Basic Dual Battery Setup to run Fridge/Freezer questions (sorry in advance if these are dumb)

Fletchxj

New member
Hey everybody,
Go easy on me as I've had some experience running basic electronics for car stereos over the years, but I'm just now getting into heavier power applications. I've been searching all over this forum and others as well as watching Youtube videos and rather than getting a really clear answer, I find myself more confused. Hence why I'm posting this and risking being flamed, :D.

Any ways, I bought this fridge/freezer a few days ago after taking a 4 day trip with some friends. The benefits for my trips seem pretty obvious and I'd like to keep it, but since purchasing it, I've also learned that my standard starter battery is insufficient to power the fridge for long (duh). I've now come to realize that my friend was using an AGM battery to run his fridge. Which, now that I've done some research, makes perfect sense. So, I'm at a cross road. I'm about $300 in for the fridge and still able to return it to Amazon. Or, I could go with a dual battery set up that solves the fridge problem and gives me some peace of mind regarding long distance solo trips.

Here are the questions I can't seem to find answers to:
  1. I heard in a Youtube video that I watched that you would have difficulty charging an AGM battery in conjunction with a standard starting battery. Something about them being different so it wouldn't ever fully charge the AGM battery, leading to shorter life and/or failure. Is this true? I was thinking of building my own battery isolator as shown in this thread: LINK.
  2. Second, the whole "group" thing on the batteries is getting me. I understand that Group 31 are larger than group 24 for example, but I'm not sure whether I'd be able to run the fridge for more than 12 hours on a group 24 vs. 31. I guess what I'm wondering about is capacity based on group size (and whether I can even charge a smaller group AGM battery with the above dual battery isolation and my standard starter battery). For example, 12 volt group 24 batteries are around $100-125. But a Group 31 is around $200-$350. I'm just wondering if a Group 24 would be sufficient to keep a fridge running overnight while saving me the cost since I'm not trying to run a ton of accessories. I'm sorry if these are basic questions, but as you can probably tell, I've done some research and it's just made things worse, haha.
  3. Alternatively to all this, I guess that another alternative would be just to replace my primary battery with an AGM battery and possibly add a solar charger for peace of mind? Something like THIS?
Any ways, I think that's it. Thanks in advance to anyone that makes it through this novel. Thanks in advance for any tips you can give me. I really appreciate it! (Pic of truck for attention).

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Winterpeg

Member
Yeah, you'll at the very least need to replace the standard main battery with an AGM starting battery. If you can cram in a grp31 then that would be what I recommend.

That will do just fine for a couple nights if you don't have the fridge set too low (fridge essentially.... not a freezer at that point).

Also, you will need to change out your Alt-S fuse to allow your alternator to provide more volts as AGM's like a higher voltage to keep em happy.

If funds allow... dual battery system would be the way to go. It sure simplified my life. But you will need to use the same type of batteries... ie: 2 AGM's.

For my second battery I had to go with a grp24 as I couldn't fit a 31 where I could put the extra tray. So I have a grp31 and a grp24 with an isolator.
 

luthj

Engineer In Residence
Generally speaking most vehicle alternators do not charge at high enough voltage to fully charge a deeply cycled battery. Thus drastically reducing their life. You need to evaluate your vehicles charging voltage. If its sufficient, a simply charge relay will work if you drive frequently. If you don't drive often, you will need some solar to keep your fridge running.

Group numbers refer to the size of the battery. Thats all. There are very few good deep cycle batteries in the "group" sizes, and the few that exist are expensive.

If you are going to be abusing a battery by running it low often with a fridge, you are better off buying a cheaper flooded battery. It will be much cheaper, and you can equalize charge it to recover some capacity lost to sulfation.
 

DiploStrat

Expedition Leader
Take a deep breath and relax. No one is going to flame you; we've all been there. (Did I tell you 'bout the time I used a diode isolator and killed a battery in the Andes? How 'bout the time I used a starter relay? Bored yet?)

