Hi folks,
I bought a worn out battery pack from an NHW11 because I knew it had at least one failed cell. The following shows the voltage as function of time as my MRC 989 battery charger tried to go through a refurbishment cycle after adding distilled water:
The proof is how quickly the voltage jumps up during the charge and later drops as soon as the discharge cycle begins. But worse, during discharge the dead cell began working like an electrolysis cell, generating hydrogen and oxygen gas that bubbled out of a small leak in one of access holes that failed to seal with JB Weld epoxy:
One problem with the epoxy sealed hole is the original hole is covered up preventing drill-out and resealing. So instead, I decided to try using stainless steel tee nuts, stainless steel screws and rubber gaskets. This will allow easy electrolyte replacement in the future:
The tee nuts were installed using a soldering iron and pre-drilled, access hole. These are test samples that I'll use for stress testing as well as 'blue printing' the battery module.
The epoxy-hole method failed because when I tried to close the hole, the cell assembly was still out-gassing. This meant the epoxy at one end failed to flow into the hole and set:
You'll notice the hole on the right, next to the B+ terminal is not filled with epoxy.
Going from left to right, the first cell has a fully closed epoxy hole; the over pressure valve, the temperature probe cup, the first tee nut, second tee nut, and the last cell with the incompletely sealed, epoxy hole:
There are dimension and process issues to test as well as pressure testing. But if the system works as expected, we may be able to return used NHW11 cells (and possibly NHW20) to like new.
BTW, the electrolyte, KOH, is a very powerful, caustic lye solution that will dissolve organic material like skin, eyes and clothing. It is risky to take a battery apart and requires: (1) eye protection, (2) face and breathing protection, and (3) skin protection. Treat the guts as if they are burning hot, lava and you'll have the right attitude:
I don't recommend repeating this experiment but I figured posting some of the results is better than ignorance. Toyota has a patent on how to refurbish a Prius battery and my experiments are aimed at understanding how well it works. But I also realize how dangerous this is and would recommend those who have some interest understand what has been accomplished and what remains.
So far, I have yet to 'recover' a cell. These initial experiments are aimed at the mechanical aspects of the problem. Soon enough, I'll get a chance to test the rest and then environmental regression testing. This means I need to make sure it works from below freezing to nearly boiling.
Toyota has a battery refurbishment patent but the method I'm following differs in these aspects:
- plastic welding vs. permanent, stainless steel ports
- KOH electrolyte vs degassed distilled water
- weight electrolyte vs. level measurement
Otherwise, we are after the same end result, returning the battery module to a significant part of the initial capacity.
Bob Wilson