How to calculate battery capacity

[kovalsmt]

Hello all
I have 4 six volt 110 amp hr lead acid house batteries 3 yrs old in the coach. Will be leaving the coach for three days with the Norcod El/propane refer running
Do I calculate battery capacity saying 440amps X (6 or 12 volts) = kWh’s
Using 12 volts I get 5280 kWh. With the fridge using say 3 amps/hr at 12 volts = 36 kWh’s X 24 hrs = 864 kw per day X 3 days = 2592 kw which says my battery capacity is sufficient but if I use 6 volts in my calculations my refer will be hot and batteries will be dead after 3 days.
I’d appreciate any help checking my calculations. I’m in Alaska with day time temps running 50F to 72 F..

42’ Monaco Camelot 2009
425 hp Cummins ISL
F-150 towd
Thanks again

[twinboat]

You don't have 110 AH 6 volt batteries.

They are 190 or more AH.

2 6 volt batteries in series doubles the volts but not the AH capacity.

You probably still have about 400 AH but that's at the 20 hour rate, meaning drawing 5 amps per 100AH for 20 hours.
To stay in that rating, you will need to draw 20 amps total. If you draw more amps, you have less capacity.

If your fridge draws 3 amps at 120 volts, it's going to draw 30 amps at 12 volts.

[TacomaJoe]

Two batteries in series will be 12V with 110Ah, parallel that string with the other two and you have 12V with 220Ah. Now the kicker, you don't want to drain them to zero but want to keep above 50% depth of discharge so we're back to 110Ah useable. At 3A draw, it looks like 36 hours.

If it's a propane refrigerator, I wouldn't think the draw would be 3A DC. That may be the 120VAC current.

[KenZ71]

Trying calculate out what it should be in my opinion is pointless. Battery age, fridge efficiency, etc all will have dramatic impact.

If me I would do a couple test runs then average results and hope for best.

[andy29847]

Originally Posted by kovalsmt

Hello all
I have 4 six volt 110 amp hr lead acid house batteries 3 yrs old in the coach. Will be leaving the coach for three days with the Norcod El/propane refer running
Do I calculate battery capacity saying 440amps X (6 or 12 volts) = kWh’s
Using 12 volts I get 5280 kWh. With the fridge using say 3 amps/hr at 12 volts = 36 kWh’s X 24 hrs = 864 kw per day X 3 days = 2592 kw which says my battery capacity is sufficient but if I use 6 volts in my calculations my refer will be hot and batteries will be dead after 3 days.
I’d appreciate any help checking my calculations. I’m in Alaska with day time temps running 50F to 72 F..

42’ Monaco Camelot 2009
425 hp Cummins ISL
F-150 towd
Thanks again

It’s not that simple. Here is an article that provides info and includes a calculator..

https://changingears.com/weight-calculators/ez-battery-life-calculator/


RVs almost exclusively use 12v for dc power. When you have an RV with 6 volt batteries, the batteries will be paired and the pairs will be wired in series to make 12v. Multiple pairs of 6 volt batteries will be wired together in parallel to increase capacity.

[Ray,IN]

You might find this website informative: https://marxrv.com/12volt/12volt.htm

[Isaac-1]

Just a practical reference point here for you, when my coach had a pair of 6V golf cart batteries, I once left it in a shop getting the brakes fixed for 5 days, and the Dometic absorption refrigerator was still running on propane when I picked it up, though they were getting low.

[SJ-Chris]

Originally Posted by TacomaJoe

Two batteries in series will be 12V with 110Ah, parallel that string with the other two and you have 12V with 220Ah. Now the kicker, you don't want to drain them to zero but want to keep above 50% depth of discharge so we're back to 110Ah useable. At 3A draw, it looks like 36 hours.

If it's a propane refrigerator, I wouldn't think the draw would be 3A DC. That may be the 120VAC current.

You can safely discharge your 6v GC batteries down to 80% discharge. 50% is a myth that certainly doesn't apply to GC batteries (they can handle deeper discharge).

[SJ-Chris]

Originally Posted by kovalsmt

Hello all
I have 4 six volt 110 amp hr lead acid house batteries 3 yrs old in the coach. Will be leaving the coach for three days with the Norcod El/propane refer running
Do I calculate battery capacity saying 440amps X (6 or 12 volts) = kWh’s
Using 12 volts I get 5280 kWh. With the fridge using say 3 amps/hr at 12 volts = 36 kWh’s X 24 hrs = 864 kw per day X 3 days = 2592 kw which says my battery capacity is sufficient but if I use 6 volts in my calculations my refer will be hot and batteries will be dead after 3 days.
I’d appreciate any help checking my calculations. I’m in Alaska with day time temps running 50F to 72 F..