None of this is hard, but there are options which can make it complex.

If you want to know about lead acid batteries, read this:https://cookfb.files.wordpress.com/2016/09/battery-charge-slides.pdf

But you should really start here: (You can certainly run a cooler; some of us run microwaves and espresso machines - it is just a matter of scale.) https://cookfb.files.wordpress.com/2016/09/plan-electrical-system.pdf

Finally, after the $50 thread, you may enjoy this: https://cookfb.files.wordpress.com/2016/09/dual-battery-slides-2016.pdf

But to your questions:

-- You can certainly charge a lead acid battery - whether flooded (FLA) or absorbed glass mat (AGM). You just have to know how to charge a battery properly. (See my first paper - note the part about voltages.) In an isolated battery set up, there is no reason that the starter and the camper batteries be the same; I use a 24v flooded starter battery with a 12v AGM camper battery - it is just a question of how you isolate them.

-- There is nothing magic about an AGM; the magic comes from having a properly sized, deep cycle battery. There IS a difference between starter and deep cycle batteries.

-- "Groups" simply refer to physical size. For your use, the number you want is the capacity of the battery, normally expressed in Ampere Hours (Ah). More is better, limited by your ability to carry the battery in your relatively small vehicle.

-- Solar is ALWAYS good for two reasons. The first is that lead acid batteries take a long time to charge and thus you will probably turn off your engine before the charge is complete. The second is the obvious, that you can recoup power while camping.

Two final notes:

-- In an SUV, you are probably better off to go directly to a lithium iron battery as they are typically physically smaller and lighter.

-- And if you do, you will probably be better off using a battery to battery charger (aka B2B or DC-DC).

Hope this helps.
 

DiploStrat

Expedition Leader
Wouldn’t he run into problems at cold temperatures?
Yes, if the battery is outside of the heated space AND charged while below freezing. LiFePO4 batteries can discharge while below freezing. Probably safe to assume that he won't be charging if the inside is below freezing - but it is possible - starting the engine after a cold night.

As I said, it ain't hard, but it can get complex!:rolleyes:
 

Fletchxj

New member
Thanks for all the replies so far! Extremely helpful. Here's a little bit more info on how I typically overland/camp. I often move around (usually no more than 1 night parked and then I'm driving to a new location the next day). Occasionally, I'll stay for 2 days, which the solar would help with and I could always run the truck for a bit if need be.

The option that seems to make the most sense for what I need is an AGM battery (Group 31) to replace my current starter battery and the solar to help top off the battery when camping to allow the fridge to stay running. Again, I'll never be in one place for more than 2 days and 1 night, so does it seem reasonable that I could run the fridge and keep it on that whole time, especially with the solar charging throughout the day? Of course the fridge has an auto shut-off so I *shouldn't* have to worry about it killing the battery, but I also don't want it to shut off half way through the night and ruin all my food, haha.

My new questions from the comments above are, how do I figure out what the stock alternator output is on my 2005 Nissan Xterra? Also, the Alt-S fuse, is that just a universal thing that would be easy to find in the fuse box? And what size fuse would I need to change it to? Sorry, I didn't drive the truck to work today.

So, basically, I'm looking at probably $250-350 (cheap versus Optima for example) for the AGM battery (something like this? https://shop.advanceautoparts.com/p/carquest-hd-battery-agm-premium-battery-group-size-31t-825-cca-ng31cs/10227749-P?searchTerm=group+31+battery) and then the Solar would be optional at $130 but would be a nice to have.

Thanks so much so far for the help, I appreciate it!
 

Buddha.

Lurker
Generally speaking most vehicle alternators do not charge at high enough voltage to fully charge a deeply cycled battery. Thus drastically reducing their life. You need to evaluate your vehicles charging voltage.
So if someone buys an AGM and uses it as the sole battery in their truck(and deeply discharge it at times) they won’t get the expected life out of it because the trucks charging system might not be up to the task? How does one evaluate the trucks charging voltage? Just measure battery voltage with a running engine or is there more to it?
 

luthj

Engineer In Residence
Is there some specific reason you need an AGM battery? For this type of application the extra cost isn't worth it.

