You may hear it pretty often. Maybe you’ve tried it yourself. You had your e-cig on one side of you with a fully charged battery, and your vaporizer on the other side with a battery completely spent. Your eyes darted from one to the other, since you want to use your vape so much more than your e-cig. Then you give into temptation. Reaching over to the e-cig, you pop out the battery and place it into the vaporizer. Somehow, it works and you enjoy your vape like nothing ever happened. But, surprise! Something did happen. Whether you believe it or not, e-cigarette and vaporizer batteries aren't the same. While they may seem very similar, the're some key differences that can result in chance bad events occurring if you use them interchangeably.
Though e-cigarette batteries and portable vaporizer batteries use the same type of ion to be able to recharge (lithium ion), the concentration of these ions are different. Traditional vaporizers have far more lithium, as users of these devices are expected to be more likely to pull harder and deeper than e-cig users. The e-cig users generally are associated with previous regular cigarette smokers, who prefer smaller, finer pulls. Therefore, battery size and power storage can differ significantly between the two.
Another key difference is the mechanism the battery uses to connect to the rest of the vaping device. Since e-cigarettes are small, sleek, and designed to emulate natural cigarettes, the battery must connect in an indented manner. Conversely, vaporizer batts don't have to worry about attaching a certain way as long as the device aesthetic remains. Therefore, the actual mechanisms in the way the two batteries connect is different. While they're likely to be able to connect somewhat and power a decent amount, it's very likely that the actual power received from the batt is far less than what you'd get if used normally.
On the point of power efficiency, batteries are meant to be used in the most optimal way. By transplanting an e-cigarette battery to a traditional vaporizer or vice-versa, you're using batteries sub-optimally. This means you're drawing more but receiving less, and therefore draining the battery faster than you would normally. Repeatedly doing this will dramatically decrease the battery’s lifespan.
Finally, though highly unlikely, there's a chance that the battery could melt or even explode. By straining the battery and using it in a non-standard way, conditions the battery has never been tested under may occur. These untested conditions could result in serious consequences. This reason alone is why e-cigarette and vaporizer batteries shouldn't be interchanged.
Though many vapers do swap batteries with no harm or noticeable change in power, it's far wiser to keep e-cigarette and vaporizer batteries distinctly separate. The batteries will thank you for keeping their lifespan normal and for not putting them under extreme, abnormal stress.