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

 

Really interesting pictures, Bob. I don't ever intend to do that, but fun to see pictures and data of your experiments. Please be very, very careful!
It'll be interesting to see if you get to the point of resurrecting a dead cell. We'll have to start calling you Dr Frankenstein.

I see the charge cycle is labeled as 0.2C, and the discharge is 6A. What is the C rating of this cell/module? (I'm curious what the discharge rate is in C units)

 

Hi,
Indeed, it has to be done carefully with safety first. Concentrated KOH is ever bit as hazardous as sulfuric acid.
I'm curious about failed cells but am really concentrating on recovering the capacity of failed modules. To really analyze a failed cell, I would need to carefully dissect one in a safety cell with optics, fine probes and a set of reagents. It would be fascinating but a poor use of time:

• 38 modules - NHW11 traction battery pack
• 228 cells - NHW11 traction battery cells

Every module returned to service with significant return of capacity is another replacement for a full-up, traction battery with one or two failed modules. The other 36-35 modules continue to provide service until the next weak module failure. If we are talking about 2-4 years that is a good return on investment.
Fresh, these modules are rated at 6.5 Ahr, which is what I'm using for the upper rating. However, the modules from the failed pack that don't have failed cells are measuring 2.2, 2.0, 2.2, 2.2, 2.0, 2.3, 2.2, and 2.3. So what I'm using is:Right now, I have two under test. One had an early tee nut failure to seal and showed out gassing. I'll have to rework that tee nut installation. Later, I need to stress test these tee nut modified modules under high current source and sink loads, ~75 A, and temperature extremes, <0C and ~50C. I'm not reworking these modules to have them fail in the winter or the first hot summer day. But how we access the cell for electrolyte replacement doesn't matter if the AHr rating isn't high enough.

The second module is showing signs of increased capacity. The initial 2.2 Ahr appears to have reached 3.2 and continues to increase. This slow increase in capacity makes sense as the KOH ions migrate into the areas recently wetted by distilled water. Using a KOH solution instead of distilled water would more rapidly return the cells to a higher capacity but I'm not ready to take that step, yet.

There remain questions about how the pressure relief valve works. We don't know the pressure threshold and this needs to be measured. I also need to learn more about KOH mechanical properties since there is evidence of electrolyte leakage around the "+" terminals, the ones treated in the resealing TIS. The NHW20 modules are substantially stronger and this and the condition of well worn, NHW11 modules suggests it was needed.

Bob Wilson

 

Do you have the patent number for the Toyota patent you mention? I'd like to look it up and see exactly how they are doing it.

 

Patent 6,936,371

 

Hi Bob,

Have you ever seen patent 6,573,685?
" Method of replacing secondary battery"
It says do cumulative 450% cycling and/or store battery in more than 50C environment for 5 days to recondition the NiMH battery.

I would like to recommend you do the reconditioning it before modify it.

Ken@Japan

 

Hi Ken,
I just learned about it this week. Fortunately, I've got inventory to test. <grins>

FYI, my latest cycle shows a discharge capacity of 5.3 Ahr from an initial 2.2 Ahr. The trend is still upwards so I'm continuing the discharge-charge testing. Also, I'm looking at round-trip efficiency and finding it is improving. This makes sense if the internal resistance is going down. Once the capacity levels off, I'll post the data and graphs in the usual places.

The second module has been sitting idle, clamped to the first, for the same period of time. I'll repair the leaking tee nut and do an initial calibration test. Then I'll review this patent and try the 50C test, which makes sense because the higher temperature encourages faster ion migration.

Thanks,
Bob Wilson

 

Bob- I enjoy the pictures. What am I looking at in post #1?

Did you take a hack saw and cut open one module to look inside?
Is the case just plastic?

In the Fords, we have 250 "D" cells. I am hoping if I ever find a defect, I can just swap out one ( or a few ) of the "D" cells.

But how would one, or a handful of new D cells behave with over 200 "old" cells? But I also suppose if I go to a junk-yard, I could find "aged cells" so they would be similar performance substitutes.

I read you had trouble with your drill holes not sealing.
May I suggest keeping the holes as small as possible ( 1mm ) and using a syringe to add electrolyte? Then maybe you would have more success sealing the hole. Maybe you could use a hot wire, or soldering iron to "melt" the tiny 1mm hole closed again?

-John

 

Correct. I had one module with a failed cell so the graph shows the voltage profile after adding water as an electrolyte replacement. But the dead cell never recovered and I used that module to find out what the internal structure looks like.

I clamped the module to a structure, put on safety goggles and protective clothing and decapitated it with a power saber saw. There were sparks and steam. The lower part, the cell assembly, is sitting in a large bucket full of water. I'll drain and refill until the KOH has been leached out of the electrodes.

The YahooGroup, Mk_1, is discussing this problem since they also have "D" cells. I've thought about the problem and suspect the same loss of H_{2}O reduces the capacity of their cells as it has in mine. But from the few sketches I've seen of "D" cells, a similar technique may work with them.

All of our batteries have a pressure relief valve so during periods of over-charging or excessive drain, some water in the electrolyte converts to O_{2} and H_{2} gas that escapes. The remaining water leads to a concentrated KOH solution that probably drives some of the KOH to form crystals. The difference between our batteries is the "D" cells have a steel case.

My current thinking is a small, shallow hole could be drilled in the cap with some risk of metal filings staying inside the cell. Then using a vacuum and valve setup, degassed, distilled water added to the cell. The small hole can then be sealed using the same 'spot welding' techniques used to affix the metal tabs.

It remains a possibility and I bought some basketball, air filling needles for such an experiment. But I'm not ready to give up on the tee nut since this leaves a permanent, electrolyte refilling hole closed by a gas-tight nut. This would allow periodic servicing of the battery modules to replace the lost water.

I have data that strongly suggest 'new' batteries improve the performance of our Prius but we lose this capacity within 3-4 months. The first is the monthly mileage chart from a Dept. of Energy fleet study of the 2001 Prius. The second is a monthly mileage chart by Patrick Wong who had an NHW11 traction battery assembly replaced (under warranty.) But now I'm seeing a mechanism for this in my current battery refurbishment experiment:

You'll notice that the round-trip, efficiency increases as the battery recovers its capacity. This probably occurs as the internal resistance decreases.

I am now more convenced that a capacitor based, buffer for our battery packs makes a lot of sense. What I don't know, yet, is how much energy and what rates this capacitive buffer needs to handle. But it is entirely possible that this could significantly extend our battery life and let us use a smaller capacity battery pack.

Bob Wilson