How to calculate battery capacity

[Kid Gloves]

Originally Posted by SJ-Chris

Relax dude! lol...... Discharge your batteries to whatever you want. Have fun.

-Chris

Was there anything in particular in my post that compelled you to tell me to relax? I found that to be a peculiar response to some legitimate questions.

[SJ-Chris]

Originally Posted by Soppy

In the remote off grid solar full time home systems I've designed , I use a DOD of 25% to a max of 50% . This gives a good buffer in case there is little or no sun for a couple of days . This will also give the best battery life since the system is cycled daily.
For weekend use cabins , I will use 80% DOD since there will be fewer cycles on the batteries , and they will have a few days to full charge before the next use.

Full time home system.... I agree 100% with your approach.

[SJ-Chris]

Originally Posted by Kid Gloves

Was there anything in particular in my post that compelled you to tell me to relax? I found that to be a peculiar response to some legitimate questions.

Yeah, your post was pretty aggressive (whether intentional or not). "Walk the walk if you’re going to talk the talk."....to some those might be considered fightin' words.


Sincerely Kid Gloves, it's all good. Apologies if anything I said came off the wrong way. I'm easy going and wish the best for everyone.

Originally Posted by Kid Gloves

Why wouldn’t you follow your own advice and discharge to 80% DOD? You said it could be done safely and it’s no problem. Now you tell us that you don’t do it because you want lots of buffer. Walk the walk if you’re going to talk the talk.

Already explained in above post.

Originally Posted by Kid Gloves

What is your preferred method for determining when a wet cell has reached 80% DOD?
OCV? Specific Gravity? Do you conduct a 20 hour load test a couple of times a year to determine capacity and then utilize a battery monitor to measure consumed energy?

I use a battery monitor which seems pretty accurate.

Originally Posted by Kid Gloves

What amount of time do you allow for the batteries to charge after depleting them to your comfortable level of SOC?

I have 2x the amount of necessary solar to charge my batteries, so they get recharged every day and while in storage so they are never dead.

Originally Posted by Kid Gloves

Are you aware that Trojan recommends maintaining a SOC above 60% when operating a battery below 32F?

I was not aware. But I try not to camp when below freezing.

Originally Posted by Kid Gloves

You’ve stated that those who suggest following a 50% DOD guideline are perpetuating a myth and you’ve promoted an 80% DOD protocol, which you do not follow.

Between roughly 30%-80% depth of discharge, a battery has approximately the same amount of LIFETIME AHs it can deliver. So use them however you want!


I have solar and it keeps the batteries charged virtually all the time. I check water levels every couple months. I think we can all agree that what KILLS batteries is a)Draining them to zero and letting them sit for months (ie while in storage) repeatedly, and/or b)running them without water. I'm confident my batteries will last 5, 7, 10yrs. AND.....if all else fails, I'll spend $220 for another pair of 6v GC batteries and start again. Who cares.


Enjoy (and use) your batteries people. Unless you are full-timing, you will probably never use up all your cycles.


-Chris

[Kid Gloves]

Thanks for the reply. Nothing I said was intended to be aggressive. That said, some folks don’t take kindly when told to relax, or to be told what they should or shouldn’t worry about. Know what I mean? Basically, having someone tell them what or what not to do can be like fingernails on the chalkboard. Especially when it’s a personal choice and there isn’t a right or wrong way to do that thing.

As you’ve suggested, this battery issue isn’t really worth getting all uppity about now, is it?

[Fridge Defend]

Originally Posted by Kid Gloves

Thanks for the reply. Nothing I said was intended to be aggressive. That said, some folks don’t take kindly when told to relax, or to be told what they should or shouldn’t worry about. Know what I mean? Basically, having someone tell them what or what not to do can be like fingernails on the chalkboard. Especially when it’s a personal choice and there isn’t a right or wrong way to do that thing.

As you’ve suggested, this battery issue isn’t really worth getting all uppity about now, is it?

Aggressive?

With a great deal of knowledge put forth on subjects, like folks that present charts and supply accurate technical knowledge, seems that a scientific playground monitor is needed at times

I’m curios to know why you would start this thread

The answer from the mind of wisdom and experience!

[cruizerEd]

Originally Posted by 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.

This is completely correct.

Your battery condition is not known.
You don't know your exact current draw.
You don't know your loses.
You don't know your duty cycle. The fridge draws X watts when running but how long does it run in any 24hour period
You don't know what other parasites you have that you don't have the time or ability to chase down and eliminate.
Even if you could exactly know the loads for the typical day you still wont know what you batteries can actually deliver given that they aren't new.
So set everything up and let it run till you get to the minimum acceptable voltage.
As for depth of discharge, anything shot of draining them completely to zero volts one time probably wont significantly shorten their life.

[Kountry_star]

Guess I'll throw out my unprofessional, math for dummies 2 bits worth.

Last summer we left our motorhome's Dometic fridge running on 2 6 volts (6+ year old) batteries for 4 days (72-83 days 45-60 nights). Propane was full. Coach fan was set for 76 low fan (no idea if it turned on or not). Fan for fridge's cooling coils was on auto. After 4 days 5 nights fridge was cold the ice was solid, batteries where not dead (lights in the coach worked fine), propane still looked full. Generator did light off a bit slow. Anyway that's how my "calculations" worked out for us.
.

