4
Cell strings connected in parallel
Using constant voltage chargers and ensuring that the
connections made between the charger and the batteries have
the same electrical resistance, no special arrangements have to
be made for batteries in parallel.
Although no special circuit arrangements are required, where the
parallel connection is made at the charger or distribution board, to
avoid out of step conditions, the bus bar run length and the area of
cross section should be designed so that the circuit resistance
value for each of the strings is equal within limits ± 5%.
Charging
Float voltage
The recommended float charge voltage is 2.28 volts per cell at
20°C.
Following a commission charge and after 6 months continuous
charge at the recommended float voltage, individual cell voltages
will stabilise within ±4.5% of the mean applied voltage.
However, immediately following commissioning and for the initial 6
months of continuous float charge, individual cell voltage values
outside the above tolerance may be observed without adverse
effect. There is no relationship between a cell’s float voltage and
its discharge capacity. Cells are perfectly capable of giving their
discharge capacity even when outside the ±4.5% range.
After 6 months service, should any individual cell or monobloc
show a continuing reduction or increase in voltage outside the
above limits over 3 successive monthly periods, Enersys should be
contacted for advice.
When the average ambient temperature deviates more than ±5ºC
from the reference temperature (20°C), it is necessary to adjust
the float voltage as follows:
Temperature
Float voltage range per cell
0°C
2.33-2.36V
10°C
2.30-2.33V
20°C
2.27-2.30V
25°C
2.25-2.28V
30°C
2.24-2.27V
35°C
2.22-2.25V
40°C
2.21-2.24V
45°C
2.19-2.22V
50°C
2.18-2.21V
If the charger does not permit an adjustment of float voltage in
relation to the temperature, it is possible to set a float voltage
value according to the temperature ranges as indicated in the last
table of this publication.
Charging current
A discharged VRLA battery will accept a high recharge current,
but for those seeking a more economical charging system a
current limit of 0.08 C
10
or 0.1 C
3
(A) is adequate.
Fast recharge
Increasing the charge voltage to 2.40Vpc with a current limited to
0.1 C
10
or 0.125 C
3
(A) can reduce recharge times. Fast charge
should be stopped after approximately 10 to 15 hours. In order to
achieve a normal service life, this charge regime must not be used
more than once per month.
Float charge ripple
Excessive ripple on the D.C. supply across a battery has the effect
of reducing life and performance.
It is therefore recommended that voltage regulation across the
system, including the load but without the battery connected,
under steady state conditions, shall be better than ±1% between
5% and 100% load.
State of charge
The battery state of charge can be determined approximately by
measuring the open circuit voltage after the battery has been at
rest for a minimum of 24 hours at 20°C.
Voltage
State of charge
2.14Vpc
100%
2.10Vpc
80%
2.07Vpc
60%
2.04Vpc
40%
2.00Vpc
20%
Open circuit voltage variation with temperature is 2.5mV per 10°C.
Discharging
PowerSafe VX batteries must not be left in a discharged condition
after supplying the load but must immediately be returned to float
recharge mode.
Failure to observe these conditions may result in greatly reduced
service life.
Publication No: EN-VX-IOM-001 February 03
www.enersysinc.com
