Battery Tech Part 1

How Batteries work

 

If you are more interested in specifics of Tesla batteries, that’s mostly in part 2.

 

When dealing with electricity, you have some terms that need defining:

Voltage – This is a potential difference between two points. You can think of it like a height in the physical world like a hill or a mountain.

Current – This is what flows from point to point. In the early days of electricity research nobody knew what was flowing, but it was later discovered to be electrons. You can think of this like a river of water in the real world.

Resistance – This resists the flow of electrons and generates heat. You can think of this as similar to friction in the physical world.

Power – This is calculated by multiplying the Voltage and the Current and you can think of it like the power of the river caused by the amount of water combined with the height of the drop. This is measured in Watts.

Energy – This is Power times time and is measured in Watt-Hours.

 

When you start looking into batteries you see a lot with Voltages, Current, Power and Energy. The biggest confusion is between Power and Energy in part because both have something to do with Watts, but they are different measures. When you are looking at what is going on instantaneously, you are concerned about Power (Watts), but when you are looking at the total storage capacity, you are concerned with Energy in Watt-Hours.

 

Power and Energy have less relationship than you might expect. Some batteries can hold a lot of Energy (Wh), but they can’t produce much current at any point in time. Alkaline batteries are this way. They can last a very long time in something like a clock or other device that uses low current, but they do not work in a high current application.

 

You can also have batteries with low capacity (Energy storage density), but they can deliver very high currents for short periods.

 

Batteries fall into two broad categories: Primary Batteries and Rechargeable. Primary batteries are use once and discard and rechargeable can be put into some kind of recharging system and recharged. We’re all familiar with alkaline batteries that are the world’s most common Primary Batteries. Almost all cars built in the last 100 years have had a lead acid rechargeable battery to start the car and keep it’s electrical systems alive when the car is turned off.

 

We have other rechargeable chemistries like Nickel Metal Hydride and Nickel Cadmium, but Li-ion is the type of rechargeable battery that is getting the most attention these days.

 

Inside a battery, you need an anode and a cathode. Each is made of a different material and the type of material used by the anode and cathode determines the voltage. Different materials have different electrode potentials, that is they contribute a certain voltage from neutral when combined with another material. Lithium has an electrode potential of -3.04V, which is one of the most negative potential materials in existence.

 

When designing a battery chemistry a lot of factors beyond just the chemisty come into play. There are chemistries that may work to extent in the lab, but they are impractical to put into production. At this time there are no other combinations using calcium or stronium that nake a better battery and/or more cost effective than lithium.

 

To get a voltage for a battery, you need a second material to make the other terminal. For example nickel-cadmium batteries use nickel hydroxide for the cathode (positive terminal) and cadmium for the anode (negative terminal). Some kind of electrolyte is in between the two to allow atoms to easily transfer from one to the other.

 

When a battery is in a circuit, electrons are released from the negative terminal (anode) that makes the anode more positively charged and the cathode at the other end of the circuit gains electrons and becomes more negatively charged. Inside the battery, positively charged atoms move from the anode to the cathode. When the cathode become saturated with positive ions, the battery is discharged.

 

 

For a non-rechargable battery this process is only one way. There is no way to get the atoms separated again unless you recycle the battery and reprocess the materials making up the battery. With a rechargeable battery, applying an outside power source to the battery reverses this process. Some rechargeable batteries can be discharged and recharged many times, while others are more fragile and break down a bit each time the battery is recharged. Some rechargeable batteries have other limitations on charging. For example NiCd batteries tend to develop a “memory” if they aren’t fully discharged before recharging and will begin to think it’s “empty” when it reaches that level of charge instead of empty. Other batteries like Li-ion don’t like to be fully discharged and don’t like being charged to 100% and then allowed to sit with no current draw.

 

Other characteristics of the battery determine how much energy the battery stores. For many batteries, the main factor is the physical dimensions of the anode and cathode. Li-ion batteries have other factors that can affect the energy density stored in the battery.

 

Lithium-ion battery tech might be the most complex thing in electronics today. The public (and a lot of media) lumps all Li-ion batteries as one thing, but in reality there are many types of Li-ion batteries and they can be quite different. Li-ion is an umbrella term for a whole family of batteries, each with a different application.

