For the most part, even on the first fill, any of these type tanks will begin to gurgle, there is a little variation, but it often begins at what appears to be half a tank. However, when turning it upside down to fill, it quickly becomes evident that it's more like two thirds empty. Because the viewing area (the see through part) is well below the full mark on most tanks, large & small.
If one takes a good close look at the KangerTech EVOD tank (& it's clones/lookalikes), one can see the rubber washer sticking up above the bottom of the viewing area. When one is close to that area, there's going to be lots of gurgling, as the tank is no longer maintaining pressure. This pressure is why one can turn a full tank upside down & provided the coil is good, there will be no leakage.
With a full tank & no coil issues, if there's gurgling, the battery likely needs charging. One reason why to keep a multimeter on hand. It doesn't have to be an expense one, but these days, digital is best. Mine was $7 shipped from an eBay seller, though Harbor Freight Tools also carries the same one of the Cen-Tech brand (it's actually their product).
Cen Tech 7 Function Digital Multimeter Multi Tester DC AC Voltage Battery Test | eBay
To test an ego style battery, turn the multimeter on & make sure the setting is on 20 DCV, place the negative (black) lead to the outer threads. Then place the positive (red) lead to where the coil seats, the small round part, then press & hold the power button & view the multimeter. If it's below 3.7 Volts, it's best to recharge (the same with IMR batteries). Prolonged or repeated usage with a low battery will cause it to have a shorter lifespan, so will repeated "shortcharging" (5-10 minute charge to get 30 minutes of use).
On this model, to perform battery testing, there are two leads, red & black. There are 3 inputs on this multimeter, on the bottom right corner. I don't know what the top port is for. The black lead plugs into the bottom port, the red in the center for battery testing, this wasn't included in the instructions, I learned it on a YouTube video.
Hope this helps someone.
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