Battery Applications

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Battery Applications

Could you run your cell phone on a 9 volt battery? How many D cell batteries would it take to start your car? How far would a Tesla run if it was powered by AA batteries? To begin to answer such questions, it is important to understand how battery cells are organized into battery packs to provide different amounts of current and voltage for applications ranging from cell phones to electric cars.

What is a Battery Cell?

A battery cell contains a cathode, an anode, electrolyte, and a separator. Each cell has a specific voltage and current, which are determined by the battery chemistry. For example, an alkaline-manganese battery is made up of a single battery cell with 1.5 V. A lithium-ion battery cell, by comparison, is 3.6 V. See more in battery basics.

Overcoming Resistance

To understand how batteries function, it is important to understand what they are trying to overcome: electrical resistance. If we think of electricity flowing similarly to water, resistance is a constriction that slows the flow of water. For instance, if a sprinkler is running and someone steps on the hose, the added resistance in the hose reduces the amount of water flowing from the sprinkler. The more people who step on the hose, the greater the resistance.

Everything that makes up an electric circuit, including the wires, microprocessors, lights, speakers, and other devices being powered, have differing levels of internal resistance. Just as a water tank must have sufficient pressure to push enough water through a hose, a battery must have sufficient voltage to overcome the total resistance of a circuit and supply enough current to power it. Batteries fail or have to be recharged once the voltage drops below a threshold and can no longer supply sufficient electrons quickly enough to power the circuit.

Batteries, themselves, however are not perfect sources of electrons. Batteries also have internal resistance. There is resistance in the internal wiring in the battery. The battery's chemistry is also a source of resistance, due to factors such as the battery's age, state of charge, and temperature, among other factors. In general, single-use alkaline batteries have much higher resistance than rechargeable batteries.

This explains one of the big limitations of a homemade battery. Although we can get good voltage, we cannot get much current out of a homemade battery due to its high internal resistance. Compared to the highly refined materials used in a commercial batteries, the zinc screws, copper wire, and vinegar in our home-made battery have high resistance.

Increasing Voltage and Current

Although a AA battery is good for lighting up a small flashlight or powering a remote control, a single AA battery isn't capable of powering a large flashlight or running an electric car. Most applications require higher voltages or more current than a single battery cell can provide to overcome the resistance of a circuit, increase the runtime of a device, or improve its performance.

To achieve higher voltages and currents, individual battery cells are assembled into battery packs. For instance, the common 9 V alkaline-manganese battery used in smoke detectors is made up of 6 battery cells. A car's starter battery is usually made up of 6 lead-acid battery cells. And the Tesla Model S electric car requires more than 7000 lithium-ion battery cells!

There are two ways that batteries can be connected together: connecting them in series increases the voltage; connecting them in parallel increases the current. To understand how battery packs are assembled, the metaphor of a water tank is useful.

Increasing Voltage

To increase the voltage, batteries are connected in series. If we think of a battery cell as a water tank, this is accomplished by stacking the water tanks on top of each other, thus increasing the pressure of the water delivered from the tap at the bottom.

wiring batteries in series increases voltage

For batteries, this is accomplished by connecting the negative terminal of each battery to the positive terminal of the next battery, and using the positive terminal of the first and the negative terminal of the last battery to complete the circuit.

Increasing voltage is important, because higher voltages are necessary to overcome the internal resistance of many electronic circuits or more demanding applications. If the voltage is too low, a flashlight may be dim, a cell phone might not power up, and an electric car would accelerate very slowly (if it functioned at all!).

Increasing Current

To extend the capacity of a battery pack, batteries are connected in parallel. If we think of a battery cell as a water tank, this is accomplished by keeping the water tanks at the same elevation, but connecting their taps together. The pressure of the water doesn't change, but the flow will be three times as long or three times as fast. For batteries, this is accomplished by connecting all of the negative terminals of the batteries together and all of the positive terminals of the batteries together.

wiring batteries in parallel increases capacity

Batteries, however, are not perfect sources of current. Every battery has some internal resistance that slows the flow of electrons. One of the important advantages of connecting batteries in parallel is that it reduces internal resistance. For instance, four AA batteries connected in parallel will have only one quarter the resistance of a single AA battery. As a result, batteries wired in parallel can supply more current.

