A Power Bomb Expansion, also known as a Power Bomb Tank, is an upgrade that allows Samus Aran to carry more Power Bombs in her suit. They are featured in every game except Metroid, Metroid II: Return of Samus, Metroid Prime Hunters, Metroid Prime Pinball, Metroid: Other M and Metroid Prime 3: Corruption. Power Bombs can destroy Bendezium, Denzium and destroy or do serious damage to most creatures. In Super Metroid, a single expansion increases Samus' Power Bombs by 5, in the Prime series and Metroid: Samus Returns by 1 and in Metroid Fusion and in Metroid: Zero Mission by 2.
The 2D Metroid games used the term "Power Bomb Tank", however it seems to have been replaced by "Power Bomb Expansion" in the Prime Series. Samus is capable of carrying 8 in the first Metroid Prime, as she gains 4 upon acquiring the Power Bombs, and there are 4 more expansions to be found scattered across Tallon IV. In Metroid Prime 2: Echoes, Samus can carry up to 10, as she starts off with 2 and has to collect 8 more scattered throughout Aether.
In Prime, the Item Acquisition Fanfare plays when collecting a Power Bomb Expansion, rather than the Tank Jingle heard when acquiring other expansions. This is likely because there are so few of them. Echoes changes this, and the Tank Jingle is heard when a Power Bomb Expansion is obtained.
Metroid Fusion manual
"Increases the number of Power Bombs Samus can carry."
Metroid: Zero Mission manual
"These tanks increase the number of Power Bombs that Samus is able to carry."
Echoes Inventory data
Power Bomb Expansion
The Power Bomb Expansion units increase the amount of Power Bombs you can carry.
Each Power Bomb Expansion increases the amount of Power Bombs you can carry by 1 unit.
You can fully reload your Power Bombs at Ammo Stations. Your gunship has this capability as well.
Metroid: Samus Returns
"Power Bomb Tank acquired. Power Bomb capacity increased by 1.
Metroid Prime 2: Echoes
|Power Suit||Energy Tank • Aeion Tank • Reserve Tank • Energy Part • E-Recovery Tank • Power Bomb Expansion|
|Beam||Universal Ammunition Expansion • Beam Ammo Expansion • Accel Charge • Ship Grapple Expansion|
|Missile||Missile Expansion • Super Missile Tank • Ship Missile Expansion|
|Keys||Chozo Artifact • Alimbic Artifact • Octolith • Dark Temple Key • Sky Temple Key • Energy Cell|