One of the great challenges facing auto makers when they’re designing hybrid and electric cars is how to store energy for their combustion-free engines.
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Lead acid batteries

Invented way back in 1886 and consisting of lead-based electrodes in a sulfuric acid solution, lead acid batteries were the first rechargeable batteries made by man. Being large and heavy, they’ve never been well suited to consumer devices, but given the tonne-plus combined heft of a car’s body, engine, transmission, seats and so forth, their 15kg or so weight is pretty insignificant.

Lead acid batteries are also able to give a big burst of current, which is perfect for jolting an engine into life. They have not only saved many an arm from the perils of manual cranking, but also power many safety features we now deem essential, such as headlights, tail-lights, on-board computers and safety systems too numerous to list out. Additionally lead acid batteries power the radios, satellite navigation systems and smartphones we’d find it hard to live without on the road.

Despite being old hat technology they’ve been used extensively in electric vehicles, although the vast majority are electric golf buggies. For these light weight, speed limited machines, the low cost of lead acid batteries make them a compelling choice.

Earlier road-going electric vehicles used lead acid batteries as well. 26 of them were employed to power the first iteration of GM’s much talked about, but ultimately ill-fated, EV1. Depending on who you believe, this was either a ploy to hobble the company’s half-billion-dollar investment or a prudent choice given the price and availability of more advanced battery technologies.

With lead acid batteries, the EV1’s range was optimistically claimed to be between 110 and 160 kilometres. Those without a hypermiler’s mindset usually eked out around 80km. A recharge required 15 hours from a standard American 110V power point; a GM 220V fast charger cut recharging time down to a mere three hours.

All up the EV1’s suite of batteries weighed around 530kg, meaning that the tiny aluminium-bodied two-seat car tipped the scales at around 1350kg; a contemporary Toyota Camry weighed just 1100kg.

NiMH battery pack for the third generation Toyota Prius

Nickel metal hydride

By the end of the EV1’s life, GM had updated it to utilise newer nickel metal hydride (NiMH) batteries. Not only did they improve the EV1’s claimed range to between 160 and 225 kilometres, they were also 35kg lighter and could be recharged in around eight hours.

While NiMH batteries have fewer toxic elements — lead and cadmium are thankfully not present — they do feature electrodes made from exotic nickel compounds utilising cerium, lanthanum, neodymium or praseodymium. These elements are often referred to as rare earths – not because they are rare (quite to the contrary, in fact) but because they're extremely dispersed throughout the Earth's crust and difficult to extract.

Aside from use in the GM EV1 and later Toyota RAV4 EV, NiMH batteries in automobiles have been largely limited to hybrids. Through careful computer management of the charging levels and working temperature of its NiMH batteries, Toyota has been able to exceed the eight-year warranty period for the vast majority of the battery packs installed in its hybrid cars.

Lithium-ion battery pack for the Chevrolet Volt


Thanks to Toyota’s current dominance of the hybrid scene and its preference for NiMH batteries, these batteries can be found in most of today's electrified cars. That may change soon, though, as competitors are gravitating towards lithium-ion batteries, because they are — you guessed it — capable of storing more charge while being lighter and requiring less space.

Part of the reason for this is that lithium is both light and highly volatile or reactive. This makes it great for batteries, but poses some safety challenges. Given their ubiquity in smartphones, laptops and tablets, fires and explosions have been few and far between. Those that do occur tend to be well publicised, such as the explosions leading to a recall of Sony laptop batteries and the fires aboard the troubled Boeing 787.

Lithium-ion batteries have to be produced to high standards as contaminants inside the battery can lead to short circuits, rapid overheating, and sometimes fires and explosions. As Tesla learnt after a few Model S fires last year, automotive lithium-ion batteries require plenty of shielding to prevent debris and other objects entering the battery and causing a fire.

Sealing has to be comprehensively tight as well because lithium has a fiery relationship with water. Whereas most other batteries use non-flammable water-based acids, this isn’t possible with lithium-ion batteries so lithium salts are often used instead. This creates headaches not only for automotive engineers, but also fire crews who need to be trained to deal with this new type of fire.

