Trickle Charging

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.