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.
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2 comments:
Hi, I am building an AC EV also. see http://a4x4kiwi.blogspot.com
I am currently thinking about how best to balance my SLA pack. There will be 50 12 volt batteries I need to take care of.
I was wondering if you could share the details of your BMS schematic and programming?
If the boards could be adapted for 12 volt use I would be interested in purchasing 60 units.
Thanks and regards,
Mal.
I can share in general how the BMS works:
I use an Atmel attiny25 on each cell controller to read the cell voltage and control a MOSFET that bypasses current. The microcontroller talks to a master controller on an isolated two-wire serial bus.
I'm afraid this wouldn't work directly for a SLA setup, though, as the microcontroller is connected directly to the battery, and the max voltage it can take is 5V. You could use a regulator to run the micro, but then you'd have to add some divider resistors for the voltage sensing.
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