Do you know a lot or even only a little about battery capacity and its testing? Let us explain a bit more..

A battery shall be tested for it’s actual capacity by extracting electrical energy from it over a duration of time. The duration of time shall be considered to be sixty minutes. The amount of energy to be removed from the battery will be equal to the ampere hour rating of the battery. The point at which it is considered to be at it’s limit of electrical discharge shall be for lead-acid batteries 1.67V per cell and for nickel cadmium batteries 1.0V per cell. So for either type of battery having a nominal voltage of 24V the minimum discharge voltage point would be 20V. The test would not return the actual capacity, but the batteries capability to produce output equivalent to it’s ampere hour rating as a minimum. A battery is considered fit for use if it’s capacity is more than 80% for a one hour discharge, which is equivalent to a full discharge for a minimum of 48 minutes.

A true capacity tester will maintain a constant current of the ampere rating of the battery over the full discharge period, irrespective of the change in battery terminal voltage. Because of the dynamic variance of the battery voltage during the discharge cycle, the use of a static means of discharge is not appropriate, such as fixed resistances. This is because the current value would diminish as the battery voltage reduces. This can be simply observed by applying Ohms law. If a 24V battery was to be discharged at 10A a fixed value of resistance equal to 2.4Ohm would be used. At discharge start the battery voltage may be higher than nominal voltage. It may start at 25V for example. This would mean that at the start of the discharge the current drawn would be 10.46A. This would fall to 8.33A at the near end of the discharge as the voltage approached 20V. It can be seen that the capacity test is not performed at the correct conditions specified. A true dynamic capacity tester would change the load resistance in accordance with the battery voltage to keep the discharge current constant. Therefore at discharge start the load resistance would be 2.5Ohm and the load resistance would change dynamically throughout the discharge to maintain the 10A required, in effect having a resistance value of 2Ohm at the 20V terminal voltage.

How do our products help?

The Intelligent Charging range of battery charger/analysers use a dynamic computer controlled load resistance. On capacity test the computer continually monitors the battery voltage and discharge current, and will alter the load resistance to maintain a constant load current. This is a true capacity test which is a uniform method of checking a battery’s capacity. A resistive method would be subject to variance between battery technologies, where the voltage fall may vary between batteries. We must also consider one important aspect of the capacity test in that most component maintenance manuals specify a constant current at the ampere rate for one hour.

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