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LEAD_ACID_BATTERY_TEMPERATURE_COMPENSATED_CHARGER

2014-11-11 15:31  
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LEAD_ACID_BATTERY_TEMPERATURE_COMPENSATED_CHARGER
The charging characteristics of lead-acid cells are strongly linked to the ambient temperature. To prevent over-or undercharging of the battery during periods of extended low or high temperatures, temperature compensation is desirable. This circuit incorporates a low-dropout linear regulator, temperature compensation, dual-rate charging, and true negative ground, and consumes zero standby current. The LT1086 linear regulator is used to control the charging voltage and limit maxi-mum charging current. If the input supply is removed, Q1 and Q2 turn off and all charger current paths from the battery to ground are interrupted, resulting in zero shutdown current draw. Diode D1 provides reverse current flow protection for the regulator should the input fall below the battery voltage or be shorted. The temperature compensation employed in this circuit follows the true cur-vature of a lead-acid cell. Temperature compensation is provided by RTH, which is a Tempsistor, in parallel with R3. Changes in temperature alter the resistor divider ratio of the regulator. The match is within 100 mV for a 12-V battery over a range of -10 to +60℃. The best location for the Tempsistor is directly under the battery, with the battery resting on a pad of Styrofoam. Dual-rate charging is implemented by comparator LT1012, which senses the charging current through current-sense resistor RS. When the current is greater than 10 mV/RS, the high-rate charging voltage is 14.4 V at 25℃; when the current is less than this threshold, the float charging voltage is 13.8 V.