All I want to do is power a DC fridge for a couple of days with my batteries

[nimblemonkey]

Hi all- Newbie here trying to understand what I need to accomplish running my Engel MT45 (max. 2.5 amp draw) with 2 -6V AGM 200 aH GC batteries.
Before I ordered the fridge I spoke to a rep from Engel (just before Christmas) and was told I would need a 60 Watt solar panel to run the fridge. Someone else who mfg. solar panels (and was well versed in the Engel line) told me 90 Watt would be better. Then I posted a couple of questions on another forum about using a 100 Watt flexible mono crystal panel to recharge my batteries and they told me I would need a minimum of 2- 100 watt panels. My simple solution for replacing always running to get ice for the cooler as I travel around the US has turned into confusion.

Here's what I want to set-up: I have a Frontier King Cab (extended cab) which I have removed the jump seats and plastic molding behind the front seats in the cab to give me a little more room to install a platform on which the fridge will sit. I want to run wire from that location back into the truck bed where the two batteries will be wired in series to give me 12V - the fridge is DC, using a swing compressor and is supposed to be extremely efficient, and quiet. I have been looking at getting an MPPT 3 stage controller/charger to maximize the energy I get from the Panel(s) (originally I intended to just get 1-100 Watt panel based on what Engel told me)- Since the batteries will be installed under a platform I built in the truck bed (under a hi-rise camper shell), I am trying to keep the weight down and maximize storage under the platform as I will be sleeping on top of the platform occasionally. the controller will be mounted above the platform right over the batteries so I can monitor what is happening with the panel and the batteries.

I decided on the 200 aH batteries because if 50% DOD is the max you should safely discharge your battery, then I figured 100 aH should get me through 2 days of rain or no sun if I am boondocking somewhere. I hope you can help me figure this out- I know this should be simple but I feel pressed for time on this as I am leaving in about 1 month and wanted to have everything put together with a dry run under my belt before I left. Also, the solar panels will not be permanently installed on my truck cap. I want the option of moving them around my campsite to get the most available sun or putting them on top of my camper shell if the sun is directly overhead. also, they will be stored flat on top of the platform while I'm driving- I can plug the fridge into an accessory outlet while I'm driving and run it off the alternator.
Here is link to the panel and the fridge specs:

http://www.grapesolar.com/photo-flex-100w.html

https://www.engel-usa.com/products/fridge-freezers/engel-mt45-ac-dc-fridge-freezer

One more thing- I am trying to purchase products that are mfg. in the USA- I know the panels are a lost cause, but there are a couple of controllers made in the US and the batteries are Trojans- I would've liked sunXtender but couldn't find a local provider for those and shipping, even for AGMs was outrageous. What do you suggest? Am I on the right track with this set-up? So far I only have ordered the fridge- it came in this week but I haven't picked it up yet- will do that early next week.

[Sunking]

You do not need any solar period to do what you want. All you need is a Battery Isolator to have your vehicle alternator to charge the batteries. Your vehicle alternator can generate more energy in 15 minutes than a 100 watt panel can generate in 2 days.

Why would you want to waste good money on solar for that job? A good battery isolator cost $50 to $75 installed under the hood out of site.

[nimblemonkey]

I may not be driving my vehicle for several days and want a way to charge up the batteries. Are you telling me that I can turn my car on for 15 minutes (not drive it anywhere) and fully charge my 2-6V 200 aH batteries with the starter battery in my truck? Really?? Should I talk to an alternator service guy locally about this?

[Sunking]

I may not be driving my vehicle for several days and want a way to charge up the batteries. Are you telling me that I can turn my car on for 15 minutes (not drive it anywhere) and fully charge my 2-6V 200 aH batteries with the starter battery in my truck? Really?? Should I talk to an alternator service guy locally about this?

Simple math my friend.

A 100 watt solar panel made for batteries generates roughly 5 amps, and on a very good sunny summer day mounted on to of a vehicle in the broiling sun will be exposed to roughly 3.5 Sun hours which means 5 amps x 3.5 Hours = 16.5 Amp Hours. In winter 1/2 to 1/3 that much or less than 8 amp hours.

