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Agilent AN 250-2
Lab & Industrial Power Sources
Application Note
Battery Charging/Discharging lead-acid cells, many manufacturers recommend
that the charging current be reduced as the charge
nears completion. This taper charge can be accom-
Introduction:
plished by inserting a small resistance in series
The charging and discharging of batteries has
with one of the load leads between the power sup-
become an area of careful study in the aerospace
ply and the battery being charged.
and automotive industries as well as many others.
Precise control of the charging and discharging
characteristics of batteries may be necessary in Constant Current Charging:
applications ranging from satellite design to bat- The automatic crossover between constant voltage
tery development and evaluation. and constant current exhibited by most Agilent
power supplies make them ideal for battery charg-
Although sufficient for many consumer applica- ing applications. Using this feature, a battery may
tions, the comparatively poor charging characteris- be charged at a constant current until the maxi-
tics of readily available "battery chargers" may not mum charge voltage is reached, at which point the
provide the precision necessary in many critical supply will revert to constant voltage and continue
applications. To achieve a high degree of accuracy, to supply a trickle charge current sufficient to
stability, and regulation, the DC power supply maintain full charge. (See Figure 1.) Thus, the
approach can be utilized. Battery discharging may charging operation can be unattended after pro-
be accomplished with these same DC power sup- perly setting the charging rate and the maximum
plies with little or no modification to the basic unit. charge voltage, then connecting the battery
to the output terminals of the power supply.
Several different types of battery charging and
discharging techniques using Agilent Technologies
DC power supplies will be examined in this appli-
cation note.
Charge Methods:
One of the most commonly used approaches to
charging batteries is the "constant current method"
sometimes referred to as a "slow charge" method.
This name arises from the fact that it typically
requires 14-16 hours to fully charge an Ni-Cd cell.
The charge rate for this method is typically 0.1C,
where C is the nominal ampere-hour rating of the
cell. At this charge rate, standard nickel-cadmium
sealed cells can tolerate an overcharge for extended
periods of time without damage or serious per- Figure 1. Constant Current Charging
formance degradation.
The other charging method that will be discussed
is the "taper charge" method. When charging
Protection diode CRp prevents a charged battery Battery Discharging:
from discharging into and damaging a supply if the Agilent constant voltage/constant current DC
supply is turned off. The diode should be capable power supplies are also useful when batteries must
of withstanding currents as high as IC. be discharged at a constant current. Connecting
the supply as shown in Figure 3 and following the
Constant Current Charging Procedure "battery discharging" instructions makes unat-
The procedure for setting the charging rate and full tended constant-current discharge possible. Using
charge voltage on constant voltage/constant cur- this procedure, the power supply output will auto-
rent power supplies is as follows: matically shut-off when the battery voltage reaches
a. Turn both the VOLTAGE and CURRENT controls zero; thus, preventing reverse charging of the battery.
fully counter clockwise (CCW).
b. Place a short circuit across the output terminals The Power Supply operates in the constant current
of the supply and rotate the VOLTAGE control mode, delivering an output current which is estab-
fully clockwise (CW). lished by the front panel current control. The con-
c. Rotate the CURRENT control to the desired stant current (ID) delivered by the supply forces
charging rate as read on the front panel ammeter. the battery to be discharged. The discharge rate, in
d. Rotate the VOLTAGE control fully CCW and ampere-hours, is determined by ID. The constant
remove the short circuit. current flowing through dropping resistor Rx pro-
e. Rotate the VOLTAGE control to the desired full duces a constant voltage across the resistor. Resis-
charge voltage as read on either the front panel tor Rx is selected such that its IR drop is larger than
voltmeter or a more precise DVM. Remember to the initial battery voltage (EB). This prevents the
set the voltage 0.7 volts more than the required supply from sinking any current by keeping the
full charge voltage to compensate for the drop output voltage positive. The minimum output volt-
across diode CRp. The unit may then be con- age from the supply will be (IDRX)◦ Jabse Service Manual Search 2026 ◦ Jabse Pravopis ◦ onTap.bg ◦ Other service manual resources online : Fixya ◦ eServiceinfo