Hi

With insights from the "Prius_Technical_Group" group, I'm working on reconditioning individual cell assemblies for NHW11 batteries. For example, the cell assembly in the photo has an ending capacity of 3.2 Ahr, enough to match the cells in a used pack that has suffered one or more failed cell assemblies:

The automated battery conditioner first discharges the cell assembly to 1 VDC. It then charges it up and records the capacity. Three full cycles appear to give consistent results. NiMH charging generates gas so the battery must be clamped to prevent the sides from bowing out and destroying the cell. The VOM has a capacitance tester and the 13.3 nF is the clamp load, no additional pressure from the cell under test.

The batteries have interlocking 'cups' and 'dimples' and using just a flat board would lead to local, mechanical stress on the sides. So a pair of plastic shims cover each side providing strain relief for these raised surfaces. One board has a capacitive, load sensor to detect excessive gas pressure:


This approach takes usable cell assemblies from a salvage battery or one with bad modules (preferred) and reconditions the working cells. Someone with a failed battery pack can get replacement cell assemblies, ~$30-40 each, and replace just failed cell units. This won't be a 'new' battery pack but a reworked pack, good enough to keep the Prius rolling for a lot less than the $2,300 of a whole battery pack.

Questions? Comments? Concerns?

Bob Wilson

After April 3, use e-mail to contact me:

Wow. Excellent research. I think this thread will become popular in the near future as the original Prius batteries approach their useful lifetime.

A few questions:
* If I remember correctly, NIMH batteries release this gas as they approach their max charge. Could you add circuitry that monitors the battery temperature (via thermistor) and reduces charging current as the temperature increases to prevent gas buildup?

* Also, I remember that NIMH battery cells (for consumer use) have a catalyst in them that removes the gas pressure and produces heat as a byproduct which greatly reduces battery damage in the event of overcharging. Do the Prius NIMH cells have this catalyst?

* How hot do the cells get during the reconditioning?

Why are the manufactures not using this method to repair HHW11 packs. Is it too labour intenssive or not condusive to making money? Thanks for the research, Bob. H

It turns out the charger I'm using uses a dV/dt test to determine when the charge cycle is ended. It also has a thermistor that also stops charging if the battery reaches a maximum temperature, currently I'm using the lowest threshold, 100F, but none of the batteries have gotten more than 2-3F warmer. The dV/dt detector is great.

My understanding is they do have the catalyst and I believe it works. Before I got the good charger, I used a car battery charger, current limiting resistors and VOM. This was not sensitive enough to detect the dV/dt, ~5 mV. But was able to detect the over charging was starting but this was using just plain, end-boards, nothing with a strain relief or load sensor. Heck, the capacity measurement was a spreadsheet that I manually entered the amps and time to calculate the aHr. But when I took that first cell out the clamps, the sides bowed out from the generated gas. I was appalled.

I checked the battery and it had a full charge so I shipped it and let the customer know all they had to do was discharge the battery to the same level as the adjacent cells and go to town. I offered to replace it if there was a problem and mentioned the sides bowed out. However, he didn't see the bowed out sides, which tells me the catalyst worked.

I now have a charging setup with a load cell that I can use to determine if gas has been generated. If so, I'll keep the cell clamped down until the pressure subsides. As long as the sides don't flex, they should handle the stress easily.

Bob Wilson

After April 3, use e-mail to contact me: