If the actual hold-up time required is between 15 and 150 seconds, electrochemical double-layer capacitors can compete in terms of price.


• • • UPS & STANDBY POWER • • • Temperature


Lead batteries are sensitive to temperatures that are too low or too high. During charging, the hydrogen can freeze at temperatures below -10°C, which is harmful to the battery. For such cold applications, pure lead batteries can be used instead. Since lead batteries leak hydrogen in temperatures above +45°C, it is recommended that lead batteries be mounted outside of the control cabinet.


Hold-up time depending on the buffer current of a battery


Fundamental differences in terms of lifetime and resistance to temperatures Lifetime


In ideal conditions and at an ambient temperature of 20°C, lead-acid batteries often have a long but limited life. According to Eurobat, a lead-acid battery has a lifetime of 6-9 years. A few but relevant factors can affect the lifetime of a lead- acid battery. Pre-aging, which occurs during storage, must also be taken into account. Temperature, however, has the greatest impact on reducing the lifetime of lead-acid batteries. For example, an increase of 10°C can halve the specified lifetime value (see graph 3).


Electrochemical double-layer capacitors are resistant to cold temperatures and can withstand temperatures as low as -40°C. This makes these components ideal for outdoor, mobile or solar and wind power applications. Electrochemical double- layer capacitors provide full performance even up to +60°C, which makes them suitable for use inside a closed and unventilated control cabinet. Unlike lead-acid batteries, capacitors do not emit hydrogen and there is no need to vent the switch cabinet, as required by EN 50272-2 for lead- acid batteries.


Lifetime expectancy of capacitors depending on the temperature


How to calculate the energy content of batteries


and capacitors The nominal capacity of a battery is given in Ah (ampere-hours). Together with the voltage, this gives us the approximate energy content:


Energy (Wh) = Voltage (V) * Capacity (Ah) Note: different measurement units are used - kWs or kJ:


energy densities. However, it is equally important to consider the actual hold-up time required: While UPS modules with capacitor storage buffer in the range of seconds (see graph 1), UPS modules with batteries guarantee an uninterruptible power supply of up to several hours (see graph 2).


Lifetime expectancy of a battery depending on the temperature


Capacitors are permanently built into the device and cannot be replaced. However, if the design is correct they will have the same lifetime as the power supply itself. Typically, the lifetime of a power supply is more than 10 years in most applications.


Therefore, electrochemical double-layer capacitors are generally maintenance-free. Electrochemical double-layer capacitors have a nearly unlimited storage time and they do not need to be recharged regularly. Additionally, putting a system into operation later than anticipated or not using it for a longer period of time does not lead to any unwanted surprises.


Hold-up time depending on the buffer current of a capacitor


electricalengineeringmagazine.co.uk


Similar to the batteries, the temperature also has an influence on the lifetime of capacitors: At an increase of 10°C, the lifetime of the capacitors is halved (see graph 4) because the warmer it is, the faster the electrolytic capacitors will dry out.


The capacitor voltage decreases continuously during discharge. Specifying the energy in Ah (ampere hours), as is normal for batteries, makes no sense, so the energy is specified in Wh (watt hours) or in kWs (kilo watt seconds). Often, one also finds the indication kJ (kilo Joule). 1 kJ corresponds to exactly 1 kWs. A value of 1kWs means that 1kW of energy can be extracted for one second or 100W corresponding to 10 seconds. The energy in a capacitor is calculated by the following formula:


Energy (Ws) = ½ CU2


This describes the energy when the capacitor is completely discharged to zero volts. However, in practice, converters cannot operate to zero volts, so it is not possible to use up the full energy. Caution should therefore be taken here. Buffer modules are classified by rated energy, not usable energy. When determining the hold-up time, always use the data sheet values or diagrams and not the simple approach shown above.


ELECTRICAL ENGINEERING • MARCH 2023 17


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