The table above shows the time needed to recharge
following a 100% DOD cycle, depending on the
maximum charge current available. If the DOD is less
than 100%, the total recharge time will be less. To
prevent overcharge, the duration from T1 to T2 (i.e.
the time at 2.41 Volts/Cell) should always be limited to
4 hours, and then the rectifier voltage returned to the
recommended float voltage.
If the battery temperature increases above +25°C (+77°F),
it is recommended that a thermal compensation of -2
mV/cell/°C be applied to the boost voltage. Conversely,
if the temperature decreases below 25°C, the boost
voltage should be increased by 2 mV/cell/°C. The graph
below shows values for an individual cell:
2.4
Recharge Power
Depending on the charging system, recharge times can
vary greatly following an outage. If the recharge power
is relatively low, it will take longer to fully recharge
a battery. It is recommended to provide a regular
overcharge factor of between 103% and 106% SOC after
a discharge.
The following chart shows the time required to fully
recharge an NSB UPS battery from a complete 100%
DOD discharge, as a function of rectifier voltage and
available current.
2.5
Ripple Current
Unacceptable levels or ripple current from the charger
or the load can have a major impact on battery internal
heat generation, cause permanent damage, and reduce
service life. It is recommended to limit the continous
ripple current to a maximum of 5 amperes per 100 Ah
rated battery capacity.
3 Discharge
3.1
Storage and Self Discharge
During storage, lead-acid batteries will gradually self-
discharge. It is recommended that battery OCV be
maintained above 12.1V at all times in order to avoid
irreversible capacity loss. The rate of self-discharge
increases with increasing temperature. For example, a
battery at 25°C will drop from 90% to 60% in 15 months,
whereas the same unit at 40 °C will take just 6 months. As
a result, maintenance charging needs to be performed
more frequently at higher temperatures.
3.2
End of Discharge Voltage (EODV)
In the event of a deep discharge, a recommended
minimum end-of-discharge voltage (EODV) should be
used to avoid over discharge. A value of 1.80 Volts/cell
is common, but this value should be adjusted based on
the discharge rate. The following table provides a guide
for EODVs at various discharge rates.
Discharge Rate (hours)
EODV (Volts/Cell)
20
1.85
10
1.80
5
1.75
1
1.70
.25 (15 min.)
1.67
EODV can be controlled using a Low Voltage Disconnect
(LVD) as part of the load circuit.
Form: SES-541-01711
Issued: 12-08-16
ECO-101191
