AGM Batteries for an RV - Which/Where/How/Why? (Discussion)

[dave_in_delaware]

OK, I'd like to run a few ideas and questions past everyone here, to get opinions, tips, suggestions, do's, don'ts, etc.

So far in my Homebuilt RV Project, I'm figuring I'll need a few 12v AGM batteries for the "house" batteries, which will power all the lights, fridge, pumps, fans, etc. (and I'll be using solar panels and the engine's alternator for charging). I'm thinking I'll be purchasing something locally from a battery company just down the street (convenient), like:

Deka/"Astrolite" Group 31, 105 ah AGM (69 lb)
Deka/"Astrolite" Group 4D, 198 ah AGM (129 lb)
Deka/"Astrolite" Group 8D, 245 ah AGM (135 lb)

Obviously, the higher the ah the better, but with that also comes a lot more weight (and cost). Are there other brands or sizes I should be looking into? What is better - more smaller batteries or fewer larger batteries? Space becomes a key factor I suppose.

From what I've read on many RV sites and forums, the general concensus about AGM bateries is that they are maintenance-free, spill-proof, and do not vent any gasses (unless charged incorrectly). Assuming this is true, could they be placed under a couch or in a storage cabinet and not vented to the outside? Would this be safe if someone sleeps on the couch that the batteries are under? Or is what I read incorrect and they NEED to be vented because they do vent a little gas normally??

I've also read plenty about the temperature affecting the performance of batteries. I've been assuming that I'd have the batteries inside under the couch so they're warmer, but depending on the outcome of the above issues (venting), perhaps they'll need to be placed outside.

If they're outside the van (RV), I know many people who have mounted their batteries inside a battery box (usually behind the rear axle) or battery "hanger" frame made of steel angle. Since I live in Delaware (which gets all four seasons, including 105

[Sunking]

Dave I can answer those questions. As for capacity it is pure physics you have to accept energy density of 40 wh/Kg or 18 wh/lb. This is why you hear so much about lithium batteries with densities up to 250 wh/Kg or 113 wh/lb. Weight and capacity are tied to each other.

As for storage, under a coach is fine. If you ever smell anything like rotten eggs, ventilate the place. Heat destroys batteries, cold lowers capacity but preserves batteries.

[dave_in_delaware]

Dave I can answer those questions. As for capacity it is pure physics you have to accept energy density of 40 wh/Kg or 18 wh/lb. This is why you hear so much about lithium batteries with densities up to 250 wh/Kg or 113 wh/lb. Weight and capacity are tied to each other.

Yeah, I figured they were tied to each other and it was just the nature of the beast. I've read some posts on RV forums about lithium batteries, but they don't seem to be used all that much. My guess is the price might be a factor? I have no idea though. I haven't done any research on them.

As for storage, under a coach is fine. If you ever smell anything like rotten eggs, ventilate the place. Heat destroys batteries, cold lowers capacity but preserves batteries.

You say "under a coach" but you meant couch, right? So that's to say that the AGM's really don't vent any gasses and they're perfectly safe to have inside the van?

The larger AGM batteries that I'm considering are not only heavy, but they're kind of tall. I'm debating in my mind which scenario would be better: put them inside under the couch/bed, or put them outside under the van. I suppose either location has its pro's and con's. But if they're under then I'd have to poke more holes into the van to get the wiring inside yet I'd save that much more space for storage. But if they're inside they'd have a little more capacity in the winter yet I'd lose that space to giant batteries. Hmmm.... Is it a 50/50 guess, toss a coin?


Are there any other brands of AGM that I should be looking at?

Thanks for your input, Sunking! I appreciate it.

[Sunking]

Yeah, I figured they were tied to each other and it was just the nature of the beast. I've read some posts on RV forums about lithium batteries, but they don't seem to be used all that much. My guess is the price might be a factor? I have no idea though. I haven't done any research on them.

You are correct it is the price, and to some degree a lack of equipment to support them

You say "under a coach" but you meant couch, right? So that's to say that the AGM's really don't vent any gasses and they're perfectly safe to have inside the van?

Yes I meant couch, my bad spell checker. AGM's take some pretty serious over charging to make them vent. It would almost be impossible for the limited solar panel wattage you will be running to over charge them. There would have to be a catastrophic failure with the charge controller, and take a few days without using the batteries.

Alternator is another story and can over charge them if the voltage regulator fails

The larger AGM batteries that I'm considering are not only heavy, but they're kind of tall. I'm debating in my mind which scenario would be better: put them inside under the couch/bed, or put them outside under the van. I suppose either location has its pro's and con's. But if they're under then I'd have to poke more holes into the van to get the wiring inside yet I'd save that much more space for storage. But if they're inside they'd have a little more capacity in the winter yet I'd lose that space to giant batteries. Hmmm.... Is it a 50/50 guess, toss a coin?


