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aesmith · 23-09-19, 07:53 AM

I switched to my winter bulbs this weekend, and while I was at it I did a quick measurement of voltage drop. Very rough and ready so pointless to record exact figures. I don't have a low ohm meter worked on voltage, which after all is what the bulb gets. Figures were taken without the engine running but with the run switch off.

Voltage across the bulb measured 9.5 volts
Voltage at battery just over 12.5V
Voltage between battery +ve and bulb +ve was just over 2V

At first I thought these contradictory, but then remembered that the positive feed to the bulb is the one that passes through all the switches and therefore more connectors. So it's quite feasible that it loses 2V on the positive side and only 1V on the return.

With hindsight I wish I'd done these tests with the engine running, but even if the voltage drop was no higher it sounds like the bulbs will be getting around 11 or 11.5V in normal use.

23-09-19, 07:53 AM	#44
aesmith Member Join Date: Mar 2013 Location: Aberdeenshire Posts: 966	Re: Osram Night Breaker Laser H4 I switched to my winter bulbs this weekend, and while I was at it I did a quick measurement of voltage drop. Very rough and ready so pointless to record exact figures. I don't have a low ohm meter worked on voltage, which after all is what the bulb gets. Figures were taken without the engine running but with the run switch off. Voltage across the bulb measured 9.5 volts Voltage at battery just over 12.5V Voltage between battery +ve and bulb +ve was just over 2V At first I thought these contradictory, but then remembered that the positive feed to the bulb is the one that passes through all the switches and therefore more connectors. So it's quite feasible that it loses 2V on the positive side and only 1V on the return. With hindsight I wish I'd done these tests with the engine running, but even if the voltage drop was no higher it sounds like the bulbs will be getting around 11 or 11.5V in normal use.