Help on how to rig up 2nd battery

Thaks Mainey, sounds easy!
Can you recommend a brand of AGM?

Mainey I've read your post again and forgot to ask how the fridge and lights are connected to the AGM? I want the fridge to be working while driving which it does now. Would it now be running on the AGM not the cranking battery?

JT
in that case place the 'agm' battery near the fridge, maybe in a cupboard, wire the fridge direct to the 'agm' battery, BUT….. place a fuse in the positive cable between the 'agm' and the fridge adjacent to the battery, also have an inline ON/OFF switch that's easy to get to so when you turn OFF the gas, so you can turn ON the fridge with-out moving away from where you are at the time, that way you don’t forget to turn on the power.

Obviously you will need a fuse at each end of the positive (+) cable near each battery terminal, must be politically correct here :-)

The lights and pump can also be connected to the same fridge cable with a connector box, after the fuse but before the on/off switch.

I see your on the small island so would not have a clue what is available to you down there, a good warranty would be a clue.

Maîneÿ . . .

John, I have preferenced my reply with;
" Reasonable and *MOST ECONOMICAL* " because the original post stated..... “Need some advice on the best (and most economical)” as a requirement.

John you say; “You could run the fridge directly of the supply wire coming from up front, then connect a 12v-12v charger (also called a battery to battery charger) between that line and the battery. These gadgets increase the charging voltage for the battery. Lights and other gear, *but N0T the fridge* would then be connected via fuses to the battery”

John, if the fridge was connected only to be powered off the alternator, when you turn off the vehicle you loose power to the fridge too.

John you say; "the current drawn by the fridge will drop the voltage reaching the trailer, no matter how huge the cable. Because of this voltage drop, if the battery is being charged from that same line, it will never be charged anything like fully"

John, not really correct, the alternator is capable of delivering say 50 Amps, the fridge draws say 18 Amps, the amperage supplied by the alternator/regulator is adjusted to suit various conditions, this does include the extra draw of the fridge.
The voltage drop will be associated with the cable being too thin or faulty/loose inline connections.
If, for example there is a (very exaggerated) 1.0 volt loss in the charging cable when measured at the aux battery, then the battery is still being charged @ 13.2 volts at least, even after also running the fridge.

John you say; “There is also a risk that you may leave the fridge running on the auxiliary battery when the engine isn't running, which will very quickly run the battery down”
John, not if the 12v ON/OFF switch is placed where I've specified, because you can turn gas ON and 12v power OFF from the same position, you don’t have to move away to do either of them, there is no remembering problem, often associated with aging or just simply forgetting either.

John you say: “My alternative is to run two cables from front to trailer, one for charging the battery, the other for running to the fridge. This way the fridge current doesn't affect the voltage reaching the battery”
I don’t agree with this, reason being BOTH cables are powered from the same power source, the alternator/regulator, which adjusts the current to suit the situation, as mentioned above.

Maîneÿ . . .

John has stated it perfectly.

Mike so you obviously agree "to run two cables from front to trailer, one for charging the battery, the other for running to the fridge.
This way the fridge current doesn't affect the voltage reaching the battery"

When both cables are coming from the exact same 12v battery connection in the vehicle, will you explain "how fridge current doesn't affect the voltage reaching the battery" please :)

Maîneÿ . . .

Just noticed a typing error in my last paragraph. Doesn't change any of the facts, but may have confused :

I meant to say " In either case you need a relay (controller) in the engine bay to connect the trailer wires only when the engine is running. In the 2 cable system you need 2 relays. (If the 2 cables shared a single relay, the line running to the battery and the one running to the fridge would be joined together in the engine bay. The FRIDGE would therefore be permanently connected to the aux battery and quickly flatten it.)"

I accidentally said " The auxilliary battery would therefore be permanently connected to the aux battery and quickly flatten it." which doesn't make a lot of sense!

Cheers

John

John,
it obviously made sence to Mike

Maîneÿ . . .

And decide!

As above, the 3-ways are very heavy on current. I assume you would only be running it on 12V whilst mobile and hence whilst the alternator is able to charge....

When stationary, it's gas all the way, unless mains is available.

If relying on solar for the charging, it is cheaper buying an all electric fridge-freezer (though with panels hitting $5 per Watt, it's getting better).


For a simple battery isolator, tell the auto-elec to use the charge light circuit to power an isolation relay. That will connect the batteries when the alternator is charging, otherwise they will be independent.
And yes - fuses at each end near each battery. (That goes much further than mere politics.)


As to whether to run 2 lines or one thicker line, or a dc-dc converter... ah, well...
My preference is a single heavy cable - especially in this case where you are only powering a heating element and not a compressor etc.
Something like 14G should drop about 1V per 10 meters for a 10-12Amp load which is probably too large a drop to charge from the alternator....
Whereas 4G will drop 10-times less - ie, 0.1V per 10m.

dc-dc converters will overcome the voltage drop, but at the expense of a higher input current in addition to conversion losses (say 10-20%?).