Wednesday, September 26, 2012


So a couple of months ago I went back to Mexicali, Baja California, Mexico. Which is not in SoCal even though most people think that whenever I mention "Baja California" -_-. Anyway, I stayed there for a few days only so I could attend my sister's high school graduation (she's going to Princeton! wooo). During those days, I learned that one of my friends, Juan Bustamante, had a cousin who was throwing away a tiny motorcycle that wasn't working anymore. I went over to see if I could fix it or at least figure out what was wrong. The first things I checked were the battery and the motor. I don't really have any tools back home besides a screwdriver and a wrench so there wasn't much I could do but luckily my Juan had just bought a multimeter so we probed the battery and found that the 18v battery only had 4v across it. Which made sense considering the lead acid battery had not been charged in about four years. To check if the brushed DC motor still worked I had to place enough voltage across it make it overcome its own inertia  and have it start spinning. I didn't have a variable power supply so instead we got eight AA batteries and taped them in series to make a 12v "pack". The motor did spin so I decided to keep it for a future project (or maybe just to take it apart and use its stator). After we diagnosed the problem, Juan decided that he was never going to use it anymore so he offered to give it to me. I was unsure about whether or not I should bring the pocket bike with me back to MIT considering that it's more than 2500 miles away and it might be a problem to bring on the plane. So I did a quick Facebook post to see what my friends thought about it.
Clearly, they thought it would be awesome if I brought it and motorized it so I could race against Adrian who built his own electric pocket bike, MilliCycle

Mount without pocket
The first thing I had to do was figure out how to mount the motor onto the frame. The original motor was a brushed motor and it had its mount holes on the bottom so the frame had a mounting plate for it. I decided to take advantage of this so I took some scrap metal I found in N52-318 (room where the MIT Electric Vehicle Team meets). It was an aluminum C-Channel so my ex-roomie Roberto used the bandsaws and drill presses in D-Lab to help me cut off one of the sides and make mounting holes for both the frame and the motor. We later realized that we did not take into account the aluminum between the motor and the pulley so we had to go down to MITERS to mill a pocket for it.
Motor mount with pocket (it's on the other side)
motor controller attached with more scrap aluminum
I wanted TinyCycle to be way more powerful than RailScooter so I used a motor twice as big as the one on RailScooter. It's a Turnigy SK3 6374 Brushless Outrunner. It's 192kv and has a resistance of .016 ohms. More importantly, it has more power than I really need on such a small vehicle but meh, it should be fun. For the battery pack I soldered two custom 5s2p battery packs in series. This means I have a 33v pack on TinyCycle; much bigger than the 24v battery pack on RailScooter. The motor controller is also different. RailScooter uses a sensored kelly controller whereas TinyCycle uses a sensorless 250watt jasonTroller. The jasonTroller was clearly TinyCycle's limiting factor so I plan to swap it for a more a 500watt jasonTroller. Hopefully that will give me enough power to do wheelies!
bicycle mount in D-Lab

battery pack sketchily mounted with zip-ties.

TinyCycle's Maiden Voyage with the 250w controller

Garage Run with  a scooter, two go-karts, a quadrotor, and a tiny motorcycle
So many little EV's!
Garage Run from the Shane's quadrotor's point of view
Cute British Accent Asian Girl
So apparently, tinyCycle is a chick-magnet. A couple of TDC brothers and I were riding it around dorm row last Saturday night around 1am. Which is also the time MIT frat parties are over. Several girls walked over toward us and started asking me about tinyCycle. 
I even got the above girl's phone number :) 

Tuesday, September 25, 2012

How to Make a Battery Pack

      First go on the internets and buy some LiFe cells. I specified LiFe cells because those are the ones that I used. You can buy some directly from the A123 website. I used A123 lithium ion cells for my battery packs. They carry 3.3v nominally and 2.2aH. Use a voltmeter to make sure that the voltage across every cell is 3.29v-3.30v. Make sure that all of your battery cells are at the same voltage. Your battery pack is as strong as its weakest cell so if some of your cells have 3.29v and others have 3.3v your battery pack will die when the 3.29v cells die, regardless of whether or not your other cells still have charge.

So many batteries!
 Then you should use a hot glue gun to glue together your batteries. I did a 5s2p pack which stands for five in series and 2 in parallel. This means that each battery pack has a total of 16.5v (5x3.3v) and 4.4Ah (2x2.2Ah). I first glued together five pairs of batteries and made sure that they all had the terminals facing the seam direction.

