Motorcycle Battery Level Indicator

Motorcycle Battery Level Indicator

This battery level indicator offers (2) incandescent lamps that light up progressively as the voltage increases. It is an adaptation of the 12V battery level indicator and is suited for motorcycle application. The reference voltage is from a 6.2V zener regulator and an LM2903 dual comparator switches the outputs. High current 2N4403 transistors drive the lamps.

Threshold voltages

  • PL2: Greater than 12.5V (50%)
  • PL1: Greater than 13.5V (100%)


Motorcycle Battery Level Indicator

Motorcycle Battery Level Indicator BOM

Motorcycle Battery Level Indicator BOM


This circuit was suggested by a reader. His requirements are as follows:
Two threshold voltages (12.5 & 13.5V)
Incandescent lamps (approx 70mA @ 12V)
Fuse protection

Circuit Operation

D1 is the voltage reference zener. Tied to this is a string of divider resistors (R2-5) that set the two fixed voltage levels. R6 & 7 form a voltage divider to that reduces the battery voltage by a factor of 3. U1 is an LM2903 dual comparator that compares the voltages from the two dividers. To obtain the 80mA lamp drive capability, the comparator open collector output drives a PNP emitter follower transistor (actually a composite transistor connection, considering the open collector transistor of the IC).


Adjust R2 for 4.5V at U1-3, and then check for 4.17V at U1-5. U1-2 should run at Vbat /3.

What I learned

Something is learned from virtually every project and this one was no exception. The first was that incandescent lamps draw sufficient current to cause a ground loop voltage between the power supply and the indicator. This caused oscillation that resulted in a sloppy threshold –yes, comparators can oscillate! Adding C1 & C2 corrected this problem. Second, I learned that my ancient HP has significant output resistance when the meter is set to measure output current (actually the meter shunt resistance) –this too caused a sloppy threshold. Third, is that incandescent indicator lamps are all but obsolete –this is due the tremendous influence of LEDs. I really had to dig through my junk to locate two incandescent indictors.

Fuse protection (no fuse required)

I opted to make it short-circuit proof by adding a resistor in series with the positive rail of U1. As a result, even if the IC shorts (as they sometimes do), R8 supports all the voltage safely as it runs within its 250mW power rating. D2 provides reverse polarity protection in which the 70mA reverse current is blocked so it cannot destroy the IC. Everything else has series resistors that offer protection, and the incandescent lamps also have a finite series resistance.


For those who wish to use LEDs

LEDs are now the indicators of choice. To adapt the board for LEDs, remove R9, R10, Q1, Q2, PL1, PL2. Connect the LEDs between the comparator outputs and the cathode of D2 –use an appropriate LED series dropping resistor such as 1K or so. The actual resistor value may be selected for desired brightness.

One Comments

Join the conversation!

Error! Please fill all fields.
  • Mark_The_Circuit_Noob

    Greetings Mr. Keith–Thanks for drawing this up–I really appreciate it.

    One question, but first an apology; I failed to note your presumption that I was utilizing incandescent lamps. I’m not surprised considering the power draw that I requested. Sorry–I should have caught that in your previous post.

    A brief background so that my request makes more sense:

    The two panel lights that I wish to use will be mounted in the top a a headlight housing. The original lights were small incandescent lamps mounted under glass lenses that point more or less perpendicular to my line of sight while on the bike. Testing their function in direct sunlight proved them to be essentially invisible in use. I replaced them with the brightest LEDs that I could find for that particular socket, but they were still not visible in direct sunlight.

    I was able to find two LED-tipped bolts that fit well in place of the original lenses. I frosted the glass domes of the LEDs with etching solution so they have a nice 180 degree glow rather than a directional beam. Bright enough to see in sunlight, but not distractingly bright at night. Easily visible without taking my eyes off the road.

    So my concept is to utilize a yellow (60mA)LED at the 12.5 volt threshold as a battery status lamp with the ignition on or the bike running. A green (70mA)LED at the 13.5 volt threshold will allow me to monitor the charging system while riding. The alternator/rectifier is supposed to provide 13.5 volts minimum @ 5000 RPM (and approximately 14.4, I believe, at lower RPMs). This lamp will let me know if the charging system performance falls below factory specifications.

    Sorry to be so long winded, on to my question:

    Because of the high power draw of these two LEDs, should I use this schematic as it is, or should I remove R9, R10, Q1, Q2, PL1, and PL2 and substitute a resistor for each LED?

    Thanks for your help–it’s always refreshing when people share their knowledge just for the sake sharing!