I am using Li-ion batteries in my F19. No problems so far and the batteries last a long time in the detector. However, the batteries do not work very good in my pinpointers, especially in the carrot. I use this exact battery:
https://www.amazon.com/EBL-Self-Discharge-Lithium-ion-Rechargeable-Batteries/dp/B00EQ3U2AA
I have 8 batteries but really only needed 4. I do have a tendency to over do things. I also have a battery charger that will charge all alkaline batteries. The same applies to recharged 9V alkaline batteries as they work well in my detectors but not in my pinpointers. I think it has something to do with a voltage regulator circuit being in the detectors but not in the pinpointers. at least I think so. The aa and aaa alkaline batteries do recharge very well and work perfectly in my other detectors that use aa batteries and the batteries can be re-charged up to 5 times. Sometimes, after a charge and sitting for a week or so, there have been instances where a recharged alkaline battery will leak so I keep an eye out for this and toss any battery that leaks. They can become a mess if not delt with. Oddly enough, the Copper Top batteries seem to be the ones that leak more often and do not last as long as other brands, and even cheaper brands. I have purchased very few batteries in the last two years. Some of my friends save their used alkaline batteries for me and that keeps my supply fresh.