Help me understand: How does a fridge not drain away your battery?

[adaml]

your wiring is inadequate, so its shutting off at a higher voltage than you set it at.. you didn't seem to do it intentionally, so your personal experience is largely irrelevant as its not a universal truth that everyone here is going to use your gauge and length of wiring with similar loads, if they put in adequate wiring the'll have the opposite experience.

Wow. Not only do you know that all fridges kill your battery (or maybe they all shut off too early, still not sure which...), but you also know the specifics of my wiring system. FWIW, I'm running 8 gauge wire about 8 feet but I'm sure you'll come in here to tell me something is wrong with that.

 

[jadmt]

been running an ARB 50 in my jeep for about 3 years. I can park for 3 days and it has always started never tried longer than 3 days. I don't think my arb 50 has ever kicked off from low voltage. I run the standard ARB harness direct from the battery to the back of the jeep. It can get to 100F but also cools off at night. Fridge is kept at 34F.

 

[dreadlocks]

Medium on the ARB Fridge is 11.4v which it shuts off at


This is a batteries state of charge for a given voltage:

So 11.4v cutoff on your starter battery is draining it down BELOW 10% SOC, again I'm stoked your managing to crawl back out of the woods doing stupid ******** like that.. but I know for a fact my vehicle will not even try to crank over at 11.4v.. not to mention the severe abuse this is doing to the cells and those batteries wont last.

 

[adaml]

Medium on the ARB Fridge is 11.4v which it shuts off at

View attachment 612862
This is a batteries state of charge for a given voltage:

So 11.4v cutoff on your starter battery is draining it down BELOW 10% SOC, again I'm stoked your managing to crawl back out of the woods doing stupid **** like that.. but I know for a fact my vehicle will not even try to crank over at 11.4v.. not to mention the severe abuse this is doing to the cells and those batteries wont last.

You know, a few interesting things happen when you look for that chart you've got there. First, you find out there's a number of other charts out there with wildly different numbers and, of course, different battery designs. Second, you get websites like this (https://marinehowto.com/under-load-battery-voltage-vs-soc/) which reference your dandy chart there and say, "This image is an epic fail on our customers part, despite his very good intentions."

 

[dreadlocks]

keep on reading there buddy..

load voltages as this chart was likely developed from. His battery bank was comprised of the identical battery being tested here.
If we cross reference this chart to the 20 hour discharge rate, under-load voltages, in this test they line up pretty closely but if using a chart like this for a RESTING VOLTAGE you will essentially be murdering your batteries.

The fridge is load, the LVD cuts off when fridge is running its not a resting voltage, and your argument is using that graph is an epic fail is because its worse than that.. not better, so are you not seeing the fallacy in your argument?

 

[adaml]

keep on reading there buddy..



The fridge is load, the LVD cuts off when fridge is running its not a resting voltage, and your argument is using that graph is an epic fail is because its worse than that.. not better, so are you not seeing the fallacy in your argument?

But the battery really isn't at 11.4 because my wiring is insufficient, right?

And, fwiw, what I'm really getting at is "A Voltage to SoC chart should only come directly from the manufacturer of your batteries, or through physical testing of your own batteries, as I have done here. You should avoid the use of use generic charts off the internet."

Look man, you keep doing you. Obviously, you feel a certain way about this and are going to twist whatever is out there to fit that narrative. That's cool. By the fact that you've stated you have a "very expensive setup", it's pretty clear how you feel on this issue. I'll continue to run my way and carry a small jump-pack. It's worked for me and many others without issue and saved a lot of time, money, and headache.

 

[dreadlocks]

If you really got 8awg at 8ft (really 16ft because the negative conductor) then its really at 11.5v (0.1v drop under a fridge load) if nothing else at all is running off that circuit, if you put anything else on it.. then all that changes, as the loads go up the drop gets bigger if the wiring remains the same.

My first fridge a decade ago was a very minimal and cheap setup, and it murdered batteries left and right and left me with a bunch of spoiled food and other things.. so yeah, I spend enough time out boondocking the cost of buying a new bank every season quickly started pushing me to build a more robust setup capable of self-sustained operation indefinitely and didn't leave me with completely trashed batteries a week into a 3-week trip, again.

Fridges are insanely variable in performance, what works for one person who is on the road every day wont work at all for someone who moves every few days a couple hours down the trail.

 

[ChasingOurTrunks]

So assuming a fresh battery (I'm getting a brand new battery installed this saturday), how long can you actually run a fridge before it cuts off? With a mid-range fridge like the Indel B, is it safe for an entire day?

I do always have a jump pack in the car already.

Good call on the jump pack. As @dreadlocks and @Herbie said, that's a really useful bit of kit. The lithium jump packs are really tiny (smaller than a VHS tape in a lot of cases) and can save you in a pinch, and they are below $100 these days so they are cheap insurance.

