In this lesson, we go over the most profound method of storing electricity and discuss the different battery technologies out there, their use and some of the challenges the battery industry faces. You will learn what types of batteries are used in cars like the Prius, why it is controlled to only use 15% of its capacity and you’ll learn what the effect is of temperature on batteries. First of all, lets see how they work!
How batteries work
A battery is a device that converts chemical energy into electrical energy. Some batteries are rechargeable and can do the reverse as well. After this paragraph, we will focus on rechargeable batteries, since these are the ones used in all electric vehicles. To explain how batteries work, we dive into some ‘Good Old’ chemistry and use the most common non-rechargeable battery as an example: the Alkaline cell. Don’t worry if you don’t know anything about chemistry, we will explain everything!
As you’ve learned in previous lessons, an electric current is basically a flow of electrons, negatively charged particles with zero mass. You can compare an electric current with wind, the movement of air (the electrons) from high pressure to low pressure (the potential difference, you know this as: voltage). So the electrons want to flow from the place where there is plenty, to the place where they’re needed. This is exactly what the battery does, it creates a voltage by introducing two chemical reactions called “RedOx reactions”. One half-reaction releases electrons (Oxidation), the other needs them (Reduction, hence RedOx). Batteries have a liquid solution (electrolyte) and a solid conductor (electrode) involved in the half-reaction on both sides. By simply connecting the supply and demand side electrodes with a conducting wire, you allow these electrons to move. And that’s where you can extract power! Do you remember:
Power = Voltage x Current?
Many different types of batteries exist, they differ in the materials that are used for the electrodes and electrolytes. The RedOx reaction for an alkaline cell is as follows:
Oxidation side: Zn (s) + 2 OH- (aq) --> ZnO (s) + H2O (l) + 2 e-
Reduction side: 2 MnO2 (s) + H2O (l) + 2 e- --> Mn2O3 (s) + 2 OH- (aq)
Lets look at the oxidation side first: One Zinc atom (Zn) reacts with two hydroxide ions (OH-) and releases two electrons (2 e-). In this process, the two hydroxide ions (in total two oxygen (O) and two hydrogen (H) atoms) release one oxygen and become one water molecule (H2O). The oxygen connects with Zn to form ZnO.
The reduction side takes the two electrons that were released and turns one water molecule into two hydroxides. The leftover oxygen atom connects two MnO2 into Mn2O3.
The voltage difference is set by the so called ‘reduction potentials’ of the oxidation and reduction reactions. This depends on how strong the reducing and oxidation agents are. For alkaline this is 1.5 Volts (compare to 3.6 Volts for Li-ion). If you want to know more about that, visit this website.
Now lets take a closer look at two different types of rechargeable batteries that are used in electric cars, NiMH and Li-ion. We will compare them on characteristics that are important in their use for electric vehicles: energy density, charge efficiency, self-discharge rate and cycle life. We will focus on the current state of the technology that is found on the market and disregard their past inferior versions.
NiMH Nickel-Metal Hydride: a “Sensitive Battery”
NiMH batteries are the batteries found in most hybrid cars. In 2008 already, more than 2 million hybrid cars (e.g. Toyota Prius, Honda Insight, Ford Escape Hybrid) were using NiMH technology. We call NiMH a very sensitive technology though, and that is because its self-discharge rate and cycle life depend significantly on the storage temperature and depth of discharge.
Lets take a step back into the chemistry of batteries and think about how storage temperature affects self-discharge. Most of you probably know that chemical reaction rates increase with temperature. The higher the temperature, the more and harder the collisions of molecules and therefore a higher probability of a successful reaction (see Arrhenius equation). When the batteries are stored, we don’t want any reactions to take place, so it would therefore be better to keep the temperature low. Some people throw the batteries in the freezer when they’re not using them. This is not a myth! It actually lowers the self-discharge rate. Besides the temperature sensitivity for self-discharging, NiMH also deteriorates quicker when stored and operated at elevated temperatures. So guess what the Toyota Prius is equipped with, to aid its NiMH battery pack? Yes! A thermal management system! In other words: battery cooling/heating to keep it at the optimum temperature during use. The minimum and maximum temperatures at which NiMH can discharge are -20 and 60 degrees Celsius.
So the NiMH batteries we find on the shelves now, have fairly high self discharge rates. A month after a full charge, the batteries have discharged themselves ~30% from internal resistance current draws. After that, the self-discharge rate drops to about 10%. NiMH is therefore unsuitable for applications like clocks, remote controls, etc. For cars like the Prius that are used daily, it’s not that big of a deal, especially because it has a State-of-Charge (SOC) management system that keeps the battery between 45% and 70% charged. In the last few years, more NiMH batteries have appeared with somewhat lower self-discharge rates, but those all came with the sacrifice of energy density. The very latest research efforts have led to improved self-discharge rates of ~2% per month (see here) .
NiMH batteries are very similar to NiCd batteries: they use the same material for the cathode (NiOOH). The difference is that instead of using cadmium for the anode, NiMH uses a hydrogen-absorbing alloy. This makes it more environmentally friendly in its disposal scenario (Cd is very toxic) and it also results in a 2 times higher energy density. Right now, NiMH has phased out NiCd batteries because of the improvements made and environmental regulation (mostly European) that has prohibited the use of cadmium in electronics. However, similar to how NiMH has won over NiCd, Li-ion technology seems to be taking over more of NiMH applications as well. The reason for this will be discussed in more detail in the next paragraph!
Benefits of NiMH
The main issues with NiMH are:
Radar Diagram of Lithium Cobalt Oxide. The graph shows relative performance compared to other li-ion chemistries (see batteryuniversity.com).
Li-ion and the Range Challenge
The biggest challenge for batteries involves increasing their ‘specific energy’ or 'energy density'. As we mentioned before, the battery energy density is only 1% of the energy density in gasoline. Below is an overview of the energy density of several batteries that are available right now. As you can see in the figure, the lead acid batteries have a rather low energy density, while the LCO is pushing the envelope with 170 Wh/kg.
Overview of the energy density in Wh/kg for different battery technologies. From: Batteryuniversity.com
But mark our words: one day, electric vehicles will have competitive ranges with conventional gasoline powered vehicles. The need for longer battery lifetimes in laptops and phones has stimulated enormous development in battery technologies, and Li-ion in specific. Besides an increase in record energy density from 0.088 kWh/kg (in 1991) to 0.270 kWh/kg of Li-ion batteries, their price has dropped ~30% in the last few years http://batteryuniversity.com/learn/article/battery_statistics. It should be noted though, that the record energy density is much higher than the energy density in actual batteries (the LEAF has ~0.09 kWh/kg, Tesla: 0.124 kWh/kg). So lets see what needs to happen for EV’s to have a 400 mile range (the range of a gasoline guzzling Jeep Grand Cherokee). If the battery-to-wheel efficiency of cars remains the same (~4 miles per kWh), and we allow for a 300 kg battery pack, we would need 0.33 kWh/kg batteries. That’s twice the energy density of what LCO has now, and that includes the auxiliary systems required. If we look at how the record energy density has changed over the years, we see that li-ion has improved remarkably over the years (see figure). But some people believe that Li-ion is reaching its maximum potential and that there is a need for a completely new storage technology or an innovative breakthrough in an existing one.