Ooooo that smell....

I've rebuilt Lithium Ion battery packs before, but I've never used the technique I tried with this most recent one. At work, we have more than a few Li-Ion packs that are unable to be shipped for various defects. It's sad to let these cells go to waste, but it's sometimes unavoidable. My most recent project required five bad packs to complete. Two would die.

When I rebuild Lithium-Ion packs, I have the luxury of new cells. This is a requirement, in my mind, since there's no practical way around the 2-3 year design life of the cells before the internal resistance goes too high for most applications. If I'm going to go to the trouble of it all, it has to be a long-term thing.

Normally, when rebuilding battery packs, I would leave the welded tabs attached and just solder them together in the proper configuration. This works in most cases, but it takes a long time to make everything fit just right. For this rebuild, I tried a new technique I'll explain in the main part of this write-up. Also, when I rebuilt packs in the past, I made sure the parallel cells are matched, but left the series side of things to the charge circuitry. It worked for the Dell and Apple systems I've done rebuilds on, but not for the Toshiba. Here's the story.

I knew I was getting into uncharted territory when I decided I was going to solder directly to the cells I was using for the rebuild. This is risky business since high temperatures are the mortal enemy of most battery types, more-so for Li-Ion technology as evidenced by the rash of recent recalls (that "r" shit was unavoidable).

To remove the tabs from the old cells, I used an X-acto razor. I went through two blades for this. Removing the strips from the new cells used another blade. the positive ends didn't require much dwell time as they aren't in full contact with the guts of the battery. The negative cans, however, require efficiency and a lot of flux. I found a multi-point approach works best, tinning a few small spots with a bit of cooling time between them. With about six small spots completed, I add more flux and scratch the battery can with the tip in a circular motion until the entire contact area is tinned. Then, I quickly press the battery can against something metal to drain the heat. It worked four out of five times. The fifth time, I ended up with an internal short and hot cells that I quenched under a drinking fountain and took outside.

With my new cells prepared, I desoldered the original strips from the control board and tinned them for attachment. I taped the banks together and quickly soldered them by holding my iron against the straps to melt the solder. The resulting joint wasn't smooth, but it was a good joint. I used the razor knife to cut the excess solder off the top of the connection to avoid fit and finish problems.

With the pack rebuilt and totally not RoHS compliant at this point, I put the whole thing back together and let it charge. Charge it did. And I ran it down that night, charged it up and ran it down again. I noticed the next day that the system would disown the battery pack during charging, but still would run off of it. Leaving the system on standby overnight would normally be easy for such a system, but I found that it was dead the following morning. I took it to work and found that the center bank was reading about .1VDC and had some fluid leaking out that smelled like a combination of alcohol and acetone. Okay, time to replace the cells in that bank.

I found another suitable donor and cannibalized the cells, once again leaving charge balancing up to the system. Didn't work. The end banks charged fully and the system disowned the battery pack again. Opening the pack, I found that the center bank was reading 3.5VDC while the end banks were around 4.2VDC. Not good. I left it like this for a day while deciding what to do.

The next day at work, I used a mass of LEDs to drain the cells on all banks down to 3.2VDC and then charged the pack again. It worked, but there was more of that fluid again. I don't know what it is, but it seems only to come from the center bank. Regardless, the battery pack discharged just fine and recharged for a while before I shut my system do to head home early for lack of work.

The new 12-cell battery pack I ordered from an eBay seller arrived soon after I went home so I charged that overnight. Today, I opened the rebuilt pack to check the voltages and the center bank is hanging around 3.95VDC while the others are 4.1VDC. Given the pack only had a 30% charge, I expected it to be unbalanced, but time will tell. There was more weird-smelling fluid from the center bank again, but nowhere near as much this time. I hope it will stop coming out eventually.

I plan to use the new battery pack only when I'm going mobile while the rebuilt pack will be used when I am plugged into the wall or the new pack is warming up. The new pack will be stored in a fridge when not in use.