The even slightly more clever way to do it would be to use something like the Tiny13 microcontroller and a low on-resistance FET to pulse power to the LEDs. You could have it sense the battery voltage and adjust the duty cycle to give you consistent brightness as the batteries discharge, but without the need for an inductor, since you really don’t need to drive LEDs with DC.

However, that’s probably getting more fancy than is strictly necessary :)

Three cells will give you about 12.3-12.6V fully charged and about 9.0V discharged. That might shorten the life of the LEDs, or kill them. It depends on how they’ve put the system together.

I’d be tempted to go with the concept of less cells and you might find that then the full brightness setting will be about right.

Come to think of it – how do they do the reduce brightness? PWM? That would be the clever way to do it. If you can fiddle with the PWM circuit, or just replace it, then you can do my favourite trick with LEDs which is to run them at higher than normal voltage but with low duty cycle. It *appears* very bright while using a lot less power. That might allow you to safely run them off 12V worth of cells – if you limit the maximum duty cycle to prevent them from overheating with the extra voltage.

I assume you’re good with electronics and won’t need my help but if you want some advice on how to build a PWM controller I’ve built a great many :)

I really love these little Atmel Tiny line of microcontrollers. They’re something like a buck each, and contain PWM (and in some cases ADC). It just isn’t worth building a discrete PWM circuit when you can just use one of those and program it to do whatever you want. For example I used one as the LED brightness controller in a VU meter I built. The LEDs in the VU meter have a common anode driven by a P-channel FET, and the Atmel Tiny13 control the FET gate. It’s constantly reading the ADC, and changing the FET gate duty cycle to be proportional to the voltage reading it gets off the ADC. Then I just connect a pot between ground and VCC, with the wiper connected to the ADC sense pin. So you can choose your frequency, and use the pot to adjust duty cycle between 0 and 100%, with all of three parts (the chip, a bypass cap, and the pot).

Did you see that Radio Shack was clearing out their version of the low self discharge cells? 2xAA packs were around $2 each.

Ed