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Why Your RTC Battery Fails After 6 Months – And How to Fix It in Hardware
Have you ever encountered this scenario—after a power outage lasting several days, the equipment is restarted, and the system time is inexplicably displayed as January 1, 1970? Or, six months after product shipment, customers are reporting that their RTC batteries are depleted and frequently requesting replacement? Even more insidious is the situation where, after industrial data acquisition devices have been running in the field for months, the uploaded data timestamps become completely scrambled, rendering the entire batch of data unusable for analysis. Tracing the root cause often points to the same source: a flaw in the RTC backup battery circuit design, or substandard PCBA manufacturing quality.
These problems may not be apparent during the prototype verification phase, but once mass production begins and products are shipped globally, microampere-level leakage current, unreliable solder joints, and inappropriate component selection will start causing trouble. The cost of fixing these issues is far higher than making the design correct in the first place.
This article will comprehensively analyze the key technologies of RTC backup batteries from three perspectives: solution selection, circuit design considerations, and mass production manufacturing considerations. Furthermore, based on the development trends of global electronics manufacturing services, it will discuss how to choose a professional PCBA service provider to ensure that the design intent is perfectly realized when the product moves from prototype to mass production.
What is a Real-Time Clock (RTC)?
RTC stands for Real-Time Clock, a clock-based timing module used to read time and help maintain the current time. Even when the main power is off, the CPU continues to operate, and it can still track real-time time. RTC uses a 32.768kHz oscillating crystal for timing. It stores seconds, minutes, hours, days, and years. Integrated circuits often include alarm clocks, timers, and calendar functions. These are primarily used for embedded purposes.
Electrical Characteristics Requirements for RTC Backup Batteries
Designing a battery backup PCB involves much more than just a battery, especially for an RTC circuit. In other words, while designing, one has to think of ways in which it can save power. Similarly, one will have to enable it to alternate between both powers. Above all, there must be a minimum leakage current that draws from the battery.
Power Consumption
An RTC is configured for very low power in backup mode so that the backup energy source would last for many years without replacement. The RTC model and configuration employed may result in power consumption ranging anywhere from a few hundred nanoamperes to a few microamperes. When in this state, all unnecessary functions are turned off, and the clock circuitry only remains active. The power consumption of the entire installation can increase considerably due to a variety of causes. These include leakage current from nearby circuitry and components, PCB flux contamination and inefficient switching circuitry. To prevent excessive power dissipation, it is essential to ensure a zero leakage path and use ultra-low power switching components. This will allow the battery to last longer. The RTC will remain stable and accurate for years to come.
| RTC IC | Typical Backup Current |
| DS3231 | 0.84 µA (typ) at 3V |
| PCF8523 | 0.22 µA (typ) at 3V |
| MCP7940N | 1.1 µA (typ) at 3V |
| RV-3028-C7 | 0.045 µA (typ) at 3V |
| ABX00006 (Arduino) | 0.22 µA (typ) at 3V |
| RX-8025T | 0.75 µA (typ) at 3V |
Voltage Requirements
The operational voltage of RTCs varies by device and is usually in the range of 1.2V to 3.6V. Many RTC devices consume very low current and can operate from a coin cell (for example, a 3V lithium cell). The backup voltage must remain within the RTC’s operating range; otherwise, a power failure may corrupt the entire memory. It is important to note that if the voltage supply drops below the RTC’s minimum operating voltage, it will either stop or drift.
Similarly, drift can also occur due to variations in voltage. Due to voltage variation, the oscillator frequency varies. Consequently, the RTC power switching design needs to be done very carefully. The circuit must have low-drop components that can maintain a stable voltage at the RTC during backup operation.
Comparison of Three Implementation Schemes for RTC Backup Batteries
The design of an RTC (Real-Time Clock) backup power supply essentially boils down to this question: what kind of energy storage element should be used to keep the clock chip running after the main power supply is disconnected? Current mainstream engineering practices focus on three solutions. There is no absolute right or wrong choice; the key is to consider the specific requirements of the application scenario regarding hold time, operating temperature, maintenance-free operation, cost, and circuit complexity.
