Sentinel batteries are an extremely useful resource for you in Doom: Eternal. You need it to unlock numerous locations on your Fortress of Doom to gain access to more power-ups and a handful of customization appearances. You can find three of them on the Cultist Base level.
The first one on the level is likely the most difficult to access of the available trio. You have the chance to grab it right before you enter the base. If you miss it by entering the base, you can only access it after you complete the level and unlock the fast travel mechanic, which you receive at the end of every mission. After you clear the area of demons, turn around to face the entrance you came in from. Turn to the left, and you should see a small platform you can grab off to the side. You need to double jump and dash to it, and you should find a breakable wall to the side. Break it, and there’s a punchable lever you need to hit. This lever unlocks a gate hidden underneath the pathway you were fighting. Dash over to the lower path, use the jump pad to jump into the air, and then dash to the sentinel battery hanging over top of the area.
The second sentinel battery is pretty easy to find. You want to look for it shortly after you defeat a horde of demons while fighting on an open platform, where the priest pulls up the drawbridge to trap you on it. Fight through the horde of demons, and then continue forward. Before you go on an elevator further into the base, turn to your left to grab that battery.
The third and final sentinel battery close to the end of the level where you’re looking for your Super Shotgun. It’s the location where those lifts are going up and down, and you have to stand in the middle of them access hidden pathways. Climb to the top of this area where you have to fight the Revenants. After you kill them, go to the lift on the right side of the area, and wait for the opening to appear. Step inside of it, wait for it to raise, and then proceed through the hidden door. It will drop you into the room with a bunch of shield chips and the sentinel battery.
There are a lot of sentinel batteries scattered throughout Doom: Eternal. Make sure to acquire as many of them as you can to unlock new appearances for Doom guy, and helpful tokens to fully enhance your weapons and suit.