Design of a Battery Canister

[AdrianHe]

Hello everyone,

I want to share my design of a 36 cell battery canister with constant output voltage. A more detailed documentation and design files can be accessed here: GitHub
Improper use of Li-ion batteries, including short circuits or mishandling, can pose serious risks, potentially resulting in fires or explosions.

Requirements:​

• Constant output voltage of 12V​
• Max. power output of 180W​
• Runtime BZ400h (115W): 3h -> Capacity: > 345Wh​
• PWM output control​
• Option for single and dual output​
• No exposed o-rings​
• Compact design​

Design:
The picture below shows the three main parts of the battery canister: the lid, the battery pack, and the canister. The batterypack is made up of 36 Sanyo NCR18650GS Li-ion batteries. They are arranged in a 6p6s configuration providing a total energy capacity of 447Wh. The pack has a nominal voltage of 21.6V. Two custom designed PCBs are used for the protection of the battery pack (BMS), the voltage conversion, and the management of inputs and outputs. The first PCB (BMS Stage) sits on top of the battery pack, it consists of the primary and secondary battery protection, a fuse and the cell ballancing circuitry. LEDs display the state of charge of the pack. The second PCB (BUCK & MCU Stage) is mounted to the lid. A synchronous buck step down converter is used to convert the pack voltage to a constant 12V. A STM32 microcontroller is used to manage the input and outputs and monitor temperature and voltages. Two radial o-rings form the seal between the canister and the lid. A spring plunger locks the lid to the can.
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Figure of section view and PCBs:

 

[AdrianHe]

Pictures of the assembly:

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[AdrianHe]

Testing:
To test the watertightness of the cannister, it is set into a home build preassure chamber, filled with water and pressurized to 8bar. After two hours, the inside of the canister is checked for leaks.

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Performance of the step down converter:

Due to conduction, switching and inductor losses, the buck converter does not run at 100% efficiency. The diagramm shows the efficiency at different input voltages (battery voltage) and under different loads. The step down converter is optimized to reach higher efficiencies with increasing loads. These values match the efficiency values described in the datasheet of the buck converter.

 

[grantctobin]

Nicely done

 

[Whitrzac]

Very nice.
Is the output voltage adjustable? Running heating garments at 15-16v can give more warmth for colder dives.

The outer shell is a bit thinner than any other "modern" pack I've seen, but I'm unsure how much it matters.
I have a pile of old Sartek stuff that was made with schedule 80 PVC and good to 300ft.

 

[AdrianHe]

Very nice.
Is the output voltage adjustable? Running heating garments at 15-16v can give more warmth for colder dives.

The outer shell is a bit thinner than any other "modern" pack I've seen, but I'm unsure how much it matters.
I have a pile of old Sartek stuff that was made with schedule 80 PVC and good to 300ft.

The nominal output voltage is 12V, can however be easily increased to 12,7V. A different output voltage can be achieved by changing the resistor divider of the voltage converters sense pin. I think I'd be cooked to a nice, well-done roast at 16V in my Santi heated undergarment

The shell thickness is 4.5mm, I've verified the stresses by FEM analysis and a pressure test to 8 ATAs. In hindsight, I would have increased the thickness a bit. A lot of commerical packs need the thicker wall for the axial o-ring seal and to screw in the nielsen clamps.

 

[SquareJ]

Amazingly well thought out and implemented!