Optimising battery designs with supercapacitors

By Tim Parker   |   April 30, 2018

The role of the battery in product design has become increasingly important. In fact, you could say it has provoked a change in design methodology, albeit with a bit of help from legislative bodies. In the past, size, cost and performance were the three main metrics that had to be balanced in every design. Now, there is an increasing demand for all three to be optimised and, in addition, for the device to be as energy efficient as possible.

In some cases, energy efficiency, in the form of battery life, is the most important overall factor in a design, such as in the case of mobile phones. But, portable mobile devices are not the only technology that relies on battery power. They are used heavily in many other areas, including electric vehicles (EV), power generation and distribution, and power back-up systems such as uninterruptible power supplies (UPS).

In particular, transportation relies heavily on batteries, and this dependence is set to become more important in the future as EVs become more prevalent. EVs are important in the context of climate change. Transportation accounts for 23% of the current global energy-related greenhouse gas emissions and is growing faster than any other energy end-use sector according to the Lima-Paris Action Agenda.

Although the great majority of vehicle journeys are short, many drivers prefer to have the assurance that a longer range brings. Battery anxiety is just as much of a problem for EVs as it is for mobile phones. The main ways of extending EV range are building better batteries, making the vehicle lighter and producing a more efficient electrical drivetrain. Engineers are working hard to solve all three of these issues, but there may be an easy way to make the vehicles lighter, without affecting the performance.

In EVs, the battery is not a single entity. In fact, in the Tesla Model S, the battery is made up of 7,104, 18650-sized, Li-ion battery cells. The cells are connected, both in series to give higher voltages, and parallel to provide power. There are 16 modules in total in the pack: each module contains six groups of 74 cells wired in parallel. The six groups are then wired in series within the module. The battery pack itself weighs 540 kg. Lightening the battery load would give the vehicle range a boost. However, removing batteries would either lose capacity or torque, unless there was a way to replace the missing capacity.

Often in EVs, the power allocation is higher than it needs to be to cope with times of high power demand. If there was another way to provide that excess power, then the battery could be specified to a figure that was closer to normal operation, either reducing the number of individual number of batteries and making the EV lighter, or using the batteries to provide extra capacity for the EV. Either way, the range of the EV would be extended. Supercapacitors are a relatively new type of component that could offer a reduction on weight without affecting performance.

Supercapacitors are components that have a much higher capacitance than normal capacitors, and can therefore hold a much higher charge than electrolytic capacitors – up to 100 times more energy by mass. Electrically, they offer attributes of both electrolytic capacitors, and rechargeable batteries. If they are used beside the normal EV battery, it would be possible to charge them between times of high power demand, and discharge them when the power is required, such as when the vehicle is pulling away from a stop sign. Supercapacitors are superior to batteries in charge and discharge rate, and they have also a higher charge/discharge cycle, making them ideal for the role. They are also more environmentally friendly than batteries, using less harmful chemicals in manufacture and operation, and therefore don’t need the same disposal precautions at the end of their operational lives.

Removing the battery from high stress situations also improves the safety of the system, and extends the operational lifetime of the battery. Safety is one area that cannot be neglected when discussing batteries. Battery failure can be catastrophic for a product; the Samsung Galaxy Note 7 is a good example. EVs are not exempt from this danger; a Smart for Two EV recently caught fire while charging in Essex, UK, destroying the vehicle and damaging property.

When specifying components for EVs, there are no shortcuts. Overcharging, especially, can be a major risk that can cause thermal runaway and lead to fires. Only the supplied battery charger for the type of battery should be used.

Battery temperature should be monitored constantly by sensors, such as TE Connectivity’s Platinum range (left), to ensure cooling systems are operating effectively. These sensors give high precision over a very wide temperature range with near linear output, In the event of a crash, safety circuits need to be employed to isolate the batteries, just as they are used to deploy airbags.

Cabling and connectors also matter when dealing with high power circuits. Connectors should be rugged, keep out ingress from dust and water and be non-flammable. For example, ITT’s Trident series is specified up to IP67, VDE certified, flame retardant and can handle up to 500 volts and 40 amps. Trident connectors also have centring pins to ensure correct alignment.

Supercapacitors can also provide the high power role in applications other than EVs. They can work beside Li-ion or lead-acid batteries in vehicles like forklifts, allowing the batteries to operate the vehicle during drive phases and providing the extra power required when the forks are in operation. Again, this allows either a longer operational period, or smaller batteries. Smaller batteries could be preferable in this application, especially when using lead-acid batteries, as there is less electrolyte to vent, and less danger of explosion.

For an application like an EV or forklift, Eaton’s XLR supercapacitor storage module (left) would be ideal. The high reliability modules consist of 18 supercapacitor cells that can operate over a wide temperature range (-40°C to 65°C). The module weighs only 14.7kg, and can be incorporated into systems requiring up to 750 volts. The modules are intended for systems requiring high current charge and discharge cycles in demanding environments.

AVX also has supercapacitor modules that can be used for EVs, forklifts and other demanding applications. The company’s 48V SCM series supercapacitor modules (bottom left) offer excellent pulse power handling characteristics through a combination of very high capacitance and low ESR. They can be used alone or in partnership with primary or secondary batteries. They provide extended back-up time and instantaneous power pulses as required.

Transportation is only one use for supercapacitors: other battery-driven applications, such as UPS, can use supercapacitors to supplant or enhance batteries. Uninterruptable power sources are used in the case of a primary power source failing. The energy stored in the UPS can be discharged almost instantaneously to keep the power flowing to the load until the secondary power source kicks in. This power is needed normally for just a few seconds or minutes. If there is no secondary source, the UPS can shut down the protected equipment safely, protecting it from damage.

Traditionally UPS have used lead-acid batteries, but because supercapacitors can discharge much more quickly, the risks to equipment from any breaks in power are minimised. They also charge back up much quicker than batteries after an outage. The lifespan of the supercapacitor can be almost infinite, while batteries have a service life of a maximum of ten years. They are smaller, safer more environmentally friendly and need less external components to operate.

Eaton also has a supercapacitor module targeted at UPS applications. The XLM module holds 69-watt hours of energy and can discharge 7,800 amps to enable safe shutdown of systems, or smooth voltage sags, spikes, and dropouts.

Summary

Supercapacitors have the ability to assist batteries in heavy duty applications by providing the extra power required at high demand times. They can charge and discharge energy much quicker than batteries, and are greener, smaller and lighter than the batteries that would be required to provide that occasionally needed power. In some applications, such as UPS, it may also be possible for the supercapacitor to replace batteries completely with no real downside.

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Tim Parker

With over 20 years' experience in the electronics industry, working for Micromark and then Avnet, Ti...

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