I have a 5.5A flashlight powered by 2x 18650 laptoppulls(1800mah, 4-5 years old, 100-200 cycles)
After 2.5 minutes at 5.5A, I took out the batteries and they were hot. I could only hold them for around 2 seconds before I had to let go. Not super hot, but hot enough to be worried.
What should I do?
I plan to get some 26650’s for this light. Idk what chemistry I should get. ICR INR IMR? What chemistry are laptop pulls?
When you pulled them out [of the laptop battery] what was the voltage? What’s the lowest voltage they’ve ever been discharged to? Have you measured their internal resistance?
Sounds like you got extremely lucky, honestly I wouldn’t use them again, especially no where near 5.5A.
Just found the best cell of a recent bull. It has nearly original capacity (2200) and lowest internal resistance - 51 miliohm. It was in a pair discharged to 0.8V. It even holds the voltage after being charged yesterday (still 4.15V)
If the batteries were used in parallel in the flashlight, then each delivered (approximately) half the current. As long as the batteries are charged to the same voltage, they would "share" the load. But if the batteries were used in series, they both had to deliver the full current. They would each have to contribute the full load, to add the voltage. That could cause disaster if you don't use very good batteries.
The flashlight drew 5.5 A. If the batteries were used in parallel then each would only have to deliver half of that. If used in series the would both have to deliver the full 5.5 A. The power used was never mentioned, only the current. I assume that the current was measured before the driver. However, that might be wrong...
Was your laptop battery pack brand new and never ever charged? If so, then it can be okay to use cells with much lower initial voltage. It's still a really good idea to test them, but chances are that they're okay. If not, then several of us are going to be in a lot of trouble because we've collectively bought hundreds of brand new never charged laptop battery packs with cells that initially had alarmingly low voltage. I'll get some discharge charts up soon on the batteries I bought.
I'm going to disagree with Ryan about current not being split when used in parallel. I don't see how it could be any other way. Overall resistance must be lowered too, which explains why the same driver can pull 7.5 amps with one battery, and then over twice as much with four of the same batteries used in parallel. If I'm wrong, I'd really appreciate if someone would explain the math so I can make sense of it.
That’s a buck driver supplying 5.6A (after modding) to the LED.
It shouldn’t be drawing too much current from the cells in series, about 3.0A or so.
Even if the cells have high internal resistance, they shouldn’t be getting that hot. I highly recommend you to measure the tailcap current, because there seems to be more going on.
With this method both measurements are made with a load on the battery, this makes it possible to measure the resistance closer to the actual working point.
The formula to calculate resistance is this.
Note: measurement 1 is the high resistance, i.e. low current measurement.
Did you research the type of batteries you pulled…do they have any markings you can Google to see if you can find any info on them…most laptop pulls are mild current ratings 2-3C which means 1800mAh at 2C wouldn’t be near the 5.5A capability you need, w/ safety buffer of an amp or two higher needed to run safely for any length of time
