Battery State-Of-Charge Chart | 12 Volt Battery Voltage & Specific Gravity

Battery voltage (chart below) can help determine its state-of-charge. However there are caveats to this (listed below).

The ‘State Of Charge’ (SOC) of a battery is a measurement of how much energy is remaining (percent). It’s like a fuel gauge. Measuring and knowing the SOC of a battery or battery-bank is useful when applying towards alternative energy, or any other situation where you need to know its condition.

There are several ways to determine a battery’s SOC.

1. Measure the battery’s chemistry (specific gravity) with a hydrometer (accurate method).

2. Measure its voltage with a volt meter while open-circuit, no load (general approximation).

3. Track the current flow in and out of the battery with a ‘shunt’ and associated metering circuit (common with alt-energy systems).

(( Battery Hydrometer ))
View the most popular hydrometer on amzn

I put together the following battery state of charge chart which indicates the state-of-charge (percent) as it relates to battery voltage or specific gravity. Voltages and Specific Gravity are listed for a 6-volt or 12-volt battery, and battery banks of 24 and 48 volts.

The chart is listed below. But first, a few important notes and caveats…

How I Made The Battery State of Charge Chart

How I determined the voltage values:

I researched as many battery manufacturers that I could find regarding their own published SOC data. Some were slightly different from each-other with regards to their SOC values. However I averaged all of them together to come up with a chart which represents what I believe to be a good GENERAL indication.

Battery Voltage Measurements Are Just Approximation

 Note: Voltage measurements are only approximate to determine SOC. Measuring battery voltage is NOT the most accurate way to do this (there are variables to consider). But a good generalization. A more accurate method is to measure the specific gravity of each cell within the battery. However for many batteries this is difficult or impossible (AGM batteries, for example). Many (most) alt-energy systems incorporate a DC-shunt which keeps track of SOC by monitoring the current flow in and out of the battery or battery bank, which is a very accurate way to track state-of-charge.

Measure While “At Rest” “Open Circuit”

Note: For best accuracy when measuring battery voltage, the battery must be in ‘open circuit’ condition (at rest, or ‘resting’). This means that the battery must NOT be under load and it must NOT be charging. To be somewhat accurate, the battery should be in that condition for an hour or two before taking a measurement, while for a more accurate measurement you should wait 6 hours up to 24 hours.

Battery Voltage vs Temperature

Note: Battery voltages are temperature dependent. In fact, good charger systems (alt-energy systems) have temperature compensation built-in. The voltage data in the chart below is that of manufacturer spec. sheet listings (vicinity of room temperature).

Hydrometer Tip

Note: If testing specific gravity (deep-cycle flooded/wet batteries), when drawing a sample from the battery, fill and drain the hydrometer several times first before settling upon a measurement.

Keep Lead Acid Batteries Above 50% State of Charge

Note: For longer battery life, lead acid batteries should remain in the green zone (40% or more SOC). Occasional dips into the yellow may not be harmful, but continual discharges to those levels will shorten battery life considerably. Generally speaking, the less you discharge the battery before recharge, the longer the battery will last. Most alternative-energy systems are designed to keep the battery bank at least 50% or higher.

This is NOT the Charging Voltage

Note: The 100% voltage is NOT the recommended charging voltage (which will be higher, and multi-stage). See your battery manufacturer recommendations regarding charging.

BATTERY VOLTAGE – STATE OF CHARGE CHART

Open circuit voltage, or specific gravity per cell

>> Download full size PDF (Battery State Of Charge)

If you know your exact battery, you might find related voltage information on their website. But the chart above will provide a general view for state of charge.

Tip: Impressive battery technology… LiFePO4 (Lithium iron phosphate). This 12 volt battery weighs only 23 pounds. Service life 8 to 10 times than the standard lead-acid batteries, 2000+ cycles. Good choice for RV & camper, solar, etc.. Can be deeply discharged without issues like lead-acid.

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(amzn)

[ Read: Lead Acid Batteries – State Of Charge versus Freezing Temperature ]

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32 Comments

  1. Thanks Ken. Just thought it might be helpful to add that one tool I’ve found useful is the little floating balls thing for checking individual cell status in lead acid batteries.

  2. Thanks for the PDF chart.

    If the gravity of each cell stays relatively the same (usually all in the green) does that mean that I don’t need to equalize that battery?

    The reason I ask is: I have 90 watts of harbor freight panels, run thru a sunsaver controller that charges 2 wally world deep cycle batteries and the gravity has almost never been below the green level, and I never really have had to add much water. I mainly use this system for 12 volt water pumps,a few LED’s and cfl lights and occasionally power tools. In other words a heavier load for shorter periods.

