Beginnings of a battery pack

I've finally entered the home stretch with the BMS system, and have 3 final prototypes installed on cells for testing. Cat for scale.

Stickers!

I am spending Christmas in Wyoming with family, so no work on the car lately. I have been debugging the BMS code, and designing a sticker for the car:
The equation is the differential form of Faraday's law, which is one of Maxwell's equations, which describes induction. If this seems like something that belongs on your electric car, I can send the full size image.

Battery charger and other stuff

It has been over a month since I last posted to this journal. I have been very busy at work, where we just launched our first product- a miniature XRD/XRF instrument for identifying minerals in the field. Even so, I have made some progress on the car, mostly buying parts and working on the next version of the BMS. I am now the proud owner of a PFC-20 battery charger from Manzanita Micro. This charger is nice because it can output the 408 volts that my pack will require. One drawback is that it is not isolated, which means that the negative side of the battery pack is connected to one leg of the mains. This should not be a safety issue, as all the other equipment I am using is fully isolated.

Oh yeah, I said next version of the BMS. Although I'm sure the voltage regulator style BMS that I already developed would perform just fine, I have gone ahead with the Eierlegende Wollmilchsau (literally, an egg-laying woolly milk pig-- a German expression that can better be translated as "Swiss army knife") version. This version has a small microcontroller (an Atmel ATtiny25) on each cell that monitors the cell voltage and communicates with a tiny linux computer over an optoisolated i2c bus. Each microcontroller can bypass current, either on its own or on command from the computer.

The advantage of this system is that it gives individual information on the state of each cell. Also, if a cell controller fails, the computer would immediately notice, and alert the driver that the pack needs service.

The other positive aspect is that each cell controller will cost less. Microcontrollers are truly amazing. For under a dollar, you can get a chip that is much more powerful than, say, the Tandy Color Computer 2 that was my first computer.

Mr Fusion

I've started buying the safety equipment needed to deal with a 300 Volt battery pack with something like 15 kilowatthours of energy (roughly enough to run our house for three days.) In addition to some fancy insulated screw drivers to keep from shorting things out while I'm working, I bought this monster fuse. It will go between the battery pack and everything else. Lens cap for scale.

British racing green

Well, I've finally fallen into the biggest trap for people who have electric car websites-- I've been working on my website instead of my car. At least reading my journal (I'm allergic to the word "blog") won't make your eyes hurt so much anymore. Plus, it's now authentic British racing green.

BMS, round two

I received and assembled the first three samples of the BMS boards. Everything looks good. I did some thermal tests, and each unit should be able to bypass one amp with a heatsink installed on the main transistor, and about twice that with a fan in addition. If I use the BMS to cut back on charging current when any cell goes over voltage, the bypass current should only be a few tenths of an amp, and I'll be able to dispense with the heatsink. That would be nice, as the heatsinks are a fair amount of work to install. Here's the top of the board, with heat sink installed:And here's the back. The cool thing about making your own layout is that you can write whatever you want on the board:


Only 93 to go...

Ol' sparky

I have finished figuring out where the batteries will sit. The spare tire well is almost perfectly sized for the pack. I will probably relocate the spare underneath the car in place of the gas tank.

While I was looking things over, I learned why British cars are infamous for their dodgy wiring.

Nothing like unsupported wires rubbing against the gas spout... and is that a twisted-together "connector"? I hope that was an aftermarket accessory. The factory connectors look pretty smashing as well:


I'm hoping these practices won't rub off on me when it's time to do the high-voltage wiring