In this article we would like to show how a battery module for Kansas State University Solar Racing Team’s upcoming car, GaAsoline, is built.

First off, a little knowledge is needed on the battery pack for the solar car. The car being built is for the Panasonic World Solar Challenge 2007. By rules they allow us 30 kg for a Lithium Ion Battery Pack. The requirements of this pack are 40 amps peak for going up hills and the like. To meet this requirement we parallel up 18 battery cells that can do 2.6A max. These 18 cells will be the module which we’ll show how to build in this article. A solar car also requires a high voltage for efficiency of the motor controller and the power trackers for the solar array. To reach a high voltage the modules will be connected up in series to reach the potential desired.

So on with the goods on building a GaAsoline style battery module. The parts needed are: 18650 style Battery Cells (in this example 18 of them) Kevlar, or other stiff and light material 5" wide shrink wrap 5 mm wide Bus Bar Electronic Monitoring Board of your choosing Nylon Bolt, Nut, and Spacer DS18S20 or other temperature sensor Epoxy

Also needed is a good assortment of tools such as ruler, tape measure, razor, heat gun, screwdriver, C-Clamp, and random blocks of wood.

To support the weight of the battery module and to secure them in place are two Kevlar strips on the top and bottom of the module. To get these strips we need to cut two 1” by 9 ¾” pieces out. Measure twice and mark your cut. Using the ruler as a guide; cut the Kevlar with a razor. This will take a couple slices along the ruler to make it all the way through the board but at least you get a good clean cut. We’ve tried cutting the board with a table saw and a band saw but they all tend to fray the bottom piece of Kevlar after getting through the core.

Cutting...

...Kevlar

Finished Strip

Next we need to secure the battery cells together with some epoxy to make two straight rows out of them that are 9 cells long. To create these 9 long rows of cells we use some small jigs to hold 5 cells together. With the cells in the small jig apply some mixed 5 minute epoxy in between the cells using a q-tip or any other small sliver of material. Once you have made this 5 long row, pull it out of the jig and put it on a flat surface. In our example the flat surface was two pieces of wood as the mat on our table was slightly warped. Take additional cells and apply epoxy to one side of them and attach them to the row of 5 already made. Keep applying cells until you have a row of 9 long. Then repeat the process till you have two rows of 9 cells that have all of its cells glued together. Let them dry for a few hours, you don’t want the rows to break apart while you’re handling them in the next step.

Small Jig

Glueing additional cells on

2 rows complete

Now is a good time to pull off the little socks on the anode of the batteries that came with them when you buy the cells. Also bend the tabs so they stick straight out. Now bring your two rows of cells together with their tabs pointing to each other. Take note of the polarity, all anodes must be on the same level with each other on both rows. As will all the cathodes need to be on the same level with each other. A nasty surprise awaits the fellow who inverts these rows and connects them together. Also take note in the picture of how the rows are brought together with one row moved a half cell length down so both rows fit in the depressions of each other. Now place wood blocks on the two sides of the conjoined rows and apply a C-Clamp lightly. This is just to hold the rows together so they don’t separate while we work. Don’t crank any pressure on the outside of these cells. That again is another nasty surprise for the foolish.

Socks

Clamped together

Take your bus bar and fish it underneath the cell tabs, but above the top of the cell. This bus bar and another on the bottom side will be handling all the current of the module. With the bus bar centered on the two rows, solder down each tab to the bus bar. Using a tool like a screwdriver to keep the battery cells tabs flush with bus bar if you need to when you are soldering. Be careful to not melt the outside plastic of the cells. With the 18650 style cells, the cathode runs all the way up the jacket of the cell to the very edges of the top. The little white ring you see on the cell and the orange skin are the only things keeping you from shorting anode to the cathode. This is not too much of a worry in this module design as the bus bar is resting on the white plastic on the top of the cells.

Bus Bar

Solder Top

Solder Bottom

Once you have soldered the top, undo the clamp and flip the module over. Repeat the process again for the bottom side. After doing so, cut off the excess of the tabs that hang over on the ends on both the top and bottom of the module. Then double check all your soldering connections. Make sure they are all secure with a small pick. Then go back over and re-solder the bad ones. A bad solder connection means a wasted battery cell.

So you have a bunch of batteries soldered into a rather thick mass. Turn your attention back to the cut Kevlar strips. Cut holes into the Kevlar so that you can recess a nylon nut into it. We just cut a small diamond into the top layer of the Kevlar, smaller than the nylon nut, and then simply forced the nut down into the core of the composite. This produced a very securely placed nut that doesn’t have any of the added weight of glue. We use this recessed nut to mount our protection circuitry to Kevlar strip using the nylon spacer and bolt.

Pieces thus far ...

Cut shrink wrap to length

Cut holes for IC mounts

To secure the two Kevlar strips to the top and bottom of the battery cells so that the strips may support the batteries we use the 5” wide shrink wrap. Cut this shrink wrap to the length of batteries, but not to the length of the Kevlar strips. In our example this was about 7”. Now before we try to put the batteries in the shrink wrap, we need to cut some holes for the nylon supports for the battery protection circuitry. Measure and do this now. We used a ruler inside the shrink wrap to keep the razor from cutting through both sides of the wrap.

Now assemble the entire module inside the shrink wrap with the protection circuitry on the outside and secured down to the Kevlar strip. Make sure the Kevlar strips are place and centered exactly like you want on the battery cells before you shrink wrap. You could use some epoxy to glue the Kevlar strips down to the cells but that’s added weight and the shrink wrap will secure down tightly enough to hold the batteries and strips in place.

A loose assembly

Applying heat

... and more heat

This is the fun part were you take the heat gun to the module. Apply even heat to the shrink wrap and not to just one area. If one spot shrinks faster than the rest it might move your Kevlar strip out of place. Also, in our example we added a temperature sensor (the DS18S20) in to the mix and had it underneath the shrink wrap when we applied the heat. This temperature sensor will be used to monitor the modules when they are in operation.

After you have shrunk the wrap on to the module, it is complete. Make many more and you have a battery pack. The bus bar leads and Kevlar strips extending out of the module can be used for rack mounting the modules in a battery box. Solder on to the leads or in our case bolt down to a main pack line. This concludes our how-to make a 40 amp peak rated battery module for our next car, GaAsoline.

Finished product