Hi, I am new to this forum and hope someone can help me. I have restored an old 1959 motorbike and would like to install some kind of warning LED to show battery discharge and would go off on charging. The bike has a small ammeter but it is vey hard to see when pointer is around zero if battery is actually charging or discharging. The 6 volt battery is charged by a single phase alternator through a full wave rectifier and 6 volt regulator. I thought of maybe making a circuit with a zener that indicated when battery is over say 6.2 volts. This would still not indicate if battery is charging or discharging.
Any help would be appreciated. I am not an expert in electronics but have had experience building simple circuits.

 

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The circuit above will light the LED when the battery voltage is above* a given level.
If, instead, you want to indicate when the battery is actually being charged you could try the circuit in the dotted box as below:

This attaches to your bike wiring at A (the bridge rectifier +ve output, regulator input), B (the IGN switch, switched side) and C (ground). Depending on your alternator characteristics you may need to tweak resistor values.

Edit: * below.

 

The circuit above will light the LED when the battery voltage is above a given level.

I disagree. The TL431 dumps current when it's sensor input is above 2.5V. My circuit has the LED on until the voltage exceeds the threshold defined by the resistors. The circuit above will light the LED when the battery is BELOW a certain level.

 

Linear Technology and Maxim make high-side current monitor chips, some with the metering shunt built-in. These can tell you which way current is moving and how much, in case you want to have more of a display than a single LED.

ak

 

I disagree.

Quite right. My bad.

 

View attachment 114934

View attachment 114934

Thank you for your quick response. I will try this circuit first.

 

The circuit above will light the LED when the battery voltage is above a given level.
If, instead, you want to indicate when the battery is actually being charged you could try the circuit in the dotted box as below:
View attachment 114935
This attaches to your bike wiring at A (the bridge rectifier +ve output, regulator input), B (the IGN switch, switched side) and C (ground). Depending on your alternator characteristics you may need to tweak resistor values.

Thank you Alex_t. This looks good thouugh I will have to change it around a bit as bike is pos earth.

 

bike is pos earth.

I was going to take a bow for guessing what you want in one try, but then you say it's a positive ground.
Can you convert the TL431 circuit by yourself?
(All you have to do is take the ground symbol off the bottom of the drawing and move it to the top, left corner.)

 

I was going to take a bow for guessing what you want in one try, but then you say it's a positive ground.
Can you convert the TL431 circuit by yourself?
(All you have to do is take the ground symbol off the bottom of the drawing and move it to the top, left corner.)

It is a simple circuit so should have no trouble changing polarity thank you. On looking closer as the current sensing circuit supplied by Alex_t, I see a problem as it has a connection between the rectifier and regulator. I cannot access this point as the rectifier and regulator are both in one sealed unit.

 

Here's a positive-earth version of my circuit, which connects at A to the alternator output (rectifier input), at B to the ignition switch (switched side) and at C to ground :

 

Here's a positive-earth version of my circuit, which connects at A to the alternator output (rectifier input), at B to the ignition switch (switched side) and at C to ground :
View attachment 114970

Thank you Alec, I will try that one.
Cheers
Lloyd
NZ

 

Hi Alec, I built your circuit on a small piece of Vero board and works well - thank you. The LED still glows faintly when motor is running. I guess Q1 is not turning off fully. I don't understand the rest of the circuit to attempt changing any components, but it is OK as is as faint glow of LED is not visible when out on the road in day time and I never use the bike at night.

 

Good to hear it's working. You could try doubling the value of C1, or connecting a 1k resistor across the LED, to reduce the faint glow.

 

would like to install some kind of warning LED to show battery discharge and would go off on charging.

depending on the levels of discharging you wish to activate on, an led + a resistor will do that.