Are My Batteries Really Full? How Can I Be Sure?

September 5, 2013 · 1 comment

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Trimetric 2025 Battery MeterYou would think that knowing your batteries are completely charged is straightforward. However the state of charge a.k.a. SOC of your battery/battery bank it is not even close to being simple. This short article will explain why.

Here are some of the common industry practices:


Using the common “at rest” voltage to judge the chargedness (obviously I know that is not a word) of your battery bank is not even close to being accurate. If you are not familiar with the “voltage at rest” method, I will explain it quickly.

The at rest voltage of your battery bankFirst thing you do is let the batteries rest for at least 6 hours although 12 hours would be better. Letting the batteries rest means adding nothing and taking nothing away…no loads and no charging. Once batteries have rested, record the battery voltage. Using the chart below you can estimate the amount of charge in the batteries.

Battery Voltage Battery State of Charge
12.65 100%
12.45 75%
12.24 50%
12.06 25%
11.89 0%

This method is not foolproof as all batteries have different at rest voltages, the at rest voltage changes with age and the temperature of the battery bank can make a huge difference in voltage readings. The colder the battery, the lower the voltage.


A battery hydrometer is a useful tool for determining battery state of charge.Another more accurate (while much more time consuming) method of determining the state of charge (SOC) is to use the specific gravity (S.G.) of the electrolyte in your batteries and comparing with a chart provided by the manufacturer. Again the batteries must be at rest for at least 6 hours and preferably more. It is best to sample at least a few of the cells of a few of the batteries and average them. You also want to get your electrolyte from the inside batteries of a string as the outside batteries (batteries closest to the battery cables)  tend to get more charge then the inside batteries.

Specific Gravity Battery State of Charge
1.265 100%
1.225 75%
1.190 50%
1.155 25%
1.120 0%

This method may not be accurate as the specific gravity can vary in each cell of the battery and even in the same cell. The batteries closest to the battery cables will usually test better than batteries far from the cables. The last variable is the temperature. The S.G. will change a lot with differences in battery temperature.

Please do not use the above tables for your batteries as the numbers above are only for a specific battery at a specific temperature.


100 Amp Hour 12 Volt BatteryAnother method of determining how full your batteries are is to track how many amps go in and how many amps come out.

For example:

Suppose you have a full battery that is rated at 100 AH (amp hours) at 12 volts. This battery would have 1200 watt hours (1.2 kilowatt hours) of available power in theory. This is calculated by multiplying the AH rating by the voltage or 100 amp hours X 12 volts.

You remove 40 amp hours or 480 watt hours. You could then assume your battery is 60% full or 60/100 AH left. If you then charged the battery with exactly 40 amp hours, the battery would now be 100% or full. Unfortunately, this is not the case for the following reasons:

  1. Batteries are not 100% efficient. There are losses when the energy is extracted from a battery and losses when energy is added to the battery. A brand new lead acid battery such as a Trojan T105 or L16 is about 94% efficient. This means it would take 104 amp hours of charging to put 100 amp hours into the battery.
  2. A battery’s AH rating changes depending on how fast the battery is being discharged. The smaller the load on the battery, the more amp hours the battery will produce. A Trojan T105 for example, can produce 225 AH (1350 watt hours) if discharged over 100 hours but only 185 AH (1110 watt hours) if the same battery is discharged over a 5 hour period.
  3. tm2020The higher the charge rate (or amps) the less efficient the charge will be. If you charge the T105 at 50 amps you might only get 45 amps in while if you charge the same battery at 20 amps you will likely 19 or so amp into the battery. This is due to the battery heating up while being charged. All heat is energy loss.

Using cumulative amp hours is likely the best system we have available today as that is how the Bogart Engineering’s Trimetric and Pentametric meters operate as well as Outback Power’s Flexmax FN-DC, Magnum Energy’s ME-BMK and any of the other higher end battery monitoring systems. If you spend some time adjusting the battery efficiency setting on your monitor you can get your monitor to be very accurate.


The only way you can be positive your battery bank is full is when your battery’s voltage remains at the recommended bulk/absorption voltage while applying a charge equal to 1-3% of the battery’s C20 rating. What is a C20 rating and why should you care?

Although this sounds very complicated, it is not. When looking at the specifications below take notice of the C20 RATE and the BULK/ABSORPTION VOLTAGE.

Trojan T105 Charging Guidelines

Let’s look at the specs of a Trojan T105 as per the manufacturer:

C20 RATE: 225 AH

Now we are going to add a second battery to make a 12 volt system. This is done by wiring the batteries in series.

C20 RATE: 225 AH

Using Two Trojan T105s in Series to Make a 12 Volt SystemNow we start charging. In the beginning we apply enough current to bring the voltage up to 14.7 volts. To prevent damaging the batteries we do not want to charge at more than 30 amps.

How do you know the maximum charge rate? Click the previous link and go to STEP #1.

It is usually about 10% of your battery bank’s c20 rate. What is a C20 rate?

Once we reach 14.7 volts, we can gradually reduce the amperage (while still holding 14.7 volts) until we get down to 2.25-7.75 amps (1-3% of C20 or 225 AH). When the battery bank only requires about 5 amps to keep the voltage at 14.7, it is 100% full.

Using s Trojan L16 for an off grid homesteadLet`s try another larger battery bank. This time we will use Trojan’s L16 batteries.

C20 RATE: 370 AH


This power system has 8 Trojan L16 370 AH batteries in series to produce 48 volts as in a typical Outback Power, Magnum Energy or Xantrex off grid setup. Why is the amperage (AHs) still only 370 when there are 8 batteries?

C20 RATE: 370 AH

Now we apply current to the battery bank until the voltage reaches 58.8. As the battery fills up it will require less and less current to maintain the BULK/ABSORPTION VOLTAGE. When only 1-3% of the C20 RATE (3.7-11.1 amps) is required to hold the voltage at 58.8, the battery bank is full. Any charging after this time is unnecessary. However if your charging source is renewable (such as wind, solar or microhydro) it doesn’t hurt to float charge your batteries.

The Trimetric meters, the Pentametric meters, Outback;s FN-DC and Magnum Energy’s ME-BMK all have functions that will track the SOC of your battery bank by monitoring the BATTERY VOLTAGE compared to the AMPERAGE required to hold that voltage. While they use the Coulomb (cumulative method) to measure battery state of charge, they also use this method if programmed properly.

If you have any questions about batteries and battery charging please contact us or leave a comment below…

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peter treen May 25, 2015 at 7:39 pm

i wish to say thank you, im at last getting to understand things better, after wasting money thinking i had it right but far from it, im on a 48v sistem 4klw inverter charger, and solar placs that can give me 25amps + zantrex c40 when the sun is right, i consume very little as i thought but when reckoning up at aprox 16 hours a day in summer more in winter, i have just put in 4 agm 250 AH bats, at around midnight i shut of the power at rest reading around 49,6v after around 5 hours rest each bat reads aprox 12,4, think that would be 25% discharge, so i think it would be wise to invest in another 4 bats giving me 500AH bank, to help during the winter time, and also installing a bts, on the bats,
thank you once again for your explinations on batterys, peter.