© Siemens AG, 2009
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6. Battery Modules
The maximum backup time cannot be calculated from the Ah capacity. The obvious forumula -
"I x t = capacity in Ah" or "Backup time t = capacity in Ah : charging c current" generally leads to incorrect
results.
The formula applies only in case of very low discharge currents of a maximum of 0.05C (i.e. 0.16A for a 3.2Ah
battery module, 0.35A for a 7Ah battery module, or 0.6A for a 12Ah battery module); in this case, the theoretical
discharge time of 20 hours still results here. (See the characteristic field in Diagram 6. Discharge characteristic
0.05C does not fall to approximately 21.5 VDC until 20 hours have passed.)
At 0.1C, it is only 9 hours according to the characteristic (instead of the theoretical 10 hours); at 1C, it is only
approximately 22 minutes according to the characteristic (instead of the theoretical one hour or 60 minutes); at
3C, for example, it is only 1 to 2 minutes (instead of the theoretical 1/3 hour or pprox.0 minutes). The cause for
this is the charging current dependent voltage drop at the inner resistance of the battery (at the 10-fold charging
current, for example, the approximately 10-fold voltage of the inner resistance of the battery drops) and the
extremely "non-linear chemistry" of the battery at loads between 2C and 3C (see the characteristic field in
Diagram 6). At charging currents between 0.05C and 2C, the battery voltage (already during the first second of
the load) falls relatively linear to the charging current to 25.1V at 0.05C and to 23.7V at 2C. The difference
between 1C (immediate fall to 24.3V) and 2C (immediate fall to 23.7V) amounts to only 0.6V. Between 2C
(immediate drop to 23.7V) and 3C (immediate drop to 21.6V) the extreme non-linearity does not appear. The
voltage does not drop by a further 0.6V (as in case of 1C to 2C), but rather by a whole 2.1V ! A further increase
of the charging current from 3C to 5C does not lead to an extreme drop in the battery voltage (by at least
several more volts), but rather just to and additional voltage drop of 0.2V (21.6V at 3C, 21.4V at of 5C.
The "battery chemistry" is thus extremely non-linear. For this reason, we recommend that the backup time be
determined using the characteristic field (Diagram 6). The selection table (Table 11) can be used to make a
rough determination of the battery module.
6.1 Battery Module Selection
Charging
Current
Battery module
1.2 Ah
Battery module 3.2
Ah
Battery module
7 Ah
Battery module
12 Ah
Battery module 2.5
Ah (high-
temperature)
(6EP1935-6MC01) (6EP1935-6MD11) (6EP1935-6ME21) (6EP1935-6MF01) (6EP1935-6MD31)
1 A
30 min
2.5 h
6 h
10 h
2 h
2 A
11 min
45 min
2.5 h
4 h
45 min
4 A
2 min
20 min
45 min
2.5 h
20 min
6 A
1 min
10 min
30 min
1 h
13 min
8 A
-
4 min
20 min
40 min
9 min
10 A
-
1.5 min
15 min
30 min
7 min
12 A
-
1 min
10 min
25 min
5.5 min
14 A
-
50 s
8 min
20 min
4.5 min
16 A
-
40 s
6 min
15 min
4 min
20 A
-
-
2 min
11 min
-
- Not permitted
Table 11: Selection table for battery modules and mains failure backup times
Note:
During the determination of the mains failure back times, the discharge duration of completely discharged battery modules
until the battery voltage drops was based on 21V and an ambient temperature of 25°C.
