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Jun 10, 2011 NEWSwire
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Lith-ion, Supercapacitors to Replace Lead Acid Batteries

Dr. Peter Harrop examines the reasons why lead acid batteries will be surplanted by more promising energy storage systems.

Published: 10-Jun-2011

It is like waiting for a bus. You wait and wait then two come at the same time. Alternatives to short lived, toxic lead acid traction batteries, with their indifferent performance, have been sought for over 100 years. Now both lithium-ion batteries and supercapacitors are near to replacing lead acid traction batteries in the many traction applications where they are still used.

Popularity of lead acid

Lead acid traction batteries are the favourites for light commercial vehicles, golf cars, surface boats, material handling vehicles such as forklifts, airport ground support equipment GSE, e-bikes and mobility vehicles for the disabled. Better alternatives have been sought for twenty years and now, like waiting for a bus, two come along together. They are lithium-ion batteries and supercapacitors ie ultracapacitors.

Various benefits

Actually, these vehicles do not form a coherent group when it comes to their needs for traction energy storage. It is true that they are all tolerant of limited range. People still buy them even when their range is only 5-20 miles. However, the weight of the lead acid battery in a golf car, surface boat, GSE and large mobility vehicle keeps the centre of gravity low so they do not fall over: with land vehicles the weight can also assist grip on the road.

Where weight is a pain

Contrast e-bikes and small mobility vehicles that can be lifted into a car or into the home: here the weight is literally a pain. Most e-bikes are bought by people that live in apartments and they want to carry the bike or at least the battery up to the apartment at night to charge it and avoid theft. Most e-bike designs mount the battery high on a pannier or amidships where weight actually makes it top heavy. One bright spark at a recent British Electric Bike Association meeting in the Houses of Parliament referred to "A brick on a butterfly".

Impediments of lead acid

The disparate reasons for wanting to get rid of lead acid traction batteries include the nuisance of having three sets for a forklift – one in use, one charging and one cooling down. Then there is the handling of acid, the short life, the lead pollutant – even though recycling is very thorough – and the poor energy and power density which makes them big, grabbing space in the vehicle that is wanted for other things. Different vehicles exhibit these problems to a different extent. For example, a mobility aid for the disabled can cause a serious crisis if it is itself disabled by running out of electricity through failure or discharge.

Bikes need on-board chargers

E-bikes include bicycles converted to electric power, partially or wholly, and e-scooters where the driver's feet are on a platform, with no pedals. Here something smaller than the lead acid battery frees up space and weight for the convenience of an onboard charger – something already taken for granted in all mobility vehicles. An on-board charger reduces range anxiety because you are much more likely to be able to charge en route and on arrival if all you need is a domestic socket. On the other hand, GSE is intensively used these days, being shared between airlines in many cases, and extra power, for instance for pulling a wider range of aircraft and extra range expressed as less frequent charging ie downtime, are of interest. Reliability also matters more and more.

Tolerating frequent stop start

Then again, many commercial vehicles need to tolerate a high rate of stopping and starting. From now on, most conventional cars and commercial vehicles will automaticsally switch off the engine when they stop for any reason. Some of these so-called micro hybrid vehicles are genuinely electric vehicles in that the regenerated electricity briefly powers the wheels not just the more onerous starter battery function. Here the lead acid battery is often used but it struggles with low temperatures and tough duty cycles.

Tolerating high currents

Fast charging stations must connect to more tolerant batteries or the battery is damaged or destroyed by the thunderbolt. The new energy harvesting shock absorbers being trialled in commercial and military vehicles can also suddenly pump many kilowatts into batteries and lead acid usually cannot cope. The move to the lower cost, more reliable AC traction motors and therefore regenerative braking without extra equipment hammers batteries too. Here come the drop in replacements.

Drop in lead acid replacements sometimes

Physically, it will be possible to have drop in replacement lithium-ion batteries for mobility vehicles but they will then be unstable in most cases and a broader wheelbase or other stabilisation may be needed. The exceptions will be electric wheelchairs and those very lightweight folding or dismantling 3 and 4 wheel mobility scooters that fit in even small cars. The others will need redesign, say with a wider wheelbase or steel or, ironically, lead weighting. After all, the obese are often perched high on these mobile seats.

Rapid adoption of lithium-ion for e-bikes outside China Contrast bikes where that BEBA meeting showed about twenty electric bikes, all with customised lithium-ion batteries. This illustrates how, outside the main market China where price and reliability are dominant criteria, the lithium option is rapidly taking 100% of the business.

Standardised drop in replacement

Indeed standardised lead acid replacements have arrived. K2 Energy Solutions (K2), a Nevada technology company, manufacturing and selling rechargeable battery cells, packs and systems, has introduced an additional battery that broadens its market-leading lithium ion and lithium iron phosphate (LFP) product line.

Known as K2B24V10EB, K2's latest LFP rechargeable battery is rated at 24 volts 9.6 Ah, making it a replacement for buyers of lead-acid seeking a lighter, stronger, more robust solution allowing longer run times. It has been developed for electric wheelchairs, mobility assistance devices and other applications currently served by lead acid, usually in a standardised motorcycle or car type format, golf cars being another example. It weighs 5.5 pounds (2.5kg) and the sales pitch points out that it does not contain expensive and hazardous heavy metals or dangerous chemicals typical of other chemistries, making it a more environmentally benign battery. Results show that, when used properly, the batteries charge faster and last three to five times longer than conventional batteries, and hold their charge even after being in storage for long periods of time.

Supercapacitors

If the burgeoning lithium-ion scene is not enough for the lead acid people to worry about they should consider supercapacitors, referred to as ultracapacitors by those using them across electric batteries in buses. Supercapacitors help with fast charge and discharge of lithium-ion batteries in particular but they have not stored enough energy per unit of volume or weight, or even stored it long enough, to impinge on traction batteries themselves. The only exception has been companies such as Sinautec embedding charging coils every few miles along a bus route so the bus can run solely on the more reliable, longer lived supercapacitors. That has never become mainstream, not least because of the hideous cost of digging up roads.

However, supercapacitors have been rapidly improving. They tend to have no toxic materials or elements subject to price hikes such as the cobalt in most lithium-ion traction batteries used today and double the life of lithium-ion batteries and treble the life of lead acid batteries is typical. Their previously rapid self discharge and poor energy density have been improving. Many new applications open up every step of the way such as mobile phone cameras with long distance flash.

Now lead acid traction batteries are in the cross hairs

Researchers at The University of Texas at Austin's Cockrell School of Engineering have synthesized a new supercapacitor carbon with a continuous three-dimensional network of highly curved, atom-thick walls that form primarily 0.6-5 nm width pores. Supercapacitor cells constructed with this material yielded high values of gravimetric capacitance and energy density with organic and ionic liquid electrolytes.

This energy density is approaching the energy density of lead-acid batteries, while retaining the high power density characteristic of supercapacitors. The processes are readily scalable to industrial levels, says Professor Rodney S. Ruoff, materials science and mechanical engineering.

Comprehensive update

The latest on supercapacitors, lithium-ion and lead acid batteries in use in all forms of electric vehicles is intensively covered at the forthcoming Electric Vehicles Land Sea Air" in Stuttgart Germany June 28–29. There are even presentations by the leader developing supercabatteries, otherwise known as asymmetric electrochemical double layer capacitors which combine the features of lithium-ion and supercapacitors and a by large number of manufacturers of land water and air vehicles that variously employ lead acid and lithium-ion batteries and various forms of fuel cell for traction power. The emphasis is on what comes next and the scope of the event is global.

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