42’ Monaco Camelot 2009
425 hp Cummins ISL
F-150 towd
Thanks again

"El/propane" I assume this means "electric/propane"?


Will the fridge be running on propane?? If so, you have nothing to worry about as it won't use much from your batteries. Just a bit to the fridge controls/motherboard.


-Chris

[bigb56]

Originally Posted by SJ-Chris

You can safely discharge your 6v GC batteries down to 80% discharge. 50% is a myth that certainly doesn't apply to GC batteries (they can handle deeper discharge).

I'd like to know more about this. From my research I recall 50% max and if kept to a smaller discharge like say 30% there will be more cycles over the life of the battery than there will be at 50%, just like going over 50% like to 80% as you state there will be even less cycles over the life of the battery. If I recall I saw charts on various batteries showing this. Will have to look that up again.

[bigb56]

Originally Posted by bigb56

I'd like to know more about this. From my research I recall 50% max and if kept to a smaller discharge like say 30% there will be more cycles over the life of the battery than there will be at 50%, just like going over 50% like to 80% as you state there will be even less cycles over the life of the battery. If I recall I saw charts on various batteries showing this. Will have to look that up again.

Looked it up, it appears that your statement is true, this is a chart for East Penn 6 volt Golf Car batteries, you can see that the number of life cycles drops sharply between 0 and 30% but then sort of levels off with the difference between 50% and 80% being much smaller, around 250 life cycles less at a consistent 80% discharge cycle. Looks like a good practice would be to try and stay above 50% with occasional dips to 80% nothing to be concerned about. Note that staying above 25% doubles the number of life cycles as compared to 50%.
Regardless while it is true these batteries can handle the deeper discharge it is also true that they will last longer with shallower discharges, so to some degree the deeper discharges do cause more deterioration on a sliding scale.

[Kid Gloves]

Originally Posted by SJ-Chris

You can safely discharge your 6v GC batteries down to 80% discharge. 50% is a myth that certainly doesn't apply to GC batteries (they can handle deeper discharge).

The 50% DOD is a recommendation provided by virtually every battery manufacturer, who’s spec sheet I have taken the time to read. If that’s a myth, then we have a very different understanding of the word.

Since switching to Lithium, such matters, including the recommendation to fully charge after each discharge cycle, are no longer a concern.

[ArtJoyce]

I would check your batteries for the specification plate. Most likely the amp hour ratting is greater than 200 AH each. Most 6v GC batteries start at about 200 AH.

Also, a propane fridge running on propane should only draw about 1/2 amp at 12v. If there are fans and when you open the door and the lights come on you will draw more.

[SJ-Chris]

Originally Posted by Kid Gloves

The 50% DOD is a recommendation provided by virtually every battery manufacturer, who’s spec sheet I have taken the time to read. If that’s a myth, then we have a very different understanding of the word.

Since switching to Lithium, such matters, including the recommendation to fully charge after each discharge cycle, are no longer a concern.

Look at the charts of cycles vs discharge. THEN, calculate the LIFETIME AH expected for the battery. Yes, you will get less CYCLES if you discharge to 80%, but with each discharge cycle you will get less AHs if you to go 50% instead of 80%. The overall lifetime AHs is within ~10% whether you discharge to 40%, 50%, or 80%.


Also, from a practical sense, let's assume that at 50% discharge you get 1000 cycles from your 6v batteries (as an example). Do you think many people will boondock and discharge their batteries to 50% ONE THOUSAND times? At 30 days boondocking per year it would take someone 33 years before they "used up" all their 50% cycles. Think about it.... Is that actually going to happen? Nope. They are going to go bad after 5-10 years regardless for 99% of users. Thus it really doesn't matter if you discharge your batteries 80% from time to time. That's all I'm saying.


Yes, if you boondock and live in your RV all year, year after year, then you are in a different category (...probably less than 1% of people here on the forums).


Hence, it's no problem discharging your batteries 80%. Would I design a system around that idea? No, because I like to have lots of buffer. But people need to stop worrying about discharging below 50%. Or not...makes no difference to me.



-Chris