A compact car alternator is 60 amps, vans and trucks around 90 to 100 amps, but lets be real unrealistic and say you are living in a cracker box small car with a 50 amp alternator. Start the engine, let it idle and it will generate 40 amps. At 40 amps it takes 30 minutes to generate 20 Amp hours or about 2 days worth of a 100 watt panel in hot summer sun.

A 200 AH battery battery discharged to 50% DOD (100 AH) would take an alternator roughly 1 to 2 hours to fully recharge. A 100 watt panel in summer in broiling hot sun will take 6 days.

Which one will you buy?? Driving around with air conditioner on for a couple of hours, or broil in the sun for a week?

Reality kind of suks huh?

[nimblemonkey]

I was going to buy the batteries from an alternator/battery guy locally anyway and if an isolator is all I need to keep them charged, you may have saved me from some expense and hassle. I'm not happy about being parked somewhere and then have to turn the truck on for two hours to charge up the batteries- I may need that gas for the next leg of my trip; Frontiers only have a 21 gallon tank and I was going to go to some remote areas, but I like that idea better than buying a controller, and panels, and also having to store them to keep my fridge going.

The only drawback (besides the gas use) is that I was going to set this fridge up at home, too (with the panels and battery bank) and use it during the summer when I'm not likely to do any long distance traveling and don't want to add to my already increasing electric bill. I'll get a quote from my local guy tomorrow. Thanks Sunking

[inetdog]

The only drawback (besides the gas use) is that I was going to set this fridge up at home, too (with the panels and battery bank) and use it during the summer when I'm not likely to do any long distance traveling and don't want to add to my already increasing electric bill.

Actually, not a drawback at all, since the price of electricity to run the fridge at home will be a lot higher from solar and battery than it would be from the power company.
So you are in luck!
As for the gas usage for your engine, that will probably still be cheaper (although nosier) than solar.
The key is not to run the engine to charge the batteries all the way up to 100% each time. Charging to 80-90% is much faster, since the current goes down as the batteries reach full charge.
Then either drive or run the engine longer say once a week.
Important to battery health is to get a hydrometer so you can check exactly how well charged the batteries are. Trying to do that just from looking at the voltage is difficult (or impossible.)

[Sunking]

I was going to buy the batteries from an alternator/battery guy locally

well that may be a bad idea if he is going to sell you a car batteries as they are not any good for what you want to do.

I'm not happy about being parked somewhere and then have to turn the truck on for two hours to charge up the batteries- I may need that gas for the next leg of my trip; Frontiers only have a 21 gallon tank

Not really a valid argument. Your fridge is going to use more power than the panels can generate in a day, so your argument is a moot point. Even if you were to install panels you still need a battery isolator to provide the bulk of the charging.

Second point is unless your fridge uses more than 600 watt hours per day, it will not need to be recharged every day.

The only drawback (besides the gas use) is that I was going to set this fridge up at home, too (with the panels and battery bank) and use it during the summer when I'm not likely to do any long distance traveling and don't want to add to my already increasing electric bill.

No Sir not a drawback at all. Any thing you take off grid is going to cost you at least 5 to 10 times more than buying it from the power company. This is why we say DO NOT take anything off-gid unless there is no other choice. Solar would be the very last choice. It is expensive.

Look at it this way. You pull up to an intersection needing some gasoline for your truck. At the corner you see two gas stations. One sells gas for $3/gallon made from petroleum. The other sells gas made from corn and is $30/gallon. Which one do you buy and why?

It is really that simple.

[nimblemonkey]

inetdog- First, the batteries will be AGM- so no checking them since they are sealed- if they are under a platform that I will be sleeping on I can't really have it any other way; (without buying or building a whole new platform to keep the bats in a plastic somehow vented-to-the-outside box- did I mention I'm leaving in a month?)

and the battery and alternator guy is selling me those AGMs- so I think they are the right batteries.

