Maybe because they are new but the batteries are lasting long, the USB C makes it so much easy to charge on the go. I carry 3 of these with me to shoots and they last roughly an hour each on my Sony FX3

NP-FZ100 Batteries.
I can confirm these work well with Sony A7S III and A7C II cameras, absolutely no compatibility issues and the capacity matches the original OEM ones. Having shelled out almost £400 on six original Sony batteries (which are faultless by the way, just expensive) I thought I'd give these K&F ones a try, just to have a few backup batteries in my bag - and I'm happy to report they work very well...in fact I bought 4 of them. I took advantage of the special offer and got them for less than half the price of Sony ones, which makes it even better.
I initially thought the USB-C charging port was a but gimmicky but I have to admit when all your charger slots are occupied, being able to plug directly into a power bank or a V-Lock with USB port is pretty handy.
I will still go to my OEM Sony batteries first as they have proven reliability but these K&F batts are good enough to keep along side them on long shoot days.

I'm surprised how long they last, noticeable longer than other brands and a bit longer than original Sony, works with FW 3.01 on a74 without any issue...so kudos to them

Works with my FX30 with no issues!

Great for my Sony FX30 and the fact that I'm getting two quality batteries at this price is amazing. So far I've had no weird hiccups or issues. No overheating or burning smells. Turning the camera on and off doesn't come with any glitches and navigating the menu is simple. These may sound off, but when you use different batteries that aren't from the manufacturer, sometimes you can get some unsettling results through regular use - like at one point my Wasabi batteries overheating my Pocket 6K. So it's important to look for any operation anomalies as you use different batteries. These are great and I definitely recommend.

Third party batteries usually over promise and under deliver by using bad cells or similar.
This kit has 2 batteries that claim 2600mAH compared to the stock 2280 mAH. I am not going to do a torture test or say which lasts longer. I will say however, I was not left wanting more life or felt like the K&F was cheating me by misrepresenting. This could even mean this battery is actually just 2280 and I would never know but it does last at least the same amount of time.

The added benefit of having the internal charging functionality is nice and a welcome addition but a few notes about it. The little orange trays that come with these batteries is a nice to have but it blocks the charging port. The charger being internal to the battery is a concern as it simply adds more components and more heat to an already tight package.

I would still buy this kit over the $80 Sony wants for ONE battery and no charger regardless of any concerns I have about longevity.

Exceptional Power and Performance:
The K&F Concept upgraded NP-FZ100 battery boasts a generous capacity of 2600mAh, delivering 7.2V and 18.72Wh. This ample power ensures extended video production and photography sessions, making it an ideal choice for professionals and enthusiasts alike. Its no memory effect feature guarantees consistent and reliable performance, providing peace of mind during your shoots.

Fast and Convenient USB-C Charging:
Say goodbye to the hassle of extra chargers. This battery supports fast and efficient USB-C charging, fully charging each battery in approximately 2.5 hours with a 5V1.5A input. The charging indicator light clearly displays the battery's status, ensuring you're always informed and ready for your next shot.

Wide Compatibility with Sony Cameras:
This NP-FZ100 replacement battery is perfectly compatible with a wide range of Sony cameras, including the Sony FX30, A7Riii, A7Riv, A7RV, A7Siii, A7iii, A7iv, A6600, A6700, A7C, ZV-E1, A1, and A9. Enjoy seamless compatibility with no pop-up error messages, ensuring smooth operation across different camera models.

Versatile Charging Options:
Equipped with a USB A to C charging cable, this battery offers multiple charging options. Charge directly using a power bank, wall socket, car charger, or other dedicated chargers. It's also compatible with Sony and K&F Concept NP-FZ100 chargers, providing flexibility and convenience for all your charging needs.

Safe and Durable Design:
Featuring a distinctive white and gray color scheme, the upgraded battery is constructed with a fire-resistant and durable ABS and PC exterior shell. It includes a safe and reliable circuit with stable voltage and current, ensuring added safety during use. Each battery is packed with a protective cover and USB-C charging cable, enhancing its durability and usability.

The K&F Concept 2600mAh NP-FZ100 battery is a powerful, reliable, and convenient solution for Sony camera users. Its fast USB-C charging, wide compatibility, and durable design make it a must-have accessory for extended video production and photography projects. Equip yourself with this high-quality battery and experience the difference in your shoots.

What a great little upgrade for your camera battery arsenal. They fit perfectly and function perfectly in my Sony camera. No error messages. Even better is it’s usb c rechargeable. Everyone has a usb c cable nowadays so recharging is a breeze if you don’t wanna recharge from the camera body. Battery life is great. Comes with a usb c cable for charging if you don’t have one already as well, but you do lol.

The built quality is excellent. It works just like the original Sony battery, and I haven't noticed any difference in charge. The main reason I got this battery was because of the fast USB-C charging, which is excellent. I can fast charge the battery in my car. Excellent value for the money.

I cannot vouch for the capacity. I doubt it, but not because of evidence. Because it has to make room for a USB port so I find it hard to believe that there's more juice capacity than they stock battery that doesn't have a charging port.

That said, charging port means it charges faster than if I have to charge through my camera. It works and is read by both my a6700 and FX30. Very happy with this set of backup power.