UB10.245

U–Series

event, the process is completed much faster since only the energy which was taken out of the battery needs to be

“refilled”. For this reason, the buffer time cannot be calculated using the Ah capacity value.

The equation “I x t” = capacity in Ah generally leads to incorrect results when the discharging current is higher than

C20 (discharging current for 20h). The battery datasheet needs to be studied and a determination of the expected

buffer time can be made according to the following example:

Example how to determine the expected buffer time for other battery types and battery sizes:

Step 1

Check the datasheet of the battery which is planned to be used and look for the discharging curve.

Sometimes, the individual discharging curves are marked with relative C-factors instead of current

values. This can easily be converted. The C-factor needs to be multiplied with the nominal battery

capacity to get the current value. E.g.: 0.6C on a 17Ah battery means 10.2A.

Fig. 9-2 Typical discharging curve of a typical 17Ah battery, curve taken from a

manufacturer’s datasheet

9/24

Step 2

Determine the required battery current. Use Fig. 8-1 “Battery discharging current vs. output current” to

get the battery current. Fig. 8-1 requires the average voltage on the battery terminals. Since there is a

voltage drop between the battery terminals and the battery input of the DC-UPS, it is recommended to

use the curve A or B for output currents > 3A or when long battery cables are used. For all other

situations, use curve C.

Step 3

Use the determined current from Step 2 to find the appropriate curve in Fig. 9-2. The buffer time

(Discharging Time) can be found where this curve meets the dotted line. This is the point where the DC-

UPS stops buffering due to the under-voltage lockout.

Step 4

Depending on Fig. 9-2, the buffer time needs to be reduced to take aging effects or guaranteed values

into account.

Example:

The buffer current is 24V 7.5A and a battery according Fig. 9-2 is used. The cable between the battery and the DC-UPS

is 1m and has a cross section of 2.5mm

Answer:

According to Fig. 8-1, the battery current is 18A. Curve A is used since the battery current is > 3A and the

length of the cable is one meter.

According to Fig. 9-2, a buffer time (Discharging Time) of 30 Minutes can be determined. It is recommended

to reduce this figure to approximately 24 minutes for a guaranteed value and to cover aging effects.

June. 2010 / Rev. 1.1 Datasheet DS-UB10.245-EN

All parameters are specified at an input voltage of 24V, 10A output load, 25°C ambient and after a 5 minutes run-in time

unless otherwise noted. It is assumed that the input power source can deliver a sufficient output current.

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