How Cars Work · The Big Battery

Putting energy back in

To use the battery, you discharge it — pull energy out, drive somewhere, repeat. Eventually it runs low. Then you have to put energy back in. There are two main ways to charge an EV.

Slow charging uses regular wall power. You plug the car into your house, the way you plug in a phone. Inside the car is a small device called the onboard charger, which takes the alternating-current (AC) power from the wall, converts it to direct-current (DC) power that the battery can store, and feeds it slowly into the pack. Charging from empty might take 8 to 20 hours, depending on the car and the outlet.

Fast charging uses a special station that bypasses the onboard charger. The station itself converts the power to DC and feeds it directly into the pack at much higher rates — 50, 150, or even 350 kilowatts. At a fast charger, an EV can go from 20% to 80% charge in about 30 minutes.

Why doesn’t every charger fast-charge? Two reasons. Fast chargers cost a lot to build — they need very beefy power converters. And charging too fast, too often, slowly damages the battery. Slow charging at home is gentler on the chemistry, and most EV owners do most of their charging that way, overnight in a garage. Fast charging is for road trips.

What batteries don’t like

Lithium-ion batteries are remarkable, but they have weaknesses.

They don’t like cold. When the temperature drops below freezing, the chemistry inside the cells slows down, and the battery delivers less power and accepts less charge. Driving an EV in a Minnesota winter typically loses 20 to 30 percent of the car’s range, until the battery warms up. Many EVs include heaters that warm the pack while you drive, partly to fix this problem.

They don’t like extreme heat. Above about 110 degrees Fahrenheit, the chemistry starts to degrade faster, and the battery ages more quickly. EVs include cooling systems — often using the same kind of liquid-cooling loops you find in a high-end gaming PC — to keep the pack at a comfortable temperature.

They don’t last forever. Every time you charge and discharge a lithium-ion cell, a tiny amount of capacity is lost. After about a thousand charge cycles, a cell typically holds about 80 percent of what it started with. After several thousand, much less. EV batteries are designed to last about ten years of normal driving, after which the car still works, but its range is shorter than when it was new.

These limits are real, but they are not deal-breakers. They are tradeoffs. The pack is heavy. The car charges more slowly than a gas car refuels. The range drops in winter. In exchange: no engine, no fuel system, no exhaust, almost nothing to maintain, and a remarkably smooth ride.

Try It: The Lemon Battery

You will need:

• A fresh lemon (or four, if you don’t have a multimeter)

• A galvanized nail (zinc-coated, from any hardware store)

• A copper penny made before 1982 (anything older is mostly copper) or a piece of bare copper wire

• A multimeter, if your family has one — or some bare copper wire if you don’t

Roll the lemon on a hard surface for a minute, pressing down with your hand. This breaks up the juice sacs inside. Now stick the nail into one end of the lemon, halfway in. Stick the penny (or copper wire) into the other end, halfway in, far enough from the nail that they don’t touch inside the lemon.

You have just built a battery.

If you have a multimeter, set it to read DC volts and touch one probe to the nail and the other to the penny. The meter will read somewhere between 0.5 and 0.9 volts. That is real, measurable electricity, made by the chemistry of zinc and copper reacting through the lemon juice.

If you don’t have a multimeter, the easiest way to detect the electricity is to use four lemons. Build four identical lemon batteries, then connect them in a chain — penny of lemon 1 to nail of lemon 2, penny of lemon 2 to nail of lemon 3, and so on, using small bits of bare copper wire to make the connections. The four-lemon chain produces about 3.5 volts. Touch the unconnected nail at one end of the chain and the unconnected penny at the other end to your tongue at the same time. You will feel a faint, distinctive tingle. The current is far too small to hurt you. You are tasting electricity made out of chemistry.

The lemon battery works on the same principle as an EV battery: two different metals separated by something that lets ions move between them, hooked up by an outside path through which current flows. Scale it up — better materials, ten thousand cells in series and parallel — and you have a Tesla pack.

Did You Know? The battery in your phone uses the same basic chemistry as the battery in an electric car. Both are lithium-ion batteries — they store and release energy by moving lithium ions back and forth between two electrodes. The phone battery is about the size and shape of a credit card and stores enough energy to run your phone for a day. A car battery is the size and shape of a small mattress and stores enough energy to drive a car several hundred miles. Same chemistry, scaled up by a factor of about a thousand. The whole idea of a battery goes back further. The very first one was invented in 1800 by an Italian scientist named Alessandro Volta. His version was a stack of zinc and copper discs separated by saltwater-soaked cardboard. The stack made about 1.5 volts of electrical pressure — exactly the same as a modern AA battery in a TV remote. Volta’s stack worked. Every battery since, including the one in your mom’s phone and the one under the floor of every Tesla, is a refinement of his idea.

Energy in a box

So we have a battery. Hundreds of pounds of carefully arranged lithium-ion cells, wired in series and parallel to make a few hundred volts and many thousands of amp-hours, sitting in a sealed box under the floor of the car.

That battery holds energy. It does not yet do anything with it.

To turn the energy into a moving car, we need a motor.

That is the next chapter.

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