Circuit for battery type Gel Charger using lm117
Battery charging is another application uniquely suited for the LM117.
Since battery voltage is dependent on electro-chemical reactions, the charger must be designed specifically for the battery type and number of cells. Ni-Cads are easily charged with the constant current sources shown previously. For float chargers on lead-acid type batteries all that is necessary is to set the output of the LM117 at the float voltage and connect it to the battery. An adjustable regulator is mandatory since, for long battery life the float voltage must be precisely controlled. The output voltage temperature co-efficient can be matched to the battery by inserting diodes in series with the adjustment resistor for the regulator and coupling the diodes to the battery.
A high performance charger for gelled electrolyte lead-acid batteries is shown in schematic. This charger is designed to quickly recharge a battery and shut off at full charge.
Initially, the charging current is limited to 2A by the internal current limit of the LM117. As the battery voltage rises, current to the battery decreases and when the current has decreased to 150 mA, the charger switches to a lower float voltage preventing overcharge. With a discharged battery, the start switch is not needed since the charger will start by itself; however, it is included to allow topping off even slightly discharged batteries
When the start switch is pushed, the output of the charge goes to 14.5V set by R1, R2 and R3. Output current is sensed across R6 and compared to a fraction of the 1.2V reference (across R2) by an LM301A op amp. As the voltage across R8 decreases below the voltage across R2, the output of the LM301A goes low shunting R1 with R4. This decreases the output voltage from 14.5V to about 12.5V terminating the charging. Transistor Q1 then lights the LED as a visual indication of full charge.