Also, I thought that battery life was severely shortened if they are not charged up full each charge cycle-(reason for originally considering a 3 stage charger/controller) If I just top them up to 80% I wont get nearly the life out of them as I potentially could- If I'm forking over $$$ for these luxuries (the fridge and the batteries and installation of an isolator), I want them to last as long as possible- Is this a myth or reality?

Just talked to the alternator guy- My Frontier has a 110 Amp alternator and he suggested a 120 Amp isolator- $75.00 (not installed.) He doesn't do installations but suggested someone who is familiar with them and could. to be continued...

[nimblemonkey]

Simple math my friend.

A 100 watt solar panel made for batteries generates roughly 5 amps, and on a very good sunny summer day mounted on to of a vehicle in the broiling sun will be exposed to roughly 3.5 Sun hours which means 5 amps x 3.5 Hours = 16.5 Amp Hours. In winter 1/2 to 1/3 that much or less than 8 amp hours.

A compact car alternator is 60 amps, vans and trucks around 90 to 100 amps, but lets be real unrealistic and say you are living in a cracker box small car with a 50 amp alternator. Start the engine, let it idle and it will generate 40 amps. At 40 amps it takes 30 minutes to generate 20 Amp hours or about 2 days worth of a 100 watt panel in hot summer sun.

A 200 AH battery battery discharged to 50% DOD (100 AH) would take an alternator roughly 1 to 2 hours to fully recharge. A 100 watt panel in summer in broiling hot sun will take 6 days.

Which one will you buy?? Driving around with air conditioner on for a couple of hours, or broil in the sun for a week?

Reality kind of suks huh?

So I just talked to a guy who could put the isolator in and he mentioned something to be cautious of (he said he didn't know the specs on AGM batteries)- that is, if I have sealed batteries and I start the truck, I'll have 70 amps of power going straight to my battery bank (2-6V 200 aH AGM Trojans in series). If I boil the gel and create a gas pocket, I will damage the cells near that gas pocket and may ruin the battery, or at least ruin the ability to charge those cells, which will weaken the battery and shorten the life. My question is, how do I prevent that from happening? Can I put a charge controller from the alternator to the battery bank to safely charge up my batteries using the alternator re-charge method? Has anyone done that with AGMs?

[Sunking]

Need the Trojan model number to answer. Auxiliary batteries need to be matched up to vehicles.

One thing that bothers me with your statement is AGM is not a Gel battery. True Gel batteries need to have charge rates of C/20 or less which makes them useless for solar or mobile applications. But a AGM is not a Gel battery so you can understand the conflict I am having with your statement.

AGM batteries have very low internal resistances and can take very large charge currents depending on manufacture and model number. So select your battery carefully to match the application. My advice to you is forget Trojan 6V AGM if that is what you are looking at. Find you a dealer who sells Odyssey or Concorde. Those AGM's can take up to 1C charge rates. If that is not possible then you need to shop for an isolator that can regulate charge current to the Aux battery.

If all else fails you can simply use enough wire of appropriate length between isolator and battery to limit charge current to whatever value your heart desires.

Shop around and see if you can find a Concord Lifeline RV Deep Cycle 12 volt 210 AH model GPL-4DA. You can go as high as 255 AH in a 12 volt AGM, or 400 AH with 6 volt batteries. Lifeline RV Deep Cycle will accept up to a 2.5C charge rate. So for a 210 AH model means it can be charged at 2.5 x 210 = 525 amps.

[nimblemonkey]

Need the Trojan model number to answer. Auxiliary batteries need to be matched up to vehicles.

One thing that bothers me with your statement is AGM is not a Gel battery. True Gel batteries need to have charge rates of C/20 or less which makes them useless for solar or mobile applications. But a AGM is not a Gel battery so you can understand the conflict I am having with your statement.

AGM batteries have very low internal resistances and can take very large charge currents depending on manufacture and model number. So select your battery carefully to match the application. My advice to you is forget Trojan 6V AGM if that is what you are looking at. Find you a dealer who sells Odyssey or Concorde. Those AGM's can take up to 1C charge rates. If that is not possible then you need to shop for an isolator that can regulate charge current to the Aux battery.

