MAINTENANCE
DOE-HDBK-1084-95
Lead-Acid Storage Batteries
Batteries
Page 32
Rev. 0
Constant-Current Charging
Constant-current charging simply means that the charger supplies a relatively
uniform current, regardless of the battery state of charge or temperature.
Constant-current charging helps eliminate imbalances of cells and batteries
connected in series. Single-rate, constant-current chargers are most appropriate
for cyclic operation where a battery is often required to obtain a full charge
overnight. At these high rates of charge there will be some venting of gases.
Positive grid oxidation will occur at elevated temperatures or extended overcharge
times. Normally the user of a cyclic application is instructed to remove the battery
from a single-rate, constant-current charger within a period of time that permits
full charge yet prevents excessive grid oxidation.
Another type of constant-current charger is the split-rate charger. A split-rate
charger applies a high initial current to the cell and then switches to a low rate
based on time of charge, voltage, or both. The choice of switching method and
switch point may be affected by the relative priority of minimizing venting (early
switching) versus maintaining good cell balance (later switching). In some split-
rate chargers, the charger will alternate between the high and low rate as the
battery approaches full charge. Split-rate chargers are useful when the discharge
cannot be classified as float or cyclic, but lies somewhere between the two
applications.
Trickle Charging
A trickle charge is a continuous constant-current charge at a low (about C/100)
rate which is used to maintain the battery in a fully charged condition. Trickle
charging is used to recharge a battery for losses from self-discharge as well as to
restore the energy discharged during intermittent use of the battery. This method
is typically used for SLI and similar type batteries when the battery is removed
from the vehicle or its regular source of charging. Trickle charging is also used
widely for portable tools and equipment such as flashlights and battery powered
screwdrivers.
Avoiding Overdischarge
In order to obtain maximum life from lead-acid batteries, they should be disconnected
from the load once they have discharged their full capacity. The cutoff voltage of a
lead-acid cell is usually around 1.75 V. However, the cutoff voltage is very sensitive to
operating temperature and discharge rate. Like batteries discharged at a high rate will
have a lower cutoff voltage than those discharged at a low rate. Greater capacities are
obtained at higher temperatures and low discharge rates. The manufacturer should
specify cutoff voltages for various operating temperatures and discharge rates.
