Supercapacitors versus batteries

Super-capacitors are nothing new. General Electric was trying out their potential in the 1950’s, but the press today has suddenly become hot with stories that this technology will change the way we store power forever. There has even been speculation that batteries as we know them will never be the same again.

The excitement certainly does seem well deserved. They have the ability to recharge within seconds and unlike all

Sinautec supercapacitor powered electric bus
Supercapacitor powered electric buses have been used in China for nearly a decade

types of batteries which rely on internal chemical reactions and so wear out, super capacitors do not degrade over time. That means that a 2.7 volt super-capacitor today will be a 2.7 volt super-capacitor in 15 years time. All other current battery designs suffer gradual performance loss, meaning your 12 volt battery today might be an 11.4 volt battery in just 3 years time.

Perhaps the biggest attention grabber is that super-capacitors can be 3D printed, making them supremely versatile for any shape without needing to set up a production line. Causing equal excitement is their ultra-thin nature which means they could easily be integrated into clothing and other fabrics.

So should we be preparing the history books for batteries? Not quite yet.

Progress in portable power has not been a linear one. Technological advances have not moved battery performance forward on every level each time. That’s why while the lithium Ion battery that powers your phone came along in the 1990’s, the one that starts your car is most likely still lead acid and based on a design which is over 200 years old!

New usually only means better in some ways. Lithium Ion batteries are good at slowly discharging steady energy over long periods of time, but they are expensive. Lead acid batteries are good at producing large amounts of energy quickly and most importantly, they are cheap to manufacture.

The history of the battery is littered with technical breakthroughs, but at each point older chemistries survive and continue in use because while the all new concept grabs headlines it is never better in every way.

Super-capacitors are no different … at the moment. While they can charge fast, last far longer, hold more power and operate at temperature extremes most other chemistries simply couldn’t cope with, they are poor at providing constant power over long periods as the graph below shows.

Supercapacitor versus Battery charge and discharge characteristics
Super-capacitor versus Battery charge and discharge characteristics. Batteries maintain a constant voltage during discharge, super-capacitors don’t – graph by Tecate Group

In terms of power storage there is some common confusion. While a super-capacitor that is the same weight as a battery can hold more power, its Watts/kg – Power Density  is up to ten times better than Lithium Ion batteries. Its inability to discharge slowly means its Energy Density (Watt Hours/kg or Wh/kg) is a fraction of that offered by Lithium Ion.

Supercapacitor Power Density and Energy Density vs other storage forms
Supercapacitor Power Density and Energy Density vs other storage forms – Image: Tecate Group

They are also pretty bad at holding onto their charge, self discharging to half their capacity within 40 days when not in use isn’t the kind of characteristic you’ll want under the hood of your car or in your smoke alarm.

Finally a super-capacitor cell has a voltage of around 2.5 compared to lithium ion’s 3.6. You can start wiring them together, but the circuitry itself then becomes the cause of internal resistance that can reduce super-capacitor advantages.

In short there is still much that is left to be desired for anyone looking to completely replace all batteries with super-capacitors.

So why all the excitement?

Well just as the Lithium Ion battery made mobile phones possible, but did not replace car and truck batteries, the super-capacitor definitely has a role to play in portable power.

China is already using them in some hybrid buses since 2006. As the bus brakes to stop and take on passengers energy generated by the brakes is passed to super-capacitors. It is stored there while passengers board and then provides a ready source to help acceleration as the vehicle moves off.

This means the bus needs less Lithium Ion batteries (in some cases none at all), making it lighter and able to travel further on one charge. Dan Ye, executive director of Sinautec, a U.S. Chinese joint venture manufacturing super capacitor only buses, claims the vehicles can go 40% further than standard electric buses and it is 40% cheaper to manufacture.

But enthusiasm is cautious when it comes to cars. Buses stop and start all the time so there is a guaranteed regular source of energy moving from the brakes to the super capacitors. They also follow a regular route where backup charging stations can be placed should braking not charge the super-capacitors enough.

Joe Schindall is professor of electrical engineering and computer science at MIT. He notes these issues make super-capacitors “not well suited for electric-only cars”.

Super-capacitors in smartphones and laptops?

This is unlikely at the moment, because although the ability to recharge within seconds has many people drooling with anticipation super-capacitors don’t hold a steady voltage or capacity as they discharge. This is exactly what smartphones and laptops need to function over long periods and so it seems that Lithium Ion batteries won’t be toppled from their perch just yet.

When it comes to replacing other battery chemistries completely, the super capacitor isn’t going to do that just yet.

They look instead to join batteries in the portable power world and offer improvements in some areas, but nothing near the total replacement many headlines seem to imply.

The Final Showdown

In general super-capacitors are suited for applications that require fast charging and discharging capabilities where these times are measured in seconds or several minutes. For anything which requires power for longer, batteries remain the better solution.

Characteristic Supercapacitors Lithium Ion Batteries
W/kg (Specific Power)  5  up to 240
Wh/kg (Specific Energy)  up to 10,000  up to 3,000
Charge time (of a cell)  Seconds  minutes
Cell voltage  c. 2.5  3.6
Cycle Life  1 million+ up to 3,000
Operating temperature range Discharge:
–40 to 65°C (–40 to 149°F)
Recharge:
–40 to 65°C (–40 to 149°F)
Discharge:
–20 to 60°C (–4 to 140°F)
Recharge:
0 to 45°C (32°to 113°F)
Self Discharge  50% within a month  up to 3% per month

Further reading and sources:

 

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2 Comments

  1. JB

    So I am confused. you say that capacitors don’t lose their charge, then say they do…which is it?

    1. BatteryGuy

      I’m not sure where in the article it says they don’t lose their charge? They don’t wear out as fast as other battery chemistries but they do self discharge faster.

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