If all else fails you can simply use enough wire of appropriate length between isolator and battery to limit charge current to whatever value your heart desires.

Shop around and see if you can find a Concord Lifeline RV Deep Cycle 12 volt 210 AH model GPL-4DA. You can go as high as 255 AH in a 12 volt AGM, or 400 AH with 6 volt batteries. Lifeline RV Deep Cycle will accept up to a 2.5C charge rate. So for a 210 AH model means it can be charged at 2.5 x 210 = 525 amps.

T6V-AGM or Trojan 6Volt 200 amp hour deep cycle AGM (absorbed glass mat) I tried to get the Concorde SunXtender or Lifeline batteries but they aren't locally available and the shipping was totally absurd, so Trojans is what I'm getting- Are you telling me that Trojan AGM batteries can't handle a charge from the alternator as well as other AGM batteries or that what I need is a different type of isolator? with a 110 Amp Alternator in my truck, which Isolator would you recommend? I just ordered the batteries so it is too late to change that now. I am only raising the issues that the people I'm speaking with are raising- I am new to solar and never gave batteries much thought before now so I thank you for your input. And wire length or size will not be enough of a fail safe to help me regulate charging. And I'll put the necessary breakers/fuses in to prevent a melt down.
Also, I spoke with the mfg. of the fridge (Engel in Florida) again today and they still are telling me that I don't need more than a 40 Watt solar panel if using the unit as a fridge or 60 Watt if using the unit as a freezer- it is about the size of a coleman cooler (48 quart- the ubiquitous tailgate size)
And If I wanted to charge my two 6Volt batteries wired in series from AC (shore power) can I use a wall wart battery charger plugged into shore power to do this? I have one to charge my 12 volt tractor battery (it was designed to recharge a motorcycle battery). If I AM camped somewhere there is power and I am running the fridge off the camp power, not my batteries, it would make sense to take the opportunity to charge up my batteries then too. If that charger won't work do you have a suggestion for a more appropriate one?

[Sunking]

Are you telling me that Trojan AGM batteries can't handle a charge from the alternator as well as other AGM batteries or that what I need is a different type of isolator?

No I am not telling you that. What I said is an AGM battery is not a Gel battery. They are two different batteries from the Valve Regulated Lead Acid (VRLA). You are fine with the TV6-AGM.

That is great the cooler company says their cooler can operate with a 40 to 60 watt panel. All that means is you have way more battery than needed. Problem is a 40 to 60 watt panel cannot support a 200 AH 12 volt battery especially mounted on a vehicle without optimum tilt angle and true solar south orientation.

Think of your vehicle alternator when running as a 1500 watt solar panel at mid day noon with full bright sun 24 hours a day.

As for the isolator there are lots of options. You want to stay away from mechanical isolators, you want electronic. Best ones out there are made by Sure Power owned by Cooper Bussman. You want to select one made for you truck/alternator combo. Here is a good on-line source to buy them. You are looking for Group 1. Sounds like a 1202 Isolator is what you are after.

Note you control current with the gauge and length of conductor whether you want to or not control the current it will happen. It is physics my friend and you cannot cheat physics.

[nimblemonkey]

Note you control current with the gauge and length of conductor whether you want to or not control the current it will happen. It is physics my friend and you cannot cheat physics.[/QUOTE]

Thanks for the links to the isolators- I'll look into those.
I can put as much wire between the batteries and the isolator as I want- I'll just have to roll up and stash anything over the 10 feet or so I need to go from the bank to the isolator. Can I charge the batteries from shore power with a wall wart type charger?

And FWIW, I think I mentioned that I never intended to permanently mount the solar panel to the top of the truck cap- it is light enough to put on a pvc frame so I can change the angle and aim it toward the sun throughout the day from the ground. I think I gave too much info in my original post- I'll try to ask only one or two questions at time in the future.

[Sunking]

Just use 6 AWG wire and call it done. Don't worry about the length.

You can use any wall charger you want as long as it does not exceed the max charge current of the battery. With that said a Wall Wart charger is not going to be enough current to even make much of a Trickle Charger for a 200 AH battery. So no worries.