Now that you know about li-ion technology and how the DS power supply works, it's time to pick a replacement battery.

For the future, I don't plan on continuing to procure 11.1V packs and prep them as I have in the past. It's a very time consuming process for sure. Instead, I would highly recommend going with an existing 7.4V pack that is readily available off-the-shelf from Magicshine, Gloworm, or Gemini.

One of the best places to pick up one of these batteries is Action LED.

If you follow the link, you'll be taken to their battery page, where you'll see a lot of good options for the 7.4V packs, all with the "Magicshine style" connector that will plug directly into the DS-500. I highly recommend getting started with one of the Magicshine packs. They're low-cost and work well. They even have a Sanyo-based higher capacity option.

Here are some direct links (note that these are from Action-LED, which is US-based, but they are also available directly from Magicshine, which has distributors all over the world):
MJ-6038 - 4400mAH, 7.4V
MJ-6030 - 5600mAH, 7.4V (Sanyo)

If you're running the highest power flash mode (EMS mode), then you could expect around 13 and 15 hours run time, respectively. Lower power modes obviously go up from there.
Here a good matching 7.4V li-ion charger, also from Action-LED.

The 11.1V option, however, is not completely dead. There are any number of on-line battery providers (, etc.) that sell appropriate 11.1V packs that can be used with the DS lights, but you'll need to remove the old power wire from the dead pack and solder it to the new one. There is at least one ebay seller that I have dealt with in the past that sells some nice 11.1V packs.
Here are the two options that are very similar to my original 11.1V packs:

Sanyo cells, 5200mAH 11.1V pack
Sanyo cells, 2600mAH 11.1V pack

These packs even have a mating connector that plugs directly to the light, just not with the water tight seal. If you carry this battery in a zip-lock in your saddle bag, you'd be fine for most everything besides riding under water. They also have a small power plug that can be cut off. Just make sure you cut the black and red wires separately, and tape off the ends of the wires.

Initially, I was concerned that the 7.4V pack would put too much strain on the power supply in the headlight, but after doing some extensive testing, I can confidently say that my existing headlight owners can also switch over to the 7.4V packs. The only drawback here is that you won't be able to run on level 5 steady for as long with no air-flow before the temperature cut back to level 2 occurs.

You're probably asking yourself... can I just plug in a 7.4V pack to a light that was originally configured to run off of 11.1V? Yes, you won't hurt a thing, BUT you will quickly find out that the light will "think" that this 7.4V pack is a severely run-down 11.1V pack and consequently shut down immediately to protect the battery. To make the 7.4V packs work properly, you need to re-define the status threshold voltage levels in the controller, and this will be the subject for part 5.