ReInVolt Pack Evaulation

[bwilson4web]

Hi,

I wanted to share the summary of this thread:

ReInVolt PHEV Plan - Page 3 - PriusChat Forums

SUMMARY

ReInVolt Capacity - there is no evidence that NHW20 modules suffer similar degredation that is seen in the NHW11 modules. Independent measurements by the Dept. of Energy, 'oldnoah' and myself have documented the loss of original NHW11 module capacity. In contrast, the ReInVolt pack has double the Ahr capacity of my fully functional, 133,000 mile, NHW11 pack. Suggestion: we, the NHW11 community, need a better method of measuring traction battery health. (My conservative measurement based estimate is 6.46 Ahr but the data suggests on the high-side of 7-8 Ahr.)

ReInVolt Battery Electrical Quality - the module-pair balance at ~0.08 volts is less than half of my existing pack balance, ~0.20 volts even after sitting unused four weeks. (This is the advantage of a depot rebuild. With a large inventory of NHW20 modules, they have the ability to match module capacity that an individual buying just pairs of NHW20 salvage packs will lack.)

ReInVolt thermal characteristics - there is no evidence of any problem cooling the ReInVolt modules. The improved, lower internal resistance of the NHW20 modules significantly reduces the rate of heating compared to NHW11s under descending hill, forced charging. (The newer modules have two, welded tabs between the cells and this significantly reduces the I**R losses. They are also more firmly constructed.)

ReInVolt case - the first case had a bolt hole misalignment and the installer of my pack reported some evidence of 'bent as if once dropped.' I didn't see it but then I didn't install it either. Regardless, case fit remains a challenge when dealing with used parts and shipping on a pallet. There was never any question about whether or not it would fit.

* * * end summary * * *

To measure the pack capacity, I calculated the dV/minute at a near constant load, ~65 A. I then took the highest measured voltage under load, ~294 V., and a projected 19*6 ~= 114 V end at 1 V/cell. Dividing the dV/dt rate into the voltage difference gave me an estimated discharge time to 1 V/cell which I multiplied by the current, about 10C. Fixing the units, I came up with 6.46 Ahr as the lowest possible capacity of the ReInVolt traction battery. The data suggests it is closer to 7-8 Ahr leaning towards 8 Ahr.

FYI, using a similar approach for the original, NHW11 traction battery suggests ~3.5 Ahr capacity. There is nothing wrong with the original pack. Rather, I felt this was an excellent time and a reasonable price to take 'battery failure' off the table. I'll also get to resume my earlier, traction battery experiments. <grins>

I now have to race getting a sustaining charger on the old pack pending building out the frame to hold it over the ReInVolt pack. Then comes the more fun task, the full battery management and interface to the ReInVolt pack. Obviously, the project is only 25% done. Still, it has been a lot of fun.

FYI, feel free to comment on any of the postings over at PriusChat. Just include the URL to the specific page so I can understand the question.

Thanks,
Bob Wilson

[MetroMPG]

Another wonderful & thorough test, Bob. Thanks for posting.

My only suggestion: maybe briefly explain what a ReInVolt pack is for some context here. (Though the answer of course is over at the PriusChat thread, and could likely be guessed from parts of your summary here.)

I'm looking forward to your PHEV experimentation. Hope you'll post a link to your thread once you get going.

cheers-
Darin

[bwilson4web]

Hi,

Thanks, I often get lost in what I'm doing and forget that folks aren't mind readers. Some background information is important.

The 2001-03 Prius, NHW11 model, introduced the first prismatic, NiMH batteries in a hybrid electric vehicle. The first generation NHW10 sold only in Japan used strings of "D" cells. But like many first generation products, field use revealed some weaknesses:

• weak terminal seals - under high temperatures and internal pressures, the seals can leak electrolyte.
• single cell-to-cell connectors - located at the top of each cell, this concentrated the heat in a 'gas' space leading the higher than necessary temperatures.

Notice the black material around the terminal. This is the rubber seal that has melted and oozed out around the terminal. This provides a path for latent gas escape under high load and high temperature operations:


These latent defects would not be a problem unless someone operated their NHW11 Prius in hot climates, speeds over 65 mph and in hilly terrain where the exothermic charge cycle can 'heat pump' the battery to high temperatures and generate gas.

With the 2004 model NHW20, Toyota and Panasonic changed the design by making the case stronger and putting a second, cell interconnect near the bottom of each cell. This distributes the I**R load and with the stronger terminal seals all but eliminates the loss of gas, H{2} and O{2}, that dries out the electrolyte:


Taylor Automotive buys the traction batteries from salvaged NHW20s and ZVW30s (2004-current) and surveys the modules to establish an inventory of matched modules. They then rebuild NHW11 traction battery packs using the much improved NHW20 modules.

Now most folks wait until their NHW11 traction battery fails, usually one or two modules and then go after a replacement. But it occurs to me that an alternate approach is to replace the NHW11 traction battery and 'retire it' to the much easier life of a PHEV battery:

• active charge balancing - so all modules are paired for optimum performance
• wider dynamic charge/discharge range - the Prius battery controller is supposed to limit charge to 80% to prevent over charging the strong modules and a discharge level of 40% to prevent a fatal reverse charge. But in a PHEV, the battery no longer has to respond to driving conditions as much as provide additional energy to reduce fuel consumption
• tailored charging - instead of 'drain me till I'm dead', a more strategic approach that seeks to minimize inefficient fuel burning makes a lot of sense. So cold engine starts and warm-up receive extra energy until the ICE becomes efficient. Then the remaining PHEV battery energy is saved for the next cold engine start where it can do maximum good. Also, a custom battery controller could store excess kinetic energy that the existing traction battery may not be able to handle.

There are some other things I'd like to do but first I need to pay attention to the basics and get the NHW11 battery mounted where I want it . . . after some refurbishment including balancing the charges. <GRINS>

Bob Wilson

ps. Is that 'brief' enough?

[MetroMPG]

I appreciate a quick read.

Are you blazing this particular trail - first PHEV boost pack in your generation of the Prius?

[bwilson4web]

Probably not the first. Just the latest with the benefits of a lot of "lessons learned."

Bob Wilson