Hopefully someone could help me with this. I'm learning myself, gradually, but this is something I just don't understand so I really need it cleared up.

I'm basing this question on the Fenix range, just because I have some for reference and because they're good examples for me to compare, but it applies to any light.

The Fenix P1D. Maximum output listed as 180 lumens from a 3.0 volt CR123A.
The L2D. Maximum output also 180 lumens, but from 2 x 1.5 volts AA, total 3.0 volts.

From what I understand (and I could be wrong of course) alkaline AA batteries start to dip in voltage once they are used. As far as I know, Nimh and lithium AAs do not dip in voltage, but again I'm quite probably wrong.

So, my question is this. How do AA lights like the L2D manage to keep the same output until the end of the life of the AAs? Is it that the digital regulation is that good, or is it actually the case that any AA light such as the L2D will become more dim as the batteries are drained? This is especially relevant to single AA lights such as the L1D. surely that will start to become more dim after just a short amount of battery use?

Thank you.

A typical LED needs more than 3 volts to run brightly. A light that uses one CR123A or one or two AA cells therefore has a booster circuit that converts the supply voltage to the voltage that the LED needs. You could consider it like a transformer that steps mains voltage down to a lower voltage, only in this case it is running backwards.

Now the brightness of the LED is a function of how much power is supplied to it, and power = voltage x current.

Therefore the voltage converter circuit can compensate for lower battery voltage by taking more current, thus keeping the power the same. This is called regulation.

Some boost circuits work better than others, but most of them stop regulating so well when the voltage drops too far. So typically you will see less light from 1 AA cell than from 2 AA cells, and possibly less light from 2 AA cells than from one CR123A cell. This all depends on the particular light and how well the circuit has been designed.

Ah, thanks for that. I always assumed that if the battery was 3v then that was exactly what the LED wants and gets, no more no less. It makes more sense now.

Using that info, I suppose a 2 x CR123A light would be very efficient right down to the end as it is starting with 6v total so there is no boosting needed, just regulation?

Originally Posted by Niconical
Using that info, I suppose a 2 x CR123A light would be very efficient right down to the end as it is starting with 6v total so there is no boosting needed, just regulation?
Not exactly, no. You see, 6 V is much more than an LED needs, so the circuit this time has to throttle the current somehow to prevent overloading the LED. It turns out that any circuit that has to boost the voltage, or throttle the current, will have some losses in operation, so the efficiency will never be 100%. Even regulation has a cost. Really good circuits can get above 90% efficiency, which for practical purposes is as good as you could wish for.

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