18650 DIY battery pack questions

[Guy's Dropper]

I'm making a mountain bike light for someone. See http://www.candlepowerforums.com/vb/showthread.php?p=3199156#post3199156
I have 12 18650 batteries, some of which I plan to use as a power source. I have a cheap charger that will charge individual batteries, but then I will need to set up the battery holder so that the batteries can be easily removed. Should I instead build a charger into the battery case?

Most importantly, is there a cheap way of making/buying some sort of indicator so that he does not overdischarge the batteries?

 

[TooManyGizmos]

I think nobody knows ............ sorry .

This will bump-it for another shot .
.

 

[Norm]

I use one of these to monitor the Li-Poly battery in my HID.
LED Battery Voltage Indicator Kit

This tiny circuit measures just 25mm x 25mm and will provide power indication and low voltage indication using a bi-colour LED, and can be used in just about any piece of battery operated equipment. Current consumption is only 3mA at 6V and 8mA at 10V and the circuit is suitable for equipment powered from about 6-30VDC. With a simple circuit change, the bi-colour LED will produce a red glow to indicate that the voltage has EXCEEDED a preset value. PCB, bi-colour LED and all specified electronic components supplied.

 

[Linger]

2)batteryspace sells many pcb's for diy battery packs.

1) individual charging, while a bit more of a PITA in the long run, is the cheapest and easiest to create. But he'll want to have a DMM to manually check etach cell's voltage and maybe to some individual charge /discharge on single cells to re-assemble it as a balanced pack each time. To make it plug and go, he'll need a more complicated charger with balancing taps and temperature probe, but these will be bundled into a single connection so it's easier just to connect the charger.