'Under load' means you have an atomiser on top of your mod. Which is actually the entire point of owning the thing!
If you measure the voltage of your battery off the charger and it says say, 4.15v ( no load) and you then use that battery in your Reo and you put probes on the positive and negative posts of your atty and press the fire button...suppose it says 3.95v, then your voltage drop "under load" is 4.15 - 3.95v.
Quite how knowing this benefits your vaping experience depends on a few things, but most important are the general condition of your batteries and the coils you vape on. If you run very low resistance coils, you can expect a fierce drop - and that's the reason that materials like copper, silver, gold are used as contacts in mods. They have a low resistance to electrical voltage compared to steel or my head.
And i wish it were that simple. But of course, nothing ever is. Because as your battery ages and gets recharged and discharged, it also builds up an internal resistance of its own, just because the chemicals it's made of decompose and that resistance has to be overcome before it can power your coil, which makes it more inefficient as it ages.
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