Out boat uses twin 12v batteries but has one engine with one charging output. The boat has a selector switch for battery1, battery2, both or Off.

If you select battery1 or 2 then that battery will be charged but the other one won’t unless you use the Both setting, but then you run the risk of one battery discharging into the other and you end up with two dead batteries.

I know there is a good solution from a Canadian company called Blue Sea which is an automatic charge relay that will send charging current from the engine to both batteries when it’s over a predefined voltage (about 13.3v), but will automatically open the relay and isolate the batteries when the voltage drops (i.e. when the engine is stopped).

This allows you to have one battery for engine starting and ‘house’ battery that runs the VHF, stereo, amplifier, chart plotter, sonar, etc.
The only issue is it’s pretty damn expensive (along with anything with ‘marine’ in the name) so I was wondering if the motorhome dudes use something similar because I’d be surprised if you run everything off just one battery.

Look for “split charge relay kit” on ebay.

V common set up for campervans. I have one in mine.

You can get low amperage ones for less than £25

http://www.ebay.co.uk/itm/CAMPER-VAN-SELF-SWITCHING-VOLTAGE-SENSING-SPLIT-CHARGE-RELAY-KIT-12V-30-AMP/371716528268?_trksid=p2047675.c100005.m1851&_trkparms=aid%3D222007%26algo%3DSIC.MBE%26ao%3D2%26asc%3D40656%26meid%3D2c252db4ac6f42dd8883e783514e8dc4%26pid%3D100005%26rk%3D3%26rkt%3D6%26mehot%3Dpp%26sd%3D222460980235

Split charge relay is a very basic way of doing it . You’ll end up with batteries that are about 80% charged ( stick a volt meter on a nicely recovered battery off the smart charger and do the same on an alternator charged one) and they are less than kind to the batteries so you’ll end up sticking them on the smart charger to condition. Them more.

Look for a battery to battery charger. Takes the current and turns it into a nice controlled current and charges your batteries in a 3 stage set up like a good smart charger would.

You will pay more though.

Simplest answer is a diode splitter like this (worth reading what Mr. Sterling has to say on the subject, too).

Ideally with alternator (or smart charge controller, if you have one) converted to battery voltage sensing.

Thanks guys….. something to look into.

STOP

Battery one = engine – start engine on this

Once running switch to both When stopped switch to battery 2

That is the entire point of that switch you are trying to bypass

IME a diode loses power and is not a great solution hence they used this switch way back before electronic set ups negate this

There is no need to buy anything simply employ manual override by selecting the right battery –
FWIW I preferred to charge one battery then the other – alternators are poor at charging and this gives more charge for reasons I wont explain
Use the switch properly basically [ no offence]