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Charger extends lead-acid-battery life

Fran Hoffart, National Semiconductor Corp, Santa Clara, CA; Edited by Paul Rako and Fran Granville -December 01, 2011

Design Ideas Classics
Originally published in the February 7, 1985, issue of EDN


A circuit that properly charges sealed lead-acid batteries ensures long, trouble-free service. Fig 1 is one such circuit; it provides the correct temperature-compensated charge voltage for batteries having from one to as many as 12 cells, regardless of the number of cells being charged.

Charger extends lead-acid-battery life figure 1

The Fig 1 circuit furnishes an initial charging voltage of 2.5V per cell at 25°C to rapidly charge a battery. The charging current decreases as the battery charges, and when the current drops to 180 mA, the charging circuit reduces the output voltage to 2.35V per cell, floating the battery in a fully charged state. This lower voltage prevents the battery from overcharging, which would shorten its life.

Read more design ideasThe LM301A compares the voltage drop across R1 with an 18-mV reference set by R2. The comparator’s output controls the voltage regulator, forcing it to produce the lower float voltage when the battery-charging current passing through R1 goes below 180 mA. The 150-mV difference between the charge and float voltages is set by the ratio of R3 to R4. The LEDs show the state of the circuit.

Design Ideas Classics bookTemperature compensation helps prevent overcharging, particularly when a battery undergoes wide temperature changes while being charged. The LM334 temperature sensor should be placed near or on the battery to decrease the charging voltage by 4 mV/°C for each cell. Because batteries need more temperature compensation at lower temperatures, change R5 to 30Ω for a TC of −5 mV/°C per cell if your application will see temperatures below −20°C.

When the circuit charges more than six cells, the additional voltage across the LM334 increases self-heating, so use a small heat sink and increase the resistance of R6. Likewise, use higher resistances in series with the LEDs to avoid overloading the LM301A.

The charger’s input voltage must be filtered dc that is at least 3V higher than the maximum required output voltage: approximately 2.5V per cell. Choose a regulator for the maximum current needed: LM317 for 1.5A, LM350 for 3A, or LM338 for 5A. At 25°C and with no output load, adjust R7 for a VOUT of 7.05V, and adjust R8 for a VOUT of 14.1V.

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