Allison will not go in gear

Checking battery voltage is always good practice. If possible, when under load when performing the test at hand. Even though in this case, the engine is running when owner is attempting to shift into drive, and it's likely there would be other symptoms if the alternator wasn't doing it's job, it's still good practice.

BTW, you can now buy a digital meter with Bluetooth! With the meter and the app on your phone, it's easy to read voltages when you can't do it alone. Pretty cool way to read meter readings remotely when you don't have a friend handy.