In practical use if you use same batteries and always use them together at the same time, the risk is a bit lower because modern battery manufacturing is quite consistent. But you do not know for sure and you cannot control what the user does. It maybe ok if the user uses 3 good batteries from Samsung and keep it together in the flashlight all the time, then the risk is low. But imagine if the user buys some batteries from a lower quality manufacturer, or maybe use batteries they have lying around the house. Each cell may have different characteristic and quality, such as different internal resistance. If one cell goes bad such as a small internal short, the whole battery pack can burn up.
Another problem is user mistake which can happen to anyone and is the more practical concern from me. The user may put one battery in backward, leading to an instant short and cause a huge fire. This is unlikely with the design of the flashlight since you need to use button head batteries. The batteries also need to be charged at exactly the same voltage before insert. Similar huge problem may occur if the user inserts a differently charged cell by mistake.
To OP, sorry for discussion off topic. I feel that this need to be discussed because of the inherent dangers. I do not see any easy solution for multiple-battery flashlight unless manufacturers sell a pre-made battery pack. Even so I am concerned that some manufacturers do not request binned batteries from battery manufacturers when making the pack. But I read that some flashlights like Manker and Acebeam with the new big flashlights use custom battery packs with protection circuit. I understand that forum members do not like it, but they should because it is a huge safety hazard. You will never see a Dewalt or Makita or Dyson tool that allow you use multiple raw cells.
OP revealed no details, but was it very cold when this happened? Like you left your light in the car overnight, had no power in the morning, and you brought it in to recharge?
Li ION cells can be really dangerous if you try to recharge them in freezing conditions. It’s all about chemical reactions, and THIS combination of chemicals does not thrive in the cold. Otoh, litium primaries (1.6V / 3V) can stand polar temperatures very well and therefore are very well suited for under-the-car-seat lights and the lights in your BOB.
Sometimes it pays off to visit another forum like HERE.
I don’t know if the famous military strategist and philosopher Sun Tzu had Li ION cells in mind when he said: “Keep your friends close and your enemies closer”. Keep them cells warm in winter! Inside your coat. They live longer, and therefore you do too.
I have to agree here, I no longer recommend lights with multiple loose batteries as there is too high a risk of a muggle using imbalanced batteries. Internal charging adds to the risk. This is why consumer brands such as Olight, Acebeam, and Fenix have moved towards proprietary battery packs for soda can lights.
I've ordered several lights directly from Sofirn with cells, and most of the cells are absolutely awful with resistance between 100 - 200mΩ from new. Rip the poorly welded buttons off the top, and then they're a nice healthy 15 - 25mΩ. It's highly unlikely that your light didn't have cell balance issues from new, due to this.
I've had the same issues with all button tops regardless of where they are purchased from. That's why I never buy button tops any more, just flat tops and then do a solder bump if the light needs button tops.
I'm not saying this is definitely what caused the issue, but it's something to consider.
Looks like a cell vented. Contact Sofirn, They’ll likely replace the whole thing for you without too much trouble.
I’d probably still use the other two batteries as long as they test ok in a cell discharge/charge cycle… I also wouldn’t be totally shocked if the light was still in functioning shape.
No need to charge them separately when they’re in parallel in the light.
In parallel the cells have no choice but to comply with eachother.
It would be wise to make sure all cells are the same voltage before putting them in parallel.
In fact, every new Li-ion cell should be 3.5 to 3.7 Volts from the factory, usually 3.6 Volts.
A difference of 1 or 2 tenths of Volts should not be a problem when you bridge them in parallel.
They will level out by themselves without dramatically high currents.
Much more prone to balance problems are series configurations.
A constant balancing circuit is strongly adviced for series configurations, but then you need a connection to that balancing circuit in between the cells, which is quite impossible with flashlights.
So we get away with hopefully well enough matched cells that are inserted fully charged and hope they’ll stay within eachother’s voltage range until low voltage protection kicks in.
Regarding the OP, he says the cell that looks like it vented still read 4.1 Volts or something.
It would be great if he could test the cell in a charger that has a NOR test option, see if it’s still up to specifications.
I just re-read the OP, and a question came to my mind. The SP36 has built-in charging management, doesn’t it? So, did you plug it into a 5V phone charger, to charge the SP36 cells? If so, that doesn’t seem proper; there would be two charging systems trying to charge them simultaneously. Or am I misinterpreting?
I'm always wonder, if a built in charging flashlight is said to charge 5v3a max, is it save for the user to charge with ' power supply that able to charge at higher voltage and amperage such high watt charger or a PD charger, while the flashlight manuals mostly described only particular charging rate; such 5v2a or 5v3a, since nowadays people have quick charging power supply.
When you charge cells using a charger, you never put two or more cells in series. Each cell has its own contact points in the charger. Charging two cells in series? I can’t understand why any flashlight maker would think this is a good idea…
