Dual battery setup for a fridge

Alloy

Well-known member
Agree. I didn't have the right size hole version copper connectors when I built it. It's on my list of things to do over. Worst thing that can happen is it comes off, shorts and blows the massive fuse I have where it connects to the truck battery, and doesn't charge. I don't think it will though.

Larger lugs are OK. In fact you could cut the end off and make spade terminals.......but you want to move the bend to the inside edge of the flange so contact from both the terminal and bolt/screw is close as possible to the wire. Bringing the wire in from the sides of the terminal rather than off the ends may help.....also make tightening the bolt/screw easier.
 

DaveInDenver

Middle Income Semi-Redneck
Agree. I didn't have the right size hole version copper connectors when I built it. It's on my list of things to do over. Worst thing that can happen is it comes off, shorts and blows the massive fuse I have where it connects to the truck battery, and doesn't charge. I don't think it will though.
These are handy for situations like this.

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wandererr

Adventurer
so going down the route of buy once cry once, I'm leaning towards the weise 100AH battery. The size and weight seem to be a bit in range of what I'm after... though the cheap SOB that I am, I would like it better at half the price.

I was thinking about a 75AH one, few pounds lighter, about about 25% cheaper but the way I see it: if I am running at 6A an hour (let's say it's a hot day in SoCal - which is not unheard of) for 10 hours, that's 60AH. at 12 hours, that's 72AH. I figure that not all of the night I would be running 6A (I try not to go venturing on extremely hot days - late spring through fall is cool area weather not desert weather)

So figure between 7PM and 10PM I'm running 6A/hr, between 10PM and 7A I'm running 4.5A/hr. At that point I'm looking at under 60AH for the night leaving me just a hair over 60% consumption.

If I went with a 75AH battery, the above scenario would drain it a tad low and potentially harm the battery. I could also potentially trigger the protection fault in the fridge resulting in the fridge turning off.

Now if I'm at camp and I have perfect view of the sun on the battery, based on my rough calculations I'm going to push about 8A towards the battery and if my usage goes up to 6A, I'm charging only at the rate of 2A.

Btw, the calculations above are assumptions based on WAG's of usage and sun performance.

So... am I making sense or did work suck my brains dry and I'm not longer making any sense ;)
 

jscherb

Expedition Leader
so going down the route of buy once cry once, I'm leaning towards the weise 100AH battery. The size and weight seem to be a bit in range of what I'm after... though the cheap SOB that I am, I would like it better at half the price.

I was thinking about a 75AH one, few pounds lighter, about about 25% cheaper but the way I see it: if I am running at 6A an hour (let's say it's a hot day in SoCal - which is not unheard of) for 10 hours, that's 60AH. at 12 hours, that's 72AH. I figure that not all of the night I would be running 6A (I try not to go venturing on extremely hot days - late spring through fall is cool area weather not desert weather)

So figure between 7PM and 10PM I'm running 6A/hr, between 10PM and 7A I'm running 4.5A/hr. At that point I'm looking at under 60AH for the night leaving me just a hair over 60% consumption.

If I went with a 75AH battery, the above scenario would drain it a tad low and potentially harm the battery. I could also potentially trigger the protection fault in the fridge resulting in the fridge turning off.

Now if I'm at camp and I have perfect view of the sun on the battery, based on my rough calculations I'm going to push about 8A towards the battery and if my usage goes up to 6A, I'm charging only at the rate of 2A.

Btw, the calculations above are assumptions based on WAG's of usage and sun performance.

So... am I making sense or did work suck my brains dry and I'm not longer making any sense ;)
I believe you are making sense. Your planning is towards the worst-case end of the scale in terms of amp-hours of consumption by the fridge; often you'll do a lot better than that but for the times when it is hot enough to approach the worst case I would be most comfortable with a battery around 100ah as you are thinking.
 

ducktapeguy

Adventurer
so going down the route of buy once cry once, I'm leaning towards the weise 100AH battery. The size and weight seem to be a bit in range of what I'm after... though the cheap SOB that I am, I would like it better at half the price.

