What are the different Volts? (not mAh)
Joye 510
Ego
and the ego mega(900mAh)
I'm not sure about Joye, but my 510 (standard) puts out 4V unloaded (most probably LiPo cell 4.2V)
Janty Ego 650 mAh gives 3.5V
m402 - 2.8V
But it is not just the voltage - there is much in the power delivered trough the atty
Power is equal to:
P = U * I
where
P = power (in watts - W)
U = voltage (volts - V)
I = current (amperes - A)
Current is calculated from voltage and resistance:
I = U / R
where
U = voltage (volts - V)
I = current (amperes - A)
R = resistance (ohm)
for example eGo batt. with Janty Tawa atty
I = 3.5 V / 2.5 ohm = 1.4 A
P = 3.5 V * 1.4 A = 4.8W it is 4.9 watts of heat delivered trough the atty.
The bigest problem is that not all
batteries are equal, and not all
batteries can deliver the needed current. It depends of internal resistance of the battery. Lower is better, but we don't know what kind of cells are inside.
For example, you can look at batteries for electric vehicles. Many of them can deliver impressive amounts of current. This is rated at C rate.
C rate is the rate between battery capacity (for example 650 mAh) and the amount of current they can deliver. Higher C rating means more current. There are battery cells that can deliver as high as 175C safely (Saft VL 10V Fe Super-Phosphate can deliver 1750A) - but they are hell expensive. Our batteries are cheap (for manufacturer only, 'cause you can buy battery cell of 10Ah capacity witch can safely deliver 100A at 3.3V = 330W for less than 15USD and last for 2000 cycles instead of 300 like ours do).
I hope its not to technical, I tried to explain it simple (it's way more "behind the scenes")