Wednesday, Nov 10, 2010 at 14:21
Hello Kc,
for this small a load I wouldn't even bother putting an aux in.
If the starter
battery has an RC rating of around 100 (40~50Ah) or more, then I can't see a problem with that (unless it's old and unreliable).
Since your friends will have occasional access to 240V, it's easy to give the
battery some good absorption charging at least once a week using a quality mains powered charger.
Every 4~8 weeks I'd also give it some cell equalisation @ 15.5V for an hour or so.
Next easiest thing to do is to replace the starter
battery with a spiral wound pure lead/tin AGM
battery.
This
battery type can be 90% discharged and cranking's still possible (cranking only requires less than 0.5Ah).
If more capacity is needed, just wire another one up in parallel using B&S2 starter wire.
So what are the expected max charging currents?
John's alreay mentioned the influencing factors.
Here's an example using some hard numbers:
If the
battery's ~80% discharged, its o/c voltage will be around 11.9V.
So there's almost 2V between the
battery and the alternator (some alternators go higher during the first few minutes, so the voltage difference could approach 3V).
On closing of the isolation switch, there'll be an inrush current through the
battery, only limited by its internal resistance, and by the combined wiring resistance. The internal resistance will cause the terminal voltage to go up by around 0.5V (i.e. 12.4V as soon as the charging current is applied). This leaves another 2V which need to be dropped along the wires.
Now you can apply R=E/I, and the wire resistance required works out to be 0.17Ohms.
Now you select a wire which has significant resistance, but can still carry the max current of 12A.
Then you discover this 4 mm auto wire with a resistance of 10.2mOhms/metre, and a current rating of 15A.
Divide 170 by 10.2 and the result is ~16 metres.
So this wire will definitely prevent the alternator from pumping in excess of the max permissible charging current through the
battery.
But on the other hand, it'll also make charging a very long process.
It's already been said, either use a
battery which can absorb charge at a much higher rate (5~6 times in case of the 50Ah spiral wound), or use a solar panel, or a DC/DC charger.
If selecting spiral wound, you don't have to restrict the alternator current because the alternator itself and the (heavier gauge e.g. B&S6) wiring will be the limiting factors. 25Ah can be absorbed by this
battery in just as many minutes.
Hope this is of some help.
There's more information in my profile.
cheers, Peter
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