notnarb, you need a decent set of rechargeable batteries in it. If you use off the shelf AA's you will only get about a 50% charge before the MP1550 dies. And it's not the charger's fault. Normal, generic AA's just don't have the umph to do the job. It's why I switched to using 2500mah NiMH batteries in mine. One set of those is enough to charge the NIT from dead to 100%. And if you look at the numbers, 2500mah at 1.2 volts is about right to achieve that.

1.2v*4 = 4.8v
2500mah*2 = 5000mah / 2.5 = 2000.

The NIT battery is 1700mah. If you remove 300mah for overhead, that's just about spot on to completely recharge a NIT from dead. 2.5 is the average power to charge ratio on most batteries. It varies depending on battery type (like Lithium Ion I believe takes 2.2 times the amount, whereas lead acid is 2.9 or something like that. The way to calculate it is to determine the total amount of power needed to charge the battery from dead to full divided by the size of the battery.

I've done numerous tests myself, and the charge rate is about exactly 2.5 to 1 for the NIT at 5v, 500ma, which is the standardized bar by which I do all my testing.