One for the electrical wizards:

[doug748]

I'm having problems with my bow light.

I am measuring 12.5v at the holder, which is managing to just about light the LED bulb - if you look at it very close. This is a 1v drop from the board which needs looking at. Nevertheless, on the bench I have a very weak motorbike battery at 11.7v and it lights the LED fine.
Back on the boat, I have tried two different lamp fittings and three different bulbs but still the same result.

Any suggestions?

.

 

[Aeolus]

At a guess, it's a very poor connection somewhere on the way to the bulb holder. It can achieve 12.5v with no load but once you apply a load, it can't draw the necessary power.

 

[Sticky Fingers]

Two things to check / try. Is the polarity correct? Does another 12v light work OK?

 

[geem]

At a guess, it's a very poor connection somewhere on the way to the bulb holder. It can achieve 12.5v with no load but once you apply a load, it can't draw the necessary power.

Agreed, something is very high resistance.

 

[Sticky Fingers]

Doesn’t explain why it works with the bench battery.

 

[Boater Sam]

Doesn’t explain why it works with the bench battery.

Yes it does, the high resistance is in the wiring forward. On the battery he has no resistance.

 

[Sticky Fingers]

Oh I see, yes of course. Senior moment

 

[William_H]

It would appear that you have significant resistance in either positive or negative wire btween battery and lamp holder. With bulb removed you should measure same voltage at the holder as at the battery. The volt meter does not draw current so no drop. You can connect a resistor or lamp in parallel to the volt meter leads to emulate the lamp. Typically 40 ohms or so or a small 6watt bulb.
Dim LED lamp means resistance in wiring. If you can get a connection to the wiring somewhere near the bulb holder then run another wire external from battery positive to lamp wire positive you may see a huge improvement in brightness. If not do the same duplicating negative wire. If neither improves brightness then problem must be between the point where you injected temporary power and the lamp. persevere ol'will

 

[Stemar]

Is it a "one piece" light, with built-in LEDs or a festoon LED bulb in a light designed for an incandescent bulb? If the latter, my first step would be to clean the contacts that hold the bulb.