Your question about the fresh battery is a natural next question -- but that's where it starts getting a bit complex. Folks like @dreadlocks have written out dissertation-level posts on these boards full of amazing explanations on the use of batteries, solar, and 12V power in Overland applications from a real world use perspective, informed by some pretty robust understanding of electrical theory, so cruise through his post history in particular and I think it will be educational. Another resource is Andrew St. Pierre White who has a few videos just on dual battery systems and some of the challenges with states of charge and such -- batteries aren't just "full" or "empty" as there is a max safe discharge, battery temperature, and battery design all coming into play -- there are a lot of variables beyond "a new battery can run a fridge for X hours". Here's some of Andrew's videos on the topic.





Also, something to consider -- Modern vehicles are incredibly sensitive to voltage irregularities. A low battery can often cause all kinds of gremlins, depending on the car. My opinion is that you are at a greater risk for getting stuck if you only have one battery. This can be mitigated with a jump pack, but a jump pack and a dual battery almost entirely eliminates the risk of getting stranded due to a battery issue, so it's the best way to go. That being said, as you can see from this thread, it works for some people (and worked for me, for years). But it doesn't give you much of a margin for an "oops" moment -- even with a brand new deep cycle AGM starting battery.

I just remembered another solution for you to explore. Your best bet might be to copy a system like what MyCoolman has. They sell the fridge and it comes with a separate lithium battery pack just for the fridge; Shaun Whale has this setup in his rebuilt "Dirty30". I think it's intended so that you can move the beers closer to the campfire but, you could run the fridge off your main battery while driving, and switch to the lithium power pack when parked, and then charge the lithium while driving the next day. This is in essence a "dual battery" system, but much more simple to set up and use in multiple vehicles.

myCOOLMAN Lithium PowerPack

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[Todd n Natalie]

Good call on the jump pack. As @dreadlocks and @Herbie said, that's a really useful bit of kit. The lithium jump packs are really tiny (smaller than a VHS tape in a lot of cases) and can save you in a pinch, and they are below $100 these days so they are cheap insurance.

I carry one of these jump packs as well. They do work great! Provided they are charged up. Don't forget to periodically check it to make sure it's charged up! I've made that mistake before...

 

[jdlobb]

Thank you everybody, there's a lot to process here. I don't think I can shoehorn a true dual battery system into my little Land Rover Discovery Sport, space is already at a premium. But, given everything I've read I think the prudent thing will be to buy a lithium "generator" from Goal Zero or similar at the same time, and just plan on running the fridge off that when in camp.

That begs a follow up question though. I think everything I've seen here is about the power draw in amps and amp hours. Most of the extra battery packs are rated in watt hours. What size battery would I need to safely maintain the fridge, under normal conditions (say 80 degree ambient temp) for a day or 2? Would a 150Wh or 200Wh batter get the job done? or do i need something closer to 500Wh?

 

[RichJacot]

I have a dual battery setup now but when we first got our ARB 50 I could go two days of the truck sitting and still start our '10 FJ Cruiser. We went 2 1/2 days once (Texas Summer) and I couldn't start the truck (prior to the dual battery). My battery is and has been a group 31. The 2 1/2 day example was a Diehard Platinum Marine about 3 or so years old. Since we have a dual battery setup now I've have had no need to test our Interstate AGM so I can't give you experience on a single battery. We rarely go more than the two days without moving the truck but my house AGM has always handled two full days without issues and it's a group 34 AGM marine.

I don't know if someone else mentioned it and I just missed it but since you haven't gotten the battery yet....try to move up a size if you have room and get an AGM or at least an AGM of orig. size.

A jump pack is always handy to have anyway. ;-)

 

[jdlobb]

I have a dual battery setup now but when we first got our ARB 50 I could go two days of the truck sitting and still start our '10 FJ Cruiser. We went 2 1/2 days once (Texas Summer) and I couldn't start the truck (prior to the dual battery). My battery is and has been a group 31. The 2 1/2 day example was a Diehard Platinum Marine about 3 or so years old. Since we have a dual battery setup now I've have had no need to test our Interstate AGM so I can't give you experience on a single battery. We rarely go more than the two days without moving the truck but my house AGM has always handled two full days without issues and it's a group 34 AGM marine.

I don't know if someone else mentioned it and I just missed it but since you haven't gotten the battery yet....try to move up a size if you have room and get an AGM or at least an AGM of orig. size.

A jump pack is always handy to have anyway. ;-)

good info. fortunately my discovery sport came with an AGM battery, so already have that covered. AFAIK though it's not possible to swap it for a larger one.

 

[ChasingOurTrunks]

Thank you everybody, there's a lot to process here. I don't think I can shoehorn a true dual battery system into my little Land Rover Discovery Sport, space is already at a premium. But, given everything I've read I think the prudent thing will be to buy a lithium "generator" from Goal Zero or similar at the same time, and just plan on running the fridge off that when in camp.

