Powering 3way Dometic via 12v from Tow Vehicle - Lithium Battery Updat

If you have a 40A DC-DC charger operating, any battery will be fine because the battery is not providing any of the power.
Cheers,
Peter
OKA196 motorhome

Thanks for taking the time to reply Peter, I was probably more talking about when it is parked along the trip from point A to point B.

I can't see any downsides to doing it this way, just not sure if there is something I haven't taken in to consideration.

The way I see it you are upgrading your van by adding -
1. ITECHDCDC40 DCDC charger at $349.00
2. ITECH120X 12V 120Ah lithium battery at $899.00

This is a total of $1,248.00 to really have no gain over your existing setup.

I assume that you already have a fridge supply from your car, it will be much cheaper to just run the fridge on gas when not drive and not having 240V to supply the van fridge.

Most three way fridge will keep cold for a few hours like when you stop for lunch, stopping longer, run the fridge on gas.

If the fridge was a compressor fridge with much lower 12V current needed, it might be worth doing but why fix it if it ain't broke?

What you have proposed will work fine. The ITech battery will give you around 95 a/hs of useable 12 volts power. Enough for around 5-6 hours of being parked up and just running off the aux battery. Good weight saving as well by ditching the agm. Even if you park up for 4-5 hours without switching to gas the lithium will be recharged to 100% in around 2 hours. The agm may take 6 hours to get to 100%. So long as you ensure the ITech is not pulling power from the start battery once you are parked up..all will be well. I,d probably put a 60 amp fuse near the start battery as you are protecting the cable and not the charger (which has its own 45 amp fuses.)

What is the capacity of your alternator?
A 40 amp DCDC charger will pull 40 or a bit more as long as it takes to top up the battery and if you run a CO relay as previously suggested add another 16 amps to that so could be getting close to 60 amps on top of what your car is needing from the alternator. Driving in maybe rain with wipers, lights and aircon on all adds up.
If your alternator cant produce enough power the DCDC will still keep drawing large amounts of power and leave you with a flat start battery after a few hours of driving if not a toasted alternator.
Just thought I would mention that, most people ignore it and never have a problem but if all the holes line up you could be left at a truck stop in shitty weather looking for a jump start, or worse, in the middle of nowhere.

In 2005 when traveling for 3 months plus lots of other trips over a few years had a lead acid deep cycle in the van charged directly off the 4WD's battery with a basic battery isolator in line. We had no issues with this basic setup and if stopped for around 30 minutes or an hour we just turned the gas on. Don't over complicated a simple thing and everything will be fine.

12 v gives less heat than 240, which gives less heat than gas....you would be better running the fridge on 240 and invertor than 12 volt. Use batteries and chargers as you suggest to keep it going. Result will be colder fridge. 12 volts only really keeps the temperature where it was when you switched it over...

Hi Marcus,

No offence intended, but you are trying to do something that is not recommended by any12 volt expert. Yes, what you are proposing can be done, but your absorption fridge will work as designed without any changes to its current configuration. Why complicate something that is already working?

An absorption fridge is not designed to run on 12 volt battery power for any length of time, without the 12 volt battery being continuously recharged. This is because of the high current draw of the heating element. Same applies to any electrical appliance being supplied from a battery system, including via an inverter. Other than 240 volt appliances being fed from a battery via an inverter, an absorption fridge has probably the highest current draw of any electrical appliance associated with a caravan/camping set up. A lot of newer caravans with absorption fridges have auto changeover systems that automatically switch the fridge between power supplies, depending on what they see. That is, if no charge is going to the battery, the fridge will switch over to gas, if 240 volt supply, the fridge will run on 240 volt etc.

Again, no offence intended, but in my opinion, you are trying to complicate an already workable system for what I can see as no real benefit.

Macca.