[PeterFTH]

Primer - 4 x 6v = 2 x 12 volt ... so 2x the amp hour capacity of one battery. If the Norcold is running on propane you will have no problem running the circuitry for 3 days or more, but if on 110 volts it will draw about 700 watts when running. Inverter loss is 10%. So in about a day your food will spoil. One long term solution is to have the Norcold converted to a 12vdc compressor type refrigeration - draws about 6-7 amps probably 12 hours a day (a lot of factors could make this more). A small solar array will keep it running indefinitely on propane which may be your best bet, but the gas side has many foibles - rust dropping from the ferris chimney can clog the burner which, if not at 100% efficiency (pure blue flame) will drop refrigerator/freezer temperatures dramatically. These foibles are what convinced me to convert my 1200 4 foot to a 12v compressor - reliable and makes more ice than I can use. Check out JC Refrigeration in Elkhart Indiana. It now can run on solar, batteries, alternator generator or shore power - versatile.

[ak-snopro]

Always entertaining reading posts about rv batteries. You mentioned your fridge was el/propane? If the propane side of the fridge is healthy why not use it and not worry about batteries. I'm blessed with a residential fridge and at times it turns into a pain. I wish i had a propane setup and half my battery worries would disappear. My previous rv had a propane fridge and batteries lasted 10yrs, never had to babysit them like i do now.

[twinboat]

Originally Posted by Soppy

In the remote off grid solar full time home systems I've designed , I use a DOD of 25% to a max of 50% . This gives a good buffer in case there is little or no sun for a couple of days . This will also give the best battery life since the system is cycled daily.
For weekend use cabins , I will use 80% DOD since there will be fewer cycles on the batteries , and they will have a few days to full charge before the next use.

You use 25% to 50% of your capacity except when you need to use more then that during a couple of days without sun.

So, your not actually just using 50%, your using what you need to use to keep the lights on, probably down to 80%.

[Tech Writer]

Having had dry campouts of three and four days, the two 6 volt golf cart batteries in series had no problem keeping the fridge operating and running a couple of LED lights at night. Don't forget to turn off the water heater, vent fans, etc.

[Tindalsj]

Interesting discussion, but CruiserEd is exactly right based on the original question of "will my fridge last 3 days?" Set it up and run a test (or several tests.) Don't forget that the food load inside the fridge will affect the duty cycle as well.

My experience comes from larger pleasure boats (50 ft) where I worked for a charter company for many years, and I think that (nearly) every technical point made here on all posts has some validity. However, the only thing that will matter in the end is if your food is still cool.

In my Endeavor, the Norcold draws 4x what my Dometic in my previous Holiday Rambler drew (based on pretty sophisticated battery and load monitoring equipment I formerly used in boats.) Your Norcold may be more or less efficient than mine, but the spec sheet is only a starting point at best. However, the Norcold and Dometic are very different and no assumption about one should be applied to the other without proof.

Finally, in talking about batteries- there are vastly different qualities between different brands, so never assume that your brand of "Deep Cycle" is the same as any other. Empirical evidence shows that is simply not the case.

I used Rolls branded batteries (no longer made under that name) of yesteryear lasted on average 8-10 years IN MARINE CHARTER SERVICE- which is many times more rigorous than the average single owner RV. And, they were regularly discharged to near dead by folks who had no idea what they were doing.

Anyway- my 2 cents. Good luck.

[Kid Gloves]

One of the most accurate ways to measure the capacity of a battery, or bank of batteries, is a controlled load test. This provides us with a laboratory result, which is extremely useful for those who camp in a laboratory or are able to reproduce laboratory conditions while camping in the wild. The result of that test will not answer a question such as: “For what length of time will my refrigerator continue to operate using the energy stored in my battery?” Why? Variables.

A battery from which 200Ah’s can be extracted in a laboratory, will likely never produce 200Ah’s in the woods. It could be more or less. We turn on lights, TV’s and run the water pump intermittently. These activities disrupt the laboratory conditions that gave us our precise capacity measurement.

The idea of conducting a field test has some validity in that it will provide information on the length of time that the batteries will allow the refrigerator to continue operating. However, meaningful results will not be obtained if we turn on the refrigerator, fill the compartments with cold/frozen food and then go back inside the house for 2-3 days. The batteries will be subject to different conditions when the RV is actually being used and will therefore produce different results. Even if we camp out in the driveway and attempt to create conditions similar to actually camping, we will have only learned the length of time that the batteries will power the systems on the RV. This is valuable information. However, it doesn’t answer the original question of how to calculate/measure battery capacity. I added the word measure, as calculating is only good on paper.

In my opinion, the ideal way to know and understand the unique battery capacities in our individual RV’s, is through the use of a battery monitor that measures the energy going in and out of the battery. Even with such a device, variables such as temperature and rate of discharge will impact the amount of energy the battery is capable of providing. It will, however, provide meaningful data which can be used to manage energy consumption.

[MSGDLD]

I'm late to this party, but I really appreciate this conversation. I've picked up a lot from it, particularly about DOD. I'm not going to be near as worried about going below 50% after this. A couple notes-

In April of 2014 I replaced my 2 6v Golf Cart batteries. In December of 2014 we were in Elkhart Indiana using the MH as a bedroom. (DIL w/cancer, babysitting 6 kids.) The DW and I had to go home over a long weekend. When we returned I realized I forgot to shut off the chassis and Coach switches and the DC batteries were DEAD. I about had a spaz attack thinking of replacing the new batteries. I charged them up, (generator or engine, can't remember which) and to my surprise and delight, they came back to life and are still in the coach to this day.

Regarding refrigerators, ours is a Norcold that has gone bad. I like the light usage of the batteries while boon docking, but I'm going to a 12v compressor. I don't have to be concerned with the MH being level, particularly since my driveway has too much grade to get the coach level enough, plus the fridge will cool much faster getting ready for our weekend jaunts. This thread has given me much more confidence in our ability to do this. Thanks to all.