Two cells in parallel

two pairs in parallel
small packs of 2s2p
 Now begin gluing the pairs to each other and make sure that their terminals are facing away from each other. This is because we are about to connect the pairs in series. We wil use copper braids to make this connection; I'll explain it later in the tutorial.
Two 5s2p packs; 
Positive, negative, positive, negative, positive...
 You should now be done gluing the cells together and you should be left with 5 pairs of alternating polarities hot glued together.
Use sandpaper to clean up the terminals
 Before we start soldering on the batteries we must clean the terminals using sandpaper. This is to brush away the corrosion on the terminals.
Notice the first pair of batteries have two negative terminals connected with copper braid. This is the pack's main negative terminal.
After sanding the terminals you should solder strands of copper braid in between each battery terminal. I used 12 gauge equivalent grounding braid from mcmaster. Make sure that you are connecting terminals with opposite polarity. The only exceptions are the first and last pair of batteries. Those will be your main positive and main negative terminals. This is the only occasion in which you should have a strand of copper braid connection terminals of the same polarity. To solder this braids you will require two soldering irons: a huge one as wide as your pinky and uhh... a normal sized iron. You'll use the huge iron to solder the negative terminals since they are much larger than the positive ones and therefore have more space for heat to diffuse. For the positive terminals you should just use the normal sized iron because the big one might heat up the terminal too quickly and you could damage the cell. You might want to buy some flux from radioshack or mcmaster to aid you in the soldering although it is not necessary.
Note the thin cables connecting the parallel nodes together.
 You should also solder some thin wire in between  each parallel node as seen in the picture above. Theoretically, all of the cells should be at the same voltage so there should be no current flowing through these thin wires but if there is a slight discrepancy in voltage across one of these cells. Current can flow from one cell to the other and hopefully equalize the voltage across them.
Finally finished soldering two packs.
 Now it's time to make a balance connector. Balancing is very important. It will make your battery pack way more efficient. If your cells are not properly balanced then your pack will die as soon as your first cell dies.
Balance Connector
For the balance connector you will need two things: the connector housing and the crimp connectors. This the most annoying and monotonous part of soldering your battery pack. When choosing a connector housing you gotta make sure that it has the correct number of ports needed for your battery pack. The number you need is one more than the number of cells in series in your pack. So for my 5s2p pack I used a 6-port connector housing.
Balance connector housing
XH Crimp Connector

If you don't have a crimping tool you can just solder the wire onto the crimp connector
 If you do decide to solder the wire to the crimp connector you must make sure that you use very little solder. If you use too much solder, when you try to connect your finished balance connector into the female port on the charger you won't be able to plug it in. I messed up my first connector doing this and I've heard that a lot of people make the same mistake their first time. The red wire should go from the "arrow" port on the connector housing to the main positive terminal on the battery pack. The blue wires should each go to a different battery terminal and they must be in order. So the red wire starts in the main positive terminal. The second wire (or the first blue one) should go to the negative terminal opposite the main positive terminal. The third wire (or the second blue one) should go to the positive terminal opposing the terminal you just did, and so on...
Make sure you connect the red wire on the arrow

I realize that the balance wire connections might have been confusing for people so I hope this diagram will help you guys visualize it better. The whole point of these balancing connector is that the charger has to be able to measure the voltage drop across each cell. After you're done you should be able to get a voltmeter and measure each cell's voltage by probing the inputs on the white balance connector housing.

Balance wires and main positive/negative wires complete.
For the main positive/negative wires you must use thicker wires since these wires will be taking on most of the current. I used 10gauge wire I believe. Very important: you should solder a polarized female connector to the end of the power wires. They should be polarized so that nobody can accidentally connect your batteries backward and short them. I like to use XT-60 connectors, but a lot of people use dean's connectors. Personally I think you should just choose one connector and stick with it so that you can use any of your battery packs for any of your electric vehicles. Make sure that the female end of the connector is soldered onto the battery power cables. This will make it harder for your batteries to short accidentally.
Heat shrink the battery pack
After you make sure that all of your balance wires are soldered to the right spots and everything is soldered correctly it's time to insulate the battery pack. There are many ways of doing this. You could do the legit way which would be to get huge heat shrink tubing and a heat gun to insulate the pack. Or you could do it the candace way and just wrap a ton of electrical tape around it.
Before you actually use the battery pack you must make sure that you use a balance charger to balance the cells. A balance will charge each cell separately to a pre-set voltage. However, after a pack has been balanced it will remain balanced for a while so you can just use a normal charger on it. I own the 1010B iCharger. It's expensive but it's powerful and can charge and balance up to 10s LiFe battery packs. It was recommended by several people who have built many electric vehicles, such as Charles, Shane, and Eli.

Anyway, I hope my tutorial was helpful! Good luck!