I think I should be able to grasp something from this question but I'm not sure if my case is the same or not. Would be great if I can get enlightment on this.

In my case I'm replacing incandescent bulb with a 12V (resistor-equipped) LED chip of the same socket. The lamp is for battery charge indicator (+ - battery icon that glows when ignition is ON but engine is not running then off when engine is running unless there's problem with battery charging)

I think there is some direct connection between the bulb and alternator; then this makes me think I should be more cautious when fixing this problem. I also guess every <=90s car has the same problem: we should not use LED bulbs to replace incandescent bulb just for battery charging indicator.

The LED is dimly lit when it is supposed to be off during engine running, but totally off when the engine is not running and ignition is not in ON position.

Should I use bleeder resistor? how to measure how much resistance I should use? or should I put inline resistor before the indicator LED?

If I should get answer from the aforementioned link, please kindly let me know. Thankyou for any inputs.

  • 1
    \$\begingroup\$ Yes, it's likely the same problem, and the same solution. \$\endgroup\$
    – Dampmaskin
    Sep 18, 2019 at 12:43
  • \$\begingroup\$ hi, thank you for your confirmation. But now I'm considering the answer that battery warning should not be replaced with LED, but that's more into automotive so I'll learn more about it in different forum topic. Since it's the same problem and solution, I'll just assume if I ignore the car-related concerns I'll just need to put a bleed resistor and figure out the resistance to stop the LED dimly lit when the light is supposed to be off. \$\endgroup\$ Sep 19, 2019 at 3:11

1 Answer 1


You should not replace the battery warning light with an LED.

The battery warning light is connected between the ignition supply and a terminal on the alternator, and it serves two functions. First, it provides a current to the alternator to drive the field coils and allow it to generate enough output to power the car's electrical system and charge the battery. Second, it warns you when the ignition circuits are turned on but the battery is not being charged (most commonly when the ignition is on but the engine is not running, but also in cases such as the drive belt breaking).

Replacing the light with an LED will not supply enough current to the alternator to allow it to function correctly. You must use the correct filament lamp as originally specified for the make and model.

  • \$\begingroup\$ Thankyou for the answer. I guess I was right to be more cautious when doing this. \$\endgroup\$ Sep 19, 2019 at 3:37
  • \$\begingroup\$ So if I must replace this with an LED bulb I think I need to: detach the bulb wiring to a separate socket (so the incandescent bulb runs normally like from factory) then make the bulb to drive a transistor turning on the LED battery indicator. What do you think on this? \$\endgroup\$ Sep 19, 2019 at 3:45
  • \$\begingroup\$ You could probably just put the LED bulb in parallel with the filament lamp - but why must you replace it at all? It's a lot of work for no benefit whatsoever and you may lose the ability to determine whether the warning light is actually working. \$\endgroup\$
    – Finbarr
    Sep 19, 2019 at 9:54
  • \$\begingroup\$ I didn't think of that, I keep overthinking everything. I might try that, but what do you mean by lose the ability to determine whether the warning light is actually working, wouldn't the LED turn on and off with the incandescent bulb if I put them in paralel? thankyou for your help \$\endgroup\$ Sep 24, 2019 at 3:30
  • \$\begingroup\$ LEDs can fail as well. You probably can't fit the bulb and LED into the same holder so you can only see the LED. I ask again, why do you seem determined to do something that's such a bad idea? \$\endgroup\$
    – Finbarr
    Sep 24, 2019 at 8:59

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.