144V NiMH trickle charger design

[cerickson]

I am toying with the idea of building a ZX-24 microcontroller-based smart trickle charger for the 144VDC (dangerous voltage, I know) battery in my Honda Insight hybrid car. The Toyota Prius also uses a 144V NiMH battery pack. The Insight uses special high-current NiMH D cells (low Ah?) and the Prius uses a more exotic proprietary cell type.

For those people who don't know, there is a known problem (but not acknowledged by Honda or Toyota) when "seasoned" hybrid cars are parked for a month or three without being driven. One or more of the 1.2V NiMH cells can become much more depleted than the rest and this can cause the car's battery computer to flag a temperature error and disable use of the battery pack. I assume the high current capacity of the cells means faster idle charge deterioration. The advice of Honda and Toyota is to spend about $5,000 for a new battery pack! When this happens, Prius's become DOA in the driveway. Insights convert to a "gas only" mode and keep driving, just with less horsepower and MPG. Savvy users have been "rebalancing" the charge in the 6-cell battery "stick" assemblies by tearing apart the battery and cycling the charging in each stick with a commercial power supply. Typically an ICE duratrax or a Superbrain 989.

I figure I can use a 2:1 ratio transformer to step up 120VAC to 240VAC (or whatever) and from there invert, filter and regulate down to the correct charging voltage and current for the 144V NiMH battery pack. I would like the microcontroller to handle the charging control as well as display the current battery voltage, charging current, depth of charge estimate and remaining time to full charge (or remaining time to 80%?) Also to terminate charging when the charge cycle is complete. I am guessing that thermally monitoring the battery temperature will also be desirable to better estimate charging process. What some users have been doing is to put four 48VDC power supplies in series to make a whole-battery trickle charger. It is also my understanding that the regular cycling of the battery packs by the cars is between 20% and 80% charge. My guess is that the last 20% of charge is just too expensive to push into the battery, especially if it is being used in cycle service instead of float service. If NiMH cells are the same as lead-acid cells, the last 20% takes four times as much charging current as does the previous 80%. Or so I have read. Maximizing the life and reliability of the 144VDC battery pack is the ultimate goal.

What I am hoping to find out is:

1. Some good links to web sites that describe the correct way (and circuit details) to charge NiMH batteries. Special interest in designs that could handle a 144V pack.

2. Any general advice on how to go about making a safe-to-handle, fully-automatic trickle charger that could be permanently installed on a Honda/Toyota hybrid car 144V NiMH battery pack and have a standard 120VAC male plug socket mounted on the side of the car.

3. Maybe some intrepid individual who is willing to design and sell a trickle charging kit for Honda/Toyota hybrid cars! I have no idea how big the market might be for a product like this. It is a rather specialized application.

There is a lot of background information that can be found at www.insightcentral.net

Thanks in advance for any and all replies.

And thanks for reading!