Any current source has a certain internal resistance. It participates in limiting the current through the load, along with the resistance of the load itself. In order to find it out, you will have to measure the voltage at the source under various loads, and then make a simple calculation.
Instructions
Step 1
Charge the battery fully.
Step 2
Take two loads. Each of them must load the battery with such a current that it does not exceed the maximum allowable for it. One of the loads should consume a current that is about 30 percent of the maximum allowable long-term (not short-term!) For the battery, and the other - about 70 percent of it. It is very convenient to use low-voltage incandescent lamps. They must be designed for a voltage slightly exceeding the EMF of the battery (voltage at its terminals in the absence of a load). If high-power lamps are used, secure them so that no body parts or flammable objects come into contact with them.
Step 3
Connect the first load to the battery through an ammeter, and connect a voltmeter in parallel with the battery itself. Connect both devices with correct polarity. Wait for the transients lasting a few seconds to complete. Measure the current through the load and the voltage across the battery. Write them down.
Step 4
Disassemble the circuit, then in the same way connect the second to the battery instead of the first load. Also write down the results. In both cases, measure quickly (except for the time required for the transient to complete) so that the battery does not run out of time.
Step 5
If the measurement results are not expressed in SI units (for example, the battery is low-power and the currents through the load are expressed in milliamperes), convert them to this system.
Step 6
Subtract the first voltage from the second, and the second current from the first. Divide the result of the first subtraction by the result of the second subtraction. This gives the internal resistance of the battery, expressed in ohms.
Step 7
Note that the internal resistance of a battery rises as it discharges and wears out. You probably shouldn't wear it out on purpose. But carry out one discharge cycle (up to a voltage slightly exceeding the minimum safe for it). At several points in this cycle, by briefly disconnecting the battery from the main discharge circuit, measure its internal resistance using the above method. Draw a curve of the dependence of internal resistance on the degree of discharge, expressed as a percentage.
