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

[dave_in_delaware]

Who are you guys talking too? Dave In Delaware has not logged in since last October.

I may not log in often, but I still love this site and read it.

[inetdog]

Who are you guys talking too? Dave In Delaware has not logged in since last October.

I'm mostly talking to PnJunction, I guess....
Did not notice the dates on the earlier posts.

Also talking to posterity in the form of later visitors.

[Sunking]

Who are you guys talking too? Dave In Delaware has not logged in since last October.

[inetdog]

YPerhaps 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?

Cooling with solar energy can work reasonably well (and keep temperatures more stable than ice) if properly designed and well insulated.
And for Air Conditioning, the use of solar PV to drive an efficient A/C like a mini-split can make sense.

Heating with solar energy, on the other hand, works out best when you use solar thermal to extract heat directly from the sunlight (far and away the most economical, but possibly difficult for an RV) or use the solar PV to run a heat pump rather than any kind of resistance heater. The method used to deliver the heat to where you need it is not as important (except in cost!) as the way you get the heat in the first place.

For grid-tied electric heat pumps can do better in most areas than propane. But for off-grid, your electric power cost is so much higher that propane almost always wins hand-down. Especially when you can always refill your propane tank (not a winter option for some off-gridders.)

[PNjunction]

You might find this series of tests for a floating RV to be useful:

http://www.morganscloud.com/category/electrical/

The main concern was the house batteries, not the starter battery.

The Lifeline AGM tests (read all the comments too) are fascinating, and the response directly from Justin Godber from Lifeline in the comments was interesting - just facts not a sales pitch. EQ, alternator issues, inverters, chargers, etc, and some real-world testing makes for some great info. Refreshingly honest since this guy doesn't hate or cheerlead battery chemistries - he just wanted his AGM's to work - and got them to do so.

If only I had such problems! .. in my dreams ..

[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.

[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]

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]

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]

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]

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]

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]

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]

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]

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.