Real-time clocks (RTCs) are designed to keep time continuously, even when the main system power is removed. This is made possible by a backup power source, typically a coin cell battery or, in some designs, a supercapacitor.
Choosing the right backup method is critical for reliable long-term timekeeping.
Why RTC Backup Power Is Needed
When the main power supply is removed, the RTC must continue running to preserve the current time and date.
- Prevents loss of time data
- Avoids the need to reset time after power cycles
- Enables autonomous systems
Without backup power, the RTC resets and becomes unusable until reconfigured.
Coin Cell Batteries (CR2032 and Similar)
The most common backup solution is a lithium coin cell battery.
- Typical voltage: 3V
- Very low self-discharge
- Long lifetime (often 5-10 years)
- Compact and inexpensive
Most RTC modules are designed for CR2032 batteries, which are widely available.
Advantages
- Extremely long runtime
- Stable voltage
- No maintenance required for years
Disadvantages
- Non-rechargeable
- Must be replaced eventually
Backup Current and Battery Lifetime
RTC chips are optimized for ultra-low power consumption when running from backup.
- Typical current: a few hundred nanoamps to a few microamps
- Battery life depends on RTC type and environment
Example:
- DS1307: higher backup current
- DS3231: lower and more efficient
In practice, a CR2032 battery can keep an RTC running for many years.
Supercapacitors as Backup Power
Supercapacitors (also called ultracapacitors) can be used instead of batteries in some designs.
- Rechargeable
- High cycle life
- No battery replacement required
They are typically charged from the main power supply.
Advantages
- No maintenance
- Unlimited charge cycles
- Environmentally friendly
Disadvantages
- Limited backup duration (hours to days)
- Higher self-discharge
- Voltage drops over time
Supercapacitors are suitable for short power interruptions, not long-term storage.
Rechargeable Backup Batteries
Some RTC modules include a charging circuit for rechargeable batteries (e.g. LIR2032).
Important warning:
- Do NOT use standard CR2032 batteries in modules with charging circuits
- This can cause leakage, damage or safety issues
Many low-cost modules incorrectly include charging circuits by default.
Common Problems with RTC Modules
- Incorrect charging circuits on DS1307 modules
- Poor-quality battery holders
- High leakage currents reducing battery life
These issues can drastically reduce backup performance.
Design Considerations
Choosing the Right Backup Method
- Long-term autonomy: coin cell battery
- Short interruptions: supercapacitor
- Rechargeable systems: dedicated charging circuit
Low Power Design
- Minimize leakage currents
- Use efficient RTC (e.g. DS3231)
- Avoid unnecessary loads on VBAT
Battery Replacement
- Design for easy access if replacement is required
- Consider socket vs soldered battery
Typical Backup Durations
| Backup Method | Typical Duration | Maintenance |
|---|---|---|
| CR2032 Battery | Years | Replace eventually |
| Supercapacitor | Hours to days | None |
| Rechargeable Coin Cell | Years (with charging) | Complex design |
Best Practice Recommendations
- Use a CR2032 battery for most applications
- Avoid modules with unknown charging circuits
- Use DS3231 for better efficiency and accuracy
- Use supercapacitors only for short-term backup
Conclusion
Backup power is a critical part of any RTC-based design. The right choice depends on how long the system must maintain time without external power.
For most projects, a simple CR2032 battery provides the best balance of cost, reliability and long-term performance.