Smoke detectors beep when their backup battery dies, which always seems to occur in the middle of the night (at least for me.) These backup batteries are usually a small rectangular 9V. They have become popular choices for electronics projects. If you need your Arduino project to last longer than a day, this isn’t the battery you want to use. Here’s why.
What is a 9V Battery
The 9v battery comes in a standard package about 25mm wide, 16mm tall, and 47mm long. On top there are standard snap-style connectors. The design of these connectors prevents you from attaching the battery backwards.
The nominal output for this style is 9 volts, which might explain where the name “9V battery” comes from! (The original name was “transistor battery“.)
Inside of the package, there will be different types of chemistries used.
Different 9V Chemistry Types
Batteries will have different energy capacities and max output currents, based on the stuff inside. This is why the chemistry is key. Here is a comparison of two popular chemistries: Alkaline and Lithium.
Alkaline batteries are the most common type of battery chemistry for disposable batteries. The chemicals used are Zinc-Manganese Dioxide (Zn/MnO2). For this chemistry type, 9Vs are given the designator “6LR61.”
To use actual data, let’s look at this Energizer datasheet on their 522-style. It says that Alkaline batteries provide their longest life when current draw is less than 25mA. Which, based on the life graph below, the life is only 24 hours at 25mA. (Note I’m calculating based on the mAh shown.)
Make sure you check out what happens when current is 100mA. The life of the battery drops to less than 5 hours! The life just goes down hill from there, as the graph shows.
These numbers are important for Arduino-based projects. Why? Because the Uno, when powered by the barrel jack or VIN, draws almost 50mA by itself. Just by adding a couple of LEDs your battery’s life drops quick!
A newer, and more expensive, alternative to Alkaline batteries are made with “Lithium” chemistries. Instead of Zinc, these batteries use Lithium along with the Manganese Dioxide (Li/MnO2).
As an example, let’s use the Energizer 9V Lithium datasheet. (FYI: The part number for their Lithium style is LA522.)
Here we can see service life for 25mA is about 30 hours. At 100mA the Lithium’s life is almost 8 hours. So there’s some improvement in life.
The real improvement, though, is the max current draw.
The Lithium battery can supply 1000mA, or 1A, of continuous current. (Alkaline struggles to provide 500mA.) Although, you can see in the graph the life of the battery drops to less than an hour at that current level.
Even with an “expensive” Lithium 9V battery, an Arduino-based project will only last for 1 or 2 days of continuous use. As a side note, a handful of AA (LR6) batteries will last 4-5X longer than their 9V counterparts.
Don’t be lulled in by the seemingly high “9 volt” output of a 9V battery. You may not be getting nearly as much life as you expected. An Arduino Uno by itself could empty a 9V in just a matter of hours.