I was recently asked for tips regarding battery pack design for a collegiate kart. There's a lot that goes into that answer, so I thought I'd write it up and share the knowledge. See below.
Since collegiate evGP teams do not have a spec battery or even battery type/configuration, teams are left on their own to design and develop a battery pack. I’d say that this is the area that requires the MOST design and development when building your kart.
You’ll first need to determine either a battery brand or a cell type. Overall, the cost of a single 4.2kWh battery pack can range from $2k - $4k. So many teams will approach a battery manufacturer for sponsorship. If this is the case, you will use whatever battery cells they give you.
If you get to choose the battery cell completely, first decide on whether you’d like an 18650 type cell, a pouch type cell (Li Polymer is usually pouch, Turnigy is a popular brand), or more likely some sort of brick module that has 1-8 cells inside and is meant to be assembled to other bricks.
18650 type cells are the most common. They are the cells Tesla has used for year. More on this below.
Many Purdue teams use the Boston Power Swing 5300 cells via an 1s8p configurated module. Boston Power became a sponsor of evGP in 2014 and donated quite a few of these battery modules for us to build packs with. See pics below. You can see 8 cells in each module that are connected in parallel (hence 1s8p aka 1 series 8 parallel).
Using these modules was nice because they are designed to be stacked and bolted together. However we’ll talk more on mechanically putting your modules/cell together later.
CHOOSING YOUR CELL SERIES/PARALLEL CONFIGURATION
We decided our pack was going to be 14s2p configuration. (when we say 2p here, we mean 2 brick modules. Realistically the pack’s 14s16p). We chose the 14s first because we wanted our kart to be around 48v. 14 series cells at 3.7v nominal per cell (this is normal nominal voltage for lithium) equals 51.8v nominal for the pack. Pack total voltage will be 4.2v x 14 = 58.8v and min pack voltage will be 2.8v x 14 = 39.2v.
We then need to determine how many parallel cells to put in our 14s pack. If we did 14s1p, our total energy on the kart would be about [14s x 3.7v nominal x 42Ah (per module brick) ] = 2175 Wh. This is not enough energy to finish the race (as we know from previous years). Also, the limit in the rules for energy is about 4320 Wh. So this is not enough energy.
Also 14s1p won’t work because you’ll be pulling too much current from a single 1p module. If your system is 51v nominal, then you’ll pull 14000 W / 51v = 274 amps (using the 14000 W power limit in the rules). 274A from a 42 Ah cell is 6.52C. C rating is calculated by taking the amps coming from a cell divided by that cell’s amp-hour. What this rating tells you is how fast you are depleting the cell. Deplete a cell too fast, and you permanently damage it or can cause it to heat to the point of combustion. Cell specifications always offer a recommend C rate for continuous discharge and peak (1-3 second) discharge. 6.52C is way too high for these Boston Power cells.
So let’s next consider a 14s2p pack. This would provide us the perfect amount of energy at [14s x 3.7v nominal x (42 x 2 Ah)] = 4350Ah. This will definitely allow us to finish the race. But also, it will now reduce the per-brick module current to about 274 / 2 = 137A at max power since there are 2 parallel cells sharing the 274A load. This is only 137A / 42Ah = 3.26C. Another way to look at this is the total pack current of 274A / (42Ah x 2) = 3.26C. This is an acceptable C rate for the cells. We have a good configuration.
Another option we are considering testing with these modules is a 23s1p pack. I’ll let you do the math on that configuration, but it should fit within the rule’s power limit, energy limit, and voltage, and it should also be an acceptable C rate.
A NOTE ON 18650 PACKS
18650’s look almost like shotgun shells and are the most common battery cell out there. However they are very hard to assemble / manufacture at the cell level. You must tab-weld each side of the cell and do this times roughly 600 cells in a pack. I would not recommend this process. However after a couple years in light-EV product design, I’ve learned to source manufactured 18650 packs over from China. This is actually very easy and affordable. You request the cell type, the dimensions of the pack, and the cell configuration and they’ll ship you the 18650 bricks. You’ll need to request 1-3s modules with no BMS and cell tap wires coming out instead. You can design a way to connect 7 or so of these modules together to form an 18650 pack. This is because a 20s30p pack is too large for them to assemble and ship without a BMS. Message EJ if you’d like more info on this route.
I’m not an ME, so I always find a close friend to help CAD out a design to hold all our battery modules/cells. They’ll likely find it a nice challenge. Just be sure to abide by all the rules for evGP when it comes to battery packs. We’ve learned that Polycarb will be your best friend. Here’s what our Boston Power packs look like with their case/housing.