MAX762 Solar Sealed Lead-Acid Battery Charger

Sealed lead-acid batteries have a lot of internal energy loss, which is a serious problem. In some cases, the battery may completely lose its charging capacity after a few weeks without trickle charging current.

The circuit in the figure below can prevent this from happening. Two groups of 3V solar panels are connected to diodes and bypassed by diodes when they are not generating power. They are used to power the MAX762 boost converter IC1. The 762 is a 15V output version of the MAX761 (12V output) series, and is used here to boost 6V to 15V. C1 and C2 are decoupling capacitors to suppress the high and low frequency stray interference components generated by the switching type voltage regulator IC. Using Schottky diode D3, energy is stored in the form of a magnetic field in inductor L1. The internal switching signal opens the pin 7 of IC1, and the stored energy is converted to the 15V output of the circuit. The V+ (sense) input of the MAX762 pin 8 is used to keep the output voltage at 15V. C4 and C5 filtering make the output voltage ripple as small as possible. LED D4 and pushbutton switch S1 allow you to check if there is 15V output voltage.D5 and D6 reduce the 15.V voltage to about 13.6V, which is the usual rated trickle charge voltage for sealed lead-acid batteries. This matches the IC's maximum internally limited 120mA output current very well.

The size of inductor L1 is not very critical, 22μH or 47μH will work well. The rated current of the coil is not less than 1A. The switching frequency is about 300kHz.