Are there any other brands of AGM that I should be looking at?

For AGM batteries Concord makes the best and Trojan makes some pretty good ones. Stick with Concord, Deka, and Trojan for AGM.

Some Advice:

1. Try to avoid parallel strings if possible. If you have to use parallel strings limit it to 2 strings or 2 12 volt batteries.

2. If you are going to use a generator or the vehicle alternator, you can go to a minimum of 2 day reserve capacity, but dot not go below that because that would require to discharge to 50%, and when you do that you are really cutting into the life cycle. At 50% you are looking at replacing the batteries every 2 years.

3. I noticed you are shooting for a huge amount of daily watt hour usage. You need to conserve more. Of course you can use the vehicle alternator to charge the batteries, but keep in mind it is inefficient. No problem if driving every day, but if camped out for long periods you might want to consider a small generator to conserve gasoline. Idling the engine just for electric power is a lot of over kill and uses a lot more gas then necessary. It is one thing to generate a couple hundred watt hours as that can be done in a few minutes on the alternator. But I heard you talking 2 or 3 thousand watt hours. That is 2 or 3 hours engine run time. A small generator requires the same time, but far less gas.

[dave_in_delaware]

... AGM's take some pretty serious over charging to make them vent. It would almost be impossible for the limited solar panel wattage you will be running to over charge them. There would have to be a catastrophic failure with the charge controller, and take a few days without using the batteries.

Alternator is another story and can over charge them if the voltage regulator fails

It's good to know that the AGMs can be kept inside an RV.

How frequent does a voltage regulator on an alternator fail? Is there a "backup" component or addition to the system that I could install to make sure the alternator doesn't overcharge?

For AGM batteries Concord makes the best and Trojan makes some pretty good ones. Stick with Concord, Deka, and Trojan for AGM.

Will do. Thanks for the advice!

1. Try to avoid parallel strings if possible. If you have to use parallel strings limit it to 2 strings or 2 12 volt batteries.

Just so I have these terms correct: Parallel strings is when you basically wire the + to another + and then that goes to a distribution panel? Compared to series where the first + goes to the next - and then that battery's + goes to the distribution? Is that what you're talking about? Like when you use two 6v batteries and wire them in series to get 12v?? I figured every 12v RV system wires in parallel?

Or are you talking about different wiring methods like here?

2. If you are going to use a generator or the vehicle alternator, you can go to a minimum of 2 day reserve capacity, but dot not go below that because that would require to discharge to 50%, and when you do that you are really cutting into the life cycle. At 50% you are looking at replacing the batteries every 2 years.

OK, understood. I'll limit my "max days boondocking" to 2 before I need to drive around to get the alternator charging the batteries again.

3. I noticed you are shooting for a huge amount of daily watt hour usage. You need to conserve more. Of course you can use the vehicle alternator to charge the batteries, but keep in mind it is inefficient. No problem if driving every day, but if camped out for long periods you might want to consider a small generator to conserve gasoline. Idling the engine just for electric power is a lot of over kill and uses a lot more gas then necessary. It is one thing to generate a couple hundred watt hours as that can be done in a few minutes on the alternator. But I heard you talking 2 or 3 thousand watt hours. That is 2 or 3 hours engine run time. A small generator requires the same time, but far less gas.

OK, point taken about the alternator and generator needs. Thanks!

I'm still working on reducing my power needs. I'm currently in contact with a company that has a water circ pump that can double for both the sink demand water and the hydronic heat. The trick with that is it only operates on 2-10v DC, so I'm not sure how that would work. I'm also still looking into a different heater or system for wintertime. I definitely want to use as little battery power as possible.

Also, when the info for an alternator says "155 amp" does that mean it'll generate 155 amps of recharge in one hour? And at what RPM would that be? I would assume normal driving RPMs (not idle, not revving).

[Sunking]

How frequent does a voltage regulator on an alternator fail? Is there a "backup" component or addition to the system that I could install to make sure the alternator doesn't overcharge?

No not really just a dash mount volt meter and keep an eye on it.

Just so I have these terms correct: Parallel strings is when you basically wire the + to another + and then that goes to a distribution panel? Compared to series where the first + goes to the next - and then that battery's + goes to the distribution? Is that what you're talking about? Like when you use two 6v batteries and wire them in series to get 12v?? I figured every 12v RV system wires in parallel?

Correct

OK, understood. I'll limit my "max days boondocking" to 2 before I need to drive around to get the alternator charging the batteries again.

OK, point taken about the alternator and generator needs. Thanks!

No you do not understand. If you go with 2 day reserve you have to run either the generator or vehicle every day to fully recharge the batteries. You never want to go below 50%. To have 2 days boondocking, you need 4 day reserve.

Also, when the info for an alternator says "155 amp" does that mean it'll generate 155 amps of recharge in one hour? And at what RPM would that be? I would assume normal driving RPMs (not idle, not revving).

