Problem with battery pack..

[henryjay]

I have a faulty battery pack for a Magicshine bike light I am hoping someone can help with. It was working fine but just died after leaving on the charger overnight.

The battery pack consists of 4 x 18650 cells 2000mAh and a PCB (part# JZR885-A). At the battery side of the PCB I have approx 8.27V but at the light side I only have 4.81V (I believe should be 7.4V).

Does anyone know if the PCB is repairable, or what I could replace it with?

 

[henryjay]

After having another look at the PCB, it looks like one of the surface mount components is missing .. possibly R6 (name is slightly covered by the soldering)

Would this affect the output voltage? what size resistor could I try? (if this is the problem)

 

[core]

Would this affect the output voltage? what size resistor could I try? (if this is the problem)

I'm not familiar with that board, the only google result for the p/n is this thread, and I'm no circuit design expert, so take this with a grain of salt:

But logically: It's a good guess that it's either a 4.7megaohm or 1k ohm. Since it's open now and nothing is smoking, I'd start by temporarily putting a 4.7M on there first, then second try 1K. Kinda need another pair of hands for that to hold all the leads and measure voltage output at the same time.

That's just what I would do if it was my equipment.

 

[VidPro]

great pictures did you indicate anywhere what the voltage of each set of parellel cells was at ?

myself i sort of doubt that a part fell off the board, the resisters there in this application are not likly to be working hard or heating up when working properly, If they were taking major heat through, then falling off would occur.
if core figures they would be megohm or Kohm resisters there, then they sure are not for all the power flow , so they shouldnt get falling off hot. the protection PCB themselves use very tiny ammounts of power.
if it was the sence resister with less than a whole ohm thing , then it could.
it just doesnt look like something fell out, its not consistant with that look.

they can leave stuff out of different models or just leave "holes" in a PCB where the item was deemed unnessisary or even was wrong to put it there. so something missing MIGHT not be missing at all.
on the other hand these flea parts held with thin vat soldering can easily fall off, but these look Blob soldered , and not conservative with the solder.

and if it did fail or something fell off, i wouldnt fix it because it is faulty design, the protection should protect itself (so to speak) where protection has a max amps that can flow through it, so even if the output was shorted the protection should not croak. the protection has to be damaged (physically) or shorted itself. uhhh because protection has short protection for the battery pack.

then what is the voltage of each battery? because you indicate it happened on the charger.
have you put a load on the output yet? or only tested with the meter?
if the protection was kicked on, the voltage reading you get might not have any meaning, unless of course it also doesnt work when put under load (which i would assume you already tried)

What if one cell item is just overcharged beyond the protection engaging, this is not balanced, or balance charged is it? soooo, that exist a tiny possibility that one of the cell items is over voltage. hmm not likly but worth checking for before you change anything.

sooo, if you cant get it working, any of these 7.4v 2S things sold here are cheap and could be user replaced easily http://www.batteryspace.com/pcmforli-coli-mn-nibatterypacks.aspx
just get the right amp ammount for the use , not way over and certannly not under.
caveats some of the "4amp" protection only does a good 2 amps, and some "8amp" protection only does 4amps, geez dont ask me why?? but the dealer will often note it , even if the manufactures specs are totally wrong.

the output side of protection should read exactally the same as the voltage of the batteries, when there is no load, and only minimally different when there is a load, the protection is not regulation, and from what i can tell this is not a regulator.

 

[henryjay]

I think I might just buy another PCB..

In the mean time though, what would happen if I bypassed the PCB?

Can I damage the P7 LED if I ran it straight off the batteries (8.4V)??

Can I recharge the batteries without the PCB?

 

[VidPro]

the protection curcuit protects against overcharge , overdischarge , and over amps, you can certannly charge it without it, but you would have to do so without overcharging. and charging to 8.4v isnt enough, you would have to insure that neither cell set went over the 4.20 (+-.05)
when discharging you would want to make sure that either cell set is not discharged beyond 2.4V or basically 3.0-3.6v depending on the load Wherin the capacity is mostly depleated, 2.4v is the bottom end low on any spec sheet, capacity depleation is more important which occurs higher than that.

as far as driving your led, was it being driven "direct" with no regulation already?
a direct driven led can keep your battery from being discharge way to low by default, as long as you stop the discharge of the cells when the led is not very bright anymore. assuming again that the balance of the cells is ok, SOO you dont discharge them to far.
if the light uses a "driver" that boosts the voltage , then no ya really should not discharge without the protection, because any single cell can be depleated to far for its own good and be damaged from overdischarge.

there are many knowlegable people who will use unprotected cells in lights, and they just watch them like a hawk to stay within the parameters. . . then they scew up that too sometimes and know what happened.
its all in the knowing of the specs and the range the battery should not get out of. As always its good to get the data sheet for the actual cell items or battery itself, even if (yes) they are all pretty much the same. if you have the spec sheet for the actual cell items, then following that is far superior to anything i have said its in the book.

protection is pure magic safety and longevity, i could tell you how many times i have screwed up , thinking i could do as well as the protection does but that would make me look rather stupid soo dont leave home without it. falling out of the parameters will eventually make the battery an unsafe brick.