Design Article
Supercaps can be a good choice over batteries for backup applications
Steve Knoth, Linear Technology Corporation
6/5/2012 8:08 AM EDT
Background
The niche that supercapacitors (also known as ultra-capacitors or supercaps) have served in the market space between conventional capacitors and batteries continues to grow as more new applications are found. Supercaps are replacing batteries in data storage applications, requiring medium-to-high current/short duration backup power and battery backup due to sudden disconnect. Specific applications include 3.3V memory backup solid state drives (SSDs), battery-powered industrial and medical portable equipment, industrial alarms and smart power meters.
Compared to batteries, supercaps provide better power density with higher peak power delivery capability, smaller form factors, higher charge cycle life over a wider operating temperature range, and lower ESR. Compared to standard ceramic, tantalum or electrolytic capacitors, supercaps offer higher energy density in a similar form factor and weight. A supercap’s lifetime is maximized by reducing the capacitor’s top-off voltage and avoiding high temperatures (>50°C). See the Table 1 below for a comparison of key features.
Table 1: Supercapacitors vs. capacitors vs. batteries
The niche that supercapacitors (also known as ultra-capacitors or supercaps) have served in the market space between conventional capacitors and batteries continues to grow as more new applications are found. Supercaps are replacing batteries in data storage applications, requiring medium-to-high current/short duration backup power and battery backup due to sudden disconnect. Specific applications include 3.3V memory backup solid state drives (SSDs), battery-powered industrial and medical portable equipment, industrial alarms and smart power meters.
Compared to batteries, supercaps provide better power density with higher peak power delivery capability, smaller form factors, higher charge cycle life over a wider operating temperature range, and lower ESR. Compared to standard ceramic, tantalum or electrolytic capacitors, supercaps offer higher energy density in a similar form factor and weight. A supercap’s lifetime is maximized by reducing the capacitor’s top-off voltage and avoiding high temperatures (>50°C). See the Table 1 below for a comparison of key features.
Table 1: Supercapacitors vs. capacitors vs. batteries
Summary- Supercaps vs. batteries:
Batteries:
• Good energy density
• Moderate power density
• High equivalent series resistance (ESR) at cold temperatures
Supercaps:
• Moderate energy density
• Good power density
• Low ESR – even at cold temperatures ( ~2x increase at -20°C vs. 25°C)
Supercap limitations:
• Limited to 2.5V or 2.75V maximum per cell
• Must compensate for leakage differences in stacked applications
• Lifetime degrades quicker at high charge voltages and high temperatures
Early generation 2-cell supercap chargers were designed for low current charging from 3.3V, 3xAA, or a Li-Ion/Polymer battery. However, supercap technology improvements have expanded the market, resulting in a slew of medium to higher current opportunities not necessarily confined to the consumer product space. Primary applications include solid state disk drives and mass storage backup systems, high current portable electronic devices such as industrial PDAs and handy terminals, data loggers, instruments, medical equipment, and miscellaneous “dying gasp” industrial applications such as security devices and alarm systems. Other consumer applications include those with high power bursts including LED flash in cameras, PCMCIA card and GPRS/GSM transceivers, and Hard Disk Drives (HDDs) in portable devices.
Batteries:
• Good energy density
• Moderate power density
• High equivalent series resistance (ESR) at cold temperatures
Supercaps:
• Moderate energy density
• Good power density
• Low ESR – even at cold temperatures ( ~2x increase at -20°C vs. 25°C)
Supercap limitations:
• Limited to 2.5V or 2.75V maximum per cell
• Must compensate for leakage differences in stacked applications
• Lifetime degrades quicker at high charge voltages and high temperatures
Early generation 2-cell supercap chargers were designed for low current charging from 3.3V, 3xAA, or a Li-Ion/Polymer battery. However, supercap technology improvements have expanded the market, resulting in a slew of medium to higher current opportunities not necessarily confined to the consumer product space. Primary applications include solid state disk drives and mass storage backup systems, high current portable electronic devices such as industrial PDAs and handy terminals, data loggers, instruments, medical equipment, and miscellaneous “dying gasp” industrial applications such as security devices and alarm systems. Other consumer applications include those with high power bursts including LED flash in cameras, PCMCIA card and GPRS/GSM transceivers, and Hard Disk Drives (HDDs) in portable devices.
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SUPERCAPACITOR
6/6/2012 9:46 AM EDT
1)Regarding the supercap limitation of 2,5V and 2,75V maximum per cell mentioned in the article.
Vina Technology (korea) www.vina.co.kr launched a supercap with 3V from 3F to 600F.
2)Also a chinese company has a patent and is in the market a hybrid unit: Supercap Lithium battery (3,2V from 10Ah to 100Ah) can be check at http://www.cyliyuan.com/en/product3.asp
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William Miller
1/30/2013 10:52 AM EST
Every generation of capacitors naturally is better than the previous one. That's called evolution or progress. Using these supercapacitors in SSD will improve their characteristics significantly. I use SSD in my PC and I can't wait to replace it for even faster one.
_______________
William - http://www.carid.com/
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I_B_GREEN
6/8/2012 12:49 PM EDT
And how do they compare on,leakage current?
not a good choice for ultra low power sleep wake systems.
Leakage a function of capacity, so trying to replace batteries for long duration backup not a good choice from efficiency standpoint.
excellent choice for agmenting a battery for high pulse low duration help.
Can decrease battery capacity needed.
cross over of battery and super cap leakage rates needs to be graphed and desiminated.
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prabhakar_deosthali
6/10/2012 7:34 AM EDT
If Super caps can be combined with batteries in EVs , they may assist in quick energy recovery during braking and also help in quicker acceleration , in my opinion
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hm
6/10/2012 8:41 PM EDT
@I_B_Green: Yes, leakage current is parameter of concern for replacing battery in RTC application. Many application requires one year or more retention duration. Supercap leakage current can work only up to few weeks.
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DadOf3TeenieBoppers
6/21/2012 9:35 AM EDT
There are now 5000F capacitors out there with sub-milliohm ESR (IOXUS makes them). They are rated for 2.7V. Put a 2.4V zener in parallel across them to keep them from overcharging, and they can be ganged as far as the eye can see. It may take a while to trickle charge them (it will take a day and a half for a 100mA charger to do so; the current limit is to keep a 500mW 2% 2.4V zener from overheating if it carries all the current in a confined space), but they would make an ideal short term power storage.
For any facility that cannot have power shut down, they will keep power up long enough for backup generators to turn on and come on line.
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Abrahim
6/28/2012 2:58 AM EDT
the best use of SupCaps is to reduce the size of the battery which is often governed by the peak current requirements of a circuit. rather than using supcaps as a replacement for batteries (which is not advisable unless the outage is short) it should be used to supplement on certain parameters like peak/burst outputs of power.
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