How to Boost a 21700 Battery? Boost Voltage: from 3.6V (3.7V) to 5V
Blog | Published by Alex on September 24, 2025
Why need a boost converter for 21700 battery?
The 21700 battery lithium cell is a cylindrical battery with a diameter of about 21 mm and a length of 70 mm. Its capacity typically ranges from 3000-5000mAh, with a nominal voltage of 3.6V-3.7V. Due to its large volume, high energy density, and long cycle life, the 21700 battery is widely used in power batteries, energy storage systems, and high-performance power banks.
The discharge curve of a lithium-ion battery is nonlinear. As the battery depletes, its voltage drops from 4.2V at full charge to 2.5–3.0V at the cutoff voltage. A single 21700 battery cannot directly drive a 5V USB output, so a boost converter is required.
Boost conversion principle
A boost converter is a type of switching power supply that uses an inductor, capacitor, diode, and switch to step up the input voltage to a higher, stable output voltage. The working principle involves:
(1) Switch on: Current flows through the inductor, storing energy as the inductor current increases linearly.
(2) Switch off: The inductor releases energy through the diode to the output, raising the output voltage above the input.
(3) Capacitor filtering: The output capacitor stabilizes the voltage and provides continuous current.
Key parameters for a boost converter include:
(1) Input voltage range: Should cover the full charge and discharge voltage of the cell, e.g., 2.5–4.2V.
(2) Output voltage: Stable 5V USB output.
(3) Output current: Depends on the load, e.g., 1A, 2A, 3A.
(4) Efficiency η: Typically 85%–95%.
The boost modules for 21700 battery
Common chips:
MT3608:
The MT3608 is a widely used high-efficiency boost (step-up) converter IC, designed to step up a lower input voltage to a higher output voltage.
MT3608 main features include:
Input voltage range: 2V–24V.
Output voltage range: Adjustable, up to 28V.
Output current: Typically 2A (modules can provide ~1.5–2A stable output).
Efficiency: Around 93% (depends on input/output voltage and load).
Protection: Overcurrent and overtemperature protection.
Price: Approximately $1 (China Factory)
It is commonly used in small power modules, USB boost boards, portable devices, and DIY electronics projects.
TPS61088:
The TPS61088 is a high-power synchronous boost converter IC from Texas Instruments, designed for applications requiring higher voltage and current.
TPS61088 key features include:
Input voltage range: 2.7V-12V.
Output voltage range: Adjustable, up to 12V.
Output current: Can support over 3A, suitable for high-power loads.
Efficiency: Up to 96% (depending on input/output conditions).
Protection: Overcurrent, overvoltage, and overtemperature protection.
Applications: Portable devices, LED drivers, battery-powered systems, and high-performance boost requirements.
Price: Approximately $1.5 (China Factory)
XL6009:
The XL6009 is a versatile boost (or buck-boost) converter IC commonly used in DIY power modules and portable devices.
XL6009 key features include:
Input voltage range: 5V-32V.
Output voltage range: Adjustable, up to ~35V.
Output current: Typically 2A, with higher short-term peak capability.
Efficiency: Around 88%–94%, depending on input/output voltage and load.
Protection: Overcurrent, overvoltage, and overtemperature protection.
Applications: Battery boost, LED driving, adjustable power supplies, and portable device power systems.
Price: Approximately $1.3 (China Factory)
Selection criteria:
Input voltage covers the 21700 discharge range.
Stable 5V output.
Output current meets the load.
Module should have overcurrent and overtemperature protection.
BMS or PCM
A lithium battery must have a protection circuit (BMS/PCM) to prevent overcharge, overdischarge, and short circuits. Protection usually includes:
(1) Overcharge protection: prevents voltage exceeding 4.2V.
(2) Overdischarge protection: prevents voltage dropping below 2.5V–3V.
(3) Overcurrent protection: prevents current exceeding safe limits.
(4) Short-circuit protection: protects the module and load.
For a single 21700 cell boosting to 5V, a low-current BMS (1A-3A) is sufficient.
Heat dissipation and safety
At high output currents, both the boost module and the cell will generate heat.
Add heatsinks or ventilation to ensure temperature rise <60°C.
Avoid direct short circuits and high-temperature environments to prevent thermal runaway.
Using high-quality cells and modules improves system stability and lifespan.
Applications
1. Single-cell 21700 battery USB power bank
(1) Cell: 3.7V 3500mAh.
(2) Boost module: MT3608 5V 2A.
(3) Output: 5V USB, can charge a smartphone 1–2 times.
2. Multi-cell parallel/series configuration.
(1) Parallel increases capacity, series increases voltage (for 12V/24V battery pack).
(2) Single-cell boost is simple, suitable for small portable devices.
Summary
(1) A single 21700 battery cannot directly output 5V, a boost module is required.
(2) The boost module should be selected based on voltage, current, efficiency, and protection.
(3) Cell current must be calculated to avoid exceeding maximum discharge capacity.
(4) Protection circuits and thermal management enhance safety.
(5) Single-cell boost is suitable for portable power banks, multi-cell solutions can scale to high-power systems.
