I don't understand why there's battery where the submarine has fuel cell?? battery also occupies space and weights. Instead of batteries, they could add more hydrogen fuel??
My smart friend enlightened me:
Elbette! Aşağıda verdiğin bilgiyi İngilizce olarak, teknik bir forumda paylaşmaya uygun şekilde düzenlenmiş haliyle bulabilirsin:
---
Why Do AIP Submarines Still Use Batteries Alongside Fuel Cells?
I've been studying Air-Independent Propulsion (AIP) systems, especially fuel-cell-based ones, and one thing puzzled me: If fuel cells can generate electricity using hydrogen, why do submarines still carry large batteries? Wouldn't it be better to use that space for more hydrogen fuel instead?
Here’s what I’ve learned:
---

1. Batteries Handle Peak Power Demands
Fuel cells produce electricity at a constant and relatively moderate rate. However, submarines often require sudden bursts of high power, such as:
Rapid acceleration or evasive maneuvers
Torpedo launches or activating high-energy systems
Emergency surfacing procedures
Fuel cells can't instantly deliver such high loads — they respond slowly and are designed for endurance, not peak output. That’s where batteries come in. They act as a buffer, delivering large amounts of power in a short time.
---

2. Silence and Stealth
Silence is vital for submarines. Fuel cells are already very quiet, but batteries are even quieter — they have no moving parts and produce zero noise during discharge. This makes them ideal when absolute stealth is required, such as near enemy sonar arrays or in contested waters.
---

3. Hydrogen Storage Limitations
Hydrogen is difficult and risky to store in large quantities:
It’s extremely flammable
Requires high-pressure tanks or cryogenic storage
Takes up valuable internal volume
So, simply "adding more hydrogen" isn't a practical solution. Submarine space is already at a premium, and increasing hydrogen storage introduces safety and design challenges.
---

4. Operational Flexibility
Batteries can be charged and discharged quickly, and they respond instantly to changes in power demand. Even during normal operations, excess electricity from the fuel cells often goes into charging the batteries, so they work together as a hybrid system — much like hybrid electric vehicles.
---

Conclusion:
AIP submarines use both systems for complementary reasons:
Fuel cells provide efficient, silent endurance
Batteries handle short-term, high-power needs and critical stealth scenarios
This hybrid setup gives modern submarines the flexibility and survivability they need in complex underwater environments.