Originally Posted by Richard Cranium
Good for you doug -I need a few people to be more conceited than me. But again - this is mostly a waste of time - all you're doing is measuring a circuit and battery charging numbers - that not measuring the light's true current consumption. But I digress - you can continue playing.
Conceited? Perhaps, but I certainly did propose how to measure *discharging* numbers too. Perhaps you didn't understand me.
Modern LED lights control how much power they use by turning the LED on and off quickly. Multimeters don't always handle this well, making measuring the power at the light problematic. (But certainly not impossible, with the right gear.)
However, if you take a set of batteries, fully charge it, then discharge them down to 1.2 volts (to pick a figure) at a low but constant discharge rate (most R/C chargers will do this for you) you can easily determine the actual
capacity of the batteries. Since you're measuring the discharge rather than the charge, you aren't measuring the "charging numbers". R/C people have been doing this for decades to know when it's time to discard their old batteries (if your receiver batteries fail in flight, any warning you get is usually very subtle, and then suddenly it's too late and the plane crashes.)
You measure the discharge current rather than just the charging current because when you charge some of the current is lost by the battery -- and old batteries usually lose more, so you don't want to guess at the difference when your R/C plane is on the line.
So you measure the capacity of the battery, then fully charge it again, then time how long the light stays on until the voltage drops to the same point (you'll have to occasionally measure the voltage) and do some simple math -- you'll know how much current the light averages in whatever mode you just measured. Yes, there's some small errors in here -- the self discharging of the battery (use eneloops to minimize this) and the voltage sag of the battery while it's on the charger is probably higher than it on the light (as the discharge rate is likely higher, unless it's a powerful light) -- but these errors are likely small if this is done right.
Though there is an easier way -- just put a fairly large capacitor (like a farad, though that's likely bigger than needed) in parallel with the battery pack (move the batteries outside of the light if they're stored internally.) Give it a minute or so to fully charge the capacitor, then use your multimeter (cheap one are OK for this) to measure the steady DC current between the batteries and the capacitor. The capacitor will almost completely smooth out the spikes in the current used allowing any multimeter to give an accurate reading.
And really, the way that the OP was simply measuring the amount of current put into his batteries as given by his charger -- the accuracy of that is reasonable. Yes, it overstates the energy used a bit, as some is lost, but it's on the order of 10% or 20% -- probably an acceptable error for his purposes.
I did also talk about measuring the power consumed by the charger itself (if you're worried about just how big the fraction of a penny it costs you to use is) if you somehow confused the two.
And I certainly do appropriately appreciate your permission to continue playing!