What is the cut-out voltage on your fridge? If its too low, you won't be able to start the vehicle.

The alternators rated amp output has little or no bearing on the actual charge rate for the battery. A group 31 battery will rarely charge at more than 40A, and sustained will be closer to 20A. This is totally depedant on state of charge, and alternator voltage.

You need to get a meter on your alternator (one of the units that plugs into a 12v power port will work). Drive around some, and see what is averages too. Report that value back here, and we can give you some more feedback.
 

luthj

Engineer In Residence
So if someone buys an AGM and uses it as the sole battery in their truck(and deeply discharge it at times) they won’t get the expected life out of it because the trucks charging system might not be up to the task? How does one evaluate the trucks charging voltage? Just measure battery voltage with a running engine or is there more to it?
Yep, just measure the voltage. If your alternator is above 14.2V, and you don't cycle the battery deeply often, you are probably okay. If its below that, you need to take a close look at your needs. Note that alternator voltages drop as the unit warms up, and some vehicles have externally controlled alternators, which may change voltage. So its important to check the voltage while driving (from the accessory 12v port works). As well as idling, and when hot.

AGM batteries generally can't be equalized charged to reverse sulfation. So extended partial charge cycling can't be reversed, and kills them quickly. Flooded batteries can be put on an equalize charge (good wall charger) every 2-4 weeks, which will dramatically reverse the effects of partial cycling and sulfation.
 

DaveInDenver

Expedition Leader
So if someone buys an AGM and uses it as the sole battery in their truck(and deeply discharge it at times) they won’t get the expected life out of it because the trucks charging system might not be up to the task? How does one evaluate the trucks charging voltage? Just measure battery voltage with a running engine or is there more to it?
It's not the deep discharge it's that vehicle charging systems are designed for flooded lead acid and usually charge at too low of a voltage. As a result AGM batteries are often chronically under charged and lose capacity over time. I personally run a pair of Odyssey and haven't modified my stock Toyota charging system which peaks around 14.2V and quickly settles to about 13.3V~13.4V and that's about 0.25V too low. At home the truck sits on a charger and I do have a portable solar panel but if a trip is substantially driving and I am not able to let the batteries charge on the solar I do end up constantly partially charged after just a couple of days. Knock on wood I have not seen a drastic reduction in lost capacity in 4 years yet. But I suspect a cumulative few days punishing the batteries this way compared to sitting on a charger 90% of the time is the only reason why. If I had to daily drive my truck or couldn't let it sit for several days continuously conditioning I doubt they'd last.
 

Fletchxj

New member
Is there some specific reason you need an AGM battery? For this type of application the extra cost isn't worth it.

What is the cut-out voltage on your fridge? If its too low, you won't be able to start the vehicle.

The alternators rated amp output has little or no bearing on the actual charge rate for the battery. A group 31 battery will rarely charge at more than 40A, and sustained will be closer to 20A. This is totally depedant on state of charge, and alternator voltage.

You need to get a meter on your alternator (one of the units that plugs into a 12v power port will work). Drive around some, and see what is averages too. Report that value back here, and we can give you some more feedback.
Ok, forgive my ignorance here. My standard flooded battery will only run the cooler for about an hour or two and then it faults out in order to protect the battery (I have it set to the highest protection setting to preserve the battery). Some of the videos I've watched and things I've read is that the standard starting battery shouldn't really be discharged below 90% because it can damage the battery. Whereas an AGM battery (could be confusing this with something else) can be discharged longer and lower (to about 50%) which would allow the fridge to run much longer before tripping protection (I could set it to a lower protection setting since it wouldn't hurt that battery).

