Supercaps for smart locker solutions

Supercaps make digital locking systems smarter

Centrally controlled electronic locking systems are ubiquitous in commercial buildings. Such systems are often integrated into a network that provides both the power supply for the lock mecha­nisms and the control signals. Many high-security installations require battery backup, or the ability for the lock to automatically enter a pre­defined state in the event of a power failure. These requirements pose special challenges for the power supply and its control. Supercap manu­facturers such as CAP-XX specialise in this.

Pulse current for the locking mechanism

Electronic lock mechanisms rely on an electro­me­chanical actuator - as a solenoid or geared motor. In most commercial buildings, locks are hard-wired to a 12- or 24-volt power supply. Likewise, most e-locks auto­matically assume a defined state (locked or unlocked) in the event of a power failure. If this process is not desirable in indi­vidual cases, areas of the system are con­trolled differently (open or closed). Battery buffering is often used so that the lock will still function in the event of a power failure.

Since space in the lock housing is limited, the choice of battery is a com­promise between energy and power. It must have the minimum energy required to operate the e-lock for a certain period of time, but also a high discharge current to provide the pulse current to drive the actuator. This factor limits the maximum energy density of the battery.

Supercapacitors, like conven­tional capa­citors, have physical charge storage. Their perfor­mance is not limited by the speed of a chemical reaction, as is the case with a battery. They can deliver 100 to 1000 times the power of a battery and have an equivalent series resistance (ESR) in the range of 10 to 100 mΩ.

Using super­capacitors to handle high peak loads can signifi­cantly reduce the load on the battery. They then only have to be designed for the average current and can thus be corres­pondingly smaller.

What makes supercaps in closing technology, for example from the manu­facturer CAP-XX, interesting in connection with a battery is their very low leakage current (IL). This is in the range of ~1 - 2µA/F. The supercapacitor of type DMF 470mF (45mΩ ESR), for example, has a typical IL of ~2µA. The leakage current is con­tinuously drawn from the battery and can there­fore represent a signi­ficant energy loss. With only 2µA IL, a DMF470mF draws only ~17mAh/year, less than the self-discharge rate of most batteries.

Current limiting to protect the battery

Due to its very low ESR, a discharged super­capacitor can draw a high inrush current, especially when first charging at the 0 V state. In some cases, the internal resistance of the battery may be sufficient to limit the inrush current to a safe level. However, if inrush current limiting is required to protect the battery, super­capacitor current limiting must be included. Nethertheless the average current supplied by the battery must always be strong enough to fully charge the super­capacitor during the minimum time between actuations.

Supercaps allow programmable fail-safe condition

All reputable electronic lock or door opener systems guarantee a defined fail-safe state after a power failure. This function is usually realised by a factory-installed mechanical spring that sets the safe state as open or locked. This means that the safe state of the lock cannot be programmed on site, which limits flexibility. Instead of the mechanical energy in a spring, the electrical energy stored in a super­capacitor can set the lock to the secure state in the event of a power failure. This can be programmed on site, e.g. with a link read by a microcontroller.

Correctly dimensioning the supercap for the locking system

Supercapacitors, which can deliver high power due to their low ESR, have a high C-value to deliver enough energy and power to operate the lock. The following factors should also be considered when selecting a super­capacitor:

1. How much energy is needed for the actuator to finish the action?
2. What is the peak and average power?
3. What is the initial voltage to which the supercapacitor is charged?
4. What is the minimum voltage required by the closing system?
5. The voltage drop due to the equivalent series resistance (ESR) for the supercapacitor=ILOADxESR.
   Many engineers choose C = 2E/(V2init - V2 final), where E is the energy required to operate the lock and Vinit and Vfinal are the initial and final volt­ages of the super­capacitor. This calculation implicitly assumes that the ESR of the super­capacitor = 0, which leads to an undersizing of the superca­pacitor.
6. The frequency response of the superca­pacitor - if the activation of the barrier is short (<~ 100 ms), the effective capa­citance is used for the pulse width of the activation. In this case, at constant current, the voltage drop = ILOAD x ESR + ILOADx PW/Ceff(PW), where Ceff (PW) is the effective capacitance for the pulse width PW.
7. How much space do you have?
   Many applications require a slim, unob­trusive and elegant form factor. CAP-XX's thin prismatic super­capacitors meet these require­ments. Where space is not at a premium, the more cost-effective cylindrical CAP-XX cells can be used.

Depending on the output voltage to which the supercapacitor is charged, CAPCOMP offers a single-cell super­capacitor or a two-cell module. If you are using a 3V battery, the cylindrical 3V cells (e.g. GY13R0 series) are suitable. Prismatic 3V cells will also be available soon. Another alter­native is to use a low-current LDO, such as a TPS78227, which draws 500nA to charge a single cell to 2.7V. For a higher supply voltage (e.g. 5V battery | e.g. 3.6V lithium thionyl chloride), a double cell super­capacitor must be used: e.g. DMF Low ESR High Power, DMT Long Life High Temp or DMH Ultra Thin. This requires cell balancing.

CAPCOMP supports you in the development of your supercapacitor circuit.

Please contact us!