It means the alternator can deliver up to 155 amps period. There is no time element associated with it. If the alternator delivers 155 amps for 1 hour then it is 155 AH. Bu there is the physics again. If you have a 100 AH battery and deliver it 155 amps, it will gas and possible explode in a short time. For AGM batteries you have to limit the current to C/4 where C = Amp Hour capacity. More on that later.

[dave_in_delaware]

No you do not understand. If you go with 2 day reserve you have to run either the generator or vehicle every day to fully recharge the batteries. You never want to go below 50%. To have 2 days boondocking, you need 4 day reserve.

Oh, my mistake. I think I know what you mean now. I got "2 day reserve" confused with "how long without using generator or alternator" which are totally NOT the same thing. You're talking about discharging the batteries only by 20% each day so they last longer, which is why in my original calculations you had me multiply the total Wh by 5? Maybe I'm not understanding the definition of "reserve capacity."

It means the alternator can deliver up to 155 amps period. There is no time element associated with it. If the alternator delivers 155 amps for 1 hour then it is 155 AH. Bu there is the physics again. If you have a 100 AH battery and deliver it 155 amps, it will gas and possible explode in a short time. For AGM batteries you have to limit the current to C/4 where C = Amp Hour capacity. More on that later.

Hmmm. No time element with an alternator "rating"? Oh, so it depends on RPM and other conditions, but it'll deliver 155 amps eventually in a certain amount of time to eventually give 155 ah. Does the alternator go through the charge controller as well? Or maybe I'll just wait for the "more on that later" part.

Thanks for your patience with me, Sunking. I'm trying to grasp all of this and truly understand it.

[Sunking]

Oh, my mistake. I think I know what you mean now. I got "2 day reserve" confused with "how long without using generator or alternator" which are totally NOT the same thing. You're talking about discharging the batteries only by 20% each day so they last longer, which is why in my original calculations you had me multiply the total Wh by 5? Maybe I'm not understanding the definition of "reserve capacity."

Don't think we are on the same.

Lets start by saying you need 100 watt hours per day for simplicity. Now let'd say you want to discharge 50%per day. That would take a 200 watt hour capacity battery. 33% per day would be a 300 watt hours battery. 25% 400 watt hours, and 20% 500 watt hours.

Hmmm. No time element with an alternator "rating"?

No Dave and this is what you need to understand. Promise it is not hard. Amps is just a measurement of current at a specific moment in time. Watts is just a measurement of power at a specific moment in time. There is no time element to amps and watts. They are just mesurements.

Now here is where it takes a American with a Phd in mathematics to understand. Any other culture a 6th grader can master in 2 minutes.

Amp Hours = Amps x Hours
Watt Hours = Watts x Hours

So lets say your alternator supplies a battery with 150 amps of 2 hours. How many Amp Hours does the alternator deliver?

Answer 150 amps x 2 hours = 300 amp hours. Now we have a time element.

Another example lets say we have a 200 Amp Hour Battery, and we connect a 20 amp load. How long should the battery last?

H = Amp Hours / Amps = 200 AH / 20 amps = 10 Hours

Hope that helps, it is just simple algebra.

[dave_in_delaware]

Lets start by saying you need 100 watt hours per day for simplicity. Now let'd say you want to discharge 50%per day. That would take a 200 watt hour capacity battery. 33% per day would be a 300 watt hours battery. 25% 400 watt hours, and 20% 500 watt hours.

Isn't the max discharge 50%? Why would I want to go below that to 25% or 20%? Or is that a "safety net" in case the discharge goes below the preferred 50% number for some reason? So, using these formulas and needing 100 WH per day, that would mean that if I only wanted to discharge down to 75%, I'd need a 133.33 WH capacity battery, right? I think (hope) I'm beginning to "get" this.

To get more specific to my calculated numbers, say I need more like 2000 Daily Watt Hours. To only discharge the batteries down to 50% (max), I'd need a battery with a capacity of 4000 WH, which is 334 AH. Correct?

No Dave and this is what you need to understand. Promise it is not hard. Amps is just a measurement of current at a specific moment in time. Watts is just a measurement of power at a specific moment in time. There is no time element to amps and watts. They are just mesurements.

Now here is where it takes a American with a Phd in mathematics to understand. Any other culture a 6th grader can master in 2 minutes.

Amp Hours = Amps x Hours
Watt Hours = Watts x Hours

So lets say your alternator supplies a battery with 150 amps of 2 hours. How many Amp Hours does the alternator deliver?

Answer 150 amps x 2 hours = 300 amp hours. Now we have a time element.

Another example lets say we have a 200 Amp Hour Battery, and we connect a 20 amp load. How long should the battery last?

H = Amp Hours / Amps = 200 AH / 20 amps = 10 Hours

Hope that helps, it is just simple algebra.