    My other set up has about 750 watts of poly panels, run through a xantrex 30 amp charge controller charging a trojan 12 volt golf cart battery. That powers a flat screen TV, charges laptops etc. for long periods of time daily. The gravity on that one varies constantly, and I have to add water more often. It is set to equalize every 30 days, however sometimes I have to do it more than that, to bring them all back to green. I combine the batteries whenever we lose power for more than a few hours so we can run TVs, fans DVD player and lights etc. I have combined them a few times, then manually started equalization.

    I guess my question is do I need to do this since the wally world batteries always seem to be in good shape as per hydrometer readings?

    1. I built an off-grid system for my home and have expanded and maintained it myself over the years, so that is my knowledge base. My immediate thought about the differences in your two battery banks is that the wally world batteries are newer, or less heavily used than the Trojan. The Trojan sounds like it is getting old, and needs more frequent watering and equalizing to keep it functioning. I wouldn’t stop doing that although it’s a pain, unless you want to lose that battery. The plates may have become heavily sulfated enough that the equalize mode can’t get it all off into solution. It gets hardened on with heavy cycling. You should probably start saving your shekels for a new battery because that one is not going to live much longer.

      1. Also, NEVER combine different types, or age, or widely differing state of charge of your batteries when charging. That leads to overcharging of some of the batteries in the set. Overcharging (and equalizing) leads to water loss. Baby those batteries for longest life.

  3. I just wanted to let you know good work on the chart and description. I do off grid solar for a living, 25 years now and I sent a customer the link to this chart to give him an extra tool to keep tabs of his system. The one thing I would disagree with somewhat from your well written article is concerning the accuracy of state of charge meters. I dont call the voltage reading open circuit voltage, I call it “resting voltage” and if done right as you outlined it is seat of the pants but still somewhat accurate. The meters on the other hand can get way off. The standard meter for years has been the Tri-Metric meter but I finally gave up on using them because they arent very accurate unless fine-tuned beyond the level most homeowners are capable of understanding. It doesnt help that the manual makes little sense to the layman or that the meter cant read battery voltage and often keeps compounding small errors into bigger ones. The meters that you can get that take their info from the inverter or in the case of Midnite Solar from the charge controller are more accurate because they temperature compensate but they need fine-tuning as well. Just yesterday a customer called because his inverter shut down at 23 volts, a definite low battery but the SOC meter claimed he was at 81% full. I sent him a link to your chart and told him to call Magnum tech support to learn to tweak the meter. Carl

    1. Thanks Carl, and as you insinuated, there are variables, including temperature compensation measurements and others. For ‘seat of the pants’ measurements, I hope this has been helpful. I agree with the terminology that ‘resting voltage’ is a more accurate representation…

  4. This is very informative thanks. I’ve been wanting to maintain my system more accurate. So thanks for the chart good information.

  5. Saw this as I was looking for a SG to SOC chart and thought I would make a couple of comments. First the function of equalizing is just as the name implies. Specifically for series batteries. When batteries are in series they do not necessarily charge at the same rate and can become imbalanced. The equalizing charge ups the charging voltages in order to get enough voltage to the lesser charged batteries in the series chain. Yes it will over charge the higher charged batteries, which is why the equalizing charge is only applied for a short time, however if the system uses a single battery equalization is not required and can be detrimental. It would be wise to buy a desulfator unit for your batteries in order to reverse sulfation of the plates after long periods of less than 100% charge.

    Thanks for the chart BTW

    1. Equalising charge is to equalise at the level of cells, rather than batteries i.e. a single 12 volt battery consists of 6x2v cells in series and will indeed benefit from equalisation.With a wet cell battery this is easily demonstrated using a specific gravity tester.

  6. Great chart! Just installed 2 24 volt reconditioned fork lift batteries in my 48 volt system. And want to really take care of these babies. I can’t believe the difference. I don’t have to wake up to pull starting my gen set each morning. Went 8 days without using the generator and now well into my next 8 days. My last set of batteries I let go down to 38 volts before charging. I realize now I was really punishing them…
    Thanks again!

  7. Your State of Charge chart does not show for a 36v system. Will the voltage reading be the same if I use 3 12v batteries or 6 6v batteries?

    1. @ROSEY,

      Here are extrapolated numbers for a 36v system. Just bear in mind the caveats listed above.

      100% 38.19
      90% 37.86
      80% 37.50
      70% 37.11
      60% 36.72
      50% 36.30
      40% 35.88
      30% 35.43
      20% 34.98
      10% 34.53

  8. Do remember that battery voltage readings are temperature dependent. So between 50F and 80F could have a 10% difference in charge state. For really serious work advisable to take temperature of the battery acid for standard lead acid batteries.