 

Why is Li-ion getting all this attention? For rechargeable technologies, Li-ion can have the best energy density per weight of any battery type. Lead acid batteries are about 40 Wh/kg. Their energy density per volume (cu ft) is not bad, but because they use a fair amount of lead, the energy density by weight it poor. NiCd batteries are about 60 Wh/Kg and NiMH (nickel-metal hydride) batteries are about 90 Wh/kg.

 

NiMH is a special case in the battery world. It was a popular chemistry for cars in the 90s, it can produce high currents and has a high safety level, plus it’s a lot cheaper than li-ion batteries. However it is limited in car use because of who owns the patent. The battery chemistry was invented in the 1980s, and GM bought the patent in 1994 and then later sold it to Texaco just before Texaco was bought by Chevron. Among the patents Chevron bought was a patent originally obtained by GM in the 1990s for a pure EV battery pack using NiMH batteries.

 

Chevron then banned the use of NiMH batteries in pure EVs, only allowing them for hybrids. (Bring that up the next time someone says the oil companies aren’t trying to prevent alternate fuel vehicles!) Chevron’s patent on the cell technology itself has expired, but the patent on the car battery packs is valid until late 2020.

 

Because of Chevron’s blocking, EV carmakers looked to Li-ion batteries, which are much more expensive, but have energy densities ranging from 80 Wh/Kg to over 250 Wh/Kg. Ultimately Chevron has forced the market to improve Li-ion batteries and Tesla to build the Gigafactory to lower the cost of cells as much as possible.

 

As mentioned above, Li-ion batteries are a complex and unusual technology. Lithium has some inherent advantages with a high electrode potential as well as being one of the lightest materials in the universe. Lithium is the lightest solid at room temperature of all the elements. However pure lithium has the nasty attribute of being very flammable when exposed to oxygen and water, even water vapor in the air.

 

Research turned to using lithium compounds usually with a carbon graphite anode and some kind of compound for the cathode. When charged, ions of lithium are loosely attached to the carbon graphite cathode and as the battery discharges, the lithium ions cross through the electrolyte to bond with the compound making up the anode.

 

Lots and lots of research has gone into the materials used in these batteries. The only thing that they have in common is there is a lithium ion that crosses the electrolyte when the battery releases energy. Most Li-ion batteries made up until recently used all graphite for the cathode, but some recent advancements have added small amounts of silicon to the graphite which increases the number of lithium ions the cathode can hold. This doesn’t change the voltage (which is determined by the electrode potential difference between the lithium ions and the material the cathode is made from), but it does increase the energy density.

 

As with many materials with batteries, silicon has drawbacks. Graphite doesn’t change it’s physical dimensions much when it absorbs a lithium atom, silicon expands when it absorbs lithium which stresses the material and can lead to the battery falling apart a little bit on every discharge cycle and shortens the life of the battery. Recent advancements have allowed small amounts of silicon to be added without risk of destroying the battery. Tesla started using batteries with a little silicon in mid-2015 when they introduced their second generation of batteries.

 

For the anodes, the picture is even more complex. There are many known compounds used for different types of Li-ion batteries. Each has different characteristics. Each Li-ion chemistry is a trade-off between the specific energy (amount of energy it can hold), the specific power (the amount of current it can discharge at once), safety (some chemistries are more prone to catching fire than others), how it performs at different temperatures, its life span, and the cost.

 

Tesla uses a chemistry that uses a nickel-cobalt-aluminum oxide which has the best specific energy, but isn’t the best for the number of charge and discharge cycles, and it is more prone to fire than many other chemistries. Tesla has compensated for these limitations with very careful software management, good battery cooling, as well as good battery protection.

 

There have been a few Tesla fires, but they are very rare compared to internal combustion engine car fires. A study of US car fires between 2003 and 2007 found an average of 287,000 car fires in the US per year, which comes out to around 32-33 per hour. A total of 5 Teslas have caught fire world-wide in 4 years of production (3 in 2013 and 2 in 2016).

If you want to learn more about the nitty gritty details with batteries, Cadex Electronics maintains a wonderful site with more than most people would ever want to know about them:

http://batteryuniversity.com/learn/