Scheme 1：Non-Rechargeable Lithium Coin Cells
The most widely used primary power source for supporting RTC circuits is the lithium-ion coin cell battery. They are developed for extended use and have extremely low self-discharge. For example, CR2032 or CR1220 batteries are mounted in a battery holder on the PCB and connected to the VBAT pin of the RTC chip via diode isolation, providing backup power for the RTC. Due to the very low current, the battery backup time can reach several years. Generally, the voltage provided by a lithium-ion battery is approximately 3V.
- Battery holder selection is far more important than it seems. Different manufacturers and different structures of battery holders result in vastly different contact reliability. During vibration testing, some inexpensive battery holders may experience momentary power interruptions, causing RTC resets. For equipment operating under vibration, it is recommended to fix the installed battery with adhesive during production and place a 0.1μF~1μF energy storage capacitor near the VBAT pin as a power buffer in case of contact interruption.
- Current-limiting resistors are essential. Direct short circuits to the battery can cause it to explode. For example, the CR2032 has a maximum continuous discharge current of approximately 3mA. Assuming 3V, a 1KΩ resistor in series can limit the short-circuit current to within 3mA.
- Temperature affects battery life. The CR2032’s operating temperature is typically rated from -30℃ to +60℃, but its actual usable capacity significantly decreases below -10℃. Outdoor equipment using button cell batteries may experience clock hold failures in winter.
| Cell Type | Capacity | Nominal / EOL Voltage |
| CR2032 | 220 mAh | 3.0V / 2.0V (EOL) |
| CR2025 | 165 mAh | 3.0V / 2.0V (EOL) |
| CR2016 | 75 mAh | 3.0V / 2.0V (EOL) |
| CR1225 | 48 mAh | 3.0V / 2.0V (EOL) |
| CR1220 | 35 mAh | 3.0V / 2.0V (EOL) |
Scheme 2：Supercapacitors
For sealed devices where battery replacement is inconvenient, or in scenarios requiring maintenance-free operation over a wide temperature range, supercapacitors are a more reliable choice.
A typical circuit consists of two Schottky diodes, a current-limiting resistor, and a supercapacitor. When VDD is powered on, current flows through diode D1 and the current-limiting resistor R to charge the supercapacitor C, typically reaching nearly 3V within one hour. After VDD is de-energized, the supercapacitor supplies power to the RTC VBAT via diode D2.
Supercapacitors charge quickly and have extremely high cycle life in applications. Therefore, they are ideal for frequent short-term RTC backups. However, their high self-discharge rate makes them unsuitable for long-term use. Voltage drop during discharge may need to be regulated to ensure reliable RTC operation, adding a degree of complexity.
Key advantages of supercapacitor solutions:
- High temperature adaptability: Typical operating temperature -40℃ to +70℃, far exceeding that of button batteries. They are practically the only choice for automotive electronics and outdoor instruments.
- Extremely long cycle life: Over 500,000 charge-discharge cycles, with a theoretical lifespan of over 10 years.
- No environmental disposal issues: Lithium-free, eliminating concerns about waste disposal compliance.
- Supports SMT soldering: Direct reflow soldering is possible, eliminating the need for subsequent manual battery insertion and ensuring better process consistency.
Disadvantages also need to be acknowledged:
Power loss retention is only in the days range. If the device needs to maintain its clock speed for several months without power, then coin cell batteries or rechargeable batteries are more suitable. Additionally, supercapacitors are larger and more expensive than coin cell batteries.
Scheme 3：RTC Chip + Rechargeable Battery Combination
Some dedicated RTC chips (such as the Renesas ISL1208 series) integrate current-draining charging functionality, allowing them to be used with rechargeable lithium manganese silicon coin cells (such as the Panasonic MS series). The chip charges the battery with a small current when powered on, and the battery automatically takes over when power is lost.
