Getting constant voltage

[Nigel Goodwin]

But what happens if you're unlucky and you've got one chain containing all the LEDs with low voltage drops and another chain containing LEDs with high voltage drops?

Presumably the combination of three diodes in series is enough to overcome that problem? - like I said, get out your LED's and meters and try it!

 

[justDIY]

our goal is to make this as complicated as possible for the OP, here is my suggestion:

to achieve uniform output from all your leds, you need to run them in a strictly series chain, no parallel. since you only have 3.6v to work with from a battery, you'll need to step up your voltage to provide enough bias voltage to push the current through 4 (or more) leds.

the texas instruments tps61040 is an easy to use switching boost regulator that can be configured to operate in a constant current mode. you will need one of the ICs (easy to use sot23-5 package), a schottky diode, a small inductor and a few passives.

here's some info on the boost converter:
https://projects.dimension-x.net/archives/98

now since you're dealing with Lithium Ion batteries, you need a safe charger. I recommend the MAX1551 dual input Lion/Lipo charger. In an easy to use sot23-5 package, the 1551 offers both USB and dc wall adapter charging inputs. You can charge your wireless lightbar from your PC, or from a wall adapter. The 1551 requires no active external components, a few capacitors are recommended for decoupling input and output voltages.

More info on the 1551:
**broken link removed**