I was thinking about a 75AH one, few pounds lighter, about about 25% cheaper but the way I see it: if I am running at 6A an hour (let's say it's a hot day in SoCal - which is not unheard of) for 10 hours, that's 60AH. at 12 hours, that's 72AH. I figure that not all of the night I would be running 6A (I try not to go venturing on extremely hot days - late spring through fall is cool area weather not desert weather)

So figure between 7PM and 10PM I'm running 6A/hr, between 10PM and 7A I'm running 4.5A/hr. At that point I'm looking at under 60AH for the night leaving me just a hair over 60% consumption.

If I went with a 75AH battery, the above scenario would drain it a tad low and potentially harm the battery. I could also potentially trigger the protection fault in the fridge resulting in the fridge turning off.

Now if I'm at camp and I have perfect view of the sun on the battery, based on my rough calculations I'm going to push about 8A towards the battery and if my usage goes up to 6A, I'm charging only at the rate of 2A.

Btw, the calculations above are assumptions based on WAG's of usage and sun performance.

So... am I making sense or did work suck my brains dry and I'm not longer making any sense ;)

These calculations would be at worse case scenario, like if you were to load the fridge with warm bottles of water and tried to freeze them on battery power. Then the fridge would be running on 100% duty cycle at max current until everything has come down to the set temp. In normal use, I've seen the duty cycle vary between 0%-65% depending on ambient temperatures and set temperature. During the cooler night time temperatures, with the contents already at the set temperature, the fridge will barely cycle and may only use 0.5-2 A/hour, sometimes less. At 100 degrees ambient temp with the fridge set on freezer, it may cycle 30-40 minutes every hour and average about 4-5 Amps/Hour. If you're driving during the day, then your alternator would be supplying the majority of the power during peak times so power consumption overall isn't that bad.

Even though a 100W solar panel should put out something in the 8A range, I've never seen mine go above 6A. That's in full sunlight during the brightest part of the day charging a deep cycle battery. It's probably due to a variety of factors, like the charge controller, battery type, and state of discharge, and wiring. Whatever the reason, it can keep up running the fridge at full power only in the few hours around mid-day, at any other time it's a net loss. My panels are fixed on my roof, which means unless I'm willing to move the car around to track the sun, I wouldn't count on more than 4-5 hours worth of good sunlight. That's good enough for my use, between the alternator and solar I can keep the fridge running for weeks. But on solar alone I think it'd only be good for 3-4 days.
 
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wandererr

Adventurer
These calculations would be at worse case scenario, like if you were to load the fridge with warm bottles of water and tried to freeze them on battery power. Then the fridge would be running on 100% duty cycle at max current until everything has come down to the set temp. In normal use, I've seen the duty cycle vary between 0%-65% depending on ambient temperatures and set temperature. During the cooler night time temperatures, with the contents already at the set temperature, the fridge will barely cycle and may only use 0.5-2 A/hour, sometimes less. At 100 degrees ambient temp with the fridge set on freezer, it may cycle 30-40 minutes every hour and average about 4-5 Amps/Hour. If you're driving during the day, then your alternator would be supplying the majority of the power during peak times so power consumption overall isn't that bad.

Even though a 100W solar panel should put out something in the 8A range, I've never seen mine go above 6A. That's in full sunlight during the brightest part of the day charging a deep cycle battery. It's probably due to a variety of factors, like the charge controller, battery type, and state of discharge, and wiring. Whatever the reason, it can keep up running the fridge at full power only in the few hours around mid-day, at any other time it's a net loss. My panels are fixed on my roof, which means unless I'm willing to move the car around to track the sun, I wouldn't count on more than 4-5 hours worth of good sunlight. That's good enough for my use, between the alternator and solar I can keep the fridge running for weeks. But on solar alone I think it'd only be good for 3-4 days.
You're right but if I plan I might as well plan for a worst case scenario.

Anyways looks like I might end up with a 115AH battery as a buddy of mine upgraded to lithium and is going to hook me up with his agm one.

Upside is then that when I get my ursa minor top I won't have to worry about charging phones running lights and so on.
 

skyfree

Active member
You're right but if I plan I might as well plan for a worst case scenario.

Anyways looks like I might end up with a 115AH battery as a buddy of mine upgraded to lithium and is going to hook me up with his agm one.

Upside is then that when I get my ursa minor top I won't have to worry about charging phones running lights and so on.

