What RPM needed to charge a dual battery set up?

[First Pooy]

My solar pannals will not be ready before my next trip,so what Rpms do I need while sitting stationary to charge the batterys and not glaze the bore I thought at least 1500 rmps the fire trucks sit at 1800 rmp

[GQtdauto]

Go by volts rather than RPM , mine puts out ^14.4 at around 800 - 900 but it's an aftermarket jobbie.

[MB]

Upgraded my multimeter recently to this combo AC/DC clamp/amp meter from Jaycar.
https://www.jaycar.com.au/400a-ac-dc-clampmeter/p/QM1563
Clamp over your positive alternator lead and you can find a sweet spot of amps versus revving its bum off!


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[mudski]

If your batteries are low, running the motor won't do alot, well, it will but depending on how far they are down it could take hours to get some charge to the batteries to full capacity, even then they wont go to full capacity. I think its around 80% an alternator a standard charging system is capable of.
But as the others have said, have a look with a multimeter. It would vary between vehicles a little I would think, depending on condition of alternator, wiring, batteries etc etc.

[the evil twin]

MB is on the money for best method.
Fire truck RPM is more to do with their PTO pumps/Hydraulics so not the best guide.

Mudski is on the money for the Alt to be a poor way of trying to recharge low batteries.
It is due to the battery charge acceptance rate and a low battery will take a fair bit of charge initially but then the rate tapers off quickly in an Alternator based system.

If you have no other choice and don't have a clampmeter I would suggest 1,000 to 1,200 engine RPM as a SWAG for best compromise but it will take hours not minutes.

[DaveGQ]

It depends more on your Dual Battery Isolator. I have the Red-Arc branded one and that closes at 13.5V and starts charging the spare. Your alternator should put out around 14.5V so even if you leave it idling, it should eventually click over and start charging the spare batt once the main batt has reached 13.5V surface charge.

Otherwise get one of these:
https://www.jaycar.com.au/6v-12v-heavy-duty-8a-car-battery-charger-with-trickle-charge/p/MB3522

They are great to have, I use mine when I haven't driven the car for a long time, and you can also leave it ON, it does a 'trickle' charge to keep the battery topped up without overcharging it or wasting too much power.

[threedogs]

Outback engineering have a honda motor with an Alternator bolted to it
that will charge any flat battery in an hour or so. Not sure if still operating
maybe someone up north would know more. dont upset your fellow campers eh

[PeeBee]

Outback engineering have a honda motor with an Alternator bolted to it
that will charge any flat battery in an hour or so. Not sure if still operating
maybe someone up north would know more. dont upset your fellow campers eh

John this is simply a 12V hitachi alternator with internal reg and driven by a 5hp petrol engine - won't do any better than the one under your bonnet. The advantge it has is if your batteries are so dead you can't start the vehicle then this will thump the charge into the battery. Lots faster than a solar panel for sure, but a bit like a souped up generator with a 12V charge outlet - yes the engel or honda are limited to say 6 - 8 amps, where the outback one will give you a hit up to the performance curve of the alternator. Very neat package I will say, and I guess its a shame no one has come up with a generator that can punch out the 240v plus the high dc current.

[threedogs]

Do you have a dual battery system and how long you going is there any driving involved?
Do you use Leds for night lighting?