Senior Neel Vakharia is a bio-electrical engineering major whose experience as a solar car driver and power-electrical engineer during the American Solar Challenge 2010 makes him the perfect team member to tell you all about the battery.
We’ll start at the smallest level of the battery, the cell:
So the battery pack is one of the three major high voltage systems on the car (the other two are the solar array and the motor). Before we can talk about the battery pack as a whole, it’s important to know a couple of key things about the individual battery cells. What is a battery? It’s a device that stores charge. But what makes it different from a capacitor, which also stores charge? The big difference is that while a capacitor’s voltage scales linearly with the amount of charge (voltage = charge/capacitance), a battery will quickly reach some nominal voltage, hang-out there until it starts reaching its maximum capacity and then quickly start increasing again, like this graph shows:
Why does this matter? Many electronic devices (like your phone or laptop) need a fairly constant voltage supply to work properly. But from the above graph you can see that batteries have a fairly constant voltage supply (unless they’re very low or very high on charge) while capacitors don’t. And the ability of the battery to produce a fairly constant voltage allows the car to run more efficiently.
So then how do we pick the right batteries? Obviously the ones that can hold the most charge, right? Well, kind of. The key criteria in picking the best battery is something called energy density. It doesn’t really matter how much charge the battery can hold, the important thing is how much it can hold per unit weight. This is because for solar cars and electric vehicles, the less the car weighs, the less energy it burns trying to move all that weight. On top of that, because the World Solar Challenge has a limit on how much the solar car’s battery can weigh, the more energy we can get per unit weight, and the more energy we have in total.
Another big concern that can rule out many batteries for UMsolar is the battery’s relative capacity at high temperatures. Because batteries are chemical devices, the ability of the battery to release energy varies depending on temperature. Since the World Solar Challenge is a race across the Australian outback, and since there are approximately 500 battery cells releasing heat within an enclosed box in our solar car, you can imagine how quickly the temperature inside the battery pack can escalate. Because of this, UMsolar has to pay attention to the smallest details on the battery’s spec sheet. The spec sheet describes how the battery ideally should work, but it doesn’t always perform this way. All batteries vary a little bit in their performance. The team does a lot of testing to verify that the batteries we’ve ordered actually work like they’re supposed to.
The last major concern is the maximum discharge rate and battery voltage. For any application, we must consider what our current and voltage demands are so that we pick the battery that can handle our application. If the battery voltage is too low, we would need to stack multiple batteries in series and if the maximum discharge rate is too low for our application, we would have to put batteries in parallel to split the current. This sets us up for the next level of the battery pack which is…
Now that we have discussed a little bit of the individual cells, we can move to the next level of organization — the module. A module of battery cells is simply a group of cells connected in the same way (series or parallel). Generally, solar car teams configure their modules in parallel because it is much easier to manufacture and manage. These modules can then be stacked in series. If you remember from your high school or college physics course, adding batteries in parallel will increase the module’s maximum discharge rate, but keep the voltage the same. We will then stack modules in series to increase the voltage. The combination of a series/parallel configuration is known as the battery pack configuration. In a nutshell, a battery pack configuration is how all the cells are organized electrically in the battery pack.
The battery pack is a combination of multiple modules.
Just like the battery cell, a big issue with the battery pack is cooling. Depending on how teams isolate their modules from other modules, overheating issues can lead to many problems. When a battery cell gets hot, not only does its relative capacity decrease, but also its internal resistance increases. Depending on how the cells in a module are arranged, this can be a problem because the cells closer to the middle or inside of the module heat to higher temperatures than the outside modules. This causes a mismatch of internal resistances, which leads to balancing issues. Basically, since the internal resistances are different, they burn off different amounts of charge. There is a set low-end and high-end voltage for a battery and if it goes outside of that range, dangerous things can happen. Outside of this range, reactions that you don’t want to happen in the battery will start to happen.
This is why adequate cooling is important so that the internal resistances stay low and decrease the risk of the modules’ state of charge growing out of balance. Certain battery management and protection systems employ techniques to bring the modules back into balance by either burning off energy from modules whose voltages are too high while charging close to 100% or actually redistributing charge to the weaker modules.
On a more basic note, fans are one of the most common methods used to cool the battery pack. The problem is how many to use? Depending on the life-span of the pack and how much energy the solar car’s battery can spare, teams generally usually use anywhere from 1 to 4 low power computer fans to cool the battery pack.
We could write a whole book about the battery, but this is just a taste of the issues that Neel and the rest of the battery crew deals with as members of UMsolar. To learn more details about the car, check back next week for another Technical Thursday featuring Mechanical Engineering major Cole Witte.