Increasing Voltage and Current

For demanding applications, such as electric cars, battery packs might include groups of batteries in parallel, in series, or a combination of both, to increase the voltage, current, and the capacity of the battery pack. As the size of the battery pack grow in size, they can give off substantial amounts of heat. Large-scale battery packs, particularly those used in electric cars, often have advanced air- or liquid-based cooling systems to keep them from overheating, which could damage the batteries or pose a fire hazard.

Battery Packs

Although you would never run an electric car on AA batteries or a cell phone on a lead-acid battery, estimating what type of battery pack would be necessary for different applications helps to highlight the advantages of different battery chemistries.

Cell Phones

The battery in an iPhone 6 is a lithium-ion battery weighing 28 grams, which accounts for 21% of the weight of the phone. The battery generates 3.82 V and has a capacity of 6.91 watt hours. In terms of use, that translates into a typical day of cell phone use. What would happen if an iPhone was powered by single-use alkaline manganese or lead-acid batteries instead?
Lithium-Ion Alkaline-Manganese Lead
# cells in series 1 3 2
Total voltage 3.82 V 4.5 V 4 V
Capacity 7 Wh 9 Wh 10 Wh
Weight 28 g 69 g 724 g
Initial Cost $20 $1.20 $28.80
Charging Cost $0.37 per year ~ $438 to replace the batteries daily for a year ~ $0.37 per year

As the estimates in the table above suggest, using single-use or lead-acid batteries poses significant trade-offs. Alkaline batteries would dramatically increase the cost of using a cell-phone, since the batteries would have to be replaced almost daily. And lead-acid batteries would substantially increase the weight of the phone.

Electric Cars

The structure of the Tesla Model S electric car battery has been reported on widely in the press. It uses a variation on the standard 18650 lithium-ion battery cell, which are used in smaller-scale applications such as laptops and portable electronics.

To power a Tesla requires a big battery pack. The ~500 kilogram battery pack is made up of 7104 18650 lithium-ion batteries. Each individual lithium-ion cell weighs 46 grams and produces roughly 2.900 Ah at 3.6 V. When assembled into a battery pack, those 7104 cells can produce approximately 345 V and have a capacity of 74 kWh.

To produce this much voltage and current requires a complex battery pack made up of 16 modules. Each module consists of 6 sub-modules wired together in series. Each sub-module contains 74 18650 lithium-ion battery cells wired in parallel. The general structure of the battery pack is outlined in this diagram:

battery pack structure

How far and fast would a Tesla go if it was powered by AA cells instead of a lithium-ion batteries? If we make some rough assumptions about the performance of AAs batteries, we can generate an initial estimate of the answer to this question based on a battery pack made up of 14,208 AA batteries (which would have roughly the same weight as the Tesla's battery pack).

Since the discharge rate of an alkaline battery is half that of the lithium-ion battery cells used in the Tesla, the actual acceleration would be 33% of the Tesla battery pack pretty slow. The range might actually be a bit better, since the alkaline batteries have a higher capacity. But since they aren't rechargeable, buying off the shelf AAs to stock up your Tesla would be prohibitive: $5683 per tank!

If you want a less expensive rechargeable battery, the obvious choice would be a lead-acid battery. But there is a good reason that today's electric cars do not rely on lead-acid batteries. If we want to hold the weight constant, we could build a battery pack made up of 12 lead-acid golf cart batteries. Although the lead-acid batteries would be about 85% less expensive and it would be easily recyclable at end of life, the lead-acid Tesla would a much slower car with a shorter range.