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3394Prius NiMH Batteries

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  • theoldcars@aol.com
    Dec 24, 2007
      Hello Bill

      I am not an expert on the Prius NiMH packs. I have though done quite a bit
      of research on NiMH batteries and have been using them for several years now.

      What Mike has pointed out to you about the Prius being optimized for
      accepting regen is true. I have read information from different NiMH battery
      manufacturers web sites. It is used as a sales tool to sell their batteries for
      Hybrid vehicles. The Prius pack is never fully charged and never deeply
      discharged. I recall it is 50 to 80 percent SOC they are maintained within.

      Heat is a real killer of all NiMH batteries regardless of the battery mix or
      use. In the Ranger and RAV4 they will stop charging if the batteries are not
      cool enough. The S-10 and EV1 use A/C to keep the NiMH batteries from
      exceeding 86 degrees during charging. It is the last 20 percent of a full charge
      that NiMH batteries temperature really takes off. As I see it the Prius
      batteries were never to go the last 20 percent of a full charge. The packs were
      never intended to have to deal with this much heat.

      Also the NiMH battery does heat up under large current draws. They also heat
      up even more during hard draws in a lower discharged state. This is not
      because the Prius pack was never to deliver the power for full EV use. The more
      power you pull from them the more they will heat up. You could very well kill
      these batteries during both charging and discharging this would destroy them
      even faster.

      The used Prius packs may be a real buy however I doubt they would provide
      long life if used as an EV battery. Unless you really oversized the pack and
      kept them within their design temperature range and power draw. I am certain
      the results would be very disappointing.

      It would be interesting to know more about Mikes testing.

      Don Blazer
      1998 S-10 NiMH

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