That begs a follow up question though. I think everything I've seen here is about the power draw in amps and amp hours. Most of the extra battery packs are rated in watt hours. What size battery would I need to safely maintain the fridge, under normal conditions (say 80 degree ambient temp) for a day or 2? Would a 150Wh or 200Wh batter get the job done? or do i need something closer to 500Wh?

Another good question. I'm getting deeper and deeper into stuff that I'm at best an amateur at, but the way I understand it is that Watts = Volts x Amps, and that math can be re-formulated to calculate for any of the variables because they are all related to one another. In your case, you are adding a variable of time, and from what I know, the formula in that case would become Amp Hours x Volts = Watt Hours. I just did a quick google and I think that's correct.

I'll steal this guy's numbers and say that your fridge needs 40 Amp Hours per 24 hour period as an assumption -- your fridge manufacturer should tell you the specifics of your fridge. That means that:

40 amp hours * 12v = 480 Watt Hours. So, sounds like 500 watt hours is closer to where you need to be.

Again, this is VERY much napkin math for me and I'm not at all sure if I'm doing this right, so perhaps someone with a bit more wisdom than me can chime in. My instinct is that just because a battery has XYZ watt hours, I don't think that directly translates into usable energy in all cases -- like my auxiliary battery is rated for 100 Amp Hours, but I think that applies to a "full discharge" measurement. It's not good for a battery to be fully discharged like that, so my 'useable' power is really only about 50 Amp Hours max -- which is the number I actually need, but I still had to buy the 100 AH battery to get it. I am under the impression that lithium batteries (like what you'd find in one of those compact power packs) do not have this limitation and if it says XYZ amp hours or watt hours on the box, it should give roughly that performance.

FWIW, this calculator confirms my above numbers. And its on the internet so it must be true

 

[Cabrito]

I'm a long time amateur fridge user. I run it off the car battery under the hood. I use old fridges without the fancy shutoff option so I just keep an eye on the voltage. @dreadloks has given some really good spot on info based on his usage experience.

These things cycle on and off so they aren't constantly draining the battery, but weather conditions are a big factor. I've shortened the life cycle of a few batteries in my time, but learned to keep up on it. I'm also playing around with cheap solar options (jury is still out for me on this experiment)

My tricks are:
Turn it off at night and don't open it. Don't forget to turn it back on! No problems here for me. Food still cold in the am.

Run the car for 15 minutes every day if it's really hot out and the fridge is cycling on and off a lot. This seems to help, but some alternators will not be charging much at idle. Sometimes I do this for peace of mind even if it's not helping that much.

Use a small ice chest for drinks- this has been huge for me since I have a small 35l fridge and it reduces the need to open the fridge over and over. After a few days I move the drinks into the fridge since there is more room after eating the food, and the ice as melted. Then I use the small cooler for garbage/recycling/dirty dishes, or just replenish with Ice and drinks. The small ice chest is also something to sit on, and it's usually outside for easy access to drinks.

 

[dreadlocks]

all the trail runners in trucks/rtts that spend all day long w/an engine running posting great results w/fridges and minimal battery are kinda what threw me way off on my first fridge on a trailer w/a dedicated house battery, I greatly underestimated how much energy my battery would get from the engine and was in a huge energy deficit that just got worse with time, I was modeling it off folks with an energy surplus.. I was wanting a fridge so I could go backpacking for a week w/out my food getting spoiled in a ranger station parking lot since nobody was there to put more ice in the cooler.. I wanted a fridge so when my long-haired hippy ass tried to cross an international border the inevitable deep search of my property was not going to end with a bunch of raw food floating open in a cooler of water since they didn't put it back up properly... everyone has their needs and expectations of investing in a fridge, and its disappointing to not even come close to reaching em.. I just try to temper people's expectations when it comes to fridges and energy supplies, just because you read someone is doing fine don't mean you will because you are not him.

It all came down to basic math I failed to do at the start, I only spend my first day and last day doing a full day of driving.. because I go out for many weeks at a time I cannot maintain that kind of driving in tow, I'd only be willing to drag the trailer a few hours down the road every few days because I didn't want to come back home needing recovery time from my vacation... that 4-5h of driving intermittently would never charge my battery back, at full charge rate (which I was not at) it'd of needed 7-8h of driving to get it from discharged to full, all it was doing was extending the inevitable cut off.. so now I have to rely on Solar and got a backup for the solar (Genset) when the sun's not playing ball.

Since those days Lithium tech has reached our hands and now we can charge batteries up from empty to full in 4-5h extremely easy (my LFP charges in 2.5h), so if I were going to try that again.. using my engine as my primary charge source, I'd definitely need Lithium and then I could had achieved recharging off the alternator w/my style of boondocking.