Well now I feel dumb. I understood all of that. I guess I was having a blonde moment earlier. LOL. I think I was bringing up the time issue because I wanted to know how long I'd have to drive/idle the van to recharge the batteries.

I'm guessing there would be different charging rates based on the efficiency of the alternator, efficiency of the wiring, charge relay, RPM of the engine (higher while driving, lower at idle), and probably other factors? So even though it's a 150 amp alternator, the batteries wouldn't be seeing all of it?

[Sunking]

Isn't the max discharge 50%? Why would I want to go below that to 25% or 20%?

Dave you got it backwards. When I say discharge 20% per day means depth of discharge leaving you 80% capacity left in the battery.

Well now I feel dumb. I understood all of that. I guess I was having a blonde moment earlier. LOL.

Don't feel bad, you have lot's of company. Just yesterday a chap from northern Wales England got all upset with me because he has no understanding of watt hours and cannot understand why he cannot use 180 watt hours per day, when all he can generate is 60 watt hours per day. I told him he only had 2 Sun Hours in th eUK winter and he replied back: " I do not live on the North Pole, I live in the UK, and thanks for nothing"

[dave_in_delaware]

Dave you got it backwards. When I say discharge 20% per day means depth of discharge leaving you 80% capacity left in the battery.

Oh, OK! So my calculations/numbers were correct but I was thinking about it all backwards. So basically, the less you want to discharge the batteries the more capacity you need. It seems a little backwards to me (conceptually) at first but it makes sense mathematically.

So for my 2000 WH daily need, if I want to go down to 50% discharge I'd need 4000 WH capacity (= 334 AH), so I'd need two 198 AH batteries, but if I wanted to only go down to 25% discharge I'd actually need 8000 WH (= 667 AH), which would probably mean more like four 198 AH batteries? Is that right?

Don't feel bad, you have lot's of company. Just yesterday a chap from northern Wales England got all upset with me because he has no understanding of watt hours and cannot understand why he cannot use 180 watt hours per day, when all he can generate is 60 watt hours per day. I told him he only had 2 Sun Hours in th eUK winter and he replied back: " I do not live on the North Pole, I live in the UK, and thanks for nothing"

Well, I promise not to get mad at you and huff away thinking you're insane. I'm hoping YOU don't feel that way about me, since some of this stuff doesn't make any sense to me at first. I am trying to learn it as quickly and painlessly (for you) as I can. I do appreciate your patience.

[Sunking]

Oh, OK! So my calculations/numbers were correct but I was thinking about it all backwards. So basically, the less you want to discharge the batteries the more capacity you need. It seems a little backwards to me (conceptually) at first but it makes sense mathematically.

So for my 2000 WH daily need, if I want to go down to 50% discharge I'd need 4000 WH capacity (= 334 AH), so I'd need two 198 AH batteries, but if I wanted to only go down to 25% discharge I'd actually need 8000 WH (= 667 AH), which would probably mean more like four 198 AH batteries? Is that right?

OK you got it now.

Well, I promise not to get mad at you and huff away thinking you're insane. I'm hoping YOU don't feel that way about me, since some of this stuff doesn't make any sense to me at first. I am trying to learn it as quickly and painlessly (for you) as I can. I do appreciate your patience.

Dave actually I am happy to help, as long as you are willing to listen and learn. It is the folks who refuse to listen and learn I do not have patience with. Energy is pretty simple, and works like a checking account. Some people have a hard time believing if you only make $50/day but spend $100/day it will not work. With energy there is no credit or loans. Once you used it, it is gone and there ain't squat you can do about it. You know if you are driving your car and it runs out of gas, wishing or hoping is pointless, your done.

[dave_in_delaware]

OK you got it now.

Yippee!!! I'm so glad. Now all I need to do is figure out how to conserve more power so I'm not using so many WH. My biggest power-eaters are the refrigerator and the heating system. Perhaps I should re-re-think my decision on using a hydronic heating system and go with a more "traditional" forced air furnace? And/or get rid of the refrigerator idea totally and just carry a super-efficient cooler and buy ice when needed? Hmmm.... I know I need to change something, because buying two 198AH batteries is going to be tough enough on the RV budget, I don't want to even think about needing four of them!

Dave actually I am happy to help, as long as you are willing to listen and learn. It is the folks who refuse to listen and learn I do not have patience with. Energy is pretty simple, and works like a checking account.

I've been putting everything I've learned here into an Excel spreadsheet and using it to calculate my usage and system requirements, and to make design decisions.

[Sunking]

Yippee!!! I'm so glad. Now all I need to do is figure out how to conserve more power

Yeah that is what I think we said on page 1 post 1

[dave_in_delaware]

Yeah that is what I think we said on page 1 post 1

Yeah, for sure. I'm working on it!!! LOL.