Monday, October 1, 2012

Big update part 1 of 2: Scooter progress!

I feel sort of guilty about writing a blog post when I don't actually have any substantial progress done, so I've had a bit of a blogging hiatus while classes started up and my projects slowed down. However, at this point I've definitely gotten far enough to justify a blog post (well, a two-part one, you'll see a logical separation point here), so I'll just pick up where I left off last time:

The battery!

Well, as mentioned previously, I decided to go with a 12S 3P (12 cells in series and 3 of those in parallel, for 36 cells total) LiFePO4 chemistry battery, cells courtesy of an A123 donation to the MIT Electric Vehicles team. I started off just grabbing a big pile of battery cells and metering them to ensure they were at or close to their nominal 3.3V potential, then running hot glue down the sides and sticking the pack into the shape I wanted. Next, I started glomming solder down on the ends of the cells where I'd be connecting them.

You can also see I've started to add thick copper braid to the cells on the left, which serves as the power line through the battery. I went ahead and connected each set of 3 cells in series, then soldered on small (24 gauge) balancing wires which were color coded for my own convenience. These wires allow the cells to be balanced individually to the same voltage when charging, and prevent degradation of your battery (or possibly exploding cells) which would result from trying to charge the pack while some cells were at significantly lower voltages than others.

I hot glued everything down to prevent it from shifting around
I then soldered on the main power outputs at the very front and back of the pack, and finally flipped everything over and repeated the same process using the braid to form the power line, only instead of balancing outputs I placed wires across the parallel points to help the battery discharge evenly.

You can see the gray wires equalizing the cells in parallel
Finally, I obtained some of the biggest heatshrink tubing I've ever seen, and slid the battery into it:

Half an hour later (okay, maybe a slight exaggeration...) with a small heat gun:

VoilĂ !
A battery!

Only... it turns out I forgot the middle power outputs, which basically break the battery up into two separate packs that can be charged individually at half the voltage of the entire pack, enabling balancing by most hobby chargers. Oops. Time for some battery-surgery.

Here's the heatshrink cleanly separated at the top
I pulled the pack back out of the heatshrink, soldered on the other two sets of power output wires, and slathered hot glue over everything.

Finally, I applied a new section of heatshrink, charged it up (in something on the order of 20 minutes. Turns out this ~6.6 Amp-hour pack can happily charge at 15-20A within ratings), and was possibly a bit over-eager to ride around with the extra power. The result?

Notice how the brake no longer looks functional? Yeah, the brake was no longer functional.
I successfully made everyone at MITERS cringe by mounting my battery exclusively with Gorilla tape. Don't worry, 100% legit engineering here.

In the meantime, I actually received my slightly more powerful shady Chinese brushless motor controller in the mail, and immediately proceeded to snip off most of the useless cables on the box (powered brakes? side-lighting? Hah!), disassembled the case, and stuck a nice new coating of solder across the built-in current limiting resistor, halving its resistance and drastically increasing the output of the controller.

Current limiting resistor circled red
I reassembled the case, applied motor controller to scooter, applied battery to motor controller, and guess what? Everything actually worked! My scooter received an immediate upgrade to scary-fast, and at 6.6Ah, the battery will last much longer as a result. Assuming an average consumption of about 600W going at 20mph, the ~43V pack should last for about 28 minutes, or a little over 9 miles.

That's all for Part 1, but I'll throw in a sneak peak for next time right here:

Expect Part 2 to come in the couple weeks or so!