Building a charger to revive lifepo4 cells

A couple of months ago when I was at a scrapyard in MI, I picked up 29 cases of these K2 energy LFP26650P lifepo4 cells with each case containing 20 cells. If you don’t know anything about lifepo4, the chemistry is much safer than lithium ion and typically comes with a higher power density and higher cycle life. The real downside is their energy density, which is much lower then a standard LiCo cell.

These cells in particular are 3.3v 9.4wh with a maximum continuous discharge rating of 40 amps per cell. These were an absolute steal for the price I got them.

But there’s just one problem, a BIG problem. These cells we basically “dead” when I got them and I plan to resell them. I can’t be selling 0.9v cells to customers for a battery which is supposed to have a nominal voltage of ~3.0v.

This is going to require some ingenuity.

Not shown, but I did test one of the really dead cells using an icharger 208B, and the cell tested at 95% of it’s original capacity. Contrary to popular belief, lithium can easily be recovered from a low-voltage state. Lifepo4 and Lithium manganese chemistries are especially good at recovering back from a low-voltage state. So no just because these cells have entered the “no-go zone” of 0-3v, does not at all make them dead. You just have to recharge them from this state and and run another capacity test. Big clive actually made a good video about this awhile ago:

What it comes down to is this: Lithium cells can be recovered if they haven’t been sitting for too long at a low voltage. If they have been sitting at that low voltage for a LONG time however, they can be problematic. These cases have a date code of June 2019, so that was quite recent. Back to the lifepo4 charger.

Now I can’t just solder these cells, because then it would destroy their resale value as well as eating up a bunch of solder. I was thinking of using an iron bar with neodymium magnets running across the bar but that solution may not yield the best connectivity. What I am going to try next is this:

These are 26650 cell holders, and I just ordered 40 of them. I plan to solder them together in parallel to create a gigantic 1S40P battery that could be used to charge two cases of cells at a time. since a very small portion of the cells energy lies between the charge voltages of 0-3v, we won’t have to push very much current into all 40 of these cells to get them back up to 3v, maybe 3v @10A max for all 40 cells. I’ve already got one of these little D3806 buck-boost converters here that I can use as a crude switching power supply, rather than buying yet another thing that I don’t need and hurting the bottom line.

once this stuff arrives in the mail, I will be building a very unusual looking lifepo4 recharge for getting these cells back up to 3v. More will come when the parts show up.

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