At the beginning of May, my beloved 2004 Toyota Prius started complaining. All of the sudden, it threw up all the warning lights you never want to see. For anyone who has been lucky enough to never have seen the red triangle of death, nor the dreaded “VSC”, it looks something like this (although potentially less blurry in real life):

That is the first sign that something is very wrong. It has happened to me in the past, when the 12 volt battery died the first time. This time, unfortunately, the 12 volt battery was fine. Not being thrilled to part with money to have a mechanic, or even worse – the dealer, plug in their diagnostic tool, the last time I ran in to trouble I picked up an ODB II reader (~20$ on Amazon). It is rarely particularly definitive, but it at least gives me some sense of comfort knowing anything beyond that I couldn’t have reasonably done myself. It also helps me mentally prepare for the order of magnitude of the repair bill. I connected to it with the free version of the Torque app for Android (Google Play Store Link), and the error code it spit out was P0A80. Googling this lead me to discover the ominous description for that particular error – “Replace Hybrid Battery Pack”. I’ve heard it referred to as the “HV battery” (high voltage), or the “traction battery”, or just the “hybrid battery”. Any way you say it, it’s the big one in the back, under the rear seats. The advice I came upon readily via Google all agreed – this was not an error you could just ignore and keep driving with.

I didn’t want to pay for a tow truck (there’s a theme here…), so I ignored the warning lights and kept driving. I dropped it off at my mechanic’s and caught a ride from my girlfriend. When I got home, I entered the research phase. Not really knowing where to start with this problem, I read the Wikipedia entry for the Prius, which states:

The Second Generation Prius contains a 1.310kWh battery, composed of 28 modules. Each battery module is made of 6 individual 1.2v 6.5Ah Prismatic NiMH cells in series forming a 7.2v 6.5Ah module. Each module contains an integrated charge controller and relay. These modules are connected 28 in series to form a 201.6v 6.5Ah battery (traction battery), also known as the energy storage system. The computer controlled charge controller and battery management computer systems keep this battery between 38% and 82% state of charge, with a tendency to keep the average state of charge around 60%.

So… 1.2v 6.5Ah Prismatic NiMH cells. At least that’s a starting point.