    Secondly pulling voltage down to less than 50% remaining capacity permanently devalues the battery. Which explains why batteries that “go dead” never recover fully. Except with some vigorous efforts at reversal.

    Thirdly using anything other than absolutely pure “battery water” (no minerals) will permanently damage the battery. With proper care and feeding quality made lead acid batteries not subject to abuse can and will last up to a decade. Much depends on plate thickness. More is always better. Installing a low voltage battery alarm can save your bacon.

    1. pls, I want to ask… I have a project I’m working on which will run concurrently on battery (12v 200ah deep circle battery). Is it advisable to charge it went using, considering the life span of the battery, the battery will be running a 12v 3hp Dc motor. victor from NIGERIA

  9. Hi I’m Tommy from Indian wells golf resort in California, i have a fleet of 160, they get used alot in season, now its summer we are slow, i have them in rotation, always have, when a certain section stays in for 2 days the battery percentage on the gps units will be 100%, a day or 2 later drops down to 85 to 80% or lower because they haven’t been used, is that good for the battery? Or is it fine

    1. Not sure if I understand your question, however I would suggest that if fully charged batteries are just sitting there not being used and they are dropping to 85% in just 2-days, then those batteries are probably end-of-life.

  10. Very good topic. Ive had when rebuilt 12v it stayed at 12.90 13.00 v ive had nothing but trouble with other rebuilt battery’s so while i had difficulty in finding temp or standing volt pages this is very thankful

  11. Thanks for the chart, great help to look after cells. I found on ebay a 9$usd 12V 24V 36V 48V Battery Charge Automatic Charging Controller Protection Module
    Set the low on voltage
    set the high off voltage

    Pulls a relay on and off when it reaches the set points and turns on a mains powered charger to protect over discharge

  12. is this for all battery types?
    I have Trojan GEL battery 225Ah when I unplug the charger, it reads 13.2v after 24 hr.

    1. @OJ, Generally speaking, yes. But please read through the caveats in the article.

      That said, if you let it rest for 24 hours with no load on it, the voltage reading will give you a pretty good idea.

      My numbers came from averaging the voltage lists from various manufacturers (each of which were fairly close to each other).

      Your 13.2 reading is quite good. Do you trust the voltage meter that you’re using for accuracy? (that’s another story…)

  13. Thanks for the PDF, newby to solar power. Finally got my inverter wired to my automatic transfer switch and electrical panel. Running a few appliances just to test it. I have 4 100 ah sla AGM batteries in series/parallel for 24 volt and warned to make I didn’t discharge them less than 80% is night time my solar charge controller is done for today,so I can’t start charging back up till the AM.

  14. It would be nice to have a 10a under load voltage chart too. I have 4x6v gc2 golf cart batteries in series and I get 25.21v under 5a load at 24v. My shunt meter says the batteries are at 91%. The batteries are 3 years old costco interstates.

  15. What is the state of charge of the following specific gravity:1.287, 1.243, 1.128, 1.200, and 1.135 reading from Hydrometer?

  16. I am using a trace C12 solar charge controller. It has been keeping my lead acid battery at 13.2 volts, this is at the minimum “HVD” setting. I have heard this is too high of a voltage to keep a battery at! What is going on here? Thanks.

  17. Thanks Ken,

    Just wondering if you can tell me what would be the best cutoff voltage for running something from a car battery when the engine is off.

    Specifically, I plan to add an option to keep my USB charger powered when the car is off, but don’t want to risk flattening the battery.

    1. For batteries used mainly to start cars, it’s best to not use more than 10% – much less than 50% quoted for other batteries – of its reserve capacity before recharge to keep the battery in good shape. A typical car battery is around 40AH which means you shouldn’t drain 4AH (about 50 watt hours) before charging again – so perhaps do not go below 12.5 volts or so. This is because car starter batteries are very weak at full discharge/flattening. Just flattening a car starter battery once causes significant damage unlike batteries designed for it like traction batteries – the ones meant for electric golf carts and solar battery systems which were designed to accept flattening because it’s bound to happen.

      So now we know how much we can discharge – this isn’t a whole lot of energy. The question is how much does the charger use? Depending on the charger it may have high quiescent power usage which would be bad. Also its conversion efficiency counts significantly towards how much you’ll use to charge a phone. Of course the act of charging a phone will use much more power, but you probably can charge once or twice before you must recharge the car battery to stave off damage.

      How often do you start the car or use charger to recharge?

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