Understanding and measuring AA-Batteries
After the introduction to multimeters in my last story, it is time to use it for something useful. In this story I will give you an introduction on batteries and how to measure them. There are many different types of batteries. Different forms, different chemistries, different connectors etc. Trying keep it simple I will talk about one of the most common form factors, the AA battery. You will see that even with this constraint, there is a lot to talk about.
What are AA Batteries
AA Batteries have many different names depending on your region and other factors. They are also called “Double-A”, “R6”, “15”, “UM-3” and some other names. Unfortunately, battery naming is very confusing with most types of batteries, so if you want to be sure that a battery is the right kind, look it up online and if unsure, compare the specifications (more on that later).
An AA-Battery is around 50mm/1.96inch long and has a diameter 14mm/0.55inches. As there is some variation in these dimensions, it is possible that one type of AA battery almost does not fit into a certain device (e.g. eneloops are 14.3mm and do not fit well into some lego battery boxes).
AA-Batteries usually have a Voltage of 1.2–1.5 Volts but can go up to 3.6 Volts for some special types. The voltage depends on different thing, the two most important are the chemistry that is used and whether they are empty or full. The chemistry battery is written on it. Whether a battery can be recharged or not depends on this chemistry. Alkaline (usually) cannot be recharged whereas NiMh (Nickel–metal hydride) can be recharged. Always read on the battery whether it is possible. Recharging a non rechargeable battery can cause a fire!
A small disclaimer: Never use a 3.6 Volt Battery in a device which is not constructed for this type. It can damage or destroy your device. Fortunately, these batteries are not very common and can normally not be bought in a normal store.
What to measure from a battery
As we’ve learned above, batteries can have different Voltages. If you compare electricity to water, voltage corresponds to the pressure of the water in a tube.
For most practical purposes there are the 1.5 Volt batteries which cannot be recharged and 1.2 Volt batteries that can be recharged (we ignore the 3.6 Volt ones and others). These voltages will go down with use. This could give you an indication how empty a battery already is.
Some devices only work with batteries which are above 1.2 Volts so they will probably never work(well) with rechargeable batteries. These devices expect a certain minimal Voltage (or pressure in the water analogy) and if this is not present it no longer works. A non-rechargeable battery which no longer works in such a device may still work for some time in a device that does not care. Just put them in another device and you can “finish them”.
If you like graphs, you can have a look at the following picture it is taken from the datasheet of a battery to show how fast the voltage goes down. It shows different curves for different devices that draw from 5mA (e.g. a remote control) to 100mA (a toy with a motor) and after how many hours you can measure which voltage.
Measuring an AA Battery
Now that we know what we are looking for, let’s finally measure some batteries!
To measure a battery, set your Multimeter to Volts. First try with a new non-rechargeable battery. These is the type you find in every supermarket. They are often Alkaline or Zinc-carbon batteries. It should be written on the battery, that it is not rechargeable.
Put the black probe on the flat part of the battery (marked with “-”) and the red probe on the one with the “+”. The multimeter should show you a value between 1.7 and 1.5 Volts. If you hold the probes the “wrong” way round, nothing bad will happen, the multimeter will just show a negative voltage.
This voltage will decrease with the amount of use a battery sees. The voltage usually drops down to 1.5 Voltas after some time. It will go down until the battery is “empty”. As mentioned above, empty might depend on the device you are using. Some devices only work down to e.g. 1.3 Volts, others will run until the battery is completely drained and can no longer deliver any current.
If you have a fully charged rechargeable battery (e.g. NiMh) they will measure a Voltage around 1.4 Volts. This Voltage will drop to around 1.2 Volts slowly with usage. Such a battery can go down to about 1.0 Volts. If you measure a Voltage that is way lower (e.g. 0.4 Volts) this battery is dead an can usually not be recharged.
A word on capacity
To finish this story, I would like to talk a moment about battery capacity. The capacity is measured in Ah (Ampère-Hour) or mAh (milli-Ampère Hour which is 1/1000 of an Ah). This specifies, for how long you can draw a certain current. If, as an example, you have a battery with 2000 mAh, you can draw a current of 500mA for 4 hours (4*500 = 2000).
It usually does not matter what capacity a battery has for a certain device. It will just run for a longer time if the capacity is higher. Often the capacity is written on the battery but not always. Capacity can not be measured directly. To measure it you have to empty a battery and see how much was “in there”. For non-rechargeable batteries this is useless, as the battery is then empty. If you are interested in a certain type, search online for someone who did the test (e.g. on youtube). For rechargeable ones you can do the same or use a battery charger which has this function.
Conclusion
Batteries are a complex subject, there is a lot to learn about. There are some basic concepts such as Voltage and Capacity and many different things to known on a specific battery type. A multimeter can help you to judge whether a battery is empty or not or if a device can handle low voltages.
If your device supports lower voltages, rechargeable batteries can be a good way to save some money. A charger can be bought for around 20$/Euros and a four-pack of AA batteries costs 15$/Euros. You can recharge them hundreds of times. I personally have almost no non-rechargeable batteries in my home.
