This page of the guide to Star Wars Jedi: Fallen Order suggests how to increase the number of BD-1's healing charges. This droid will accompany the main hero from the first chapter onward. This knowledge can be very helpful in keeping the main character alive.
BD-1 has the ability to recover lost health points. This is also the main method of healing of the hero, excluding meditation points. Initially, the droid has only two healing charges - this may not be enough for larger battles or fights with challenging bosses.
You can increase the number of healing charges by finding Stem Cannisters. You don't get them automatically by making progress. Instead, you need to look for them yourself - they are collectibles/secrets. New cannisters are stored in the yellow chests - see the picture.
Each time you find a new yellow box and open it, the number of available healing charges will increase by 1.
The first yellow chest can be found on Bogano, the "starting" planet. You come across it on the way back to the ship, after visiting the Vault and listening to the hologram. A large hole in the ground can be found near the structure resembling a stone amphitheater. Jump into it to land near the yellow chest.
The second yellow chest is on the planet Zeffo visited in the second chapter. You can find it shortly after landing on the planet. It is inside a cave you can access by straying from the main path. You also have to jump to a few rocks. The cave is very dark, but Cal can light his way with the lightsaber.
Locations of other yellow chests can be found in the Secrets section.