How to Check Battery Voltages    


One of the most fundamental skills in electronics and robotics is checking battery voltages. For many this skill is second nature and you may wonder why it requires mentioning. I have found, however, that many students have never used a voltmeter and this tutorial is for them.

I suggest that my students check their battery voltages in the following situations:

  1. After they've been in storage for a long time.
  2. After their devices have been running for a while. Robotic applications will drain batteries more quickly than, say, data-collecting apps.
  3. If their microprocessor, sensors, and/or motors are behaving erratically.
  4. If they suspect that their batteries have been damaged somehow.

Table of Contents for this Page

  1. Checking Individual Battery Voltages
  2. Checking Battery Pack Voltages


Checking Individual Battery Voltages
To check the voltages of individual batteries, follow these steps:

  1. Gather your supplies. Besides the battery in question, you'll need a voltmeter or digital multimeter (DMM) as shown in Figure 1. Of course, we'll also need wire test leads to plug into the DMM. I keep my leads hanging on a wall in the lab as shown in Figure 2. While the color of the wires are immaterial, it is common to use one red one and one black one. (Red is usually used for the positive (+) terminal and black for the negative (-) terminal.)

  2. Plug the red and black test leads into the DMM as shown in Figure 3. The black lead should be plugged into the COM or "commom" socket. The red wire should be plugged into the socket marked as "V W mA", which are the units of voltage (volts or V), resistance (ohms or W), and current (milliamps or mA).

  3. Turn on the multimeter. Since we are measuring voltages from a battery, we'll turn the dial to one of the DC Voltage settings. DC stands for direct-current as opposed to AC or alternating current. Batteries deliver steady DC voltages. Wall outlets in your house, for example, deliver alternating current.

    For this exercise, turn the dial to the 20VDC setting, as shown in Figure 4. This will allow us to measure DC voltages between 0 and 20V. (Note that we could have also chosen the 2V setting since AA batteries are rated below 2.0V. (FYI: Alkaline batteries are rated at 1.50V and NiMH rechargeable batteries are rated at 1.25V.) The reason I suggest the 20V setting is because we'll use that setting when we test the voltage of the entire battery pack.)

    The DMM should read 0.00V as shown in Figure 1. If not, the test leads are reading residual voltage from the atmosphere and it is probably close to 0.00. Touch the test leads together and they the meter should be zeroed.

  4. We are now ready to measure the voltage of a battery. Touch the red lead to the battery's positive (+) terminal and the black lead to the battery's negative (-) terminal. It's best if you can touch the sides of the leads to the terminals, as shown in Figure 5. (Try and avoid touching the terminals with the leads' sharp points as shown in Figure 6.)

    As shown in Figure 5, the voltage of the battery is 1.28V, which means this battery is charged. If the battery is low, say around 1.1V, then it's probably time to recharge the battery. If the battery voltage is very low, i.e., below 0.5V, it is likely that the battery is damaged and must be replaced. NiMH batteries are rated at 1.25V, but they can hold up to 1.33V right after they have been charged. If left alone for a couple of days, they will drift down to between 1.25V and 1.29V where they will hold steady for many days.

  5. When you are finished measuring your voltages, be sure to turn the multimeter off! This is shown in Figure 7.

  6. Here's a helpful hint: some DMMs (like the one pictured on this page) come equipped with a Hold button. (In these images, the Hold button is the black triangular button on the left and below the DMM's LCD screen.) If the Hold button is pressed, it freezes the reading on the screen. To make another measurement, press the Hold button once again.

Click on the images below to enlarge them.


Figure 1. The digital multimeter (DMM)
complete with wire test leads.


Figure 2. I keep my test leads hanging
on a wall in the lab. Notice, students, that
the wires are neatly separated by color!


Figure 3. Plug the test leads into the
COM and VWmA ports on the DMM.


Figure 4. Turn the DMM on by turing
the dial to the 20VDC setting.


Figure 5. When measuring the voltage
of your batteries, try and touch the sides
of the leads to the battery terminals.


Figure 6. This is a no-no. Try to avoid
using the sharp tips, for you will often get
an erroneous reading.


Figure 7. When you are finished,
be sure to turn the DMM off!

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Checking Battery Pack Voltages
Often is is desirable to measure the voltage of the entire battery pack. This is easy to do and you don't even have to remove any batteries! Here's how:

  1. Gather your digital multimeter (DMM) and test leads, as described in the previous section.

  2. Plug the red and black test leads into the DMM and set the meter to 20VDC. This is also explained in the previous section.

  3. To measure the voltage of the entire battery pack, touch the DMM's test leads to the battery pack's contacts as shown in Figure 8. (The battery pack shown in this picture uses 6 AA batteries.) To know where to touch the leads of the DMM, follow the read and black wires going into the battery pack. The red battery wire connects with the positive terminal to battery #1. Touch the red DMM lead here. The black battery wire connects with the negative terminal to battery #6. Touch the black DMM lead here.

    This shouldn't be a mystery. Remove all the batteries from the pack and trace the path made that would be made by electrons as they flow through the battery pack. The connection starts with the red battery wire and ends with the black one. Notice that path winds in and around, connecting all the batteries end-to-end. We say that the batteries are connected in series.

  4. Since all the batteries are wired in series, the total voltage of the battery pack is the sum of the voltages of the individual batteries. The batteries pictured here are NiMH rechargeables, which, when fully charged have a potential of between 1.25V and 1.33V. Therefore, a fully charged battery pack with six batteries should have a voltage reading between 7.50V and 8.00V. As you can see from Figure 9, this battery pack is fully charged.

    But the battery pack shown in Figure 10 is not fully charged. Showing a total voltage of only 2.22V, it is likely that this pack has one or more damaged batteries and each battery should be checked individually. Sure enough, upon closer inspection we find that one battery is totally dead (see Figure 11) and should be discarded.

     
    • See Appendix A in my book to learn how to properly handle NiMH batteries.

  5. Of course, when finished, be sure to turn the DMM off as is shown in
    Figure 12.

Click on the images below to enlarge them.


Figure 8. To measure the voltage of the
entire battery pack, touch the DMM test leads
to the battery contacts as shown above. Note
the battery wires terminate at these contacts.


Figure 9. This battery pack is fully charged.


Figure 10. This battery pack is not charged.
Due to it low voltage, it is likely that at
least one battery in the pack is defective.


Figure 11. This battery is defective.


Figure 12. When you are finished,
be sure to turn the DMM off!

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Copyright 2006, Chris D. Odom. All Rights Reserved