If you interupted the switch circuit for the alternator (it runs through the amp light), this can cause the alt to not charge. A dynamic test would have shown the alternator having no output.
Does your amp light work? It should prove-out when you turn the key to accessory. If it never comes on, the bulb is bad or the circuit has been interupted and your alternator doesnt know that it needs to start charging.
Sounds like you did not put the cluster back correctly. If the switch circuit is not hooked up, (like JT mentioned), then your alternator will not charge.
You could always run power to the exciter wire on the alternator and see if charges.
Have you tested the voltage at the battery and see how many volts you are getting when the car is running?