Charger with temperature compensation. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Charger with temperature compensation. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Chargers, batteries, galvanic cells

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The charging characteristics of lead-acid batteries are highly dependent on the ambient temperature. To prevent undercharging or overcharging batteries under conditions of prolonged exposure to low or high temperatures, it is advisable to use temperature compensated chargers.

The proposed circuit is assembled on a linear regulator with a low voltage drop across the regulating element. It includes a temperature compensation circuit, has two charge modes, a "true" ground bus and does not consume current in standby mode. The integrated linear regulator LT1086 is used to regulate the charge voltage and limit the maximum charge current.

(click to enlarge)

If the input supply voltage is turned off, transistors Q1 and Q2 turn off, all possible circuits for discharging the battery through the charger output to ground are broken, which ensures zero current consumption in the off state. Diode D1 protects the integrated regulator from reverse current if the input voltage is less than the battery voltage or if the input terminals are accidentally shorted. The thermal compensation used in this circuit corresponds to the temperature characteristic of a lead-acid electrochemical cell and is provided by an RTH thermistor, branded as Tempsistor. A resistor R3 is connected in parallel to it. The transfer coefficient of the resistor divider of the stabilizer depends on temperature changes.

The compensation accuracy is better than ±50 mV for a 12 V battery in the temperature range from -10 to +60 °C. It is advisable to place the Tempsistor element directly below the battery, which is placed on a rectangular piece of foam rubber. The choice of one of the two charge modes is performed by the LT1012 comparator, with the help of which the charge current value is estimated from the voltage drop across the current sensor resistor Rs. When the current exceeds 10 mV/Rs, the fast charge mode is activated and the voltage will be 14,4 V at 25°C. When the charging current is below this threshold, the trickle charge voltage will be 13,8V.

Stabilized charger with a charge current of at least °C / 4

Емкость batteries, A/h	Chip	Maximum charge current, A	current threshold charge, mA	current sensor (shunt), Ohm
<3	LT1117	0,8	20	0,5
3-6	LT1086	1,5	50	0,2
6-12	LT1085	3,2	100	0,1
12-24	LT1084	5,5	200	0,05
24-48	LT1083	8,0	400	0,025

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