I have a cheap Magicshine clone light. With these lights, is the circuitry that cuts off the light when the battery pack is nearly discharged built into the light itself, or the battery pack? Or maybe both?

I was thinking about buying a water-resistant 4-slot 18650 battery holder designed for bike use, but one I was looking at doesn't appear to have built-in undervoltage protection. I can buy protected cells to put in the holder, which is fine, but could also try reusing some unprotected cells that I have. But I don't want to risk over-discharging any non-protected cells I may use.

The appeal for me is to be able to put the cells in a high quality charger that I know will charge them correctly individually each time. I know I can get 4-cell battery packs cheaper than the holder + batteries....

Thanks!

I have two such lights, a one and five CREE XML-T6 bike headlights and a couple of battery packs. Neither light comes apart easily. The battery packs I own are exactly the right size to be made from four and six 18650 cells and there is no extra room for electronics. My bet is that the controller circuit is behind the LED module in the lamp head. Frankly I never use either light as I quickly became disillusioned by the battery pack and cord being a PIA to deal with. You might take your question to BudgetLightForums.com. There is a group of dedicated light modders there Flashlight Modding and DIY Parts | BudgetLightForum.com who would surely know the answer to your question. I generally use a CREE XML-T6 flashlight for my front light. Flashlight and secure flashlight holder (w/o 18650 battery) were well under $10. No cords, easy on and off, very secure, and plenty bright.

You can expect booth don't have any circuit. Magicshine is already cheap for the delivered quality, so a copy can't be good.
Stick with protected 18650-cells and your good cell charger and you won't get any problems.

Well, I'm sure the light does alert you via a red LED, and cuts the output, when the battery pack is low. I guess it will shut the light off entirely at some point, although I haven't ever tested that. Thinking about it some more, that functionality must be built into the light, and not the battery pack. That might be enough protection on its own for the batteries, but protected cells would be good insurance.

Thanks. I think you're right that the controller circuit is in the light.

I agree that these lights are something of a PIA. I usually use a Serfas True 500+ on my handlebars (very easy on/off) and a single-18650 cell flashlight on my helmet. I have the clone light partly for backup purposes and partly for when I want longer run time before changing the batteries. Also, I can bring my bike into a secure area of my building at work, so I can just leave the light and battery pack on the bike during the day. These clone lights aren't great in some ways, but I think they're useful enough to keep one around.

mtbr has a great light forum with many MANY threads on batteries including people taking them apart and building new ones like you are talking about. I'd suggest you go there instead of here.