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  1. #1
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    dead battery pack q

    Hi,

    My cheap Amazon light stopped working, so I dug into the bundle of 4 cells--there's 5+ volts going into a circuit board, 0 volts coming out. What is that board's function? Is it essential? It's stealing all my voltses...

  2. #2
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    You need the little circuit board - bad things will happen if you connect the battery pack directly to the LEDs. The function of the circuit is to convert the voltage of the battery into a nicely regulated constant current for the LEDs, as LEDs are current controlled devices and their light output is pretty much a linear function of their current. Put too much voltage across them and they'll draw way too much current, either damaging the LEDs (if you're lucky) and/or overheating the battery pack (not good with lithium). In theory LEDs could be controlled with a regulated constant voltage but it's not easy to do as a small change in voltage leads to a big change in current/brightness and there's a temperature coefficient to the LED voltage-to-brightness relationship that needs to be accounted for.

    Phile, check for loose wires, a dodgy switch or bad solder joints on the components as those would be the most likely faults. Charred components are also a bad sign, although some can become a little heat-discoloured with no ill effects.
    M¦dgemagnet

  3. #3
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    Huh, I get voltage with one test lead upstream of the board and one test lead downstream--and can get that same voltage on either of the downstream wires (pos/neg), trying one at a time. Then I broke the board's connection to the middle of the pack, but left it connected to the battery pos and neg, and both downstream wires then had voltage. The back of the board looks fine. Can the board be replaced with an off-the-shelf part? I assume running the light without the circuit is a fire risk?

    dead battery pack q-board.jpg

  4. #4
    Drinkin' the 29er KoolAid
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    Grassington was confusing an LED driver with the protection board in your battery pack. They perform completely different functions.

    The protection board is important for the longevity and safety of lithium ion batteries. It prevents them from being over charged and over discharged, both of which are bad for the batteries and can lead to potentially dangerous failures such as cells venting/exploding violently!

    You mention there is 5 volts going in, but nothing coming out. If it really is 5 volts then the cells are pretty much completely discharged and the protection board is doing its job by not letting power continue to flow and further discharging the batteries.

    If you think you need a replacement protection board you can get it from places like eBay but you need to get the correct one the battery configuration. From looking at that photo it appears to be the typical 2S 7.4v variety. Something like this should work as a replacement: 2S Li-ion Lithium Battery 3.7v 18650 Charger BMS Protection Board 5A 7.4V 8.4V | eBay

  5. #5
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    ^ A picture is worth a thousand words, innit. Kwarwick is quite correct, that board is the battery protection circuit and plays no part in the LED driving apart from linking the batteries to the LED driver circuit and/or charger when everything is healthy. It shouldn't be bypassed - aside from the risk of fire, you really don't want to breathe in the fumes from a venting lithium cell, they're nasty.

    Phile - have you tried putting everything back together and plugging in the charger? It might be that the protection circuit is OK and it's just the undervoltage lockout kicking in and cutting the juice, as kwarwick mentioned. You can tell the cells are charging OK if the voltage starts to rise. They'll start off at 2.5 V per cell or thereabouts (the point at which the UVLO trip disconnected the LED load) and will slowly rise to a plateau of 4.1 V per cell (ideally) or 4.3 V per cell (max for safety). The cell voltage will drop when the charger is disconnected - this is normal.

    Many chargers have a safety feature that prevents them charging cells that are over-discharged, and 2.5 V per cell is about the threshold for this so there's a chance the charger may automatically disconnect its power if it detects a low cell voltage. It might be possible to nudge the cell voltage over this threshold by slowly and gently warming them up a bit (keep it below 50 °C/120 °F though, because lithium). I don't recommend boosting the voltage from an improvised charger because - aside from the risks of damaging the cells this way - lithium cells can be permanently damaged from over-discharge and can become unstable and unsafe. This is the reason chargers also have an undervoltage lockout. 2.5 V per cell might be salvageable, but something like 1.5 V per cell is a definite candidate for the battery recycling bin.
    M¦dgemagnet

  6. #6
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    Quote Originally Posted by Grassington View Post
    Phile - have you tried putting everything back together and plugging in the charger? It might be that the protection circuit is OK and it's just the undervoltage lockout kicking in and cutting the juice, as kwarwick mentioned.
    Oh, that would be a tidy fix. I think the charger may be dead--should it put out any voltage without a battery attached to it? It isn't doing so.

    Thanks a lot, both of you! Kwarwick, you were correct about the cell layout. Nice to know that circuit can be replaced cheaply.

  7. #7
    RAKC Industries
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    Btw, 5ish volts is the protection cut off, usually cut off at 6V. So you need to make sure all leads are reconnected (that little metal tab stick out is one) and recharge.

    Sent from my SM-G950U using Tapatalk
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