Battery charge versus remaining usable charge

"I have new batteries in my 2007 NL 10-2QRR and charged them to 12.4v last week."

12.4 isn't my idea of a full charge, and after four days/three nights it couldn't be 12.3. Something is wrong with your numbers.

I'm charging it up again, but 12.4 seemed low to me too. My other truck and SUV were closer to 12.7v.

My RV was a PITA until I put a good solar system on it, now I never have to worry about it

That's my thoughts on the issue, I will never have a RV without solar again.

I had the battery switch to off and it was down to 12.2v. I didn't realize it wouldn't charge in that position, though it does seem obvious now. It's possible I had the switch off when charging last week so it was where the dealer installed it.

I'll watch it and see if it charges tonight. That said, any thoughts on if a delta of 0.1v at 12.3 is the same usage time as at 12.7v? I assume it's just something I'll have to watch and see like a fuel gage in a car.

Charging voltage should be 14.2 to 14.8 volts.
Then after resting disconnected 24+ hours the battery should hold about 12.65 to 12.75 volts.

Battery monitor is the best way but a voltmeter should suffice.

It's pretty hard to assess the State of Charge from voltage measurements. The recent load on the batteries, current load, temperature, and age all affect the measurement. If you are interested in tracking the actual state of charge, there are meters made for this purpose that track current in and out.

Very roughly, on flooded cells, 12.7 - 12.8 is fully charged (measured after being disconnected from everything for a couple of hours). 11.5 is getting near the bottom. 12.2 - 12.3 might be half. But these could be way off due to the above mentioned factors.

I go by a chart like this one when camping off-grid...



I also have a DC VOLTAGE and DC CURRENT homemade Battery monitor setup right at the main battery switch.

When my battery drops down to the 12.0VDC level I have a pretty good idea this is close to the 50% charge state. It is NOT absolute but good enough for me to stop using my batteries until I get them re-charged back to at least their 90% charge state. My system performs very well performance wise at the 90% charge state level on the batteries. Almost as good as a full charge state.

My parasitic drain all the time is around 1-2 AMPs and between 6PM and 11PM each evenings I will be pulling sometimes up to 22 DC AMPS current or so running the things we want on during the evening. This is probably higher than most campers do but this is what we do... We can almost do everything we run at an Electric Site with the exception of NO AIR CONDITIONING or NO HIGH WATTAGE MICROWAVE...

My 255AH Battery Bamk does just fine with the daily load and usually makes it down to around 12.0VDC by 8AM the next morning. This is when we can use our 2KW Generator connected to the trailer shore power cable and allow our PD9260C converter/charger re-charge our battery bank back to its 90% charge state in a three hour generator run time. This is also when we make our grind and brew coffee for the day and use the skillet sometimes cooking up something good... Generally we are allowed to run our generator after the 8am time frame where we camp..

Been doing this routine since 2009 and it is all second nature now camping off the power grid for us...

My original 2009 installed GP24 12V Interstate 85AH batteries sort of came to their end-of-life last season and we are presently upgrading the battery bank as we speak. Going to go with two groups of 6V 225Ahs Golf Cart batteries (T105s) in series that will give us around 450Ahs capacity from our battery bank.

Hoping to add solar panels with this install so that I won't have to run the generator so long each morning. After the batteries DC Charging current tapers back to around 6-8 AMPS for each battery I can shut down the generator and just let the solar panels continue charging while they are in the high sun. I really want my solar panels to produce 17-20AMPS DC current when in high sun but can get in a good 6 hours or so of charging time so should get the batteries back up to their 90% charge rate pretty easy...

Lesson learned for me is I must start each day/night run off my batteries with at least a 90% charge state otherwise it will get dark on me around 9-10PM each evening..

I have also learned that if I let my batteries get below the 50% charge state they will start doing internal damage to the batteries and eventually won't hold a good 90%-100% charge state very long... After loosing one battery for this reason I watch them very close when camping off the power grid now...

I have also learned that I can only do around 12-14 of the 50% to 90% charge states and then I must do a god slow 100% charge time run which will take around 12 hours plus time to get a good 100% charge. This is when we head for the house as most of the time I would not be allowed to run my 2KW generator that long of a time at the camp sites.

This old man just can't go buy batteries every three or four months...

Roy Ken

A fully charged 12 volt battery "resting" voltage should be at 12.6 volts.
Something is amiss with your voltage meter, the battery or the charger.
Or you never gave the batteries enough time to charge to full power.

bkenobi wrote:
is there any way to know how long a set of batteries will last based on initial voltage?

That is pretty much impossible.

Voltage alone is a very poor indication of "state of charge" when a battery is partially discharged. It won't tell you how many amp hours are left

What you need is a Xantrex LinkLite or LinkPro, which factors in Puekert's equation to determine amp hours

bkenobi wrote:
This seems like a silly question, but I know it's actually complicated and may not be something anyone can really answer. That said, is there any way to know how long a set of batteries will last based on initial voltage?

I have new batteries in my 2007 NL 10-2QRR and charged them to 12.4v last week. We went on our first trip last weekend and over 4 days/3 nights we dropped to 12.3v. We were very conservative on consumables since we are new to TC's and only used the furnace, water pump, some water heater, and lights (mostly LED). I know some equipment only works down to a minimum voltage (11.5-12v I think), so I'm not sure if that means we could go 4x longer or if we would be more/less.

Any thoughts?

This is a really good question, I don't have the knowledge to even attempt to answer it but I think this explanation might help.
The question is much more involved than just measuring voltages.


What shape are my batteries in?