This is a simple circuit that will indicate a low voltage on a 12V lead acid battery. Many that have golf carts, small EV’s, RV’s, or solar power banks for homes have a number of 12V lead acid batteries to maintain and this circuit will help protect your investment while not using these batteries. When stored, batteries will self discharge, and if left for too long, will self-discharge to a damaging level. This circuit will light an LED when the battery voltage decreases to 11.6V. When the LED turns on, you’ know it is time to charge the battery to prevent the battery from dying on the shelf.
Only a few parts are needed in the small but useful circuit. The BC557 PNP transistor controls the lighting of the LED. The BC557 transistors’ base is biased by the 10V zener diode. As long as the battery voltage stays above 11.6V, the zener keeps the base of the BC557 transistor high. When the battery becomes discharged, the zener stops conducting and the base bias goes low, and BC557 begins conducting, and the LED alerts you to the low battery voltage. You can use a variable resistor to fine tune the low voltage indication, and/or use a set resistor when you have a good value figured out.
|RefDes||Name||Value||Part Number||Digi-Key Part Number||Description|
|R1||RESISTOR||1K||RMCF2010FT1K00||RMCF2010FT1K00CT-ND||RES 1K OHM 3/4W 1% 2010|
|R2||VARIABLE||5K||EVM-3YSX50B53||P3Y502CT-ND||TRIMMER 5K OHM 0.15W SMD|
|C1||POLARIZED||10µF||EEE-1EA100WR||PCE3895CT-ND||CAP ALUM 10µF 25V 20% SMD|
|D1||Zener||10V||MMSZ5240BS-7-F||MMSZ5240BS-FDICT-ND||DIODE ZENER 10V 200MW SOD323|
|Q1||PNP||0||BC557BTA||BC557BTACT-ND||TRANSISTOR PNP 45V 100MA TO-92|
|L1||LED||0||VLMS1300-GS08||VLMS1300-GS08CT-ND||LED SUPER RED 0603 SMD|