BMS


My A123 modules got a beating at a combat robotics contest recently. They held
up fine. They were charged with a BMS on each cell. Discharge rates overlapped
my truck. These cells saw far more G-forces than normal. But I needed some
testing on the structural side of the design to make sure nothing stupid
occurred. The spot welds were of particular interest. The owner uses A123’s in
his robot. Now he’s sending 100 more for more packs to be made. Sweet. This time
however he has a different module layout he wants to try.

I used nickel as the conductor since copper is apparently next to impossible to
weld with a CD welder. I was told this and didn’t believe it. Then I tried to
for 3 days. Nope. Even as fancy as my CD welder is. Nickel is universal for cell
assembly. But not only is copper cheaper, it also is a better high current
conductor. That’s also what makes it harder to weld. It basically has no
resistance. Since I’m making over 70kw now, I have to make sure a module can
pass 100kw for the future (fingers crossed). If copper ever gets used, I’ll have
to buy/make an inverter type welder.

I just won’t be satisfied until I can spin tires on dry pavement at will!

I found on the Endless Sphere forum a BMS that matches my needs and philosophy of being simple, but with a processor. Here is the link. He talks about his design as he builds it in other threads. I just don’t see anything since late 2009 on the progress of the design. I’ve emailed him. We’ll see.

Here you can see his prototype layed out. It communicates using infrared leds. That helps get around the EMI issue.

I’ve ordered and will characterize and evaluate a minimal bms system called MiniBMS. Dimitri sells it from Florida. It’s all analog and uses a continuous loop of wire for signaling that one cell has gone too high or too low, by breaking the loop. A buzzer or any other indicator can be used to notify the driver. This single wire concept is suppose to have next to zero noise issues. I would be concerned that such a long length of wire could have inductance sufficient to create a high voltage spike when ever the loop is broken.

The only drawback is that the idle current is rated at 10ma. That uses 87.6 amp-hours per year from the pack. I consider that high. My own design uses 0.75ma at idle, or 6.57 amp-hours per year. I suspect it’s keeping an opto coupler enabled that accounts for the current draw. It won’t tell you what cell is having trouble, only that one of them is in trouble. Several loops could be used instead, breaking the pack into groups. This would at least narrow it down some. My pack will require 100 cells. So it’s important to me. It is suppose to have thermal protection on it as well to keep it from bypassing more than 1 amp.

The BMS board pictured here is $12 for each cell.

The control board pictured here is $30 for each vehicle.

The control board has an on board timer that allows an adjustable delay of the alarm enable. So if the driver momentarily loads the pack past the low voltage threshold, the alarm won’t sound. If the pack has a cell that is low for longer than the delay, then the alarm sounds. The control board uses a simple resistor and capacitor to achieve the delay.

The bms boards can fit any of the large format cells that have 6mm screws. The bms board is small and bolts to the negative terminal only. A length of wire is used to reach the positive terminal. The downside is that during assembly the board could be rotated into a neighboring cell and create a serious short circuit. So assembly has to be more carefully done than usual. More on this system when it arrives.

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