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Battery Basics

Batteries are an incredibly popular power source that are used in a huge variety of projects. If you don't want to plug your project into a wall, you'll probably be using a battery to power it, so choosing the correct battery for your project is a very important step in the design process. There is a lot of variety in batteries, and a lot of factors that go into picking the perfect battery for your project. Here's a quick rundown.

Form Factor

Batteries come in all sorts of shapes and sizes. In some cases the size and shape of the battery may be the most important factor. Coin cells and lithium-polymer batteries are great choices when a small form-factor is important. Other common battery sizes include AA, AAA, D, and 9V.

Chemistry

The chemistry that drives a battery is another characteristic to decide upon. Whether the battery is disposable or rechargeable is directly related to the battery's chemistry. A few of the more popular disposable batteries include alkaline, lithium, and zinc-carbon. As far as rechargeable batteries go, there's lead-acid (commonly found in automobiles), lithium-ion (often used to power consumer electronics), and NiCd and NiMH batteries (often take the same form as common alkalkine batteries: AA, AAA, etc.).

Nominal Voltage

A battery's chemistry will also affect the nominal voltage that it will produce. For example, an alkaline battery, like a AA or AAA, has a nominal voltage of about 1.5V, while the voltage of a LiPo battery will usually be around 3.7V. The key word here is 'nominal', as the actual voltage of the battery will fluctuate depending on how much charge it has left. A fully charged LiPo battery will produce about 4.23V, while when discharged its voltage may be closer to 2.7V.

Common batteries, their chemistry, and their nominal voltage
Chemistry Battery Size Nominal Voltage Rechargable?
Alkaline or Zinc-carbon AA, AAA, C, and D 1.5V No
Alkaline or Zinc-carbon 9V 9V No
Li-MnO2 Coin Cell 3V No
Li-ion, Li-poly Various sizes 3.7V Yes
Li-poly Various sizes 3.7V Yes
NiMH or NiCd AA, AAA, C, D 1.2V Yes
Six-cell lead-acid Car battery 12.6V Yes

One more point on battery voltages: when two or more batteries are placed in series, the voltages of the batteries are added together. For example, lead-acid car batteries are actually made out of six single-cell lead acid batteries tied together in series; the six 2.1V cells add up to produce 12.6V. When tying two batteries in series, it's recommended that they be of the same chemistry; also be wary of charging batteries in series as many chargers are limited to single-cell charging.

Batteries in parallel/series

Capacity

Capacity is another of the more important characteristics to observe when picking your battery. The capacity of a battery is usually rated in ampere-hours (Ah) or milliampere-hours (mAh), and it tells you how many amps a fully charged battery can supply over a period of one hour. For example, a 2000mAh battery can supply up to 2A (2000mA) for one hour. If you know the average current your project will require, and how long you want it to run before the battery dies, you can pick out a battery that will perfectly suit your project.

When you connect two or more batteries in parallel, the capacities add. For example, four AA batteries connected in parallel will still produce 1.5V, however the capacity of the batteries will be quadrupled.

Power Density

All of the characteristics above (size, voltage, and capacity) factor into a battery's power density. Power density relates the amount of power (amps * voltage) a battery can supply to its weight, and is usually displayed in terms of watt-hours per kilogram (Wh/kg). The higher the power density, the higher the power it can supply in smaller sizes. Lithium batteries (coin cells, LiPos) can pack a lot of punch into small spaces due to their high power density, while zinc-carbon and NiCd batteries have extremely low power densities. In general, disposable batteries are more power dense than rechargable batteries.

Example Question

With the above information at hand, here's a quick quiz to test your shiny new knowledge:

1. Fill in the blanks: depending on a battery's chemistry it can be either rechargeable or disposable. Alkaline batteries are a common example of __________ batteries, while Lithium-polymer and lead-acid are very common ____________ batteries.

2. A lithium-polymer battery has a capacity rated at 2Ah. The battery is powering a circuit that consumes on average 10mA. If the battery is fully charged, theoretically how long can it power the 10mA circuit?

3. Four 1.5V alkaline batteries in series will produce what voltage?

Answers:

Spoiler Alert! Highlight from here...

1. disposable, rechargable

2. 200 hours. How'd we get there? First convert the capacity of the battery to mAh: 2Ah * (1000mA / 1A) = 2000mAh. Next, divide the capacity of the battery by how much current the circuit it powers will consume: 2000mAh / 10mA = 200h.

3. 6V. How's that work? Batteries in series have their voltages added together, so 1.5V + 1.5V + 1.5V + 1.5V = 6V.

...to here to reveal the answers. Because, you know, they're so super-secret.

Quiz Question - Billy's Battery's Capacity II

Billy needs a night light when he goes to bed but when he sleeps in his tent in the backyard he has nowhere to plug his light in. So he reprogrammed his LectroCandle to constantly illuminate the 9 LEDs (3 RGBs) on there. Each LED has 10mA flowing through it and on top of that the ATTiny85 microcontroller consumes 8mA. Billy replaces the stock AA batteries with a 1000mAh lithium-polymer battery so he can recharge it. Will the battery last long enough to ward off the bogeyman? Theoretically, how long could they last?

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