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Old 01-28-2012, 11:47 AM   #1 (permalink)
Daox
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DIY lithium BMS help

For those who don't know, I have an enginer PHEV kit for my Prius. The BMS that came with it was a total piece of junk (it has since been improved upon), so I started to develop my own to stop it from killing the batteries.

The setup right now is relatively simple. There are a couple celllog8s devices monitoring the batteries voltage. The celllog keeps track of up to 8 lithium cells at once. It has a low and high voltage alarm as well as a voltage differential alarm that are user settable. The celllog also has an alarm port that activates a transistor when an alarm goes off.

I currently have a couple celllogs monitoring the 16 cells in the PHEV kit. Using the alarm ports, they are hooked up via optoisolators to an arduino that controls power to the charger. When the high voltage alarm is triggered on the celllog it sends the signal to the arduino to shut off the power to the charger. This works great for charging, but I'd like to expand the system to control when to turn off the PHEV kit from providing power to the car (protection from over discharging). I have tried using the low voltage alarm, but it is not that simple. With variance in voltage sag due to loading and temperature there isn't a good voltage to set and forget. Under EV acceleration the voltage will sag much lower than when you're just cruising for instance.

So, I am looking for help on how to enhance this system. I see no reason why this system can't be scaled to use on a full blown car and its also REALLY cheap to setup. Right now I'm looking at two different options.

1) Some smarter arduino programming on how to identify when a real low voltage alarm is happening vs a fake one. Something as simple as the alarm has to be going off for more than 20 seconds or something. Unfortunately there are some hills you may climb in EV mode that will trigger this.

2) Add a current sensor and keep track of Ah used. This would be a much more robust option I think. For example I am using 40Ah batteries in the PHEV kit. So I'd program it to power down after say 32Ah used. I know this can be done, I'm just not really sure how to do it.

3) ??? I'm open to suggestions!

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