Sunday, March 31, 2013

Making the Battery "Packs"


Time to make a battery pack. A very oddly shaped battery pack. Before I did any soldering, I had a good discussion with Shane and read a bit about how Charles made his battery packs for his various electric vehicles. I also sketched up a method for how I can assemble the batteries and then fold them after soldering the ribbon cables. Whiteboards help.


Sketchings on the IDC whiteboards. Shane discussed the importance of balance leads.


If you want to learn how to solder battery cells into packs, a good website to read is Charles's blog and/or Victor's How to make a batter pack post. Here is an overview of how Charles taught me. 

First step: it helps a lot if you sand both ends of the cell. (For A123 cells the positive terminal is what you would normally expect the negative terminal to be, the largest metal cap). The picture below shows the positive terminal.


Sanding the metal creates more rough edges and thus more surface area for the flux to stick to. Next apply flux to help the solder stick to the battery caps.


I used a liquid flux which works well, but there are other types of flux with creme/goop-like texture that also work.


Once you apply the flux, you can then put a blob of solder on the battery cap. The longest you ever want to hold the soldering iron on the battery is about 3 seconds. That should be long enough to heat up the battery enough to get he solder to flow around. Any longer than 3 seconds and the battery could get damaged or start leaking.


Once the battery is primed with solder, you can connect them with ribbon cables and attach the balance leads. The pack for my electric orbit wheels isn't really a "pack" but rather a string of cells. Normally you would hot glue the cells into a configuration you wanted and solder the ribbon cables to multiple batteries (not just two, as I do in the picture above). 


I soldered on some XT60 connectors for connecting the two 5S packs in parallel between the front and back plates. 


To provide some sort of protection and aesthetics, I put some 1 1/4" heat shrink around the five batteries. 


One pack down, three more to go. I am about to run out of the heat shrink I got from MIT's Edgerton Center. I might have to get Amazon Prime and buy some from there, because HobbyKing will take too long.  


I drilled holes through the battery clips on the MITERS mill. I had to do so very carefully otherwise the acrylic would crack when the drill bit exited the part. I also used a higher rpm for drilling through plastic. 


The 12 mm clear acrylic battery clips don't crack as often as the 6 mm black acrylic clips. I marked all the balance leads to keep track of where they were connected to the pack internally. 

When I tried to charge up my complete 5S2P pack, I got an error on the charger. After isolating the problem with a multimeter, I found that one of my balance leads was disconnected somewhere between the end of the last battery and the JST-XH connector. I opened it up and found the spot pretty quickly. After that it was just a quick fix with some solder.

Charging up the Battery Packs after Soldering


I've been waiting on the motors for quite some time now. I started worrying that I might not have chosen Express shipping from HobbyKing's International Warehouse. So I checked, and it turned out my Dad's credit card freaked out and didn't actually place the order because it was from Hong Kong. 

It was sort of a relief knowing that they wouldn't take another month to ship (which happens if you don't choose Express), but it was also quite frustrating. I placed the order again and this time it went through. So, hopefully in a week or so the motors will show up!


Now I need to figure out how to make a ring gear with Autodesk's gear generator. Then I can cut it out on the water jet and get ready to pour a mold. 

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