Jimmy, you'll be glad to hear that after-market batteries for DJI's older drones have evolved from the dodgy DIY contraptions they once were to plug-and-play batteries whose performance exceeds the original DJI versions by a comfortable margin.
Cutting to the chase, Ali Express in China is your one-stop for DJI-compatible batteries sporting extra charge capacity that will keep your Phantom 3 S aloft for a few more minutes than would a stock DJI battery.
By way of example, the stock Phantom 3 battery is rated at 4,500 mAH while this after-market version of the same battery sold by Ali Express is rated at 4,980 mAH. That extra 480 mAh makes a significant difference in flight time, which for the Phantom 3 S could mean an additional 5 minutes of flight time per full battery charge. That 5 minutes extra flight time is a BIG deal if you plan to fly the drone for long waypoint missions. Here is the link to this battery I describe, however there are numerous vendors of similar batteries on Ali Express.
From Ali Express, I bought multiple extra-range batteries for my Mavic 1 Pro,
Mavic 2 Pro, and Air 1, all of which get a new lease of life with that range boost. Finally, I have yet to see any indications of battery failures from the numerous examples of non-OEM DJI batteries I bought from Ali Express.
Even the Air 1 batteries I bought, which are obviously a DIY job whereby newer, more powerful cells were shoe-horned into the original Air 1 battery housing, have performed flawlessly, though my highest praise is reserved for the after-market extra-range batteries for the Mavic 1 Pro and
Mavic 2 Pro, which enable both those drones to cover 13.46 miles roundtrip Litchi waypoint missions at 27mph, with flight times up from 23 minutes to 33 minutes thanks to those batteries on steroids now on sale for older DJI drones at Ali Express.
Bob's yer uncle.