In this Instructable you will be making a battery tester for 1.5 V AA or AAA battery.

Supplies

Wire stripper, a few LEDs (in case you burn), wires, plastic/cardboard box (any box), screwdriver, soldering iron (optional), two 100 ohm resistors, AA or AAA battery harness for two batteries(optional), crocodile clips (optional), two AA or AAA batteries, electrical tape, scissors.

Step 1: Drill the Hole

Use an electric drill to drill a hole in the box for the LED.

Warning: Be very patient with the scissors. I cut my self many times.

Step 2: Insert the LED

The LED is connected to two wires and soldered to two wires.

Step 3: Build the Circuit

The circuit is very simple. The switch represents your crocodile clips connectors to the battery under test. The crocodile clips are not shown in the video on the next slide. I used electrical tape.

Old PSpice simulation software was used to reduce editing time with the use of keyboard short cuts.

Three diodes are used to model to LED which has a voltage of about 2.1 V. The current across the LED is (3 - 2.1) V / (100 ohms + 100 ohms) or approximately (3 V - 2 V) / 200 ohms = 5 mA. You can try replacing the R1 resistor with a smaller resistance value or even a short circuit. However, this will eliminate the short circuit protection feature of this device. During the short circuit of terminals, the current across the LED will double to 10 mA (when R2 is shorted to zero ohms). The voltage across the LED must not exceed about 2 V. You can use a bright LED as well.

I used a small power rating for R2 resistor. However, I suggest that you use a slightly bigger resistor for R2. The current across R2 when the charged battery is connected will be approximately 1.5 V / 100 ohms = 150 mA = 0.15 A. You can multiple 0.15 A by 1.5 V = 0.225 Watts rating for the resistor. This is a high current value.

The internal resistance of the battery under test is assumed to be 10 ohms. When the battery is connected the Vtest node value increases to about 1.5 V and thus reduces LED voltage to below 1.5 V, thus turning it off. If the battery is discharged that the battery internal resistance is high, then the LED will not turn off.