I have one of the cree xm-l Chinese bike lights but am having problems with it. It is switching off after around 30mins on medium power. I tested the voltage from the battery which starts at 7.4, as expected, and the light switches off at ~7V. Is this as expected? I would have thought the voltage should be able to drop more before the light switched off?

It uses 6x 18650 batteries. I have bought some replacements already assuming it was these, then carried out the test which made me wonder….

Did you measure those voltages with the light on or off? You should be getting very close to 8.4V from a new, fully charged, battery pack before you connect it to the light. If you’re getting only 7.4V off load, after fully charging the pack, then there’s something very wrong with the pack.

As Steve says above the charged voltage without load should be closer to 8.4v as it sounds as though the pack is 2S3P (2 cells in series in 3 parallel banks), the light switching off at ~7v (3.5v per cell) is to be expected to protect the batteries, it is the LVC (low voltage cutoff) setting inside the light.

I would suspect either the battery pack or the charger if the ‘charged’ voltage is too low.

If the batteries are defective dispose of them in a salt bath and then take to the tip for recycling when their voltage reads 0V.

They’re cheap for a reason.
The batteries are usually the cheapest they can get or just batteries out of old laptops.
The Chinese lie about everything, from the light output to the battery capacity.
The light/head unit itself might be okay, but buy a battery pack from someone trustworthy, like MTB batteries or Tourchythebatteryboy.

I’ve found that it’s often the way packs are assembled that is more an issue as this will be done manually by the factory building the bike lights rather than the cell manufacturers, whos process is highly automated.

I have had several Li-Ion packs fail due to badly spot-welded tabs inside the pack or badly soldered power wires and rarely had problems with the cells themselves.

As above, the battery packs are probably made from recycled laptops and generally useless. But the lamps themselves are sometimes OK, just ignore the lumen* figures. If it’s cheaper than the cheap but reputable brands (Magicshine, moon, nitefighter, etc) then they’ve cut something out, usually the battery.

*you can make an estimate by taking the quoted battery capacity in amp hours, and the run time in hours, divide one by the other to get the number of amps they’re running at. 7.4V x that = the power. Then go to the led manufacturers website and work out how many lumens that LED actually emits at that power (you may need to look at the driver current Vs forward voltage graph to determine the current from the power figure you just derived, then a second graph to convert that to lumens, then multiply that by ~0.8 to allow for the power supply, lens, reflector etc.)

Have a word with Torchy he will sort you out.

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Thanks for the responses. The 18650 batteries are 3.7V so why not 7.4V at max output?

The 18650 batteries are 3.7V so why not 7.4V at max output? They are 3 in series 2 parallel as you suggest

You sure that the light isn’t turning off due to overheating?

3.7V is the nominal voltage of a 18650 cell. You can think of it as the average voltage between the fully charged voltage (4.2V) and the cut-off voltage (around 3V). If your charger isn’t charging the pack to 8.4V then either the charger or the cells are faulty (or the cells are old).

They should be 2 in series (7.5V) and 3 in parallel (*.*Ah). After a full charge the output voltage on the battery should read 8.4V. How are you measuring the voltage? Off load or on load as it will differ.
You said you have bought replacements? Do you mean cells to make you’re own or a complete built battery.
The guys above have covered most options but I can certainly look further into the light and batteries should you wish too.

I buy cheap lights and quality batteries and charges from

seller:big_f_d_d

@smudge I tested with the lights on, and bought the 18650 cells plus waterproof box to put them in. The originals were spot welded so probably not great.

7.4V on load on is rather low, unless your light is drawing several amps on the medium setting that you are using. I don’t know what light you are using, but most of my Solarshine X2 type lights draw around 1.25A on medium power and 1.75A on high. I get at least two hours mixed use (half high, half medium) on just two good-quality 18650 cells in series.

If you are charging the cells in the pack, have you measured the off-load pack voltage after a full charge? This should be very near 8.4V. If not, measure each cell individually to see if you have a duff cell. You should also try charging the cells in a charger that charges cells separately (if you have one) and then measure each cell individually. (With removable cell battery packs I would always remove the cells anyway to charge them.)

I had the same problem. turned out to be a bad connection due to the cable not being clamped ie just held in by the soldered connections and kept coming off. re soldered and glued the cable in so its now fine.

To rule out overheating, run the light on the usual power with a decent fan blowing over it.
If the problem persists, it’s probably the battery and not a temp issue.

7.85V fully charged no load

Without seeing and testing it sounds like you have unbalanced cells within the battery, as soon as one cell (or pair paralleled) reach 4.2V the protection circuit will trip as this is the over charge cut-off voltage. This will also work on discharge as the lower voltage cells will reach the end of discharge voltage first – Hence unbalanced cells.
All cells should have the same nominal voltage so they all charge and discharge at the same rate.

Thanks CaptainDanger, although a bad cell or cell connection might be pulling down the pack voltage the charger is also worth checking – can you measure the output voltage of the charger without load?

Thanks chaps, yep appeared to be unbalanced battery, replacements work fine. I will try to work through them to find the defective ones and keep the rest!

OP,
if you say that you’ve bought a box and separate batteries, you should always remove the batteries and charge them in one of these Charger or similar. They will never balance in one of those boxes. Therefore if you have a cell that is slightly low in capacity the whole pack will suffer.

Ok so I got some new batteries, take much longer to charge and work for 30 mins, then the light gets dimmer and simmer until it’s almost off! Why does that happen, shouldn’t the light itself switch off instead of getting dimmer? Wish I’d bought something reputable now!

Are you trying it indoors? Is it (the light) getting hot? – It might be overheating – try using a fan to keep it cooler.