In this Instructable you will be making a rechargeable battery tester for low internal resistance batteries.

I suggest that try making this device first:
https://www.instructables.com/id/Battery-Tester-8/

It is important the mention that the internal resistance of the battery in this design is assumed to be zero even for discharged batteries. I have never seen such batteries in real life. However, if the battery is connected to a voltage regulator, switching power supply or Zenner diode voltage regulator then this can significantly reduce the internal resistance.

This device is not very useful. However, if you have nothing better to do with an old three-wire connector then please continue reading.

In the circuit shown an LED is modeled with three general-purpose diodes because the PSpice simulation software is very old.

With battery having zero charges and zero voltage across its terminals the maximum current entering the battery will be 2.3 V / 100 ohms = 23 mA.

The voltage across the LED is about 2 V. (3 V - 2 V) / (10 mA) = 100 ohms.

Warning: Do not connect non-rechargeable batteries (for example - alkaline batteries)! They might become hot or even explode. Do not hold terminals to battery for a long time if you are not 100 % sure that the battery can handle the 23 mA input current. However, this is not very likely.

Because 23 mA is not a high current (equivalent to just over two LEDs when on (10 mA each)) for typical even AAA 1.5 V batteries you can short the outputs. However, this will discharge the battery very quickly.

However, you might be even able to use this circuit as a charger if the battery is made to be charged with this (23 mA maximum) current. However, the battery will not charge above 1.3 V because the LED voltage is 2 V. 2 V - 0.7 V - 1.3 V. This could be an advantage for charging NiCad batteries that must not be charged if they are almost charged (1.3 V rather than full 1.5 V) to prevent failure of those types of batteries.

Supplies

You will need: - three-wire connector, - screwdriver, - high power diode, - two 1.5 V full charged batteries, - two 1.5 V battery harness, - one charged 1,5 V battery and one discharged 1.5 V battery for testing, - one LED is needed (although I used two) or a few if you burn one, - insulated wires, - 1 mm metal wire, - pliers, - 100 ohm high power resistor, - small matrix board piece, - solder, - soldering iron, - wire stripper, - drill, - scissors.

Step 1: Build Input

Connect input as shown with screw driver.

The red and black wires are the battery terminals and black and yellow wires are the LED terminals.

I used 47 ohm resistor instead of 100 ohm resistor because I made a circuit with two LEDs. This twice the current was needed. However, this is a risky idea because then the maximum current could be as much as about 50 mA, not 23 mA. Do not do it at home!

You must attach a thin metal wire to prevent eventual wire breakage due to bending of wires when opening the box many times.

Step 2: Build Output

Connect output as shown with screw driver.

In this circuit I used old Russia (Soviet made) diode. Any high power diode would do the job.

If you use Germanium of Schottky diodes that have lower forward bias voltage the battery will charge to above 1.5 V and fail. However, if you are just testing the battery then lower forward bias diode voltage might lead to a maximum current of about 29 mA.

You must attach a thin metal wire to prevent eventual wire breakage due to bending of wires when opening the box many times.

Step 3: Attach the LED Wires

For this step you will need a soldering iron.

LED conduct in only one direction. From anode to cathode.

"...cathode lead being shorter than the other as the anode (+) lead is longer than the cathode (k).", https://www.electronics-tutorials.ws/diode/diode_8.html

The red circle is showing that you must attach a thin metal wire to prevent eventual wire breakage due to bending of wires when opening the box many times.