Sunday, January 13, 2013

The BMS is in the building.

I received my Lithiumate Lite BMS kit today from Elithion. It's smaller than I expected, which is a good thing, and it seems very well thought out and built. I put a lot of research into choosing a BMS, and eventually chose a digital one because of the increased flexibility and possibility of integrating with the computer I already have in the car. With 85 cells, the digital BMS was comparable in cost to an analog system of the same size, so it was really a no brainer.
The BMS master unit connected to AC power and my laptop
Here you can see the the main unit that all the BMS daughter boards connect to using cat5 cable. It has a number of handy features for integrating into a vehicle such as 12V outpus for interfacing with the motor controller and charger. It can drive 5V gauges for battery current, voltage, and state of charge with any scale you want. It can even produce a pulsed output for a factory dashboard fuel gauge. Aside from those features the highlight of this system has to be the software.

In addition to collecting data from every single cell once per second and graphing it, the software allows configuration of soft and hard battery limits such as when to turn on warning lights or disable the pack. It also provides graphical gauges for current, voltage, and state of charge. Although the provided application only works on Windows, documentation of the communication protocol is provided which makes me very happy. I plan on using this to allow integration with the touchscreen interface in the car as well as allowing access to the data from my smartphone. More to come once I start taking apart the charger.

Thursday, January 3, 2013

Goodbye lead!

After arriving back in SLO for the impending winter quarter at Cal Poly, I dropped the four large packs of lead batteries out of my car. After having sat uncharged for three weeks during winter break the pack voltage had sunk so low that my charger would not start charging presumably because it couldn't even detect that the battery was there. Oh well they're coming out anyway.

Here you can see all 24 of the deep cycle lead acid batteries (US battery model 31DC-XC). Each battery weighs 54 pounds by itself, add on top of this the steel frame each pack is supported by and the cables and bolts between every battery and you have yourself a quite near 1 ton battery pack. The corrosion left behind from spilled acid during charge cycles due to overfilling was formidable. I spend about two hours with a chisel and hammer breaking powdery chunks of corrosion off of the (at one point painted) steel frame in the back of the car that will house one of the PVC boxes for lithium batteries.
Looking down into the rear battery box frame. Rust and corrosion aplenty.

The next step is to measure for the second battery box that will go under the car, which now has significantly more ground clearance because the boxes underneath are gone and it is almost a ton lighter. I had originally put in heavier suspension to compensate for the added weight and it worked very well. It remains to be seen whether I will need to switch back to the stock suspension to make the ride bearable.

I've ordered the BMS from Elithion and it should be here in the next few days. That will be my next project along with modifying the charger to allow the BMS to shut it off and change it's current. Until next time... this is KK6ADS signing off.