Sometimes when I put in 3 or 4 batteries (18650), it doesn't charge one of them. I think I read in the manual that with four it charges the first three first and then the fourth at 1A. I leave them in, but it doesn't charge; it seems to be charging with the light blinking and stays there for hours

Positive that it charges a battery & does not require a pair.
The battery placement position is NOT user-friendly, resulting in uncertainty if it is placed correctly. (for 1.5 Volt batteries)
Charges very quickly (maybe 3 times faster than my old charger) but reaches a very high temperature (both on the machine and the battery) which I am very afraid is NOT good for the life of my battery.
The charging screen (indication is, as there is NO screen) is very poor in the information it provides.
Overall, I would NOT buy it again.

As a charger, it does its job well. I bought it for the 18650 batteries, but it doesn't fit the 21700 batteries as stated in the title. They don't fit and I have to charge them with a USB cable through the device I have that works with such batteries. If you want it for 21700 batteries, buy another one because you will be disappointed. That's why I give it 4 stars.

Used for 21700 4000mAh batteries.
A full charge lasts about 4 hours (simultaneous charging of 2 batteries) and it overheats quite a bit.
I now use it as a backup, I wouldn't recommend it for "big" batteries.

Very good product and it is not related to other cheaper ones that do not charge the batteries adequately. This specific one is a guarantee for what it claims to do because it actually does it.

The two buttons are useless in practice. Additionally, I have brand new AAA and M batteries that show they are charged at 60 percent. I take them out because it is unreasonable for them to be drained after one day, and I put them back in at the same time, and M shows that it is at 30 percent... This is after a year of use, charging well but slowly 18650... not worth it... disappointing.

Good charger, after a few months of use, the right side stopped working, it no longer charges from there. As for whether it charges 21700, of course it does and how it charges, I only use 21700. You just need to make sure that the batteries make contact with the metals on the poles. At first, they didn't charge for me either.

It does not charge the 21700 batteries even though it claims to do so. It charges the 18650 batteries just fine. However, it disappointed me for the first reason I mentioned.

The first one I bought, I returned the next day. It was heating up and smelled like burnt plastic. After about 9 months of use (where it was hot during charging), it reached the point of smelling like burnt plastic throughout the house. I don't know what would have happened if I hadn't heard to unplug it.
I bought the Opus BT-C3100 v2.2 and so far (1 month of use) everything is going well. Of course, it has double the price but it is problem-free.

Very reliable charger. Technology that helps prevent battery overcharging and a function that "wakes up" batteries that haven't been charged for years. It charges most types of batteries, both in terms of size and type. It is a bit large for a two-battery charger.

So far, I have been charging 18650 batteries without any problem. Definitely, the user instructions are not the best and perhaps they should issue more understandable ones.

After 2 years of use, it burned out. Fortunately, I was next to it and saw smoke coming out of the side holes next to the screen, so I immediately unplugged it. Until then, I was very satisfied with how it charged the batteries.

I don't understand the indications on the screen at all! I don't think even the person who made it understands! The battery application is not good at all with 2A!

Difficult to understand on the display, charges slower than its predecessor, and despite what it claims... you also don't know what it says and how it says it on the display. There is another one, the Efest Luc V4, which seems better for the same price.

It has a 12V plug for car charging, but they don't provide a cable. I am not satisfied - it says a lot in the advertisement that it doesn't deliver.

I put AA batteries side by side with the regular i4, and it took longer to charge them. Also, you can't choose between gentle or hard charging for these batteries, meaning to charge them with low or high current... another charger I have can do that. What can I say, maybe it does it for other batteries... but I am not satisfied..