Sunday, Feb 28, 2010 at 17:47
As I see it, this is a typical problem (charging of secondary battery) with two almost equally applicable solutions.
As it looks to me, there doesn't seem to be any isolating switch/solenoid or diode in this dual battery setup?
The second battery is just connected in parallel to the first one via 7 pin plug, switch and 6 mil wire?
If so, it would be interesting to find out what fuse there is on the second battery, as some starting current will flow through it.
To make sure things don't go bad and fry the pin 7 wire connection, the 240/12V switch ought to be on 240 while cranking, I'd imagine.
An isolation switch/solenoid basically disconnects the second battery from the first one during cranking or while the engine is off, automatically.
But it could be that the resistance of the existing 6 mil wire/plug/switch/fuse(s) is sufficiently high to limit this current to a manageable level, i.e. not blowing the fuse.
It's a hit and miss affair really.
Thus, an isolation switch/solenoid/diode offers better protection against this reverse current, with the added advantage of the second battery not being able to discharge the first one while stationary.
To your question on the batteries not receiving full charge when wired in parallel: they will receive full charge, even though there is a fridge connected to the second battery.
The thing to keep in mind is the time factors involved. Say your alternator outputs 14V to the first battery: there will be some charging current through this first battery, and also through the second battery. But because there is more resistance in the wiring to the second battery, it'll receive less charging current, hence it'll take significantly longer to have both batteries fully charged. Also, the fridge will compete with the second battery for any 'left over' current from the first battery, meaning there will be even less current available for the second battery while the fridge cycles on - extending the charging times for the second battery even more.
The alternator really doesn't bother about how these currents are divided up between loads and batteries, it'll just output 14V no matter what.
Note that the second battery's charging current is positively influenced by the depth of discharge of this battery, meaning it'll take in more charging current if it's relatively low on charge.
But this also means that the charging rate becomes very low once the battery has received more than 80% of its charge.
So this second battery will have a hard life sitting there partially discharged for much of the time.
Unless, you travel for long periods of time of course (weeks rather than hours/days) which could actually see your second battery overcharged, especially under high ambient temperatures, when the float voltage requirement could be as low as 13.3V
Nevertheless, use the on-bord 240V charger to recharge your second battery every now and then and see how it goes.
You can check the second battery voltage after 6 hours of no driving, to get an idea how much charge is left - I'd want to see at least 12.3V there on average, otherwise there would be potential for undercharging/non reversible sulfation of your second battery. And don't forget to turn your switch to 240V while stationary otherwise the first battery gets discharged as well, if I'm understanding this right.
Best regards, Peter
AnswerID:
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