Batteries do not go down in a nice straight line. There is a quick drop off from the 4.1 volts of a fully charge battery, then a long period of slightly falling voltage, then a quick fall to the end. In a mechanical mod you can keep going and damage the battery but regulated mods stop firing. Also this voltage is at rest. That means if you check the voltage of the battery with no load, the atty, on it. The light on the QF1 is based on that voltage. It stays green until a certain voltage, turns yellow, then red.
Now this is a perfect example of male answer syndrom and you might want to skip it.
Volts is a measure of the difference in electrical potential.
When a battery is just sitting there there is a voltage between the positive and negative terminals and it is steady and based in large part on chemistry. It is just sitting there producing no power (watts) and no electrons are moving.
When you add a circuit, from the negative end, through the circuit board, the atomizer, and back to the positive terminal, you are producing power (watts) and electrons are moving. The circuit board in the QF1, through PFM, Pure F...ing Magic, is able to alter the voltage and since it knows the resistance of your coil and it also knows Ohm's law it can figure out that in order to produce 8.5 watts of power for a 2 Ohm coil it needs 4.12311 volts. The board is not 100% efficient so it will need more than the 8.5 watts as input. So it starts to get power (watts) by moving electrons around. When the battery is asked to produce the electrons it does it's best but it probably can't keep up so the voltage it supplies drops, the board senses this, and through PFM adjusts what it is doing and gives you the 4.12311 volts you need and everything works great.
At some point even the PFM in the board is not able to convert the voltage from the battery, because it is low, to the 4.12311 you want, so the vape will cool down a little.
Your battery at rest might have a voltage that is in the green but when you ask for the power it might drop too far and not be able to supply the power you need.
I have an SX-350 board that shows current battery voltage and a freshly charged battery shows 4.2 volts. However while I fire it the voltage drops to 3.61 and goes back up to 4.2 when I stop. The resting voltage continues to drop as I vape and at some point, before it is discharged, it is unable to supply all the power I want and it gives up and I recharge.
The board in the QF1 is a lot simpler than the sx-350 and it probably just checks the resting voltage and does not indicate it is time for a charge when it should.
A better or newer battery will be able to produce the required wattage longer.
