Abstract

You probably use batteries to power different devices every day, ranging from toys to TV remotes, without giving it much thought. Figure 1, below, shows some common types of batteries. Eventually the batteries will die and you have to replace them with new ones (or recharge them if they are rechargeable batteries). How much do you actually know about how batteries work? This abbreviated project idea will give you some suggestions to investigate how batteries perform in common household devices.

Batteries act as a voltage source for an electrical circuit. An ideal voltage source can hold a constant voltage indefinitely, while providing any amount of electrical current. However, batteries are not an ideal voltage source. Batteries have internal resistance, which causes their voltage to drop when they are under load. They also have a limited capacity, meaning that eventually they will drain and need to be replaced or recharged. How does a battery's voltage change over time as it is used in a common household device? When does the battery "die" and cause the device to stop functioning — does the voltage drop all the way to zero, or is it before that?

To investigate this problem, you can use a multimeter, a tool that can measure both electrical voltage and current (and other things, like resistance). If you need help learning how to use a multimeter, check out the Science Buddies Multimeter Tutorial. Pick a battery-operated household device that you can leave on continuously to drain the battery (for example, a toy with a spinning motor and an ON/OFF switch would work well, a TV remote would not work well since the batteries last for a very long time). Put fresh batteries in the device, turn it on, and then use a multimeter to measure the batteries' voltage in regular intervals (see notes below). Record your data, and then make a graph with voltage on the y-axis and time on the x-axis. How does the batteries' voltage change over time? At what voltage does the device stop functioning? Can you do the same test for other devices or types of batteries?

Here are some additional notes and suggestions on taking your measurements:

• Many devices contain several AA or AAA batteries in series. A typical new alkaline battery cell provides about 1.5 volts (V). Voltages add in series, so, for example, 4 AA batteries in series should provide about 6 V. Make sure you measure the voltage of all the batteries combined in series, and not just one battery.
• The best way to measure the voltage of your batteries is when they are "under load," or installed and still powering the device. Depending on how the batteries are inserted into your device, this may be difficult to do with your multimeter's probes. If you cannot take measurements with the batteries in the device, then remove them and quickly take measurements, then re-insert them into the device. It is important to take your readings quickly and consistently, because the batteries' voltages may start to "recover" over time. So, for example, you should not measure voltage 30 seconds after removing the batteries one time, and 5 minutes after removing the batteries the next time.
• If your device has multiple batteries and you need to remove them to take measurements, then measure the voltage of each individual battery and add up their voltages to get the total equivalent voltage of the batteries combined in series.

To get you started on your background research about batteries, see the Bibliography. For a more advanced project, look up the data sheet and "discharge curves" for a specific type of battery. Devise an experiment to take measurements and create your own discharge curve, then compare it to the official data from the manufacturer.

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