PCM for Battery Pack

The batteries may be damaged....I would replace them...

http://www.all-battery.com/1S_Li-Ion.aspx

http://www.all-battery.com/polymerli-ionbattery37v2400mah3568100.aspx?utm_source=GoogleShopping&utm_medium=GDF&gdftrk=gdfV26767_a_7c354_a_7c922_a_7c30137&gclid=CjwKEAjw6sC5BRCogcaY_dKZ2nESJABsZihxe3k_vmeAx_lt7DVTQw2SIXY1UmEk8DcP7hEmg1NmEBoCbMXw_wcB

I don't know what PCM stands for, but there are several things that seem odd:

1. You say: "The unit got wet and burned the B+ trace". To me, a "trace" means a strip of copper foil on a circuit board (PCB). From what I can see, the traces all look fine.

What I think I see damaged is not on the PCB, but in the strip of metal that goes from the B+ terminal to one of the cells/batteries. If it got wet, then I suspect that strip didn't get burned (damaged by heat), but rather it got dissolved by chemical action (probably involving leakage from one or more cells/batteries).

Those strips of metal are commonly spot-welded to the end of the battery, because the battery will be damaged by the heat of soldering. Once that strip gets oxidized, either from chemical action or from heat, it is virtually impossible to solder a jumper across it. If you scrape it enough to get a clean solderable surface, then the base metal will be exposed, and it will deteriorate rapidly from the chemicals/vapors.

2. You say: "The pack consists of 4 cells, (3.7V) 2 in parallel." In the photo, I can see what appears to be two cells/batteries. Although these cells/batteries are physically mounted parallel to each other, the upper one clearly has its positive end on the right, while the lower one has the positive end on the left. Assuming you are correct that the left ends are connected to common, then the batteries are electrically in series, not parallel.

There are several small traces on the PCB, indicating that there is/are more components on the other side of the PCB. Until you know what additional components are there (and what function(s) they provide), you have little probability of getting it to work, even if you should follow SE's suggestion and replace the cells/batteries.

Perhaps I wasn't clear, my suggestion is to replace the batteries and the Protection Circuit Module...Not knowing the mAh required or the load, I left links to sources that should fill the requirements...

I found this on eBay
http://www.ebay.co.uk/itm/2-Cell-7-2V-7-4V-8-4V-Li-ion-Polymer-Battery-Protection-Board-18650-Circuit-A16-/221779998121

This will work if he has a 2s2p battery configuration. We still don't know for sure but it would make the most sense.

I don't know if the cells have internal protection, but if they do, then the wire may have failed from a short, and one of the cells popped it's protection. Or, as it was suggested, the soldering job did it in. I have soldered the positive terminal of 18650 Li-ion batteries ok, but not the negative. You have to be very quick so a hot iron is needed.

The picture does not clearly show this but the top and the bottom cell shown each have a cell underneath, so the top cell has one cell in parallel and so does the bottom cell. The charger adapter has an output of 8.1V @ 1A, so you are correct that they are in series.

The foil coming off the B+ of the PCM was deteriorated and could have been from corrosion. I examined the case closer and did not see any heat damage so that may be the root cause.

Without knowing precisely where your battery pack originated, the forum has nothing to base a suggestion of an appropriate charger.

Battery technology has come far in the ten years since this original discussion. I would suspect that an appropriate charger for the subject battery pack is no longer available through conventional sourcing.

A quick search of an internet commerce site using search term “7.4 volt battery pack” yields dozens of results, and many of these are rechargeable using an integral USB port. This seems to be the current state of the art.

Some offerings are very inexpensive… $13.00 USD for two… but should you elect to go this route be aware: The cheap ones are in the range of 600 mAh capacity.