Just for the sake of curiosity, I've been trying to determine a correlation between the working voltage of my batteries ( measured while running all of the lights in my trailer) as distinguished from the resting voltage (measured after the battery has been disconnected and has been sitting for a while). My goal is to try to determine when the battery is really at a 50 percent, since I'm told that we are not supposed to run 12V deep cycle batteries below 12.1 volts, or roughly 50 percent.So I have two questions: when they say don't run down the battery to below 12.1, that means the resting voltage, right?And second, is there an accurate correlation between working voltage and resting voltage? For example, if the battery reads 12.2 while under load, does that necessarily correspond to (let's say) 12.4 when resting? It would be handy to be able to measure the voltage of the battery while in use and come up with a good approximation of the true state of charge, without having to disconnect everything and then wait a while till the battery reaches its resting state. (Usually, when we are camping, I don't have a lot of free time to make those kinds of careful measurements -- DW always wants to hit the hiking trails asap, instead of hanging around and looking at the multi-meter.)Thanks in advance for your opinions and expertise!(By the way, these are three year old group 31s rated at 110 amp hours, and the hydrometer indicates that all the cells are doing fine. We have had no trouble with these batteries.)

You can't get a true "resting voltage" while camping, but you can get a "good enough" version of it by using what I call the "morning voltage".Take your voltage in the morning before solar can start to raise that voltage is the first thing. Next thing is to wait till after the furnace has quit for a while and before it cycles back on, and turn off any lights and fans too. Never mind the little parasite draws.Ok so that is your "morning voltage" and you can use the usual voltage vs SOC tables for your type of battery. some 12s use 12.2 for 50% Trojan uses 12.1, and so forth. AGMs have higher voltages per SOC. Hard to pin them down, but mine are about 12.4 at 50%. Whatever.Now the other voltage you see is the "loaded voltage" and this depends on the size of the amps draw at the time. The higher the amps the lower the voltage. So there is no fixed amount like you were asking about. You can get an idea of the voltage drop to expect for what you have "on" with your rig after a time using it.Then if your voltage does not "match" what you would expect, you can go looking for what is wrong. This is good practice, because you need to be able to "tell" when things are "not right".If you had a Trimetric, you could cross-check between the AH down from full, and the "morning voltage" to see if they "match" as a good way to see if all is well.

Discharging a lead-acid battery by any amount reduces its cycle lifetime, but 50% isn't any sort of magic number. In fact, for the vast majority of campers, the batteries die because of abuse or their calendar lifetime, not the cycle lifetime. Getting 500 cycles instead of 1000 simply doesn't matter - it's the difference between dry camping every weekend of the year for 5 years vs 10 years, and the vast majority of people simply aren't off-grid that much. Discharging by 80 or 90% a few times a year simply won't make a difference, as long as they're recharged soon, and it's ultimately cheaper than buying more batteries to meet some "50% rule." The 50% thing comes from off-grid daily use, where it does make a difference.The other thing which kills them is not recharging them shortly after discharge, or not recharging them fully. The latter happens with the cheap converter/chargers most campers come with. Even the $20 from China eBay solar controllers do better in most cases.

The best idea is to just stop worrying about all that stuff. If you have a continuous battery voltage monitor, just watch it as the day or days progress. You will develop a pattern and learn to live by it.In my case, I bought a two decimal point voltage meter from China for about $3 delivered. I connected it to my battery so I could see what it said. I learned to watch it roughly to see how fast it fell, and learned to top up with generator as required. Now I have tons of solar, so I never have an issue with our usage.

Yes 12.1 resting--but that means 24 hours of non use and non charging.The only way that would work for an RV is if there were twin banks, and one could deliberately be disconnected.My solution is to have more battery capacity than I would ever need. Unfortunately LI is not an option where I live due to cold. I wish I had a thousand amp-hours of capacity.

I do the same thing here just let it drop down to around 12.1 to 12.0 VDC as read on my BATTERY MONITOR PANEL.I have planned for this to happen around 8Am each morning when I am allowed to run my 2KW Generator. If it gets down to this level before 8AM I switch off my batteries and wait until morning...My Battery monitor is tied in with Battery selector switches reading each battery bank. Of course with both selected I will read the same DC VOLTAGE for Bank1 or Bank2. The DC Current is shown for the DC connection feeding my 12VDC Distribution Panel. This lets me see what is being pulled from the batteries as well as what is being put into the batteries when charging. Of course none of this is absolute so just getting close is OK for me. been doing this setup since 2009 with no issues.My three 12V 85AH Standard issue Interstate batteries lasted until 2015-2016 when they started to lose performance. I would be camping off-grid and doing the almost 50% to 90% cycles for some 5-10 times before doing a full 100% charge state when I got back home...I am hoping to get similar results from my new battery bank I am currently working on paralleling two groups of two 6v GC2 215Ah Duracell Batteries I just recently purchased. Folks are already telling me I may have to increase my 60A PD9260C converter/charger to a 80 A setup and maybe using 14.8 Boost charging voltages. Going to try with what I have and see how much more time it will add to my previous three hour smart charging time. Where I camp off grid has generator run time restrictions in place but I can add a couple more hours. I may have to drop back to just be running two 6V GC2 batteries giving me 215AHs until I come up with a 80A smart mode charging unit converter/charger. I might have enough time to do each of the battery banks groups separately...Roy Ken

