NiMH batteries are a bit tough to charge; They continue to accept a charge even when they've reached full. This energy can't be stored and is turned into heat instead -Not good when you have a whole pack of them in the back of your car! To prevent fire in the original Prius, Toyota only charge the batteries to 80% of their total capacity, since it's this last 20% of charge which can lead to overheating (or thermal runaway as it is better known).
I'd been advised by Norm and other PHEV pioneers that a final charged voltage of 238V would be the safe limit that I could charge my pack to. I would need a charger which would cut power immediately when reaching 238V and not allow any kind of float charge as used with Lead Acid batteries.
That's easier said than done when most commercially available chargers are designed to be used with Lead Acid batteries. At least, that's the case when you're dealing with a pack which operates at 238V. A few years ago, companies who made and supplied electric vehicle chargers would not have entertained the idea of making a charger for NiMH batteries which could have been purchased off the shelf. Sure, NiMH chargers existed, but were generally very expensive and out of my price range. Or they were part of a factory-built EV, such as the RAV4EV. But recently charger manufacturers have started to reaslise that customers do want to use more advanced battery technologies in their vehicles.
My luck was in. I turned to Zivan, who make the NG range of chargers. Initially I hit a brick wall - the sales team did not know of an appropriate charger curve which could be used on NiMH batteries and said to try elsewhere. Giving up, I contacted Brusa, whose very expensive charger range came with computer software to manually adjust any parameter of the charging process on-the-fly. Brusa's head office told me that I should deal directly with the UK agent. Unfortunately the UK agent for Brusa was AmberJac - the same people who charge £12,000 for a plug in Prius and, quite possibly, the reason why there aren't more PHEV Prius in the UK.
Fortunately though, one of the sales team at Zivan had passed my original enquiry email onto one of the engineers who, as it happened, had just returned from a trip to Zivan's head office in Italy, where the company had been working hard on developing new charge algorithms for more advanced battery chemistry. NiMH was one of them!
I was asked if I'd like to order a charger and after a few weeks of correspondence back and forth between Zivan UK agents Electrofit-Zappi and various PHEV pioneers I was able to order a specially adjusted Zivan NG3 charger, with an SX charge curve, cut-off voltage of 238V and a maximum charge current of 4A.