This option combines the advantages of long hold time and rechargeable, replacement-free operation, making it suitable for situations where the main power supply is frequently interrupted. However, the circuit design and BOM costs are higher. The design requires careful reading of the charging current settings and mode switching logic in the chip’s datasheet—if the chip misinterprets a deeply discharged battery as an abnormal state, it may refuse to charge or experience abnormally increased power consumption. This back-and-forth troubleshooting has been a headache for many engineers.
Summary of core differences:
| Parameter | Primary Lithium Coin Cell | Supercapacit | RTC Chip + Rechargeable Battery |
| Rechargeable | No | Yes | Yes |
| Typical Voltage | ~3V | 2.5V–5.5V (varies) | ~3V (nominal, e.g., ML/VL series) |
| Operating Temperature | -10℃~+40℃ (usable area) | -40℃~+70℃ | depends on battery type |
| Self-Discharge Rate | Very Low (~1%/year) | High (5-20%/day) | Low (~1-2%/year) |
| Backup Duration | Long (years) | Short (hours to days) | Long (months to ~15 years) |
| Maintenance Requirement | Battery replacement needed | Minimal | Minimal (no replacement needed if regularly powered) |
| Design Complexity | Low | Medium | Medium to High (needs charge management, power path) |
| Soldering Process | Rear-mounted battery holder | Supports SMT reflow soldering | depends on battery type |
| Cost | Low | Moderate to High | Moderate to High |
| Best Use Case | Long-term backup, devices rarely powered | Short-term backup, frequent charge cycles | Always-powered devices requiring long backup & no maintenance |
Key Design Considerations and Common Pitfalls of RTC Backup Battery PCBA
Diode Leakage Current
The selection criteria for diodes in power switching circuits should not be limited to “low forward voltage drop.” Reverse leakage current (IR) is also a critical parameter determining battery life.
Assume the circuit current configuration is as follows: an RTC chip consumes 1μA, a ceramic capacitor has a leakage current of 0.5μA, seemingly a total load of only 1.5μA. A 220mAh CR2032 battery could theoretically last for over a decade. However, if the selected Schottky diode has a reverse leakage current of 2μA (this value increases exponentially at high temperatures), the total circuit current becomes 3.5μA, effectively halving the battery’s lifespan. Inferior or improperly selected diodes can easily have leakage currents of 10μA or even higher at high temperatures.
Design Guidelines: Choose Schottky diodes explicitly labelled with “Low Leakage,” such as the BAT54 series. Confirm the reverse leakage current value at the highest operating temperature in the datasheet and use this as a basis to calculate battery life under the worst-case conditions.
Current-limiting resistors
As mentioned earlier, current-limiting resistors are necessary, but a common misconception is: since the purpose is current limiting, wouldn’t a larger resistance value be safer?
Resist this temptation. Take a 10KΩ current-limiting resistor as an example. If the circuit consumes 100μA of current, the voltage drop across the resistor will be 100μA × 10KΩ = 1V. For a 3V button battery, the voltage reaching the RTC pin after passing through this resistor will only be around 2V. As the battery voltage gradually decreases with discharge, the RTC supply voltage may drop below the chip’s minimum operating voltage, triggering an undervoltage reset or forcing the chip into an abnormally high-power mode—intending to save power, but instead accelerating battery consumption.
Calculation principle: The value of the current-limiting resistor should balance “short-circuit protection” and “power supply margin.” Generally, a reference upper limit is set at 0.3V, meaning the voltage drop across the resistor due to the worst-case load current should not exceed this.
PCB Layout Design
The VBAT energy storage capacitor must be placed close to the MCU/RTC chip pins. The shorter the trace, the better. This capacitor is responsible for maintaining power during the microsecond-level switching interval when the main power is disconnected. Excessive trace length introduces additional parasitic inductance, weakening the capacitor’s transient response.
The I²C pull-up resistor should be powered by the system’s main power supply and must never be drawn from the VBAT battery side. Otherwise, each I²C communication will draw extra current from the battery, and this seemingly small loss can accumulate significantly.
The charging and discharging circuits of large-capacity capacitors (such as supercapacitors) should be consciously separated during layout to avoid voltage drops across the discharge path impedance caused by the charging current, which can interfere with the RTC power supply accuracy.