Working voltage, resting voltage, and the 50 percent rule

23 Replies

MEXICOWANDERER
Explorer
May 06, 2018

This is cheaper

Easier to Install

And just as accurate
Harvey51
Explorer
May 06, 2018
Mex mentioned an amp/hour meter -- is there a good hand-held version of that, kind of like a multi-meter? I would want to use something like that on the road and also at home, so a hard-wired unit would be less useful.

No, the amp-hours can only be measured while the current is running in and out of the battery through the monitor continuously for all the hours since last full charge and setting 100% state of charge.

My cheap Chinese eBay battery monitor automatically calibrates 100% state of charge - when I continue charging from solar after it says 100% it stays indicating 100% while I make sure it really is fully charged. I simply never switch off the solar charging and the monitor stays accurate.
wa8yxm
Explorer III
May 02, 2018
profdant139 wrote:
So I have two questions: when they say don't run down the battery to below 12.1, that means the resting voltage, right?

And second, is there an accurate correlation between working voltage and resting voltage? For example, if the battery reads 12.2 while under load, does that necessarily correspond to (let's say) 12.4 when resting?

To the first question: YES it means resting

To the 2nd.. YES but the correlation is specifit to both the battery and the day of the test..

That is a battery that looses say 0.1 volt under a given load TODAY,, Well in a month may loose 0.2

It is a function of the internal resistance of the battery which increases as the battery ages..
pianotuna
Nomad III
May 01, 2018
If I had money to spend I'd put it into more solar wattage.

time2roll wrote:
I spent my monitor money on more batteries. I might be off my estimates 25% but I have 100% more power.
time2roll
Nomad
May 01, 2018
I spent my monitor money on more batteries. I might be off my estimates 25% but I have 100% more power.
brulaz
Explorer
May 01, 2018
My cheap china Ah meter is almost useless in that you have to frequently reset the 100% value. Certainly the Victron, Bogart and Magnums are better especially when used in conjunction with their chargers.

But I primarily follow BFL and other's morning Voltage check.

EDIT: Heard elsewhere from someone who believed his Ah meter rather than "resting" voltage and undercharged his batteries for months, Until he discovered the Ah meter calibration was off. Luckily his batts were LiFePO4.
BFL13
Explorer II
May 01, 2018
In a previous thread Mex pointed out another factor.

"•I would never take a thin plate AGM to less than 50% SOC regularly. Over-amp really thin plates and they will warp"

(At low SOC a battery will sustain a high amp charging rate for a longer time before amps taper, so that might be "too much" for some batteries to take depending on their construction.}

It is not so easy to learn if your AGM has plates that thin either. You could use a lower max current charger to play it safe unless you are on generator time.
JimK-NY
Explorer II
May 01, 2018
Your battery manufacturer may provide a table showing depth of discharge vs voltage for various amounts of load. If not look at page 32 for Lifeline AGM batteries: http://lifelinebatteries.com/wp-content/uploads/2015/12/6-0101-Rev-E-Lifeline-Technical-Manual.pdf

The table includes a 120 hr rate which should match minimal electrical use such as a refrigerator control panel and a propane leak detector and maybe an LED light or two. The results do not need to be highly accurate. You are only looking to be over an approximate 50 percent discharge on most occasions. Unless something major is running such as a TV or furnace, I use about 12.2v as a warning and 12.0 or 12.1 as the action level for starting up my generator or killing all the major loads.
2oldman
Explorer II
May 01, 2018
Victron makes some with bluetooth phone meters.
scrubjaysnest
Explorer
May 01, 2018
We use a Bogart battery monitor. Gives %charge, days since full charge, days since equalized, aH used plus other stuff you'll never really look at.
I like it because I've set it up to look and battery voltage and solar panel voltage. Having read several articals on the net it looks like SOC and DOD using voltage are kinda vague terms any way.