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  1. #1
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    18650 Li-ions built-in protection question.

    Hi,
    Every cell has it's own buil-in ptotection ( overcharge, discharge, short circuit, over-current ) the question is - do I need some extra (outer) protection if the cells ( two or three ) are working in parallel ?
    Will the protection of each cell work ? I don't wont to discharge them to the danger-low level. Every cell is charging separately so charging is not an issue, I think.

  2. #2
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    Quote Originally Posted by heniekkrol
    Hi,
    Every cell has it's own buil-in ptotection ( overcharge, discharge, short circuit, over-current ) the question is - do I need some extra (outer) protection if the cells ( two or three ) are working in parallel ?
    No.

  3. #3
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    ok, thanks for the help.

  4. #4
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    Quote Originally Posted by heniekkrol
    Will the protection of each cell work ? I don't wont to discharge them to the danger-low level
    For the first few uses I would suggest checking how low the voltage goes before the protection circuit stops the discharge. I remember a thread on CPF that discussed protected cells occasionally dropping below 2.7V without the circuit stopping the discharge.

  5. #5
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    My AW 17670s do not cut out at 3.0V.

    It's usually more like 2.5 to 2.7V, depending upon temperature and amp load.

    I regularly run these in a 2S configuration, and (I know I shouldn't but) I generally know they need to be recharged when the light goes off following the PCB cutting out. After two years of this 'abuse' and probably 100s of recharges, the 17670s are still working great.
    Last edited by P220C; 02-22-2011 at 10:06 AM.

  6. #6
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    Quote Originally Posted by heniekkrol
    Hi,
    Every cell has it's own buil-in ptotection ( overcharge, discharge, short circuit, over-current ) the question is - do I need some extra (outer) protection if the cells ( two or three ) are working in parallel ?
    Will the protection of each cell work ? I don't wont to discharge them to the danger-low level. Every cell is charging separately so charging is not an issue, I think.
    I assume you understand that there are protected and un-protected versions of 18650 cells. If you are building a pack with protected cells, then you shouldn't add an external protection PCB. The protection PCB inside the cell wrapper should protect each cell from over and under voltage and over current.

    If you are building a pack with un-protected cells, then you should use an external protection PCB. Professionally built packs will always use external protection PCBs for a number of reasons.

    - Cost, one PCB costs less then multiple PCBs

    - Efficiency, with a single pack level PCB you only have one set of components to measure and control current in and out of the pack. Since these components cause voltage drops, having one set is more efficient that having multiple sets. Additionally, the larger FETs used on an external protection PCB have lower voltage drops than the tiny FETs used in individual cell protection PCBs. This is particularly true at higher currents.

    - Reliability, there are less total parts that can fail on a single PCB as opposed to multiple PCBs

  7. #7
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    That's pretty clear to me, I asked about parallel conection, because as far as I know in packs connected in series, single PCB's of each cell doesn't work properly.

  8. #8
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    Quote Originally Posted by heniekkrol
    That's pretty clear to me, I asked about parallel conection, because as far as I know in packs connected in series, single PCB's of each cell doesn't work properly.
    Why wouldn't they work properly in series-connected packs????

    Cell's PCB knows nothing about other cells, so it doesn't care. It simply monitors the parameters it was designed for (e.g. cell's voltage, temperature, current, ...) *in its own cell* and executes appropriate actions if any of those parameters are outside of safe limits *in that particular cell* only. So why should it matter how cells are connected together?

  9. #9
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    Yes, You're right; I'm not an expert, for sure - I thought if the pack is 7.4V so "there is 7.4V" in whole chain of connected batteries and it will "cheat" pcb. But there isn't! Mea culpa.

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