The procedures outlined below are quick and easy tests to see if your battery and electrical system are in good shape. For these tests, I use a voltmeter, called an analog meter, with a gauge and needle set to a 25 volt range. Some prefer the digital readout type. As long as you get a good idea of the voltages with these tests, your results will be similar. Techniques will differ such as testing drains from the negative side instead of the positive side of the battery. You are still measuring current draw. A test light used for testing current in a wire or circuit can also be used in place of the voltmeter for current drain. If the test light lights up in the test then you have a draw somewhere. A voltmeter will need to be used for the other tests. Just remember to remove all watches and jewelry so that you have no metal that can touch anything hot. Never smoke cigarettes or have any flame next to the battery during these test. When charging, the battery can produce explosive fumes that could cause the battery to explode if ignited. Also wear protective goggles as a precaution. It's a good idea to drape a fender cover or blanket over the fender you are working over to protect the paint. Just take your time and you will achieve good results from these tests.
Charging System Test
To test the charging system, connect a voltmeter set higher than 12v as shown above with the engine running. If the charging system components are operating normally, the reading should be between 13.5v and 14..4 volts. At rest with the engine off, a fully charged battery will read about 12.5v.

A word of caution: NEVER remove a battery cable from the battery while the engine is running to check to see if the alternator is working or for any other reason. This is extremely hard if not fatal to an alternator and possibly the regulator. The alternator has diodes inside that may be damaged when the battery cable is removed or replaced while the engine is running. If the charging system is working properly, this test will show the 13.5v to 14.4v and you will know the alternator is charging.
Battery Drain Test
If your battery keeps draining after sitting for a short time, you may have a parasitic drain from the electrical system. To check for a power drain, connect a voltmeter as shown with the ignition off and all components turned off. If there is current then something is draining the battery. The ideal voltage reading for a classic Mustang at rest is 0 to 1.5 volts. Sometimes when you are trying to get a reading for a drain, you might see a slight reading at first. If everything on the system is turned off, then there is a residual current that needs to be drained off. Just open the doors to "turn on" the courtesy lights and then close them. You should then be ready to start your checks. To attempt to locate the drain, perform the following:

1. Remove the fuses from the fuse panel behind the dash and then check your voltage reading. If your drain is gone, then one of the fused circuits of the system is causing the drain. Reinstall the fuses one by one and check the voltage reading with each fuse. When the drain returns with a reinstalled fuse, that is the circuit that is at fault. Using a wiring diagram, trace the components on that circuit to find the cause. Unplug each component of the circuit separately and check your voltage. The faulty component will stop the drain when it's unplugged.

2. If the drain still remains after all of the fuses are removed, then the fault is in a main component such as the alternator, voltage regulator, starter solenoid, or even the starter. One by one unplug these components. When the faulty component is unplugged, the drain will stop and you will have found the cause of the drain.
Things to remember:

1. The older Mustangs did not have any components that constantly pulled current from the battery. One problem that can arise is if your Mustang is equipped with a factory clock. The clocks ran on a points and spring mechanism. A spring loaded gear would tick down the seconds until a set of points closed thus making contact and receiving a current signal from the battery. At that instant the points repel each other, the gear spins around winding the spring, and the seconds count down again. If the points are bad, they can touch and not make good contact to repel each other but still pull current from the battery. This has been a puzzler for a lot of people over the years. If the clock is causing a drain, unplugging it will stop the drain and you will know it's the culprit.
2. Don't assume any component or switch is immune to causing a power drain. In one instance, I had a stoplight switch that was shorting against the brake pedal. When I removed the switch and checked it, it checked out fine. I reinstalled it and the drain returned. I replaced it with a new one and the drain disappeared.
Jumping A Low Or Dead Battery
When jumping your Mustang from another vehicle, connect the jumper cables as shown above.
Always connect the positive leads first and remove them last.