MAINTENANCEDOE-HDBK-1084-95Lead-Acid Storage BatteriesBatteriesPage 32Rev. 0 Constant-CurrentChargingConstant-current charging simply means that the charger supplies a relativelyuniform current, regardless of the battery state of charge or temperature. Constant-current charging helps eliminate imbalances of cells and batteriesconnected in series. Single-rate, constant-current chargers are most appropriatefor cyclic operation where a battery is often required to obtain a full chargeovernight. At these high rates of charge there will be some venting of gases. Positive grid oxidation will occur at elevated temperatures or extended overchargetimes. Normally the user of a cyclic application is instructed to remove the batteryfrom a single-rate, constant-current charger within a period of time that permitsfull charge yet prevents excessive grid oxidation.Another type of constant-current charger is the split-rate charger. A split-ratecharger applies a high initial current to the cell and then switches to a low ratebased on time of charge, voltage, or both. The choice of switching method andswitch point may be affected by the relative priority of minimizing venting (earlyswitching) versus maintaining good cell balance (later switching). In some split-rate chargers, the charger will alternate between the high and low rate as thebattery approaches full charge. Split-rate chargers are useful when the dischargecannot be classified as float or cyclic, but lies somewhere between the twoapplications.TrickleChargingA 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. Tricklecharging is used to recharge a battery for losses from self-discharge as well as torestore the energy discharged during intermittent use of the battery. This methodis typically used for SLI and similar type batteries when the battery is removedfrom the vehicle or its regular source of charging. Trickle charging is also usedwidely for portable tools and equipment such as flashlights and battery poweredscrewdrivers.AvoidingOverdischargeIn order to obtain maximum life from lead-acid batteries, they should be disconnectedfrom the load once they have discharged their full capacity. The cutoff voltage of alead-acid cell is usually around 1.75 V. However, the cutoff voltage is very sensitive tooperating temperature and discharge rate. Like batteries discharged at a high rate willhave a lower cutoff voltage than those discharged at a low rate. Greater capacities areobtained at higher temperatures and low discharge rates. The manufacturer shouldspecify cutoff voltages for various operating temperatures and discharge rates.
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