Douglas Krantz - Technical Writer - Describing How It Works

How do You Calculate Charging Current?

For a generic calculation, the maximum size of battery (in Amp/Hours) that the power supply is rated to charge is divided by 48 (the maximum number of hours allowed), plus an allowance for the charging inefficiency of the batteries (30%? - depending on the manufacturer).

The charging current that you need is what it takes to get the battery from 20 volts to 27 volts in the alloted time.

Greetings Douglas,

Question: Can you please explain how to calculate the Charging Current. This is one of the questions in the annual inspection in Emergency Power Supply test.

Thank you, WA

Is this for fire alarm systems? If the question is for fire alarm systems, the final answer depends on whether it from the manufacturer's point of view, or from the technician's point of view. Also, the answer is based on what the NFPA Code (National Fire Protection Association Code) says about charging time.

NFPA Code Charging Time

Look it up in the index of the NFPA 72 "Battery - Charging" or "Storage Battery - Charging". Go to the code that's referenced there. There is mention in the Code about Float Charging and Trickle Charging, but these types of charging are required to keep the batteries charged once they are at full capacity.

Whenever the batteries are discharged, however, the NFPA Code essentially says that within 48 hours, the power supply that they are in has to fully charge the batteries.

Manufacturer's Charging Rate versus Technician's Charging Rate versus Theoretical Charging Rate

Manufacturer: The manufacturer has their system tested (including the charger) by a third party, nationally known testing facility like UL, ULC, FM, CE, etc. This is a worst-case scenario test using the maximum battery sizes. The equipment is all proprietary, so beyond knowing that the equipment passes the tests, we just have to accept that the chargers work.

The manufacturer doesn't know what size batteries are going to be used in their equipment, so their specifications will show the maximum size for the batteries. If larger batteries are used, then the power supply will take too long to charge the batteries and the charging circuitry won't meet the requirements shown in the NFPA Code.

Technician: Usually, the battery used in the field is smaller than the maximum size allowed by the manufacturer, so the measurements are going to be different.

The exact charging current is also non-linear; the exact charging current will vary through the charging cycle. If the battery is fully discharged, the charging current will be much higher than if the battery is, say, three-quarters charged. That means that the measured charging current can't be used in a simple formula.

To find out if a power supply will charge the batteries within the 48 hour period, the only way to accurately test is to use batteries that are discharged to the voltage shown in the power supply's literature (you may need to call technical support for the power supply). Charge the batteries for 48 hours, and then measure the voltage on the batteries.

A fully charged 12 volt battery is commonly 13.5 volts, a fully charged 24 volt battery is commonly 27 volts. Again, check the literature or call technical support for confirmation.

Theory: For the test that you're taking, you may have to just use a theoretically generic calculation. This would be the power supply's maximum Amp/Hour size, divide that by 48 (the maximum number of hours allowed), plus an extra current allowance for the charging inefficiency of the batteries (30%? - depending on the manufacturer of the batteries and the power supply).

(Battery Amp/Hours Divided by 48) Times 1.3

Because of the non-linear current values over the charging cycle, the different manufacturing processes for the batteries, and different designs for the charging systems in the power supplies, the simple formula shown here is only a nice starting point. The formula shown is not practical in real life.

Douglas Krantz
Life Safety
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