Tuesday, May 21, 2013

Motor Controller Part 5: The controller strikes back

Last time I was working on doing wiring on all the batteries.  I'd run out of time to finish it before our friends Toni and Warren visited us from Germany.  But between beers and while the girls were playing video games, Warren and I snuck out to get a little work done.  We started by finishing the routing of the 2/0 cables.  I got the cables securely tied up under the car so there's no chance of a stray stick snagging it.  Warren is sitting here with a phone in his hand.  He punched in 9-1, and if the car falls on me he'll just have to hit 1 again.

After finishing the large cables, it was time to finish the small wires.  You can see all the BMS wires are hooked up in the front and we got all the connectors pinned out.  I also wired up the the B- contactor and the vacuum pump.

Here's a close up of those connectors I keep mentioning.  There are three that plug into the BMS boards.  I'm also using them to initially top charge the batteries.

Here are the two clusters of wires coming out the back end of the car.  Finally the spaghetti of wires are neatly bundled into wire looms and all fitted into connectors.  Now I just need to drill a hole into the cargo cubby hole that's behind the wheel well  and stuff the wires in there.

Alright, what's next?  Hmm, can't think of anything else that needs to be done...I guess it's time to test out that controller again!  Just a reminder, I've driven the car about 3 miles on 1/3 of the battery pack.  That's about 80 volts.  Well, a lot of the reason for the custom controller is that I wanted to run higher voltage than 150v, so I really haven't proven anything yet.  An 80v 300 amp controller is pretty common out there, but a 230v nominal controller, that's special and seems to cost $2400-$3800.

Well, no more lollygagging, let's see what happens.  I flip the manual disconnect and that connects the positive side through the precharge resistor.  Next I flip the ignition to engage the B- contactor and that closes the loop and allows the capacitors to charge up.  If something blows, the voltage here probably will be less than the pack voltage, but since the 1k ohm precharge resistor is the only path for current to flow, the current is limited to 1/4 amp and the damage can be minimized.  Once that's bypassed by closing the contactor, the controller determines the amount of current, and if that fails closed it'll be a couple thousand amps.  Booya, 235 volts!

Okay, don't get too excited, it's possible that something still hasn't surfaced yet.  I opened up the manual disconnect and discharge the capacitors by pushing the pedal.  0 volts, so that checks out.  I decide to redo that cycle a couple times just to make sure there's no problem with the caps charging and discharging with no problems.  In the pic below I had opened the manual switch for a while and the voltage reading actually drained the charge a bit to 231 volts.  So now for the real test.  At this point I also hit the "start" button to close the B+ contactor and remove any limitations on battery pack current other than what the controller does.  I put the car in neutral to avoid a runaway and push the pedal...

Aaaahhhhhhhh!  Oh the humanity!  At least I've got insurance now.


Just kidding...the motor just spun like it's supposed to.  Though the picture above represents some of the nightmares I've been having over the last couple weeks.  Here's what the car really looks like now.  After slowly working my way up to it, I drove a whopping 0.1 miles to pick up my daughter from a friend's house and successfully made it home!  Woo hoo!

Next up, time to start installing instrumentation and get ready to take it to the DMV!

Wednesday, May 15, 2013

Wires, wires, and more wires

The batteries are all finally mounted in the car.  To get here I spent months designing, fabricating, fitting, modifying, refitting, and finally bolting in the battery racks.  It was a lot more difficult than I imagined it would be, but I guess that's par for the course for such a custom car.  Now that the batteries are strapped in place it's time to wire them all up.  Each of the rear battery clusters has 23 batteries, 46 bolts, 92 washers, 1 fuse, 2-4 copper bars, 22 bus bars, 3 twisted pair wires, and 24 BMS wires.  The next step in wiring these all up is to build the BMS wires.

The BMS wires serve two purposes.  When the BMS functions, a very small amount of current is flowing through the wires while the circuit board monitors the voltage across each cell.  The current flowing out of one cell is not exactly the same as all the other cells in the cluster, and I'm estimating after 3 months, some will be around 1.5% more discharged than others (about 1 Ah).  Eventually, this will cause the charger to stop charging prematurely and the cells will need to be re-balanced.  The BMS boards each have three balance charging ports, and a balance charger will send current through those little BMS wires to do just that.

Each wire comprises a ring terminal where it is bolted to the battery, a 4 amp fuse, and the wire.  To put these wires together I had to crimp the terminal to the fuse and solder it in place.  Without the solder joint, there's a chance that the crimp could come loose and cause some faults.  How do I know this you ask?  Well, I'll just say that Joe highly advised the solder joint.  So clip the fuse to size, crimp on the ring terminal, solder the fuse lead, solder the wire to the fuse, cut the wire to length, heat shrink over the fuse connections.  That should be too bad right?  Only a couple minutes and you're done.  If you add it all up, that took about six and a half minutes.  Multiply by 73 and that's how long it took to make all these dang wires!

Ok, the next step is to hook it all up.  One of the terminals on each battery is aluminum.  When you put aluminum and copper together you have the potential for galvanic corrosion.  To get around this, you put a little conductive anti-corrosion paste and rub it in with a wire brush.  After I got everything installed, this is what it looks like:

To keep things straight I got some sticker number flags and put one on both ends of each wire.  Amazingly some places want to charge you $20 for a sheet of these stickers, but luckily I found some on Amazon for just a couple bucks.

