Originally Posted by
εrv'
a followup (and not necroposting) about that post. That's the second time I get some nice fail on those pack, I know why (from the beginning) but I wanted to see how durable it could be.
Points are :
- yes, ultrafire & clones (well, clowns) are getting worse everyday. Still, they are a cheap alternative for cheap project and I'll keep using them but quality is worse than in the past).
- Cells get unbalanced very easily. So even if you pre charge the cells individually, you'll get issues, and I'm betting I'll have to replace the pack in my E11 in a year or less. I synched the battery cells before making my pack but I can't reach 8.4V after a charge
Again, that's NORMAL. You have 2 individual protection PCB monitoring the voltage on each cell, so if one is reaching the max before the other, it cuts the circuit and the other cell can't keep charging.
This is WHY you have a middle point on a 7.4V protection PCB. It's MANDATORY to use it.
Last time I got the issue, one cell leaked during the charge.
If you want to keep using those to make your packs, it's ok but :
- remove the heatshrink.
- cut the strap that is over the piece of high-temp tape (beware, don't make it touch the body of the cell, neither the cutting tool you use). Keep a long piece of the tab you cut
- unsolder the PCB at the bottom, keep the tab.
- use the tabs to put the 2 cells in serie
- cover the cells with high-temp tape (yellow, check DX to get some cheap rolls)
- wire up a protection PCB designed for 2 cell packs. I got mines at battery-junctions. Wire the B+ to the positive of the pack, B- to the other end, and BM to the middle point.
- export the wires from the output of the protection PCB
- wrap with new heatshrink (use some real one, not the crappy PVC shrink used for R/C models, it reacts too much to heat and melts when glued with hot glue).
it's possible that you'll get no voltage out of it the first time, by default the PCB is in protection state. Connect the pack for a few seconds to your li-ion charger then remove it and check the output voltage, you should read something.
Just to be accurate. It's NOT a Prof. Erv' EE blabla, I *strongly* advise you to stop using that individual cell technique for the packs or you will have sabers coming back shortly (dead packs, leaking packs or ridiculously short runtime). Issue will happen even with high end cells like AW, it's just a matter of time. R/C hobbyist know that issue very well, it's called unbalanced li-po. Most chargers charge cells individually to avoid that.
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