Battery warning lamp.

Hicap phill said:

What about fitting a ballast resistor?
Works on indicators.
Not up to date on LEDs

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dieseldog69 said:

As I understand it, the current going through the lamp to the alternator is used to "excite" the alternator, when the alternator is charging the lamp goes out.

Thing is, I ordered a snazzy little button lamp with the battery symbol on it but it is an LED, it won't draw much current at all, but the question is, will it be enough?

Want to try and iron out the niggles now while I wait for the unexpected parts delivery to arrive.

This is the wiring diagram I'm working to from Gwyn Lewis.

View attachment 147994

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Kwakerman said:

I assume this is for a second battery / alternator rather than changing the standard alt warning lamp in the dash?. Don't have the wiring diagram for a 200 (another assumption) but I have attached one for a 300 which should be broadly similar (ignore the glow plug bits as I know they will be different.

The ignition live comes in from the switch on the white cable, it then goes through the lamp and a diode (G126) and out to the alternator. The resistor (G101) is effectively in parallel with the lamp so if the lamp fails there is still a path for the power to get to the alternator to excite (start) it, unfortunately I'm not sure what value it should be, could be something like 100 ohms (the 1E3 on the diagram is probably a typo of the value). The diode is there to prevent the battery power drain through the alternator / lamp when not running.

Another couple of assumptions, if we ignore the resistor for the time being and assume that the lamp is rated at 1.2 watts and that it lights at the brightness (12v) when on then there will be a current of 100 milliamps flowing through the lamp and this equates to a lamp resistance of 120 ohms (which roughly matches the resistance of the resistor for when the lamp fails). This would appear to indicate that 100 milliamps is sufficient to get the alternator going, there is probably an element of spare capacity there so you may find that 50 milliamps could also be enough.

I would suggest that you just try the LED you get, it should be a 12v one (rather than the standard 2v one) as it is designed for that purpose so it should be ok, however if the current is insufficient then you should be able to add a 100 ohm resistor in parallel to provide sufficient current to get it started. The only issue with this approach is that you will also need to add an extra diode (as per G126) to prevent battery discharge through the resistor.

If you want to plan for the worst case then get hold of a 100 ohm resistor (1 watt rating should be ok for short term use) and also a 1N4007 diode, they should be ok to get you going as a start.

Hope that helps

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neilly said:

have you tried RS components?

Cheers

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neilly said:

have you tried RS components?

Cheers

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