The trend in current boxes is to offer fast charging via USB (sometimes up to 2.5A!), which may seem attractive at first glance, but it is in fact just a marketing argument and not a technical one. Let's try to explain. Indeed, this fast charge can only be carried out on the obvious condition that the power supply to which the said charge is connected is capable of delivering such an intensity. The label on the device will show you this. 

In order to preserve the maximum life of the batteries, it is advisable to charge them at a maximum current of 0.5A to 1A (according to Mooch, although it is possible to exceed this value at times). This means that USB charging should only be used for emergency purposes and that it is much better to use an external charger for everyday use. In the case of a box with an integrated battery, the choice is simple, of course. However, it is still possible to ensure that the power supply does not exceed 1A.

Charging via USB 

Charging via USB has many disadvantages:

- No control of charging intensity.

- Rarely balanced charge in the case of dual battery boxes.

- Overheating of the chipset, which often does not appreciate it over time.

- Increased risk of outgassing due to overheating of the batteries in a closed space.

- Premature battery fatigue.

- The micro-USB port will not stand up well to repeated plugging for which it is not designed and will prematurely give up the ghost (since this year, the USB-C standard has been imposed, with a more robust and reversible connection).

Without being alarmist - as some bad tongues may claim - here is what can happen.

Battery charger 

For small wallets, an adapted battery charger will be a much more interesting choice, bringing a certain gain in security and longevity for both the box and the batteries. This means that the purchase will quickly pay for itself. Depending on your needs, you can opt for a charger with one, two, four or even eight slots.

Generally, the charger will be delivered without a power supply. Remember that to charge four batteries simultaneously at 0.5A, the power supply must deliver at least 2A. If you use 21700 batteries, make sure that the charger accepts them as well.

Some suggestions : 

Nitecore UMS4

XTar X2