Hybrid Battery Reconditioning

Welcome to Mechanical Moments at Mercie J. I am your host, Rick Smith, certified master technician. We’re going to talk today about a battery pack from a 2007 Toyota Prius which is similar to the pack in our 2006 Lexus RX400h which is our courtesy vehicle here at Mercie J. The pack in the Prius is a nickel metal hydride pack and has 28 individual cells. Each cell is 7.2 volts, which is the same as a large flashlight with six rechargeable batteries in it. When all the cells are combined in the Prius, we come up with 201 volts which gives us enough power to motivate the car down the road. The first thing we do is start with the entire hybrid battery pack which is in a metal housing. There are connector bars which connect the cells in series which is like connecting a series of links in a chain. We know that a chain is only as strong as its weakest link, so we have to take apart the pack in order to test each link. In addition to the cells and connectors, the housing also holds temperature sensors, relays, resistors, and a computer. There is also a safety disconnect which is very important. Be aware that 201 volts has the potential to cause serious injury, so anytime you see any wiring or connector on your hybrid that is color coded orange, do not disturb it unless you have the proper training and equipment. Over time nickel metal batteries can deteriorate chemically which causes their internal resistance to increase. So the first thing we do once the pack is disassembled is measure the internal resistance of each cell and discard any cell that has too much resistance. The second thing that happens to nickel metal batteries is they lose their “memory”. In the car they are never fully charged and they are never fully discharged; treating them in this gentle manner is what gives them such an amazing long life. However, they sort of forget what their full capacity really is. So what we do is completely discharge the cells and then fully charge them. We do this cycle a couple of times which “reconditions” them to regain their lost capacity. Once we have reconditioned the cells, we hook them together in parallel instead of series. In other words, next to each other instead of in a chain. What this does is balance the voltage between all the cells so they all are exactly the same and when we put them back into the housing as a chain, all the links will have the same strength. An interesting thing with the pack in our Prius today is that none of the cells had an internal resistance issue. Cell #10 was leaking some acid which provided a path for electricity to travel from the battery pack to the body of the vehicle. This presents a safety concern, so the computer shut the car down. So on this one, we replaced the leaking cell, reconditioned all the cells, and cleaned the corrosion from the housing. Now we are balancing all of them and will put them back in the housing and we will be good to go. I hope you have found this information of value. Until next time, safe travels and blessings to you.