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

Submitted: Wednesday, Oct 05, 2022 at 10:54
ThreadID: 144915 Views:7214 Replies:8 FollowUps:15
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Hi all, I have posted a similar question about this but it was about powering the 3way fridge directly from the tow vehicle. Things have changed a bit for me and theoretically, tech has also evolved some so I would like to repose my question and get some opinions on why I am wrong :)

I understand, running 3way fridges on 12V directly from house batteries while travelling was a big no no in the past due to the high current draw (around 16A) and the limited battery capacity and recharge profiles of AGM batteries; but what about now with lithium. What would be the biggest concern about running the 3way via 12v while the battery was being fed by a 40A DCDC charger from the tow vehicle while travelling and switching it over to gas when setup is done?

Car wiring
6 B&S with 50A midi fuse on positive line connected to a 200A normally open 12v Relay (with 6mm terminal lugs) circuit closed by Micro2 Fuse tap. 4mtr 6 B&S cable to rear of tow vehicle with weatherproof 50A Anderson plug.

Caravan
3mtr 6 B&S cable from drawbar to ITECHDCDC40 DCDC charger, charging the house battery, an ITECH120X 12V 120Ah lithium battery. 3way Dometic Fridge, connected to 40A midi fuse on positive line via bus bars with bluetooth monitored shunt.
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Reply By: Peter_n_Margaret - Wednesday, Oct 05, 2022 at 13:01

Wednesday, Oct 05, 2022 at 13:01
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
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Reply By: MarcusM - Wednesday, Oct 05, 2022 at 13:15

Wednesday, Oct 05, 2022 at 13:15
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.
AnswerID: 641776

Follow Up By: Peter_n_Margaret - Wednesday, Oct 05, 2022 at 13:59

Wednesday, Oct 05, 2022 at 13:59
How long you can run the fridge when parked up is dependent on the battery capacity. An Ah is an Ah. It is not related to battery chemistry.
16A should not create significant chemical voltage sag in any reasonable sized AGM and cable voltage drop is directly related to amps and cable size so is independent of chemistry.
Calculate it here.....
VOTLAGE DROP =
[cable length (in metres) X current (in amps) X 0.0164] divided by cable cross-section in mm.sq.
A lithium would recharge a little faster when driving after a stop, but that is a minor advantage and either Li or LA should be fully charged after driving thanks to the 40A DC-DC.
Cheers,
Peter
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Reply By: Kazza055 - Wednesday, Oct 05, 2022 at 17:06

Wednesday, Oct 05, 2022 at 17:06
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?
AnswerID: 641781

Follow Up By: MarcusM - Wednesday, Oct 05, 2022 at 20:49

Wednesday, Oct 05, 2022 at 20:49
Hi Kazza, Thanks for the reply.

I already have the lithium battery as a replacement for my old dying SLA battery, I was going to add the DCDC anyway as the solar controller and charger to charge batteries while driving so there isn't really an additional cost.

All of the above was going to be done regardless for the battery and charging system... I suppose the point I am trying to get at is: Advice I had received in the past was run the fridge directly off the tow vehicle. But I was wondering if that was the best way considering what I had in place. The 12V solution was never to run for longer periods of maybe a few hours, anything longer than that and I will be parked up and on gas.

I don't have the car wiring done as yet, I have all the parts to do so, just need some clear weather to get out in the driveway and do it.

As much as I would love a flash new compressor fridge, that isn't in the budget.

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Follow Up By: Frank P (NSW) - Wednesday, Oct 05, 2022 at 22:02

Wednesday, Oct 05, 2022 at 22:02
Marcus,

You could do what you plan above, but add a change-over relay in the fridge circuit in the caravan so that when the car is plugged in and running the fridge runs directly off the car. When parked and the car is off, the fridge runs off the van battery.

You will need 6 AWG cabling from car to relay and maybe a short run of 8AWG from relay to fridge to make it work properly.

I did this in a previous van with a 3 way fridge and it was a great success.

Just remember, if you think you're going to be parked for more than an hour or so, switch to gas.

Cheers
FrankP

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Reply By: Member - Bigfish - Thursday, Oct 06, 2022 at 05:28

Thursday, Oct 06, 2022 at 05:28
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.)
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Follow Up By: MarcusM - Thursday, Oct 06, 2022 at 09:09

Thursday, Oct 06, 2022 at 09:09
Thanks Bigfish!
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Reply By: qldcamper - Thursday, Oct 06, 2022 at 07:55

Thursday, Oct 06, 2022 at 07:55
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.

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Follow Up By: MarcusM - Thursday, Oct 06, 2022 at 09:07

Thursday, Oct 06, 2022 at 09:07
Hi qldcamper, thanks for the reply.

