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  1. #1
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    Li Ion battery pack, PCB or Balance charger

    hey guys

    if i use a balance charger to charge a 3 pack 18650 pack am i right in thinking that i wont need to have a protection pcb in the pack?

    thank you

  2. #2
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    Yes but only if there is some other other form of low voltage cut out to stop you from draining the cells to far down. The bold here is because over discharge can cause a pack to heat up and maybe even burn.

    Many lights out there rely on the battery PCB to cut out when the battery voltage reaches a critical low level. Often this is not soon enough to prevent degrading the pack.

    I built a light for my brother that used a balance lead on the pack and a balance charger and no battery pcb. I had a low voltage warning that would flash red and then go solid red then sound an alarm but he ignored it and nearly started a lipo fire.

    As long as you have a low voltage disconnect and not just a warning you should be ok. A properly configured taskled.com flex driver will provide a good low voltage cut out and it warns you of the cut out before it happens.

    I have found the combo of a taskled.com flex driver set to cut out when the pack still has 30% and a balance charger to be the very best for my battery packs.

  3. #3
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    the batteries i am using are samsung icr18650-26a, the data sheet suggests the discharge cut off voltage to be 2.75V, so what you saying is to have a low voltage cut off to cut the power before it gets to this?

  4. #4
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    Close to it - for my TaskLED drivers I program 2.9v - after that much discharge on one of these, there's very little capacity left.

    Ive seen too many pre-built packs, or standalone PCB's that state 2.75v as the low cut for these cells, which is the manufacturer's lower safe limit. I don't want to get to that point if I can avoid it.

    You're talking seconds of extra run time, not minutes.

  5. #5
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    PCBs are also there to prevent overcharge and protect against shorts. Given that decent PCBs are ~$10, it's not too much of a stretch to add one for some extra piece of mind.

  6. #6
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    Quote Originally Posted by mattthemuppet View Post
    PCBs are also there to prevent overcharge and protect against shorts. Given that decent PCBs are ~$10, it's not too much of a stretch to add one for some extra piece of mind.

    I am now wondering if and how a PCB works with a balance charge? Can you just wire up the balance charge lead as a normal pack and charge like I normally would with mean and balance lead connected to charger. Balance charging with a good charge is enough of a plus that I will give up the protection a PCB offers if it means I cant balance charge with my turrnigy accucell.

    A polyswitch is usually what provides short circuit and over current protection. They can be thought of as self resetting fueses. They are used with and with out PCBs. I would like to buy a few to use my packs as there .if virtual no disadvantage. Any one know where to get them cheap? They ought to be a cheap component.

  7. #7
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    good point, I hadn't really thought of that - if you balance charge the pack you'll lose the overcharge protection while charging. It should still protect against shorts and overcurrent situations though.

    I'll have to check out polyswitches - I've heard of them but never done any research into them.

  8. #8
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    Quote Originally Posted by mattthemuppet View Post
    good point, I hadn't really thought of that - if you balance charge the pack you'll lose the overcharge protection while charging. It should still protect against shorts and overcurrent situations though.

    I'll have to check out polyswitches - I've heard of them but never done any research into them.
    Skip to the end for solution.

    Based on the way my turnigy accucell 6 works don't see why there would not be still be over charge protection. When I connect my packs to the accucell I have to attach the main leads and each balance lead. The charger monitors each cell and the mains at the same time. I assume and could be wrong that the pcb would cause my charger to end the charge if the pcb disconnected the mains.

    Experiment :

    I disconnect the mains while balance charging. - result - charge stops, charger makes beeping sound, displays flashing error message on screen - something to the effect of circuit break mains.

    Still pondering on the pcb would behave if it monitored individual cell voltage and cuts out main(total series voltage) based on individual cell voltage as well as main(total series voltage) voltage.

    Now I am thinking it will mess with the chargers ability to actually charge and balance the pack.

    Solution :

    I have thought about this the best solution is a high quality 18650 holder with the PCB and poly switches wired in to use as the pack. Add a second cheap 18650 holder with a charge harness.

  9. #9
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    The main thing you're giving up leaving out the protection PCB and using a balancing charger, is the per-cell voltage monitoring during discharge and a certain amount of the over-current protection provided by the PCB.

    Using the driver's low voltage cutoff will work ok when the battery pack is new, but is somewhat risky as the pack gets old. As the pack gets near the end of life in 3-5 years, and you've forgotten about the decision to leave out the protection PCB and aren't paying much attention anymore, one cell will tend to die first from old age. This is how li-ion pack almost always fail, the cells never go bad at the same time. That's why people are able to salvage some moderately good cells from old laptop packs. The bad cell will go under voltage very early and will not be detected by the driver because the other cells that haven't gone bad will still be in the range of 4.2 to 3.9 volts fresh off the charger. Then in theory, the bad cell could go into thermal runaway on the next charge cycle even though you're using a balancing charger.

    Using a polyswitch for discharge protection will provide some over-current protection, but not the same amount as a protection PCB. A protection PCB won't reset until the pack is charged again. The poly-switch will reset automatically and can expose the pack to repeated over-current spikes. But, I agree that the poly-switch provides the lion share of the over-current protection you'd get from a PCB.

    There are lots of different ways to approach balance charging. Some chargers actually charge through the balance leads, while others monitor the balance leads and only act if necessary by shunting a small amount of current around cells that are getting more charged than others, and some actually only monitor like a protection PCB and don't really actually balance at all. They just terminate the charge when the most charged cell gets up to it's limit.

    All of these typically work fine in conjunction with a protection PCB. If you're using a charger that actually charges through the balance leads, you will bypass the protection PCBs over current protection during charge but presumably the charger is monitoring that for each cell. For chargers that charge through the main leads, both the charger and protection PCB will be happy as long as everything is in spec. If either one detects a problem, the charge will be terminated. If the PCB detects a problem, it'll open the charge path and you'll see an error on the charger that something disconnected the main leads. If the charger detects a problem or terminates the charge normally. The protection PCB can't tell this from unplugging the pack from the charger.

  10. #10
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    I'm glad you chimed in mtbmacgyver, it's your explanations of PCB function over the years that's convinced me to use one on my next light

  11. #11
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    Yes that clears it up. I am glad to know a PCB can be used with a charger with balance leads.

  12. #12
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    So what are the best options for PCBs? There is so much talk on this forum about LED systems, but not too much about making battery packs.

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