There were several sets with identical data on offer. Why was the decision (again) in favour of the K&F Concept Set? Based exclusively on good experiences.
After changing the system and manufacturer, I quickly bought a K&F Concept Set for a spontaneous short trip. Planned as a quick, short-term temporary solution, these batteries and the charger are still my first choice almost every day.
So there was no doubt that my NIKON D5100 and D5300 would also use the K&F Concept additional/replacement batteries for upcoming tasks. As well as a new external charger. One charging slot from the original charger is also not in competition with the three slots that K&F Concept delivers here.

We also have a dual-slot USB charging port here. This gives us the option to charge via micro-USB and USB-C cable and with any suitable charger, power bank, laptop, car charging cable and “whatever”. They are therefore at the cutting edge of technology.
That is the charge indicator on this charger (as opposed to the other one mentioned) here too. For each battery slot, there is a symbolic 4-bar display for the fill level. Unfortunately, no charge level can be read here either when inserting the batteries, or a battery to be tested. This would be more advantageous when using many batteries, particularly in mobile operation.
A short, 0.95 meter long USB-C cable is included. I myself usually use the USB-C port. I charged the batteries with a 45W USB-C charger. Nothing gets warm or even hot at all. Every regulation and protection function seems to work wonderfully here. It should be noted, however, that there are only 2 contacts per charging slot. The batteries each have 5 contacts. With many batteries, communication takes place via sensors, for example with regard to the internal battery temperature.
These values are primarily used to protect very cold batteries during charging. I can live with it and charge at room temperature. The charger is lightweight (very pleasant for tours) but looks and feels very good. It is odourless, has no burrs, does not rattle and cannot be bent or twisted. Even stickers and signs are accurately attached. Installed batteries fit snugly and cannot be shaken out. TOP!
The batteries also give the same perfect appearance on the outside and are in no way inferior to the original. They fit just as well, i.e. perfectly, into the camera. According to the manufacturer and description, electrical protection circuits are supposedly integrated. My cameras don't display a warning message or interfere with functionality when using these batteries.
Capacity or runtimes comparisons are more possible under laboratory conditions and with original batteries of the same age and identical charging cycles. In practical use, I have not been able to notice any difference so far.
Overall, a highly recommended set to supplement or replace Nikon EN-EL14 batteries in a pack of 3 including a good 3-slot charger.