Posted on Feb 7, 2014

Typically, most japanese motorcycles do not have any charge control lamp at all and even on bikes equipped with such a lamp (Bosch system, e. g. BMW and Moto Guzzi), defects may still occur in a manner where the `idiot light` may not light up in due time. The circuit presented here is a simple means to overcome this problem. The device is used to

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monitor the battery voltage continuously: As long as this is around 13. 6V ( ± about 0. 8V), the charging system is very probably working properly. The circuit itself is of a rather minimalistic design, with only two LED to indicate the actual voltage range. A red LED is used to signal a voltage that is too low to charge the battery (usually a sign for alternator failure), and a yellow one that indicates overvoltage (usually due to a defective voltage regulator). Thus, it is more a "voltage indicator" than a "voltmeter". Why only two LED There are many circuits available that use e. g. three, five or more LED to indicate the actual voltage. However, all I want to know is if the voltage at the battery is in the right range to keep the charge up - if yes, I do not want any control light shining at me: "No messages are good messages" ;-) When a (low) voltage is applied, LED D1 simply lights up as current flows through it; R1 serves as current limiter. If the voltage applied to the circuit reaches some 12V, the Zener diode will start to conduct. The current through the Zener diode causes a voltage drop across R2 which will open T1 and in turn will "cut off" the voltage across LED D1. At this point, no LED is lit. which should be the standard condition with a healthy charging system. If the voltage increases further (above 14. 6V), the voltage drop across R4 will be large enough to open T2, which makes LED D2 light up. While...

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