Guide to Choosing the Right Battery Size

How to choose the right back up unit size: To choose the right battery size the following method is suggested. The 3 main requirements are, Voltage, Amps required and for how long in Hours.

• Volts: Nominate the Voltage required, say 24V, remembering that 24Vdc is the nominal voltage and that the charge Volts and battery Volts will vary (battery and charge volts for 24V will vary ffrom 18V to 28V). Please enquire for all other voltages available.

• Amps: The current required by the load in Amps.

• Time: The amount of time required for the power to be supplied to the load so that it can function well. Number of hours or part there of. Using the typical, Battery Available Ahr Output chart below the following example will help. Please consult battery manufactures specifications as data may vary from this chart. A 24Vdc system needs to be backed up for approx. for 8 hours and has a load of 2 Amps.

1. Multiply Amp x Hours (8 x 2 = 16). This makes it 16Ahrs

2. Look at the chart, at the time scale, in this case it is 8 Hrs, follow that line up and observe that it is approx. 90% of the 20 hour rated battery.

3. 16 ÷ 0.9 = 17 approx. This is the Ahr battery at the 20hr rate that is required. 2A for 8Hrs at 24Vdc requires a 17Ahrs battery backup unit.

The current time product available from a particular battery is dependent on a number of factors. The most important of these is the time over which the current will be drawn. As this time gets smaller the product of the current and time is reduced.

Batteries are usually rated in Ampere-hours based on the total available energy when used over 20-hour period. (Higher capacity batteries are sometimes rated in Ampere-hours over a 10 hour period).

The curve above shows the probable available Ampere-hour capacity for varying time periods over which the battery is drained. This is fairly indicative of commercially available sealed lead acid batteries. The battery manufacture's data should be consulted for precise figures.

Other factors are end voltage and temperature.