Despite many compromises for safety’s sake, the Nissan Leaf’s battery pack, at around 250kg, weighs significantly less than the GM EV1’s and is capable of delivering an official range of about 135km.

Capacitor for Mazda i-Eloop energy storage system

Capacitors, supercapacitors and ultracapacitors

Capacitors are everywhere in our everyday lives, although, unless you’re an engineer, they go about their work unseen and unheralded. For example, large capacitor phalanxes are used throughout the electricity grid to smooth out power delivery. Much smaller capacitors help radios tune into specific frequencies and filter out unwanted noise.

Aside from those important tasks, capacitors also store energy. While batteries do a similar thing by chemical means, capacitors store energy electrically in a field between two metal plates.

They differ in many other ways too. For example, batteries are able to store energy for long periods of time (think months) and can deliver a nice, long, steady flow of electricity. On the downside, continual charging and discharging, especially fast charges, wear a battery down, so they need to be carefully managed by their on-board computers.

By comparison capacitors typically work for years or decades with minimal performance degradation and can be charged quickly (in the order of seconds or minutes). On the flipside, capacitors are much larger and heavier than their battery equivalents, and don’t hold their charge for anywhere near as long. For instance, a smartphone where capacitors were used instead of batteries would either require recharging every 90 minutes or so, or be around eight centimetres thicker.

Capacitors also prefer to unload their energy in short, sharp bursts, which makes them perfect for devices like the flash on a paparazzo’s camera or, indeed, performance hybrids where a quick burst of energy can help with an overtaking manoeuvre. To this end they've been employed in Toyota's TS030 Le Mans racer.

In a more traditional fuel saving role, Mazda's i-ELOOP system stores energy captured via regenerative braking in an ultracapacitor. This capacitor then powers various electrical components in the car or recharges the lead-acid battery, reducing the time spent by the engine-driven alternator in performing these tasks.

Flywheel accumulator for the Porsche 918 RSR

Flywheel accumulator

An alternative to storing energy chemically or electrically is to do so mechanically. Several car makers and the odd Formula 1 team have toyed with just such an idea via flywheel accumulators.

Here energy from an internal combustion engine or regenerative braking system is used to rotate a free-spinning flywheel at speeds up to 60,000rpm. When the driver or the car’s computers determine that an extra dash of power is required, the stored energy is transmitted from the flywheel to the wheels, slowing the flywheel down. As with capacitors, flywheel accumulators are best suited for delivering a quick power boost.

As friction doesn't allow a flywheel to spin forever, flywheel accumulators aren't great at storing energy for extended periods of time. In order to lengthen the time it takes friction to stop the flywheel, it’s usually sealed in a vacuum and fitted with a system of fancy magnets. This vacuum seal isn’t perfect, though; in race conditions, where parts can be replaced frequently, this isn’t an issue, but in road-going vehicles it’s a headache that’s yet to be overcome.

So, despite being able to pack more punch into a smaller and lighter package than an equivalently powerful battery-based hybrid, flywheel accumulators have found little favour beyond racing. The Audi R18 e-tron has dominated Le Mans over the last few years with this technology, while the only production vehicle has been the Porsche 911 GT3 R Hybrid.

Compressed air hybrid Peugeot diagram

Compressed air

In 2013 Peugeot and Citroen displayed the C3 Hybrid Air concept car. Instead of mating a petrol engine with an electric motor and a battery pack, the Hybrid Air system pairs a petrol engine with a hydraulic motor/pump and a tank for compressed air.

Aside from the method of energy storage, the Hybrid Air system works much like a regular full hybrid car. There are three basic drive modes: compressed air only, petrol motor only, and combined operation. In normal driving under 70km/h the compressed air inside the car’s tanks power a hydraulic motor that’s able to solely power the front wheels, while during hard acceleration the two motors can work together. Air in the tank is pressurised either via either regenerative braking or by siphoning some power off from the petrol motor.

PSA Peugeot-Citroen claims that the Hybrid Air system will deliver fuel economy of around 2L/100km when used in a Peugeot 208 or Citroen C3-size car, which would place it between a plug-in hybrid and a full hybrid in terms of mileage. That said, a Hybrid Air car is unlikely to see production before 2016.