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  1. #1
    Five is right out
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    Resettable fuse question

    I've been looking at resettable fuses as I am using them in my battery packs, and realised that I am not sure how they actually behave.

    For example, on the 4A fuse (http://www.batteryspace.com/prod-specs/2591.pdf) there are two different parameters: a Hold current of 4.2A and a Trip current of 7.6A. From my reading here, http://en.wikipedia.org/wiki/Resettable_fuse, the fuse should be capable of passing 4.2A and then it becomes highly resistive at 7.6A.

    My question is, what happens between 4.2 and 7.6A? I had assumed that these fuses would trigger at 4.2A rather than at some higher current.

    Secondly, I'd picked 4A fuses on the assumption that if I am charging at around 2A on my 2500mAh packs, I should be using a fuse sized above 2A. Seemed pretty obvious, but I guess I should run this one by more electrically experienced people here.
    Last edited by womble; 10-13-2010 at 08:08 AM.

  2. #2
    Do It Yourself
    Reputation: Homebrew's Avatar
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    I don't think you need these as most 18650 Li-Ion cells that I've seen have a PTC fuse built in. That's one of the reasons you shouldn't directly heat solder the cells.
    Long Live Long Rides

  3. #3
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    Ah, I should have added a little more background.

    The protected 18650s that I can get locally are dodgy trustfire/ultrafire type cells, which I have little faith in. I'd prefer to run higher quality unprotected cells and build in my own fuses for current protection.

    I have constructed my own 4S packs from unprotected Sanyo 18650 cells which deliver power to a bflex driver. The cells come with tabs, so soldering them was a pretty straightforward affair. I rely on the bflex to provide undervoltage protection at the default 12V and my balance charger to not overcharge any individual cells.

    I'm toying with the idea of using thermal fuses as well, but haven't ordered any yet.

  4. #4
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    I believe the fuse acts like a slow-blow at currents between 4.2 and 7.6A . at 7.6A it trips immeadiately. It may be better to size the polyswitch closer to your actual current, so you don'y have too big of range of overrun.

  5. #5
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    The 4.2A pass current is the top end of normal operation. From 4.2A to 7.6A is 'undefined', but at some stage in there the fuse will 'trip' (ie is designed to trip before 7.6A is reached).

    18650 packs seldom use PTC resistors or polyswitches for protection as those devices have too much power loss. Instead they use MOSFET switches.

    One big switch per battery pack is generally better than one per cell (less loss).

    Common failure mode is the battery pack (and PCB) getting damp. Plastic dip or cling film can a heap.

  6. #6
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    What about one of these http://australia.rs-online.com/web/s...duct&R=5176922. and heatshrink to the battery pack They are nice and small and reset when the power is removed or fault clears itself. You can get smaller or larger amperage ones also

  7. #7
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    I put in a normal mini blade fuse, but they are super cheap and easy to replace.

  8. #8
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    Quote Originally Posted by brad72
    What about one of these http://australia.rs-online.com/web/s...duct&R=5176922. and heatshrink to the battery pack They are nice and small and reset when the power is removed or fault clears itself. You can get smaller or larger amperage ones also
    They look to have low loss & are cheap. Good find!

  9. #9
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    Thanks guys. Coincidentally I'm going to make an RS order soon anyway for some switches.

    I'm starting to think along these lines for a fuse instead though as they incorporate a thermal cutoff in addition to current. http://australia.rs-online.com/web/s...duct&R=2047382

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