How important is it to buy and install a "Grid Charger"

great idea! any specs on wattage compared to oem's?

 

Last night update:
Finished and tested repackaging Toyota's High Voltage NIMH Rescue Charger for Lexus / Highlander. It is suitable for any 288v nominal NiMH battery pack, either 40 x 7.2V or 30 x 9.6V. Topped up the 40 module pack in the storage room straight away. Beauty. Have limited number of those.
Extremely small form factor - will upload photo on site this weekend.
It draws 26 Amps from 12V DC. I use IBM 12V DC 60A Industrial Power Supply to power that, but in the field it can be used from the starter battery. Internal timer shuts off charger automatically to prevent depleting the source.

 

This may be off subject but I had an experience with a max grid charger representative on ebay and I explained to him that I had built a homemade grid charger that’s charged off at 350 mA and I can use it on both my Honda insight generation one & in my Toyota generation 2 and I couldn’t understand why his Maxx grid charger for generation two priuses couldn’t be used on a GEN three anyway apparently the guy got mad and said he was blocking me from all future transactions with his company and I hadn’t even made a purchase yet. That’s a little strange

 

See what Dr. Prius shows. Way up from 42% to almost new condition. I think the discharging is almost as important as the charging.

My fan also badly needed cleaned.

This Prius is new to me and I didn't want to go through the hassle of swapping cells.