Fridge & dual battery connectivity question

Hi
Since the fridge is a 3way , it may not be thermostat controlled.
You have 2options to prevent it flattening the van battery
[a]fit a "FRIDGE SWITCH' in the van
or
[b]an ignition controlled relay switching a separate feed from the main battery [heavy cable]
This WILL give better results including better charging of the van battery. Which is very unlikely to be overcharged if same type as main battery[type meaning all wet lead acid or all agm ]

oldtrack123,

>>>>>>>>>>>>>
Which is very unlikely to be overcharged if same type as main battery[type meaning all wet lead acid or all agm ]
>>>>>>>>>>>>>

if his alternator/regulator doesn't offer a battery temperature sensor input, it'll always generate an output voltage at varying degree of accuracy (accuracy in terms of what the battery wants to see under a certain charging-temperature).

It doesn't really matter whether both batteries are of the same type or not since the cell gassing voltage potential is always lower than the charging voltage potential, regardless of battery type. This spread varies with temperature, which is the reason why at higher temperatures (say 50 degrees plus) it becomes almost impossible to charge any lead acid battery, temperature compensated or not.
They are best charged at 20 to 25 degrees regardless of type, when the charge acceptance is already good enough, and the gassing voltage potential still high.
If you want to charge them at temperatures between 25 and 50 degrees, temperature compensation keeps gassing in check.

Temperature compensation is cheap insurance against loss of electrolyte in valuable deep cycle batteries.

If your setup doesn't offer temperature compensation, and you're using AGM or gel, it's really better to err at the lower end (undercharge), and occasionally boost charge with a quality temperature compensated precision charger.
Best regards, Peter

how rusty is your rig? ;)

Shouldn't be a problem if there is a good steel mass, providing a good current return path to your starter battery/alternator ground connection.

Best regards, Peter

SS,
Some vehicles don't have a good ground between chassis and alternator - especially for high currents. On modern vehicles, there can be a fair bit of rubber between the two - body mounts, engine mounts, and some vehicles have had problems with strange lights coming on because of poor earthing of the trailer. If you hook onto the chassis, I tend to use a spare threaded hole, tap it out and strip it of paint and coat in vaseline.

I run a 4Ga earth lead from battery to anderson plug, and the extra battery I have in the canopy plugs into it as well. I also have extra straps between chassis and body and canopy - some of this is also there because of the HF radio.

Hi Bonz

Yes the fridge will give about0.12v drop, but on top of that there is the battery charge current voltage drop another 0.12v @ least.
It is then 0.24v drop , quite significant when trying to FULLY charge a remote battery

I guess it depends on the output of the alternator, 0.3v from 14.2 doesn't matter a lot, IMHO 13.9 should fully charge a battery anyways, but even down to 13.5 should do the trick.

The voltage available for pushing charge into the battery is already pretty small. The battery's "native" voltage is a bit over 12.6V. The alternator output will be in the range 13.7 - 14.2, less when the alternator is hot. So the potential difference isn't much over about 1volt. You can easily lose a quarter of that if there are 20 - 30 amps flowing, even with the big cable and connectors planned here. I think if it isn't desirable to run a seperate wire from the front to the fridge, a dc-dc charger is probably a good way to go. Smaller ones (7.5 amps) cost less than $100, a big 30A one can be had for well under $300, about the price of an AGM battery.

HTH

John