I have found it to vary depending on the output you have your mod ser to. If I'm using my 1.8 ohm drip tip at 7 watts it will run the battery down to 3.2 or so before it cuts off. But if I'm using a dual coil carto at 14 watts the unit will shut during a draw at around 3.6 volts. The 800 mah 18350s just can't handle the draw like a bigger mah 18650.
So to sum it up, sounds like your vamo is operating normally to me. You might want to try the ultrafire 1200 mah 18350s.
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The 1200's sux - 2.5-3hr tops with a kanger @2.6 running 7w Same with 2 brand new Ultra's
Testing the Efest 800's now. First one still going @4+hrs on the same setup. Hmmmm......