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September 17, 2012

Harvesting and Re-using Lithium Cells from Laptop Batteries

For more than a decade, I’ve been using a series of HP and Compaq laptops and as a result, I had accumulated a pile of expired, apparently dead batteries composed of lithium-based cells. I decided to resuscitate a couple of the old laptops for a project in Campbell River, and needed a couple of working batteries. Rather than buy new ones, I decided to crack open a few of the old ones to see if they still had any working cells that I could re-use.

Unfortunately, unlike NiCd cells, Li cells apparently can’t be rejuvenated when they die. However, all I was looking for here was enough working cells from my pile of used batteries with enough life left to have 30+ minutes of runtime on a laptop to get me through what seem to be fairly frequent power failures at my home in Campbell River.
My faint hope was that the cells had not died uniformly and that I would find a few cells that would still hold a charge. That luckily turned out to be the case.
Before you wade into trying this yourself, note that these cells store a lot of energy (when working) and also contain nasty chemicals. Wear protective gear such as safety glasses and thick gloves to protect yourself, and be careful. Usual disclaimer: I take no responsibility for any harm or damage that might result if you try anything like this yourself.

If you’ve looked at HP and Compaq batteries, you’ll know that they are sealed plastic units, with replacement prices ranging from $40 (no-name) to beyond $150 for real HP replacements. The cases can be broken open, though. In my case, the way I did that was to carefully hacksaw a groove along the glued seam parallel to the side of the case while trying to avoid damage to the cells, and then gently pry the case apart at the groove using a large standard screwdriver. That was typically enough to get the cells out. Retain the circuit boards and case for re-use. I test-charged the extracted cells using a variable DC supply, and retained for re-use the ones that would charge to and hold at least 4.5V voltage. I didn’t bother doing internal resistance measurements or any other electrical tests.
I found enough cells that met that crude criterion to put together 2 batteries of 8 cells each, to fit the battery cases for an n610c laptop and an nc6400 laptop, respectively. The metal tabs on the cells appeared to be spark welded onto the cells, but there was enough exposed tab remaining that I could solder 14 gauge wire onto it using a soldering gun, and put together a working battery. Be careful doing the soldering; you want to avoid heating the cells much, so use heat sink clamps between the cell and heat source if you can. Note that at least for these HP laptops, the cells are set up in paralllel pairs. The n610c version looks like this:

The end result was a couple of battery packs that would get me >1 hour of runtime, basically to give the laptops consumer UPS-level power assurance. These were all used cells, and I don’t expect much from them, but since they won’t be going through a lot of charging cycles in their anticipated usage, I think that they will last at least a few years.

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