Wednesday, Oct 20, 2010 at 15:09
allright then:
start with measuring the actual starter battery voltage while revving the motor a bit.
You want to see at least 13.8V
If that's ok, you can then think about the aux battery.
You've planned using a 55Ah unit.
Because this is a cyclic application, your alternator won't charge your aux fully.
To get it close to fully charged, the depth of discharge shouldn't exceed more than 50%, and the alternator run times need to be greater than 10 to 12 hours before another discharge cycle.
Because you won't be driving that much on a daily basis (I hope!), the battery will be sitting there, partially discharged for extended periods of time.
The resulting sulphation gradually eats away on your battery capacity, which increases the daily DOD (at constant load pattern).
In other words, you walk the capacity down over the months until it won't supply the expected Ah any longer.
One way of achieving a faster charging rate and an overall higher state of charge, is by selecting spiral wound pure lead/tin chemistry.
These take in charge rapidly even at the low alternator voltage.
B&S8 wire is rated 85A @ 60% duty cycle, so that should be ok.
It is specced 2.4mOhms per metre, so expect a bit of voltage drop during the early stages of bulk charging.
But that's probably a good thing in order to limit the max charging current through your smallish aux, which shouldn't exceed 12~15A anyway.
In case you select a bigger battery, or a spiral wound AGM, you should use at least B&S6. Spiral wound AGM, rated 50Ah can be recharged by 60A+.
If the chassis ground is firm, no rust, use it for the 55Ah battery - otherwise run the aux negative back to the starter/alternator negative.
20A fuse in the ciga lighter wire is a bit heavy on the wires which would need to be of 5mm auto type (rated 25A).
Note that the ciga lighter socket itself is only good for 8~10A continuous duty, so just leave the original fuse in there.
cheers, Peter
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