Free (I assume) is a no-brainer, but for future readers of this thread, 115ah AGM battery has about 60ah of capacity per the usual thinking. A lithium battery has a much better discharge profile and can go down to close to 0%. I think of my 50ah lithium as having 45ah of capacity. 60ah should be plenty and give you a lot of extra power to run other stuff.

Also, I found some real-life fridge power usage numbers here from people who measure such things: https://expeditionportal.com/forum/threads/12v-fridges-going-mainstream.218762/

Reports are from 10ah a day in cooler conditions, to 40ah in very hot conditions. My consumption of about 20ah a day seems to fall in the typical range.
 

wandererr

Adventurer
Free (I assume) is a no-brainer, but for future readers of this thread, 115ah AGM battery has about 60ah of capacity per the usual thinking. A lithium battery has a much better discharge profile and can go down to close to 0%. I think of my 50ah lithium as having 45ah of capacity. 60ah should be plenty and give you a lot of extra power to run other stuff.

Also, I found some real-life fridge power usage numbers here from people who measure such things: https://expeditionportal.com/forum/threads/12v-fridges-going-mainstream.218762/

Reports are from 10ah a day in cooler conditions, to 40ah in very hot conditions. My consumption of about 20ah a day seems to fall in the typical range.
Thanks! That's a good article. For now the free AGM will give me an ability to figure out my usage so that if possible I can go lighter/smaller. It will also give me potentially more days between charges if conditions / circumstances don't allow for them.

Btw my calculations were based on worst case scenario assumptions .
 

86scotty

Cynic
You're right but if I plan I might as well plan for a worst case scenario.

Anyways looks like I might end up with a 115AH battery as a buddy of mine upgraded to lithium and is going to hook me up with his agm one.

Upside is then that when I get my ursa minor top I won't have to worry about charging phones running lights and so on.

If budget is at all a concern just put lithium out of your mind. Overlanding forums etc. have made it out to be the only logical choice anymore. It is not. Remember that a huge segment of this hobby have Earthroamers or spend hundreds of thousands building and playing with this stuff that they actually sleep in 2 weeks a year.

Lithium is great, I suppose. Personally I can afford it but have no interest in it. Weight is not really a concern with my current rig. Like I said, if lithium doesn't fit your budget go AGM. If AGM doesn't fit your budget go lead acid.

You will find it hard to beat the value (literal definition bang for the buck) of two golf cart batteries (2 @ 6v run in series for one big 12v bank). They last forever with only the slightest maintenance and you can easily get 200 AH for $200.

Edit: You mentioned Ursa Minor so I guess you have a Jeep and weight and space are big concerns. I had an Ursa on a 2016 JKU and put a 100AH AGM behind the passenger seat in a homemade Goose Gear type box replacing the rear seat. Worked out great.

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billiebob

Well-known member
I'm at the point of doing this. I have a 115ah battery and only powering a 5.5ah fridge in a Square Drop Trailer. After reading all about isolators and diodes and cabling I'm leaning to a 240watt solar charger which seems to be the best buy and forgetting about charging off the Jeep. The cabling run from solar panel to the fridge is about 5'. Seems the simplest solution.

I might add a bit of LED lighting and Tablet.Cell chargers but the fridge will be the primary draw.

Has anyone done this? I think the battery is huge for he needs and 240W of solar more than enough even with just 8 hours of clear high sunshine. And my only cost will be the solar panels, controller.

Something like this. Altho maybe I only need half that.

 

wandererr

Adventurer
So here's the scoop. Just spent 4 days out with the fridge and the 115AH battery and a 120 solar panel. It for th e most part worked though on the third night the fridge 'slowed'down as the battery was getting low. First night temp was in the high 60s, the other two in low 30s, days were in 90s on the drive in, the next two in high 70s low 80s.

Problem was that day 2 and 3 we did little driving so it was all solar charging and the battery was disconnected from the vehicle battery.

Probably part of the issue was that we weren't by the rig all the time and the sun kept moving.... might end up picking up a m.j other 120W panel so that I can place the two strategically and capitalize on moving sun.
 

PSea

Active member
Recently saw a new cooler/refrig that comes w/ a lifopo battery (easily replaceable), but more importantly, it lasts 5 days on the battery. sounded like a great cooler. wasn't at a premium price. saw it on youtube. might want to do a search and check it out. had anderson plugs for easy solar panel attachments too.
 

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