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Technical Note
Cylindrical supercapacitors for RTC backup power by Chris Likely, VINATech EMEA Business Development Manager
The Real-Time Clock (RTC) function is an essential part of many products from electricity meters, industrial control & automation to intelligent white goods, as they all need an accurate clock. The question is, how do you maintain the integrity of the clock when the power fails?
Traditionally the go to solutions have been 3V primary Lithium button cell batteries or 5.5V rated coin cell supercapacitors, often referred to as Goldcaps after the original Panasonic brand name. The Supercapacitor solution offered advantages in terms of environmental safety, operating life and temperature range making them ideal for high reliability industrial applications. However, to overcome the limited energy storage capability compared with the button cell batteries the operating voltage had to be increased from 3V to 5V. This change means that, today, almost all RTC ICs can operate from 5V down to 1V or less.
Fast forward 40 plus years from the launch of the original coin cell Supercapacitors and the market dynamics are changing. Panasonic have ceased production of their Goldcap, demand is greater than supply and prices are rising. Technology has also moved on, cylindrical style supercapacitors are now available with a 3V rating and low leakage current making them an ideal, lower cost, alternative to the traditional coin cell option.
In this technical report we at look at VINATech’s WEC series of electrochemical double layer capacitors (EDLCs) and VEL series of hybrid Lithium capacitors as alternatives to coin cell supercapacitors in RTC applications.
The EDLC option offers a slightly higher energy density than the coin cell alternative with a rated voltage of 3V at 65°C (derating to 2.8V @ 70°C). It not only provides reliable, long life, operation at elevated ambient temperatures it also has excellent high humidity performance. As shown in table 1 below, in addition to is its long operating life, the EDLC solution also offers the lowest cost.
Comparing the EDLC solution with the hybrid Lithium capacitor it is seen that the hybrid capacitor, with its 3.8V @ 70°C rating (derating to 3.5V @ 85°C), exhibits an energy density over 10 times greater. It also offers long operating life, small size, and ultra-low leakage current leading to a very long hold-up time. The main disadvantages of the hybrid capacitor technology being higher overall cost and a minimum operating voltage of 2.5V, which can add to the circuit complexity if a load disconnect is required to ensure the minimum voltage is always maintained.
Both the WEC series and VEL series cylindrical supercapacitors have a smaller PCB footprint than equivalent coin cell products. They are also available in ammo-pack for auto-insert assembly and can be supplied with the lead wires preformed for horizontal mount.
Table 1. Comparison of different technologies
Hold-up time Calculation
To calculate hold-up time, the following formula is used
t = C x (V1-V2) / (Iload + Ileakage)
Where, t is the hold-up time, C is capacitance, V1 is the charged voltage, V2 is the RTC minimum operating voltage, Iload is the load current and Ileakge is the supercapacitor leakage current.
For example, for WEC3R0105QG, V1 is 2.8V, V2 is 1V, assuming load current and self-discharge as 1 micro-Ampere each, the backup time is given by:
T = 1 x (2.8 – 1)/(1µA+1µA),
From which a backup time of 10.41 days is obtained.
Table 2 below shows the hold-up time for 0.22F, 1F, 70°C and 1F, 85°C rated coin cell products. In addition to the hold-up time the table also compares the relative cost, size & weight of each part. These results can be contrasted with those from table 3, this shows the same information for WEC series 1F & 3.3F EDLC products and a 10F, 3.8V rated hybrid Lithium capacitor.
The results from tables 2 and 3 show that the WEC3R0105GQ (1F, 3V) part offers ~20% greater hold-up time than the 0.22F coin cell at 70% of the price, and by operating at a nominal voltage of 2.8V the same 70°C temperature rating is achieved. Comparing the WEC3R0335QG (3.3F, 3V) part with the 1F, 70°C coin cell and the calculated hold-up time for the cylindrical part is only 55% of that offered by the coin cell, all be it at ~60% of the price. At this point it would appear to make sense to try a higher capacitance cylindrical part to try and match the 1F coin cell performance, but as leakage current is proportional to capacitance (at ~1µA/Farad, as a rule of thumb) you get a diminishing return from doing this.