I don't have the intention to run a second relay, I don't see the need to add this complication and extra draw on the alternator. My intention would be to run only the 40A draw from the DCDC and run the 3way from the battery.
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Follow Up By: Batt's - Saturday, Oct 08, 2022 at 04:27

Saturday, Oct 08, 2022 at 04:27
A 40amp will definitely pull more I blew a 50amp fuse in a Redarc only used it because I had them already then I put the recommended 60amp fuse in. I also found out an 80amp alternator isn't big enough either. My bcdc was wired to the starter battery as recommended by the book.
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Reply By: Batt's - Friday, Oct 07, 2022 at 13:02

Friday, Oct 07, 2022 at 13:02
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.
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Reply By: Member - DOZER - Friday, Oct 21, 2022 at 17:44

Friday, Oct 21, 2022 at 17:44
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...
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Reply By: Member - McLaren3030 - Saturday, Oct 22, 2022 at 07:58

Saturday, Oct 22, 2022 at 07:58
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.
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Follow Up By: Bob Y. - Qld - Sunday, Oct 23, 2022 at 11:52

Sunday, Oct 23, 2022 at 11:52
Further to Macca’s reply, some months back on a trip along the Gibb, we were having 12v problems with our van, accompanied by erratic behaviour with 3-way fridge. In Kununurra, we went to see a local fridgey, with some expertise with 3-ways. During conversation with him, he mentioned that, on 12v, these fridges lose 1° per hour. 8 hours=fridge at 10°-12°.

I’ve found that bottles/bags of ice in the fridge help to keep fridge cabinet temps a bit lower.

Bob

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Follow Up By: Allan B (Sunshine Coast) - Sunday, Oct 23, 2022 at 12:29

Sunday, Oct 23, 2022 at 12:29
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Hi Bob,

Many, many years ago I did a 1 year 'refrigeration mechanic' course at what is now known as TAFE.
At the very end of the course the teacher turned his attention to 'absorption refrigeration'. After a short description of its principles he told us to "take it out to the lawn and roll it around a bit then come back next day and by the Grace of God it may be working"...... So concluded the course.

For the life of me, I cannot comprehend how anyone would want to tolerate one of these contraptions.
With the advent of 12v compressor fridges, efficient solar panels and lithium batteries we have moved so far ahead of this ancient technology.

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Allan

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Follow Up By: Member - McLaren3030 - Monday, Oct 24, 2022 at 08:14

Monday, Oct 24, 2022 at 08:14
Totally agree with Allan’s last point, we stopped using absorption refrigerators as domestic refrigerators in the middle of the last century. Compressor refrigerators are far more efficient, you just need to have an adequate power supply. The purpose built expedition trucks that are set up to be fully self contained and self sufficient only use compressor fridges, but have adequate LiFePo4 batteries and solar charging capacity.

Macca.
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Follow Up By: Member - Warren H - Monday, Oct 24, 2022 at 14:05

Monday, Oct 24, 2022 at 14:05
On the other hand my sister has a 50yo Finch 3-way portable chest fridge that they use as a cyclone season backup. It still runs fine on both gas and 240V. It gets tested at the beginning of the cyclone season and then put away. About thirty years ago, I used it camping a few times a year. I doubt you'd find too many compressor fridges of that vintage with such irregular use still functioning. The only maintenance has been the replacement of the gas regulator and bottle, their simplicity being both a virtue and a curse.
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Follow Up By: Allan B (Sunshine Coast) - Monday, Oct 24, 2022 at 16:00

Monday, Oct 24, 2022 at 16:00
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I'm surprised that they don't have one that runs on kerosene. lol
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Follow Up By: Member - Warren H - Monday, Oct 24, 2022 at 16:29

Monday, Oct 24, 2022 at 16:29
Well for emergency use it actually makes sense, no point having something that needs a battery recharged, in the aftermath of a cyclone you likely have no sun as well as no power. What is worse than no power is no water, stored water is fine for drinking and cooking but then there's dunny flushing. In one cyclone the sewage treatment plants kept running on emergency power, but the water supply had to be shut down due to debris in the intakes, then the roads had to be cleared to get access to remove the debris. I can remember having no problem with the power out for 48h, but no running water for 4 days is much more of a pain. We had to collect buckets of rainwater and water from storm drains.
But funny you should mention kerosene fridges, my bil's father insisted for years that he would only use a kerosene fridge. Comes of decades living in bridge builders camps I suppose.
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Follow Up By: Allan B (Sunshine Coast) - Monday, Oct 24, 2022 at 16:48

Monday, Oct 24, 2022 at 16:48
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Devotion to kerosene fridges?....... Yair, the fumes get into your system and probably addle your brain. lol

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Allan

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Follow Up By: Zippo - Tuesday, Oct 25, 2022 at 15:01

Tuesday, Oct 25, 2022 at 15:01
Allan, about ten years ago I helped a high-school "outward bound" adventure group by being the advance guard to set up the huge house they rented as their base (about thirty of them). Had a hand-cranked diesel gen-set (battery always flat, the place got used twice a year!), a huge wood-fired central heater, and a LARGE kero fridge. The hardest of the three to fire up was the gen-set, the kero fridge was straightforward.
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Follow Up By: Allan B (Sunshine Coast) - Tuesday, Oct 25, 2022 at 17:13

Tuesday, Oct 25, 2022 at 17:13
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Zippo, I always avoided hand-cranking large kero fridges. lol
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Allan

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