All About UPS Battery Charging: How To Calculate Charger Size And Charging Time

These days I am examining demonstrations of 10KVA UPSes by different suppliers. My company wishes to purchase a 10KVA UPS with a backup time of a whopping 120 minutes (2 hours!!)

To cater to this long backup time, the UPS suppliers will provide an external battery bank. The issue is that all these UPSes come built in with internal batteries of 20 batteries of 12V, 7AH that are insufficient for 120 minutes backup thus external battery banks will be provided by the suppliers. One of the suppliers is quoting 65AH and the other one is offering 100AH batteries. I wanted to know whether the original chargers designed in these UPSes for 12V, 7AH would be able to support charging batteries with higher ratings.

Hence, I went on a search to find the perfect charger size for a battery and here is what I found:

According to , the charging current that the battery charger should be able to provide is:

Charging current should be 10% of the AH rating of battery.

So a company which is providing 65AH battery should provide a charger that is of at least:

Rating of battery: 65AH
10% of the AH rating of battery: (10/100)x 65=6.5
Therefore, charging current: 6.5 A

And for the supplier providing 100AH battery

Rating of battery: 100AH
10% of the AH rating of battery: (10/100)x 100=10
Charging current: 10 A

There is another formula for calculation of charger size

Battery Charger Sizing Calculator


The terms in this formula are:

AH (taken out from battery) – You can put the actual AH of the battery here but the word removed signifies that you only put the Ampere Hours that are removed from the battery after a particular time which depends upon how much the battery is discharged. According to Proper Charger Sizing for Utility / Stationary Battery Chargers by Art Salander “While it may be typical to use the battery AH rating, it is actually required that you only recharge what you take from a battery. It is not necessary to recharge a battery from “empty to full” if you are only using a percentage of the battery’s capacity. Other considerations that affect a battery size such as, design margin, age factor, temperature correction, etc. are not to be added in as part of the battery’s AH removed. These factors do not reflect the AH removed or discharge from the battery.”

Rf= Recharge Factor – This is a constant factor that depends upon type of battery
Wet Vented Lead Acid – 1.1, 1.15
Valve Regulated Lead Acid – 1.15
Wet Vented Nickel Cadmium – 1.3, 1.35, 1.4
Valve Regulated Nickel Cadmium – 1.4

T (desired recharge time) – This is the time you require to fully charge a battery. This value is typically 8 to 24 hours.

L (constant loads): These are the loads that are present on the battery all the time. In other words, a load that draws current all the time. For example a motor starting draws high current initially but drops to low once it goes into motion. This is the part that is a little hard to calculate as in my organization these UPSes are used in a variety of airports with different needs so it is not easy to calculate the constant loads without knowing what this UPS will be running. So I did not use this formula for calculation and stuck to the easier one above that takes into consideration only the AH of the battery.
So whether you calculate  charging current using the first formula or the second the charger should be designed to provide at least the amps you get through either of these formulas otherwise the batteries will  either not charge properly or will take a long time to charge.

Charging Time of Batteries

Finally, you can calculate charging time of batteries if you know the rating of battery and charging current using this formula

Charging Time of battery = Battery Ah / Charging Current

So to calculate the charging time for the supplier which is providing 6.5A charger for 65AH batteries, the charging time would be:

Charging Time of battery = Battery Ah / Charging Current=65AH/6.5=10 Hours

Similarly, for the supplier which is providing 10A charger for 100AH batteries, the charging time would be:

Charging Time of battery = Battery Ah / Charging Current=100AH/10=10 Hours

So in both the cases the batteries will take 10 hours to fully charge.

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