Soldering Process
Disposable, non-rechargeable lithium batteries (such as CR2032) can only withstand wave soldering, not reflow soldering. If the PCBA process involves surface mount technology (SMT) placement followed by reflow soldering, the batteries may be damaged or even explode at high temperatures.
This means that for solutions using battery holders, the holders can undergo reflow soldering first, but the batteries themselves must be manually or automatically installed after wave soldering. Supercapacitor solutions, however, do not have this limitation and can be directly installed on the SMT production line. For mass production that prioritizes automation and process consistency, this constitutes an additional manufacturing cost and quality control point.
Real-World RTC Application
RTC circuits with battery backup are widely used in devices, where the running time must be accurate even when power is off.
- Microcontroller-based embedded systems: Drive system clocks for real time in the absence of main power.
- IoT Devices: The sensors and smart home devices need to maintain the correct time in order to schedule events.
- Industrial Controllers: Store operational logs, timestamps, and process timing during power disruptions.
- Data Loggers: Records error-free timestamps for sensor data in use cases such as environmental tracking and medical devices
Why RTC Backup Battery PCBAs Require Professional PCBA Manufacturing Services
Having solved the issues of solution selection and circuit design, the next challenge is to translate the design intent into a high-quality, mass-producible product. For circuit modules like RTC backup batteries, which involve minute signals and long-term reliability, the manufacturing quality of the PCBA directly determines the market performance of the end product.
Leakage Current Control Assurance
As mentioned earlier, flux residue can lead to microampere-level leakage, which may be insignificant in the digital circuits of ordinary consumer electronics products, but in the RTC power supply circuit, it is enough to significantly shorten battery life. Professional factory cleaning systems, environmental cleanliness control, and ion contamination detection capabilities are the foundation for eliminating such hidden risks.
Soldering Reliability
This directly affects the performance throughout the product’s lifecycle. Battery holder solder joints and supercapacitor pads are subjected to continuous mechanical stress during transportation vibration and temperature cycling. A solder joint that appears to be “soldered” but develops microcracks under long-term stress may become the root cause behind a 1% customer complaint rate after 10,000 units have been mass-produced. Professional PCBA service providers adhere to IPC-A-610 Class 2 or Class 3 standards, employing 100% AOI inspection plus X-ray sampling to minimize this probability.
Component procurement
If counterfeit or substandard components—low-leakage-current diodes, high-quality supercapacitors, and original RTC chips—are procured, all design optimizations will be rendered useless. PCBA service providers with global genuine product procurement networks and incoming material inspection systems can cut off this risk at the source. For overseas clients, when local supply chains experience shortages, partners with global procurement capabilities can better guarantee project delivery times.
FS TECH's RTC Backup Battery PCBA Service Capabilities
FS Tech specializes in providing high-quality, one-stop PCBA manufacturing services to global clients. Headquartered in Shenzhen, we own our own PCBA factory, fully adhering to IPC standards and featuring 100% AOI+X-ray inspection coverage. In the niche market of RTC backup battery PCBAs, we have accumulated extensive experience in large-scale production and understand the unique manufacturing characteristics of low-leakage-current circuits.
End-to-End PCBA Manufacturing Capabilities
- Full BOM Component Procurement: Leveraging a global sourcing network, we ensure a genuine supply, competitive pricing, and stable delivery times for low leakage current Schottky diodes, branded supercapacitors (e.g., Murata, TDK, etc.), and original RTC chips (e.g., NXP, Microchip, IDT, etc.). We provide equivalent alternative solutions when items are out of stock.
- Integrated PCB Manufacturing and Assembly: From bare board manufacturing (supporting 1-58 layers, HDI, and rigid-flex boards) to SMT placement, DIP insertion, wave soldering, and post-soldering assembly, we provide a one-stop service, eliminating the need for multiple intermediaries for our clients.
