The ratings 650mAh and 1000mAh do not state the maximum current output of the battery. Rather, they state how long the battery will last given a particular current draw. For example, a 650 mAh battery can deliver 650 milliamps (0.65 amps) for one hour. The 1000mAh can deliver 1000 milliamps (1 amp) for one hour. So, if you're using a 3.7 Volt 1000mAh battery, it works out like this:
I = current (amps)
V = voltage (volts)
R = resistance (ohms)
3.7 volts across a 1.5 ohm atomizer:
I = V / R
I = (3.7)/(1.5)
= 2.47 amps = 2 470 mA
This is the current that the atomizer will draw from the battery. To find out how long the battery can deliver this current, you do the following:
time (hours) = mAh/mA
= (1000)/(2 470)
= 0.405 hours
Therefore, with a 1.5 ohm atomizer, a fully charged 1000mAh 3.7 volt battery will last for 0.405 hours (24.3 minutes) of constant use.
Separating this 24.3 minute time frame into 5 second "draws,"
24.3 minutes * (60 seconds)/(1 minute) = 1458 seconds
1458 seconds * (1 draw)/(5 seconds) = 291.6 draws
You can do your own math based on how often you tend to take a draw to determine how long it will last you. My own personal experience is that, with a 650mAh 3.7 volt battery, using a 1.5 ohm atty lasts me about 24 to 36 hours while using the standard 2.2 ohm atty lasts me about 48 to 60
