Protected circuits

If I understand this correctly, protection circuits can exist on batteries as well as the flashlight itself. Is this correct? If so…is it necessary or beneficial to have both?

Some flashlight s do have a low voltage protection cutoff, others don’t. Personally I like my batteries with built-in protection. Where ever the cell goes, a little protection comes along. The protection circuits may cause a slightly higher resistance but having individual protection on each cell is worth it to me. If you use multi cell lights any added protection is more needed (I would only use quality cells like panasonic in multi cell lights).
Some chemistries are safer & panasonics in general appear to more resistant to failure.

How can you tell if your light has some kind of protection? Is it found only in upper end models?

The light will start blinking for example, and it will change into a Lower output mode.

This kind of protection is not really expensive, you can buy a Driver for $2-5 which have this kind of protection…

not sure if that was what you`re asking!?

That one reason I’m fond of protected cells. Less need to check if lights have any protection & keep track of which don’t.
You should be able to check for low voltage cutoff using a pot / variable resistor. Also magnets, alligator clips & multimeter to see where it cuts off. The cell voltage recovers some when the load is removed so I believe it might look like it cut off higher if you measure voltage after instead of during use.
Some budget lights should have it. Don’t know how common it really is since I don’t rely on it.

I guess the reason I asked is that I have some rescued LG 18650 cells from a laptop and was going to use them in my UF 502B (after testing them for a couple weeks on my multimeter). The cells are unprotected, so I thought they might be OK to use in this single cell light. Was just wondering if there was a way to tell if my 502B had some sort of internal protection.

The 502B hasn’t a protection.

Often you can say:
If the light (I speak about brand models like Fenix, Olight, Xtar, Eagletac) cost less as $50 it will not have a protection. Cheap drop-ins doesn’t have a protection. I know two kinds of protected drop-ins (special driver series) and they starts with about $17-20 (only the drop-in).

You can built in this kind of driver in the 502B (have to solder) and it cost about $4-5 (if you want a full powered XM-L).

You can use unprotected 18650 in the 502 without a problem, but you have to notice the brightness. If the high mode is only a bit brighter as the mid mode you should test it with the multimeter. If the battery has less as 3,5-3,6V you should charge it.

Also, the light protection only covers low voltage, it won’t protect against a dead short - reflector across led connections etc, this would worry me more than killing a cell by dropping the voltage below a safe level.

I do however have some unprotected cells myself and some lights won’t take the girth/length of a protected cell.

Hi Krono,

Yes, there is a simple way to test the low volt cutoff. At least it is electrically simple, but a bit mechanically challenging unless you have lots of hands. Diodes allow current in one direction only, but drop a fairly constant voltage (about 1/3 volt for Schottky or 1 volt for standard) in the forward direction. You will need diodes rated for at least 3 amps (30 or 40 cents each from Mouser).

Remove the tailcap from your light. Use a diode to complete the tailcap circuit. Touch the diode end with the white band to the neg of the battery and the other end to the threaded part of the flashlight body. If you have a standard (not Schottky) diode, the flashlight will see about 1 volt less than the full battery voltage. You can estimate the cut out voltage using a voltmeter to measure the open circuit battery voltage and the diode voltage in circuit. By adding diodes you can adjust the voltage to the light in steps of 1 volt or 1/3 volt.

You could directly measure the voltage to the flashlight if you use a dowel and wire to make contact with the inside of the light where the positive battery contact is. This way you don’t have to guess how much the battery voltage is sagging. Measure from the pos contact to the tailcap threads on the light body. Depending on your batteries, you’ll want the light to shut off around 3 volts or a bit higher.

Lithium safety:
I think you are already addressing the biggest safety concerns by using a single cell light and a quality charger. DO USE A QUALITY CHARGER. I ordered several cheap chargers for charging protected cells. They vary in max charge voltage from 4.19 to 4.43 volts (different units, same model). The internal build quality is poor. (Bad solder joints and bad mechanical design.) The shut off circuit would easily fail with a single solder joint failure. In other words, bump or drop the charger in just the wrong way and blow up the next battery. Under no circumstances would I want an inexperienced or careless person using these with unprotected cells. Yes, I would do it if I could keep my undivided attention on the process, and terminate at 4.20 volts. That’s a PITA so I would not want to do it routinely. Charging to 4.43 volts might not be guaranteed to start a fire, but it is clearly a violation of spec; therefore very bad practice and considered dangerous unless proven safe!

Over discharging an unprotected cell certainly isn’t good for cell life, but I’m uncertain about safety. I’d worry about short circuiting a cell, but over discharge of a single cell might not be a safety concern at all. Recharge of a somewhat over discharged cell is apparently ok, if a slower recharge rate is used.


In light of Scaru’s comments, I’d just try to recharge them in time. If you lose a recycled cell, get another.

Scaru’s current thread on lithium use is well worth a read.

That’s why its included (and was partially written for) the one stop mod reference shop.

The whole idea of this thread is to openn up info for budding modders, if you find a usefull link, drop it in here with a short description of what it contains.