I'm sorry, but I gotta do it. What's the point of having a pristine USB port on your mod if you're not gonna use it? How often is firmware updated?
I tend to agree... I have a great looking mod. I am currently looking for a USB cover to put on it. I bought an Efest LUC V4 to charge my batteries, and I do not use the USB port on the mod unless I absolutely have to. Unfortunately, I will have to explain in a long message, so please bear with me.
Balanced charging is important for the lifetime of the batteries. If one battery has less of a charge, then every time it discharges, it will use one more than it will use the other, and one battery will get warmer than the other... that changes the discharge rate of the battery, and when it cools down, the OTHER will have more of a charge.. .it will see-saw back and forth, and the overall result will be that the batteries life will be shorter. It will also shorten the overall life of the battery as well.
The 18650 was not designed to be used as an individual battery. Unlike the lithium Ion batteries in a cell phone, These batteries have NO electronics in them to regulate them. Some do have them, but they limit the charge/discharge power, and down to a level that Ecigs do not use. Electronic cigarettes demand a higher wattage than most devices, so we have to use unregulated batteries. That means that the battery won't stop charging if it is fully charged. It relies on the charger to regulate that and to cut the power used to charge the battery when the battery is full.
Now for the scary part. If you do supply more power, the battery will overcharge, and possibly release a gasses. This is called a thermal runaway, and it happens more often than you think. the "ecigs exploding" on the news has been giving electronic cigarettes a bad rep. Most of the time it is someone using an overpowered charger on one of those EGO batteries, but it does illustrate the necessity to use proper charging techniques on an electronic cigarette or their batteries. The circuitry on a mod is geared more towards the output. I am sure they try to get things right the first time, but with a market so volatile as the electronic cigarettes, new chips are being released every few months.
There have been problems with designs that have popped up. They are constantly upating, changing, and modifying current chips. These chips have to deal with power specs/problems all over the world. The USB is supposed to supply 5v, but there is no official specifications on the Amperage. Add to the fact that power bricks for USBs are rarely what they say they are. You can have a USB power brick that says 2.1a, but it supplies more. I have another one that supplies even more than 2.1a even though it's rated at 2.1a. I cannot be sure how much power I am getting with those USB ports even though the specs are listed on the port itself. Every layer of protection is helpful. That is why I prefer to use an external charger... Efest makes Lithium Ion batteries, so they know the dangers that can occur if they mess things up. Ecigs do as well, but they are space limited. Not to mention you put your Ecig in your pocket, so who knows what kind of pocket lint or debris can get into your USB port.
My advice... get a reputable external charger. If you need more power elsewhere, charge several batteries, and keep the pairs in a protective case and swap them out when you are running low. label the batteries, and keep the batteries paired (AKA "married"). Only use the onboard charger if absolutely necessary, and if you do, always be awake and near the batteries when they are being charged. occasionally feel the warmth of the batteries and see if one battery is getting warmer than the other. ALWAYS keep an eye on the battery wrap. If there are any nicks, holes, or whatever, don't hesitate to replace the wrap.
Out of everything that can be hazardous in an electronic cigarette, the batteries are #1 on the most dangerous list. Don't skimp on that part no matter what you do.