I cut open an old carto put it on the battery and use alligator clips to connect my leads press the button and you have it.
I have an idea (a phrase that has always caused co-workers to cringe in terror). If you were to take one of the little plastic plugs out of the end of a carto, stick one probe through the hole in that, and use that one for the center post on the eGo, wouldn't that provide enough insulation barrier to avoid the dreaded "BOOM"? Just stick that end in and use the other probe on the outside ring and it should work. My hands tend to shake at odd moments so I'm not about to risk a battery by using to bare probe ends on a live battery.
Tobacco free since 6/30/10 4:30 pm! VV box mod user and amateur multimeter reader.
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Use to be an Electronics tech before I was an Net Eng. Safest way is to take one lead to a Volt meter and put a bit of tape so only the very tip of the probe is exposed then using the probes like chop sticks in one hand put the insulated tip in the hole to the center conductor and holding the other against the outside of the battery. Then use the other hand to push the button. Or get your Vape slave to help. LOL. Check it after charging and then you have a reference point. If you want to go very techi. There is a circuit you can build to check the internal resistance of the battery. When the battery's internal resistance rises that is an indication that the battery is starting to go. You have to know what it was like out of the box to have a reference point. This may be a lot more trouble than it is worth for most. But I'm a tech weenie so i love such stuff.
If any other DIY's or other nerds are out there here is a diagram of the circuit I'm talking about.
Commercial/industrial energy efficiency: Electric cell - Internal resistance of battery
To measure voltage on any device using PWM you need a Digital Multimeter that measures True RMS. These are generally not cheep if you serious i suggest getting a quality meter I suggest Fluke meters as that is what i have used for years as a electrician .
I just tested my ego twist batterys and got a low of 3.23v and a high of 4.94v.