Yeah yeah yeah, I know I'm missing one wire and I forgot to put the fuse in (if you look two pics up).  And no, I swear I didn't do the exact same thing to the battery pack on the other side...I was up until 12:30 last night getting this done, so cut me some slack!  So far, so good.  Now I bundled all the wires together, zip tied them about 700 times, and collected all the ends.  They're not all the same length anymore, so I have to retag them and cut them all down to the right length.  I've gotta be really careful though because each of these are live wires!  If I'm unlucky enough to touch two wire tips together I'll have to spend another 13 minutes making new wires plus another 30 minutes swapping them out.  That also means don't clip more than one wire at once!  So I clip, strip, then crimp a connector pin onto the end of each and now I've got three plugs to go into the plugs of the BMS circuit boards.  I've got all the wires mounted on the batteries, but only 1/3rd of those connectors have been pinned out so far.  Oh yeah, and I'm missing one wire.

I also got all the large 2/0 cables measured, cut, and crimped with lugs, so theoretically I'm all set to test out the controller on 230v.  I just have to screw it all together.  I'm a little battery weary and we've got friends visiting this weekend, so I'm not gonna push it an do something stupid in the testing phase.  So after a weekend of R&R I'll be back next week to see if I can keep the magic smoke in the controller!

Friday, May 10, 2013

Need more volts

Previously I've been driving the car around on just 1/3 of the batteries, or about 80v, but now it's time to put the rest of the batteries in there.  In the back of the car below the floor I have two battery racks that hold 23 cells.  Actually, they're big enough for 23 each, but I only bought enough for 23.  I guess if later on I want 2/70ths more range I can buy two more cells.  At any rate, the way the batteries are tied down, I can't just leave that empty space open so I need a battery "blank" to fill it in.  My father in law has a ton of scrap wood and woodworking tools, so he helped me put a pair of these together.

As part of the battery monitoring system I added the capability to monitor the temperature of the cells, three spots in each cluster of batteries.  To do that I've got some thermistors, which are resistors that vary resistance as a function of temperature.  Here's one of them soldered onto a wire.

Apparently there's a lot of debate of how to "properly" monitor the temp of your cells.  Some people attach the thermistor to the posts on the cell since they're wetted in the electrolyte.  Other people using lead acid batteries will actually put the thermistor inside the battery so it's directly touching the electrolyte.  I'm going simple and just taping it to the side of the battery like this.

Then I loaded all the batteries in those nice racks in the back and clamped and strapped them down the same way I did with the front batteries. 

Pretty clean look from underneath.  You can see the threads on the four eyebolts that are really easy to access here.  Too bad the aluminum clamps are super difficult to get to, but oh well.

Well that's it for now.  This weekend I'll start wiring it all up!

Thursday, May 9, 2013

Charger Part 3 Plus Other Stuff

I'm moving along pretty fast now and the next thing I want to do is get the batteries charged before I mount them in the car.  Once they're in the car, it'll be a lot more difficult to do without the big charger, and most of them are not balanced.  Here are the remaining 46 batteries hooked up to the mini charger.  Just 30 hours and they should be ready to go!

The batteries already in the car have already been balanced as a set, so I can charge them all at once if I have a charger big enough to do that.  Last time I started smoking the charger due to a short circuit through the inductor's magnetic field that created an inductive heater.  In this picture you can see the metal bracket that holds the inductors in place loops right through them and the screws complete the circuit with the case.  It's gotta go!

 Here you can see my solution.  I've taken a plastic pipe and used it to hold the inductors in place.  All fixed, at least I hope so!

I realized I ever showed a full picture of the charger, so here it is.  Due to some rework, the "Batt" and "AC" labels are unfortunately backwards.

The big capacitor is one of the few remaining parts to be strapped down in the charger, and here's how I plan to do that.  You can also see the fan that'll blow directly onto the inductors to keep them cool.

Apparently after this I didn't take any pictures, so you'll have to take my word for it.  I hooked the charger back up to the car and plugged her in.  After a few minutes charging at 2 amps, still no smoke so I ramp it up a bit.  Over the course of 20 minutes I get the courage to boost it to 7.5 amps into the battery pack and so far so good!  The inductors are a little warm to the touch, but not bad.  Should be better when the fan is blowing on them.

The way these batteries work, they are close to 3.3 volts over most of their charge/discharge cycle.  During charge, the voltage will get up to 3.37 volts for ages.  As they approach a full charge, the voltage will start to rise faster and faster.  Since I have no BMS hooked up, if one of the batteries happen to be more charged than the others, the voltage on that one will start to rise while the others are still stalled out around 3.37 volts.  To avoid having some batteries start smoldering, I was checking to voltages every minute or so.  The charger is programmed to raise the entire pack voltage to 3.45 volts per cell.  Once it gets to that point, the charger will back off on the current to maintain that voltage.  The current will slowly decrease and the charger will turn itself off after it drops to 2 amps.  Amazingly, that's what happened!

So that's not even close to how far I've gotten on the car since last time, but I've been picked away at this post for 3 days so it's time to POST!  Maybe next time I'll be testing the car on 230v...