Understanding battery usage and capacity

Here is what I have been doing since around 2009 in my off-road POPUP trailer...

I have this home made BATTERY MONITOR panel that has two 12VDC meter and one 0-70A DC Current meter in it. The two 12VDC monitors each of my two battery groups... Of course when both are selected then these two 12VDC meters read the same...

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I have this installed in a good spot so I can get a quick glance at what is going on. These are not absolute reading bit gives an idea what my batteries are doing...

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This is my three working GP24 85AH batteries on the tongue of my POPUP Trailer. I lost one GP24 batteries just a few months after settin up this system using my OEM single mode converter/charger and it boiled out the fluids and ruined one of the GP24 batteries. I had four each originally..

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I usually see around 24-25ADC on the Current Meter running from 6PM to 11PM each evening which varies up and down of course when we are camping off-grid somewhere and running the things we want to run before going to bed each night. My DC voltmeters usually starts out in the 12.6-7VDC range and slowly drops down as the evening goes on...

Again these are not absolute scientific numbers but just gives me a good clue as what is happening. When these numbers get down to around the 12.0VDC range then I know I am somewhat close to the 50% state of charge.

This usually happens around 8AMP the next morning and this is when I want to run my 2KW generator when allowed to run my on-board PD9260C 60A converter/charger unit. This will bring my battery banks back up to their 90% charge state in around three hours of generator run time. This is also the time we run a few of the high wattage things like making coffee for the day and cooking things etc...

Like said this is our somewhat normal experience when camping off-grid for three or days at a time... Each evening I want to start out with my batteries up at least to their 90% charge state as from experience I have found out they will produce a good 99% of their rated performance and I can can make it just fine to the next 8AM generator run time period. I have found out I can do around 12-14 of these 50% to 90% charge cycles before my battery banks start dropping off on performance. This meaning they start dropping the 12VDC voltage reading quicker each evening... Since it takes around 12 hours or so to get get the battery banks up to the 100% charge state using the smart mode chargers I will only camp out for a week or so and the 12 hour charging run when I get back home. In my case I doubt the camp ground would allow me to run my generator that long at one time.

Adding solar for me will reduce my generator run time back to around 30 minutes to an hour when I notice the DC charging from my 60A PD9260C has tapered back to around 6-8AMPS DC current. My Dc CURRENT meter will show a NEGATIVE symbol when we are pulling amps from the batteries and will read not show any symbol when we are putting DC amps back into the battery banks.

I started to go through all the math like you are doing and it really didn't make a whole lot of sense to me so after running a few days and nights in my backyard setup doing exactly what I probably would normally do at the camp site meant more to me than having the exact numbers and college answers etc...

I have done good with my old GP24 12VDC batteries until just last season when they really started dropping off on performance. This was just using three of the GP24 85AH 12VDC batteries in parallel giving me something like 255AHs of capacity...

I am currently working on a new battery bank with two groups of GC2 6V batteries in series that should give me around 450AHs of capacity. I may just alternate groups each night with this setup...

This is what I have been doing since 2009 with my setup... These are the things I went through getting my battery system up and going for my style of camping off grid...


Roy Ken

I have 2 6-volt batteries and I also measured the amps each of my 12v appliances used. Those measurements at least gave me an idea of where my major battery draws would come from. My goal was to be able to dry camp for as long as possible without having to charge my batteries and also to keep my batteries above 50-percent SOC.

We changed out all our lights to LED and we now can run off battery power for up to a week. I would use my volt meter at the batteries every day or so to test the current voltage. The voltage reading is how I would determine the percent of SOC (I taped the standard chart to my battery compartment). The week use is spending most of our time outside, not using the furnace, basically just the frig on LPG, water pump and LED lights.

My process is not very scientific but my Trojan batteries are still going strong going into our 9th year.

I don't know if the charger does that or not. It's a PowerMax Boondocker 75 from Best Convertor. When the camper comes home from it's long winters sleep I'll get my friends meter/recorder and check that out.

Look at a portable solar solution. Any solar is a bonus.

johnm1 wrote:
I'm trying to figure out what my next electrical system upgrade should be (ex: should get 2 more 6v gf batteries?)

Does your charging system hold 14.2 to 14.8 volts for 3+ hours on the battery?
If not... that will be the next upgrade.

Sorry the data is all garbled up. I tried to fix it but the forum changes it to fit ... something other than what I wanted. I did correct the bottom line though and use ~51ah/hrs a day (which makes sense now).

I live in the NE (NYS) and we have LOTS of trees where we camp. Though I'm looking at solar again to see what it might do for me. However, I don't think I can count on it doing much on account of the shading.

230 AH from the 6 volts with about half of that usable. If I'm following your numbers there is no way it used 610 Amp Hours so maybe you mean 61 AH which might be just about right. Some rigs could sit idle with the frig on and expect to see 30 or so ah used and some may use 40 to 60 ah when using a few other things. It also adds up that you got by for a couple days and used maybe 100 or 120 ah out of the battery.

What your next upgrade would be could depend on several things such as if you only go out for X number of days you might add battery capacity and charge when you get home. You could also keep the two 6 volts you have and charge with a small generator after being sure your converter and wiring are in good shape. This would possibly mean an upgrade in converter that does a higher than 13.6 charge voltage. You could also add solar and help the batteries that you have and make it through your trip.

If you are like us and usually in the south west, solar works well. If you are in the north west a lot or in heavy trees solar may not do as well. There are different ways that we all accomplish the same thing.

Well Bob - that could be part of my problem.

Using this new info ... by having 115AH usable battery capacity and, by using the correct math, I use ~51AH/day. That says I need to recharge at least every other day. Assuming I get a full/complete charge each time.

Things are making more sense now!

Thanks

Sorry - my data didn't format correctly. I'll try to fix that soon.

However, I figured out part of my problem. To get the total AH/day I used Watts * hrs instead of Amp * hrs.

No wonder I was SOOO far off!

johnm1 wrote:
Sometimes I get a brain fart and just can't get out of my own way.

What does battery AH ratings vs usage mean?

I have (2) 6vdc gf batteries with 230ah each which means I have 230ah's to use if I drain them to ... 50%. How long does that "last"? I know, VERY user dependent. No, 2x6V in series gives you 12V at 230AH or 115AH at 50% SOC.

Series adds the voltage and parallel adds the AH.