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I wanted to check the power consumption of my LED bike lights, so i connected my multi-meter up and switched the light on.

Initially the current was 1.2A, but over the next 15 mins or so the reading dropped to about 0.7A, there was no noticeable change in light output. At first i thought this was some sort of heating effect reducing the current.

But if i switch the light off and on again quickly the current would return to 1.2A and again drop off over 15 mins or so.

Can anyone explain what is going on? Shouldn't the current usage be more or less constant?

The bike lights are the generic "magicshine" LED type lights connected to 18650 batteries if that matters.

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  • \$\begingroup\$ Did you measure the voltage across the LED? Did you try with a different multimeter and see the same results? \$\endgroup\$ – electrophile Oct 5 '16 at 15:41
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    \$\begingroup\$ When you say "no noticeable change in light output" was that measured with a lightmeter? \$\endgroup\$ – Brian Drummond Oct 5 '16 at 15:50
  • \$\begingroup\$ What meters is it? \$\endgroup\$ – PlasmaHH Oct 5 '16 at 16:50
  • \$\begingroup\$ I didn't measure across the LED. Just with the MM in series with power. The change in light output is just based on my observation. But it seems to give the same light until it stops altogether. The multimeter is a Precision Gold WG 020. But to add to the unexpected behaviour: I added a second MM across the battery to measure the voltage there at the same time. Now the current reading on the ammeter no longer drops off. It now works as i would expect; as the voltage in the battery drops the current goes up to compensate and keep the light levels the same. \$\endgroup\$ – Richard Oct 5 '16 at 21:36
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From your question I have the impression that you hooked up the armature to an external source. It also seems that there is a lipo inside the light fixture.

If the above is correct than the behaviour can be explained as follows:

Upon connecting the power supply both the lamp and the battery receive current from the external supply. The lamp is on and the battery starts charging. When charging progesses the charging current will reduce (simple charging circuit no cc/cv) and the lamp keeps on.

This would explain why the current is high at the beginning drops of slowly.

The current is therefore in a large extend dependant on the battery charger inside the armature.

To measure the power consuption this way would not be correct. You would have to deduct the stored energy in the battery. Without opening the armature and separating the circuits you are not able to determine how much current is going where and consequently how much power is used for the lamp and the battery.

I looked up the Magicshine and indeed there is a lipo inside.

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  • \$\begingroup\$ There is no external power in this case. It is literally just a battery connected to an LED light via my multi-meter. \$\endgroup\$ – Richard Oct 6 '16 at 9:50
  • \$\begingroup\$ Is the battery connected to the 18650 or is it an external battery. \$\endgroup\$ – Decapod Oct 6 '16 at 10:15
  • \$\begingroup\$ Made a little mistake in my comment. It should be: Is the battery external and connected to the lamp as such? \$\endgroup\$ – Decapod Oct 6 '16 at 11:28
  • \$\begingroup\$ The battery is external. Just connected with a normal piece of wire. \$\endgroup\$ – Richard Oct 6 '16 at 15:58
  • \$\begingroup\$ Well in that case the external battery is your power supply. The lamp itself without the lipo would not take 1.2 A \$\endgroup\$ – Decapod Oct 6 '16 at 16:13
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Most likely you did not connect the meter in series to the LED circuitry.

How to measure current with a DMM

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  • \$\begingroup\$ The meter must have been in series since OP measured 1.2 A dropping off in time. \$\endgroup\$ – Decapod Oct 6 '16 at 9:02

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