simple relay. It isn't rocket science to see it won't last - a fire is about to happen! If our theoretical vehicle has a 100A alternator and the engine is running, then the batteries will make up the 300A difference required. Each battery now provides 150A, but battery 1's 150A adds to the alternator's 100A and we now have 250A passing through the split-charge relay, an even worse scenario. If we decide to winch with the engine off to prevent this scenario (or a manual switch to the relay operated to open the contacts) then battery 2 will discharge VERY quickly if it has to provide 400A on its own. So what do we do... yep, in the heat of the moment we start the engine (or make the manual switch) to get the alternator pumping some charge back into battery 2 again, remembering that the alternator is connected to battery 1. Our operator gives it 5 min to put a bit of charge in (our theoretical 50A from above), gets bored and decides to just winch anyway, with the engine still running. Battery 2 hadn't charged by a lot and can only provide, say, 50A, so battery 1 now supplies 250A, the remainder of the required 300A, to add to the alternator's 100A! This time, we have 350A passing through the relay! Again, a fire scenario!
What I'm getting at here is this:- Forget charging currents, to a certain extent, as it has been proven that they aren't the main issue. Consider the load demand that is intended to be placed on the second battery and where the charge for it comes from and then consider the worst case scenario for current drain and add the potential for further "toys" (lights etc) being added in the future, then add at least 10% as a safety margin. The safest way by far is to fit a second alternator feeding a dedicated battery, the alternator control trigger (field) being taken from the main vehicle through a relay to keep the circuits completely separate but with secondary alternator controlled by the vehicle ignition in the same way as the vehicle one. If you are never going to use the winch to its full capacity and only on a very infrequent basis, don't even bother with a second battery - just wire it to the one that's already there, maybe even consider replacing it with a larger capacity one.
I have a 110 that is used for recovery work, it has a 95Ah vehicle battery fed from a 115A primary alternator, a 120Ah fed from a 140A secondary alternator, the secondary circuit being relay-triggered by the primary. Radio equipment and all standard vehicle wiring feeds from primary battery, the winch and extra lighting from the secondary. I have NEVER had any issues with this system, and I've tried both diode-based and relay-based systems before deciding that this was the ONLY way to do it properly and safely (for my intended usage pattern).
Badger.