I took some quick lessons in volts, amps and resistance of common battery types over the past few months in order to be properly educated for building many components for my smart home project. I am sharing some of the more important lessons on battery testing here for my own reference and others to learn by.
How to test a 9 Volt Battery
Here I describe how to test a regular 9 volt square type battery typically used in smoke detectors and small transistor radios. Yes, the typical battery most of us think about when we hear the term “9 volt battery”
To test the output voltage of a typical 9 volt battery, simply set your volt meter to DC current and set the voltage at 20 volts or the closest setting to it. Applying the red lead to the positive or smaller round terminal of the battery and the black lead to the negative larger round lead of the battery should get you a reading of between 8 and 9.5 volts on a decent 9 volt battery with some life in it. THIS IS NOT GOOD ENOUGH to tell if the battery is really good however! If it is below 7 volts, you can be pretty sure that the battery is not much good anymore.
If the 9 volt battery measured 7 volts or above, then you should do a very quick short circuit amperage test to see that the short circuit current is sufficient as well. This is a true test to determine that the battery has life left in it. It is possible that a battery could give you a strong 9 volt reading using the simple voltage test just described, but still be dead for all practical purposes. If the battery is old or has bad resistance, it could still be bad and show a high voltage reading of 7 to 9 volts. To eliminate this possibility, follow these steps carefully and only if your multimeter has a 10amp setting.
Steps to testing the short circuit current of a battery
- Make sure the black lead is plugged into the com port of the multimeter as usual.
- Plug the red lead into the 10amp port that is the opposite port used for testing voltage as described above.
- Set the dial to the 10 amp position.
- hold the red lead to the positive terminal of the battery and then quickly touch the black wire to the negative lead just long enough to take a reading and that reading will be your short circuit reading. If your reading while the multimeter is in the 10amp position is between 0.05 and 0.50 amps, then your battery has some life in it. Obviously, if it’s closer to 0.05 or less, then it doesn’t have much life. When I tested a brand new cheap 9 volt battery, it had 0.5 amps, so a good brand name battery would probably be much higher, but this range gives you a pretty good idea of the working range of a nine volt battery because even the battery that held only 0.05 amps would still power a 12 volt rated bank of of 3 LED lights. Probably not for long, but it did work.
Note: while a more accurate reading can be achieved by holding the two leads onto the battery for a longer period of time, it is counter-productive because you will also be destroying the battery and in some cases even endangering yourself and others near by, so be careful and do any of the exercises on my blog 100% at your own risk! I claim no liability what-so-ever! If you don’t want to risk it, don’t.