Please, what is this socket for?
That's for charging a 3800mAh battery using the side jack. To charge the BL-5L 3800mAh battery you must purchase a: - TYT TH-UVF9 battery charger for 110VAC or - TYT TH-UVF9D for 12VDC.
- These are fully regulated chargers
- Plugs directly into side of battery, not a charger base
- The AC wall wart has a charging indicator. Red while charging, Flashing Green Red when charged.
Check point no 6.7 on this page - http://www.miklor.com/uv5r/UV5R-FAQ.php#Batteries
It talks about an extended battery like you have shown here. Also says that it uses a side jack. The port you see here is probably for the side jack.
BaoFeng BF-F8HP 100% backwards compatibility claim: In the document referenced above, "Check point no 6.7 on this page - http://www.miklor.com/uv5r/UV5R-FAQ.php#Batteries" it's inferred that since 3800mAh battery pack is compatible with UV-5R it's also compatible with BF-F8HP (based on BaoFeng claims BF-F8HP is 100% backward compatible with UV5R accessories, I suspect)...however...
There are ads on ebay that specifically stipulate their BL-5L 3800mAh is NOT compatible with BF-F8HP even though compatible with UV-5R. So, BF-F8HP compatibility is not 100% backwards compatible with UV-5R fitting accessories. I verified this by purchasing and checking. While slightly more difficult to slip onto a UV-5RTP case than the 1800mAh pack that came with the radio, the BL-5L battery packs definitely had to be firmly pushed to engage the eject tab on the F8HP. I wrote baofengtech.com and they insist BF-F8HP is 100% compatible with all UV-5R accessories. I realize we're talking a non-Baofeng accessory, but if an accessory fits a UV-5R many folks would think "backwards compatibility" would ensure it fits an F8HP.
So, the lesson of this is...don't ASSUME compatibility based on model numbers and Baofeng-ish claims...read every product description fully. The warning on this UV-5R=compatible, but BF-F8HP=incompatible was listed WAY down on the product page.