Table 2. Coin Cell Supercapacitor specifications & performance
Characteristics | 0.22F Coin Cell | 1F Coin Cell | 1F Coin Cell |
Capacitance (F) | 0.22 | 1.0 | 1.0 |
Leakage Current (mA) | <0.2 | <0.5 | <0.7 |
Size | 13.5mm dia. x 6.5mm | 21.5mm dia. x 7mm | 21.5mm dia. x 9mm |
Weight (g) | 3.3 | 9.1 | 10.4 |
Temp, °C | -25 to +70 | -25 to +70 | -40 to +85 |
Relative Cost | 1 | 1.7 | 3.0 |
Vmax | 5.5 | 5.5 | 5.5 |
VWorking | 5 | 5 | 5 |
Vmin | 1 | 1 | 1 |
Back-up time (days)* | 8.48 | 30.86 | 27.23 |
* Calculated value assuming 1µA load current
The calculated hold-up time for a 5F, 3V cylindrical device would be 17.3 days, so only a small improvement over the 3.3F device. This being because over 80% of the load is derived from the capacitor leakage current. In practice hold-up times of more than 3 weeks are achievable with the 3.3F cylindrical part but this still doesn’t compare with the 1 month that the 1F coin cell device offers.
Table 3. Cylindrical Supercapacitor specifications & performance
Characteristics | WEC3R0105QG | WEC3R0335QG | VEL Series Hybrid Lithium Capacitor3 |
Capacitance (F) | 1 | 3.3 | 10 |
Leakage Current (mA) | ~1 | ~3 | <0.5 |
Size | 8mm dia. x 13mm | 8mm dia. x 20mm | 8mm dia. x 13mm |
Weight (g) | 1.1 | 1.5 | 1.4 |
Temp, °C | -40 to +65 | -40 to +65 | -25 to +70 |
Relative Cost | 0.7 | 1 | 2.6 |
Vmax | 3 | 3 | 3.8 |
VWorking | 2.81 | 2.81 | 3.52 |
Vmin | 1 | 1 | 2.5 |
Back-up time (days)* | 10.42 | 17.19 | 77.16 |
* Calculated value assuming 1µA load current
1 – Rated voltage for 70°C operation
2 – Rated voltage for 85°C operation
3 – Currently available form 30F – 250F, 10F version in development.
Although the VINATech WEC series can be used at 85°C and 85% relative humidity for hundreds of hours at 3V, and reliably for longer durations with voltage derating to 2.4 – 2.5V, the better option for an 85°C rated device that provides long term hold-up is the VEL series of hybrid Lithium capacitors. Using a 10F, 3.8V hybrid Lithium capacitor at a charge voltage of 3.5V allows for 85°C operation, and as can be seen from table 3, above, this provides a hold-up time of 77 days for a 1µA load current. This is almost 3 times the hold-up provided by the 1F, 85°C rated coin cell solution at ~85% of the price.
Conclusion
Cylindrical style EDLC and hybrid Lithium capacitors offer a viable alternative to coin cell supercapacitors in RTC applications, offering advantages in terms of improved high temperature & humidity performance, longer operating life and lower cost.
Limitations on the available hold-up time provided by EDLC solutions, due to their relatively high leakage current, is an area that VINATech is working to improve. Initial results suggest that products with leakage current levels as low as ~0.5µA/Farad could soon be available, allowing in excess of 30 days hold-up times. In addition to this smaller size hybrid Lithium capacitors will offer more price competitive solutions for applications requiring RTC back-up for significantly more than 30 days as well as supporting 85°C operation, making them an ideal alternative to both Lithium button cell batteries and coin cell supercapacitors.
