Load testing a battery will give a good indication of its true value for a given application. Batteries used to supply power for high current demand applications like flashlights, digital cameras, and other current-hungry devices should be load tested at or near their operating current level. The actual operating current level can be checked and matched with a fixed resistive load; however, in many applications this is not easy.
A good average resistive load for AA batteries in high current applications is a 1.5-ohm, 5-watt resistor. (See Fig. 1.) This load can be a single 1.5-ohm resistor or ten 15-ohm, 1/2-watt resistors in parallel. For less power-hungry applications, a 10-ohm resistor will do for comparing one brand to another. Lighter load applications may be tested with a resistive load near the value offered by the battery-operated device.
The resistor value is easy to determine. Measure the operating current and plug the value into Ohm’s law formula–E/I. The resistor’s power requirement is determined by multiplying E x I. However, for the majority of battery comparisons, the 1.5-ohm and the 10-ohm loads will give the raw data necessary to determine which battery to use in a given application.
Keeping Track Of Time
Loading the battery is the easy part of the process; however, keeping track of the voltage versus time can be a real problem. A voltmeter, digital preferred, connected across the battery will indicate the voltage, but it will not give out an alert when the critical discharge voltage occurs.
Even while life testing batteries at the high current-discharge level, it can take over an hour to reach the critical discharge voltage. Keeping a constant watch on a voltmeter(s) for an hour is not easy, especially if an hour of constant attention is just not possible. Here’s where our electronic hobby gives us a leg up on everyone else-we can design and build our own voltage monitor.
We’ll Do It This Way
The 339 quad comparator is a very handy little IC that’s inexpensive, widely available, user friendly, and just what we need for our voltage-monitoring job. Inside the 339 IC are four independent voltage comparators, which are designed to operate from a single power supply. The output of each comparator, see Fig. 2B, is an open collector NPN transistor. A simplified comparator circuit is shown in Fig. 2A, with the negative input connecting to a reference voltage and the positive input tied to the battery under test.
Here’s how the comparator circuit in Fig. 2A operates as a voltage monitor. As long as the voltage at pin #5 is more positive than the reference voltage at pin #4, the LED will remain dark. When the battery voltage has barely fallen below the reference voltage, the LED turns on indicating the critical discharge voltage has just occurred. Now, are we to watch an LED in place of a meter? NO! NO! This is only the BASIC circuit that will be used in our full-blown monitor circuit-have some patience, please.
A Single Battery Monitor
A single battery monitoring circuit is shown in Fig. 3. A single comparator is used to monitor the battery voltage, just as in our previous circuit, with the addition of a regulated reference voltage source and an audible alert sounder. R2 sets the discharge reference voltage, which is kept constant by the 7809 voltage regulator IC. The LED and the piezo sounder remain off until the test battery voltage drops below the pre-set reference voltage. At that time, the LED and sounder turn on.
Comparing two different brands of batteries at a time can be easily accomplished by doubling the circuit in Fig. 3, with the following exceptions. The reference pot, R2, and the 7809 regulator IC need not be added for the second monitor circuit. Connect the negative input of the second comparator to pin #4 of the comparator in Fig. 3. CAUTION! Connect all unused 339 inputs to ground. If they’re left unattached, bad things can happen to your circuit.
A load box, see Fig. 4, can make it easier for life testing batteries at different load currents. Three different switch positions allow for load currents of 1-amp, .15-amp, and .068-.0123 amps. The actual resistor values may be selected for special testing currents or load applications. It is very important to keep the internal resistance of the load box as low as possible. Use large wire (#16) and pick a selector switch that is a high-current (15-amp), low-loss type. Two load boxes are required when using the dual comparator circuit.
Why not use all four of the comparators in the 339 and build a quad battery life tester? Why not, indeed? Take a gander at the circuit in Fig. 5 and that’s what you will see. Granted, with all of the battery brands available, there is no reason not to investigate as many as we can and become better educated on what is the best battery buy.
Each of the positive inputs of the 339 comparator connects to a battery under discharge testing. The reference voltage is set for all batteries for the same discharge voltage level with R5. Testing four batteries also takes four load boxes or load resistors. Always be sure that the electrical connections to the battery are solid and without ohmage loss. Invest in good metal battery holders, and if necessary apply light pressure at the terminal ends with a heavy-duty rubber band. It’s an easy task to check for resistive losses by connecting a digital voltmeter, set on the lowest voltage range, across the battery to terminal junction and across the closed switch contacts on the load box while the circuit is in the discharge state.
If more than a few millivolts are present across any of the contact junctions, check the connections and be sure they are clean and solid. Any large error here can make one battery look much better than it actually is and give it a false edge over the other batteries.
Now, let’s take a closer look at the quad-monitoring circuit in Fig. 5 and see how it operates. As previously stated, the 339′s positive inputs go to the positive terminals on the test batteries. The negative terminal of each test battery goes to circuit ground. The output of each comparator is connected to the input of a NAND gate, which inverts the low output signal to a high. This high output turns on the LED and sends a positive output voltage through a 1N914 diode to an on/off switch. On the other side of each of the on/off switches is a piezo sounder that gives out an audible tone when any one of the batteries drops below the pre-set lower voltage limit. The pooped-out battery can be identified by which LED is on and that corresponding switch can be turned off, silencing the sounder until the next battery failure occurs.
The electronic testing equipment is just a part of what is needed to come to a sound decision on which battery is the best value. Keeping accurate track of the discharge time of each battery is a must for a meaningful outcome. Also if a noname, low cost, battery does exceptionally well in the life test, then repeat the test. If the results are similar, go buy the batteries and take advantage of your electronic expertise.