Newbie circuit board on battery question

[Rich Gold]

Hi, I'm new here, I just wondered if anyone here could help. I'm looking at fixing an expensive speaker which wouldn't power up. Having opened the speaker I checked the battery which didnt seem to have any power, so I opened the battery pack and checked each cell which did have power, but then I noticed a small circuit board attached to the battery pack. When I by-passed this board by taking the positive and negative from the battery straight to the speaker then the speaker turned on, if only for a few seconds as the battery must now be discharged. Here's a pic of the battery pack:

Apologies for the fuzzy picture. The picture shows where I cut the red and black wires to the board and bypassed it. The wires went from battery to the solder points half way down the board then out at the top of the board (where the yellow wire is still attached) to the speaker.
When I connected the unit up to charge the speaker also worked, but when I stoppped charging it stopped almost straight away.
Am I right in thinking that the yellow wire is to supply charge to the battery and the board only contains a diode to allow current to pass one way? Are these packs available anywhere as a full unit to buy? It contains 4 li-ion 18650 with a total rating of 7.2v 4400mAh.

I have to be honest I'm pretty much a novice at these kind of things but thought I'd have a go a fixing it so any help would be appreciated and gratefully received.

Apologies if I've posted this in the wrong section.

 

[Mongrel Shark]

Sorry for the slow reply. That board is a protection circuit at least. (without clear pics of both sides I cant say exactly what it does but could have some charging stuff on there too).. The Li-ion batteries contain highly flammable and volatile ingredients that can become dangerous if the voltage in the cells gets too high or too low. So a protection circuit can be used to disable them permanently or until inspection. Some or all of the cells could be good. Its likely at least one cell is bad. Testing them mostly requires some dedicated gear or moderate understanding of voltage, current, and battery charging theory. There is also some risk of fire if you where to test them and do something wrong.

From your description I would say more than one cell is bad. Maybe all of them.

The yellow wire could be a sensor for the charger, or a charging input or a balance point (soldered to the junction between batterys in series). Or something else. I suspect from your limited description that its a charging input, but is it charging both strings of cells in series at 8.4v or all 4 of the cells in parallel at 4.2v?

You can get new Li-ion packs on ebay. Start with "7.2v 18650" search and expand or fine tune as required. Beware of the really cheap cells with really high AH ratings. They tend to be fake/not the advertised AH. I got some 18650's labeled "ULTRAFIRE" a while back. Not sure if brand name or product description... Also no li-ion via air freight, so pay more for local or wait 3-4 months for the boat from china....


Does this help you resolve your questions?

 

[Rich Gold]

Very helpful, Thanks for the reply.

I had been playing with it for a couple of hours and now understand far better the fire risk from these batteries, not first hand thankfully, only from watching them explode on youtube. As for the moderate knowledge of voltage and current I know a little and have an mulitmeter
The battery serial no is: AE18650cm1-22-2p2s
I've been looking to buy a replcement battery pack or to get one made up but have been unanble to find one. Do I need anything special to make one up myself? I undrerstand the tags are spot welded on arent they? Would solder do the job? Could I blow myself up while doing it? I saw 4 panasonic 18650 4400mah for sale for £10 which looked like they were right for the job but havent bought anything yet as I moved on to trying to buy a direct replacement.

I will add a couple of better pictures of the board in a day or so (it isnt here with me at the moment).

I also have ultrafire batteries here, I believe they are branded for Ultrafire led torches, which I also have a few of.

Thanks again for your reply

 

[Mongrel Shark]

Yes. The cells can be soldered although its not really recomended. Depending on skill and luck some may get harmed. If the skill is low enough fire could be a posibility.
Maybe try some really cheap ones first. With extinguisher at the ready.

Here is a pic of a similar 4 cell 7.2v 18650 pack I have. With (vented) ultrafire for scale.
I have a few of these that came with LED bike lights. I get a pretty good light and one 18650 pak for under $30. If you like LED lights that might be an option.


I doubt the packs have protection circuits. The lights are quite happy to run down to 3.5v. So I'm not sure if my ultrafires keep venting due to being under charged, or because "ultrafire" is product discription...


If you decide to make your own pack. There are cheap protection circuits on ebay. I'd recomend the $2 investment. Some can be reset/reused. They also sell 18650 holders in many configurations. Likely including 2p2s configuration... for $5 you could have a spring holder with 4 slots and a protection circuit.

Strangly I couldn't find 7.2v 18650 packs either.

 

[Rocko the fixit cat]

I think I found the battery you have on AliExpress and bought it Sept of 2018. Its an exact match! Label and all. Made by Hixon. Probably fits most Braven speaker models:

Hixon New 7.4V 4400mAh Replacement Battery for BRAVEN 850 Speaker-UL UN Certified

Price: US $36.99 free shipping

Replacement for: BRAVEN 850, J177/ICR18650-22PM
cells inside: 4pcs 18650 cells
capacity: 4400mAh
Voltage: 7.4V/32.56Wh
It was shipped from SoCal too.

 

[Externet]

What I would do is apply 4.00 V to each cell - one at a time- for about 10 minutes each without disconnecting anything.
That will bring their voltage high enough to be recognized as 'healthy' by that controller board, and the speaker can/should be recharged normally.
The performance of the pack can be diminished if cells are beyond recovery, but will give you an idea of what is going on, if were discharged beyond a point that enables the circuit to recharge.