Here's a comment from a review of the fridge on Amazon that might answer your question:
* Tried to determine the voltage drop out (voltage protection) set points but have not had consistent tests. The "L" (low) voltage point might be as low as 10.0 volts but not sure. Recommend leaving on "H" High until you experience F1 fault (protection) and lowering to "M" medium only when necessary. I would avoid using "L" low unless you know what you are doing.
 

luthj

Engineer In Residence
Okay. AGM does not equal deep cycle. Neither does flooded. There are starting, deep cycle, and dual purpose type batteries in both variants. The only true deep cycle batteries in the group sizes are expensive, like 400$ expensive. Given your usage, this extra cost doesn't make sense in my view.

Voltage cut outs also need to take into account the voltage drop from the battery to the fridge when running. This is why I suggest a 10 (or 12 gauge for short runs) direct wire to the battery or distribution block for the fridge. Many factory runs for 12V outlets are to small. Not to mention the 12V cigarette lighter ports will often drop plenty of volts on their own. You can figure out the voltage drop by turning the fridge on, and measuring the voltage at its power terminals. This must be done with the fridge running.

Here is a rough chart for SOC based on open circuit voltage. Loaded voltage will be slightly lower. Lots of variables, but your battery should start the vehicle at 50% discharged in non winter weather. I would not set the cut-out below 11.2V at the fridge. Depending on voltage drop, that would have the fridge cut-out at around 60% SOC.

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DiploStrat

Expedition Leader
You REALLY need to read the materials I referenced. We can help, but ya gotta do your part. (And yes, that is a bit of a flame.)

-- And AGM and a FLA are the same. The differences have to do with gassing and vibration resistance. Period. If you want to go with a single battery, you need a deep cycle battery, which can be FLA or AGM. Both AGM and FLA have the same discharge "rules." A deep cycle battery is more resistant to deeper discharges.

-- A voltmeter will help you determine your vehicle's base charging voltage. If it is 14v or higher, you should be fine. 13.9v or lower, you will need to do something.

-- Trusting a low voltage cutout is a bad idea - size your system to meet your needs.

-- Running the engine for a short time does nothing useful, especially not for a constant load like a refrigerator. You really need to know how many amps your cooler will consume every 24 hours. NOTE: It will be a much bigger number than you expect.

N.B. Look at Jon's (luthj) links: He has done this for real and in a smaller vehicle I have.
 
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Fletchxj

New member
You REALLY need to read the materials I referenced. We can help, but ya gotta do your part. (And yes, that is a bit of a flame.)

-- And AGM and a FLA are the same. The differences have to do with gassing and vibration resistance. Period. If you want to go with a single battery, you need a deep cycle battery, which can be FLA or AGM. Both AGM and FLA have the same discharge "rules." A deep cycle battery is more resistant to deeper discharges.

-- A voltmeter will help you determine your vehicle's base charging voltage. If it is 14v or higher, you should be fine. 13.9v or lower, you will need to do something.

-- Trusting a low voltage cutout is a bad idea - size your system to meet your needs.

-- Running the engine for a short time does nothing useful, especially not for a constant load like a refrigerator. You really need to know how many amps your cooler will consume every 24 hours. NOTE: It will be a much bigger number than you expect.

N.B. Look at Jon's (luthj) links: He has done this for real and in a smaller vehicle I have.
Ouch, :D! I read through all that stuff, but clearly didn't get it all, haha. Guess that's why I'm not an electrician. It's interesting because I think that there's a lot of misinformation out there (and I've been inundated with the bad info before coming here) so that it's burned into my brain that an AGM is a deep cycle battery, which is clearly not the case. I also heard multiple times that using a standard flooded cell starting battery wouldn't work to power a fridge (not realizing that there were deep cell FLA batteries too), hence my questions here. The voltmeter will be helpful, I'll track one down and see if I can get a good reading on the alternator.

It seems that there's going to be a decent amount of cost/rewiring in order to safely and consistently run the fridge.
 
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