The DJI Ronin-S BG37 Battery Grip is an add-on accessory for the DJI Ronin-SC / Ronin-SC Pro gimbal that provides up to 11 hours of operation. The grip houses a 2450 mAh LiPo battery that can be fully charged in around 2-1/2 hours.
No. The DJI Ronin-S is not compatible with the BG18 Battery Grip.
If you are looking for a spare battery for the Ronin-S, you want the DJI Ronin-S BG37 Battery Grip.
No. The battery can be charged by connecting it to a Ronin-SC gimbal and plugging into a power source via the USB Type-C interface.