- Professional process for matching RTC circuits: Double-sided SMT placement capability, minimum mountable 0201 package, BGA pitch 0.35mm. Extensive experience in assembling mixed-shaped components such as supercapacitors and battery holders. Strict adherence to IPC-A-610 Class 2/3 standards, 100% AOI optical inspection covering every PCBA, and X-ray inspection of BGA solder joints.
- Cleanliness assurance: Fully automated cleaning line removes flux residue; optional ion contamination testing for key products ensures that microampere-level leakage current circuits are not affected by board surface contamination.
- Functional testing (FCT): Custom-designed RTC circuit test fixtures can be developed to measure critical parameters such as backup power switching waveform and VBAT current consumption before shipment, preventing defective PCBAs from leaving the factory.
Flexible Cooperation Models
Whether you require turnkey manufacturing, mixed/kitted parts where you procure some key components and we supplement with general-purpose parts, or consigned manufacturing where all components are provided by the customer and we handle the processing, we can flexibly accommodate your needs.
Manufacturing Standards & Certifications
- Assembly Quality Standards: IPC-A-610 Class 2 / Class 3
- Quality Management System Certification: ISO 9001:2015
- Environmental Management System: ISO 14001:2015
- Environmental Compliance: RoHS / REACH / UL
- Industry Standard: IATF 16949 Automotive, ISO 13485 Medical
Conclusion
This absolute power loss on the RTC backup battery depends upon many factors such as power source selection, components used in the PCB with leaks, proper switch design and good PCB layout. By ensuring proper voltage regulation and incorporating protection mechanisms, along with layout optimization, these care electronics prolong battery lifespan while maintaining accuracy. Depending on the requirements of the system and how long backup is needed, primary lithium coin cells, rechargeable batteries, or supercapacitors can be selected. Choosing high-quality PCB manufacturing and assembly processes ensures the design is translated with precision and remains reliable over time.
FS Tech has availed expert solutions for strong RTC backup battery PCBA designs across a wide range of applications. If your project is in the proof-of-concept phase and you wish to obtain DFM (Design for Manufacturability) review at the design level; or if you have already completed design verification and are looking for a PCBA manufacturing partner capable of handling the entire process from prototype to mass production—we are willing to be your trusted choice.
FAQS about RTC Backup Battery
An RTC may also drift time, possibly from bad crystal quality, incorrect load capacitance, high leakage currents, or RV stability. For accurate timekeeping, a well-designed PCB with low leakage parts and a stable backup voltage is necessary.
Primary lithium coin cells have a shelf life of approximately three to ten years. The actual lifespan depends on the RTC current consumption, environmental conditions and leakage currents. This translates into longer battery life if components are chosen wisely and the RTC backup battery PCBA is optimized.
Supercapacitors support SMT reflow soldering, offering better process consistency than battery holders with post-installed batteries, making them suitable for fully automated production. It’s important to note that supercapacitors are relatively large and heavy, so the pad design must adequately consider vibration resistance. Additionally, the cleaning process must be thorough to prevent residues from creating leakage paths around the capacitor.
MOSFET-based ideal diode circuits lead to significantly lower voltage drop than conventional diode OR-ing and therefore are highly efficient. This promotes a constant voltage supply to the RTC and limits the amount of current drawn from the battery, improving backup reliability when running for an extended period.
The most common reason is that the total current consumption of the circuit exceeds expectations. Troubleshooting steps: Is the diode’s reverse leakage current excessive? Is the current-limiting resistor too large, causing the RTC chip to malfunction? Is there any flux residue on the PCBA board causing microampere-level leakage? It is recommended to measure the actual current consumption of each node in the circuit segment by segment. If you have difficulty troubleshooting yourself, you can send it to a professional PCBA manufacturer for failure analysis.
Three core aspects to consider: quality system certification (ISO 9001 is the minimum, IPC-A-610 Class 2/3 is a specific embodiment of assembly standards), genuine sourcing channels for key components (directly affecting product reliability), and testing capabilities (AOI full inspection is standard, BGA requires X-ray). Manufacturers with experience in RTC circuit production will understand the special requirements for manufacturing low leakage current circuits, which is a plus.
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