Data Sheet

UC10.241, UC10.242
UC-Series
24V, 15A, CAPACITOR-BASED DC-UPS
23.9. WHAT DO KJ AND KWS MEAN?
The units kJ (kilo Joule) or kWs (kilo Watt seconds) is used for specifying the installed storage capacitor size.
1 kJ = 1 kWs = 1000Ws
The stored energy of a capacitor can be calculated with the following formula:
Energy
(
Ws
)
=
C
*
U
2
2
For example: A 350F (Farad) capacitor which is charged to 2.5V has the following energy
26/27
* 2 . 5
E
=
350
F
V
2
2
=
1093
Ws
=
1
. 09
kWs
E =
C *(U1
2
− U2
2
)
During discharging of the capacitor, the voltage decreases with the amount of discharge. A boost converter is needed
to generate a stable output voltage. The boost converter needs a minimum input voltage (cut-off voltage), which
reduces the amount of energy. The cut-off voltage usually depends on the load current, the lower the load current,
the lower the cut-off voltage.
Considering this voltage range, the energy can be calculated with the following formula (full charge voltage= U1=
2.5V, cut-off voltage= U2= 1V):
=
350 * (2.5
2
−1
2
)
= 919Ws = 0.92kWs
2 2
The energy which can be used for the 24V loads is further reduced by the efficiency of the boost converter.
The UC10.241 has a rated capacitor size of 6kWs and the UC10.242 12kWs. This is the energy which can be used for
24V load at low load currents.
Calculation of the needed energy:
Example: A power of 45W is needed for 2 minutes:
=
45
=
P
*
E
t
W
* 120
s
=
5400
Ws
=
5 .4
kWs
Always check with the buffer time curve (see Fig. 6-1) if the load
can be powered for the required period of time!
May 2016 / Rev. 2.2 DS-UC10.241-EN
All parameters are specified at 24V, 10A output current, 25°C ambient and after a 5 minutes run-in time unless otherwise noted.
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