How To

How to Measure Battery Capacity?

Can one recognize the charge state of a Lead battery at the voltage? Are there any ways to measure the capacity and thus the proper functioning of your AGM, gel, or car battery with a multimeter? The answer is yes and no. At least you can roughly test whether the used assault or the battery already installed in the vehicle still brings the specified performance. After being asked how to expose a lead acid battery, it’s time for a new contribution.

Used RV or used car batteries

Lead batteries are subject to wear and tear. Unfortunately, there is no way around it. Even with the best care will eventually adjust a capacity loss. Depending on the type of battery, it can handle a different number of charging cycles. After reaching these cycles specified by the manufacturer, a lead acid battery is not yet ready for disposal. Only the original capacity may have decreased. Especially if you buy a used car battery or a used motorhome with body batteries, it would be interesting to know what those things still have on the box.

Irregular full charge, wrong chargers and frequent discharges to close to the final discharge voltage can additionally damage the battery, which can lead to total failure. Unfortunately, you only notice these defects when the lead battery (e.g., in winter) is required to perform well over a more extended period.

No matter how you look at it, when the lights go out, although theoretically there should still be enough battery capacity, you have a problem.

Measure the charge status of the battery

In the first step, I would like to explain how to measure the state of charge of car batteries or lead-acid batteries. You need a multimeter and logically a battery. But there is one small thing to note:

  • Only the real rest voltage gives a comparatively significant value to the state of charge of the cell. 
    This means that our car battery has to be charged for several hours (4 – 8, ideally 24 hours) without a charger or connected consumers. After loading, the battery voltage is generally too high. The same applies after current consumption only that now the energy is displayed too low.
  • They are always approximate.
    The determination of the state of charge based on the battery voltage is not very accurate and gives only a rough overview. Also, it depends on the battery type and manufacturer. The temperature also plays a role. All stated values assume 25 degrees Celsius ambient temperature.
  • The measurement does not say anything about the capacity of the battery. 
    Measuring the voltage is always just a snapshot. The actual performance and available amp hours cannot be determined from the voltage.

State of charge and voltages for car batteries (wet battery), gel and AGM batteries

If you have allowed the battery enough time, it goes to the measuring. What is needed is a multimeter. The values are average and may vary slightly depending on the manufacturer. As already mentioned above, the voltage values are the respective quiescent voltage. You can also check our Best AGM Battery Reviews

SOCWet batteryGel batteryAGM
100%12,70V> 12,90V> 12,85V

Measure the charge state based on the acid density

There is another way to determine the state of charge of a car battery (open lead-acid battery) without waiting time. Since chemical processes take place in the battery during charging or discharging, the current state of charge can be measured based on the acid density. Of course, this works only with open lead-acid batteries. With closed VRLA battery systems, the acid cannot be used, or it is bound in gel or nonwoven.

An acid lifter is needed for the measurement. Open the sealing plugs of the battery and draw in the battery acid with the acid lifter. On a mostly colored scale, the density of the battery acid can now be read off.

“Take care when handling battery acid. Please wear protective goggles and gloves. In case of contact with the skin wash off with plenty of water and contact a doctor if necessary”.

This is how the charge state looks with the corresponding acid density (here too, depending on the manufacturer, there may be small deviations.):

SOCAcid density at 25 ° C [kg / l]

Measure the capacity of a battery (consumption battery)

As simple as the voltage, the actual capacity of a battery can not be measured. The test takes some time and a little bit of computing work.

For starter batteries, this test is unsuitable, since it does not necessarily depend on the capacity, but rather on the ability to deliver the highest possible currents in a short time. All measurement methods mentioned here apply to consumable batteries.

First, you need the following values from the label of the battery, from data sheets or directly from the manufacturer:

  • Voltage (usually 12 volts)
  • Ah, capacity at C20 (who does not know what C20 about the capacity means, I recommend my post about RV batteries ).

Determination of capacity by controlled discharge

First, the battery is fully charged and then left on trickle charge for at least 24 hours on the charger.

If the maximum charge current drops below 1-2% of the battery capacity, it is fully charged.

Now to determine the actual capacity of the battery, we have to discharge it in a controlled manner. For our example, I assume a 120 Ah lead acid battery (C20). This means that you have 120 Ah at a load of 6 amperes. After 20 hours, the battery would be empty and the quiescent voltage below 12 volts. However, as lead-acid batteries should only be discharged to a maximum of 50%, we halve the time to 10 hours in order not to cause any damage to the battery.

Check battery capacity with a battery computer

If you own a battery computer, you can test your consumption battery relatively quickly. First, we need a consumer, which takes approximately the determined C20 current from the battery (in our case about 6 A). It can also be several, such as. As TV, light or similar. It is essential that the withdrawn current remains the same during the battery test. We can read the critical values all at the battery computer.

We turn on the consumer and stop the time. During the experiment, we should regularly check the tension. Once this reaches 12 volts, we need to inspect at shorter intervals. At the latest at 11.5 volts, we turn off the load to avoid deep discharge.

Now we can read on the battery monitor, how many ampere hours were taken. This value multiplied by 2 corresponds to the actual battery capacity.

  • If the voltage drops below 12 V after just 1 – 2 hours, our sample battery is defective and should be replaced. She would then have only 6 – 12 Ah.
  • If the Undervoltage is reached after 5 hours, the 120 Ah battery would still have about 60 Ah, etc.
  • Everything between 8 and 10 hours speaks for intact power storage.

Measure the battery capacity with the multimeter

If there is no battery computer, check the voltage with a multimeter. After connecting the load, we also measured the time until 11.5 volts are reached. To calculate the consumed amp hours, multiply the time by the consumer’s current (for example, 5 hours x 6 amps x 2).

Thus we come, as in the above example, also on a capacity of 60 Ah.

Only test if there are problems

The capacity tests I have presented are a stress for the battery. Under certain circumstances, they can give the coup de grace to a battered battery. But they should not cause any problems with a working battery, as long as you do not let the voltage drop below 11.5 volts under load. Nevertheless, I would not torture my battery without reason. When buying used batteries or to test the batteries of a used motorhome, a test may be useful.

In many cases, it is sufficient to keep an eye on the voltage of our RV battery after an original full charge (shore power, charger and at least 24 hours trickle charge). If this falls below 12 volts after a few hours, it does not need any real experimental setups to expose the battery as defective.

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