As Jeremy explained in a previous post, it depends on the battery configuration you are using.
We need to remember that the Sigelei ZMAX is a digital device and ALWAYS uses 6 volts to fire the atty using
pulse width modulation. What changes the power to your atty is the duration of those 6 volt "pulses". The 6 volt pulses reproduce the same level of energy as the comparable DC analog voltage or power level your device is set to. So for example, if you have your mod set to 4 volts, the 6 volts pulses would have less duration than if you had it on a setting that required 5 volts. The duration of those 6 volt digital pulses can be made to reproduce the same heating effect at your atty as if you were applying a straight analog DC voltage from a mech mod (within the capabilities of the device obviously).
Since this device is designed to use different battery configurations, it needs to be able to sense the input supply voltage from the battery and then make a decision on what strategy to use to power the device. Since the input voltage from the battery is never 6 volts which is what it needs, the input voltage either needs to be increased or decreased in order to power the device. To accomplish this, the device either uses a "
boost circuit" or a "
buck converter" to get the correct working voltage of 6 volts.
When using only one battery, the device uses its
boost circuit to increase the 4.2 volts from the battery to the necessary 6 volts the mod needs for those pulses. Since the voltage is increased, the current is decreased (Law of Conservation of Energy). For this reason and probably for other reasons based on the boost circuit components and how it's designed, it cannot provide more than 2.5 amps. Sigelei does not advertise this limitation and this is what can confuse people when they attempt to get more than 2.5 amps to the atty with a single battery and can't understand why they are having a problem.
When you are using 2 stacked batteries, the device uses a
buck converter to take the combined 8.4 volts from the batteries and reduce it to the 6 volts the mod needs for the pulses. Using the buck converter to decrease the voltage gives the added benefit of increasing the existing current beyond what the batteries are currently providing. Since voltage is decreased, current is increased (Law of Conservation of Energy). The increased current capability of the buck converter is what enables the device to provide 4 amps of current to the atty when the batteries are stacked.
I hope this helped eliminate any confusion, and I apologize ahead of time if I didn't explain this very well...