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Lead-acid and Li-ion battery benefits compared

Which is the better choice of battery for an electric forklift truck:
lead-acid or lithium-ion? The answer depends on your application and your priorities. Mark Nicholson summarises the pros and cons, and finds out why lead-acid is still the most popular choice.

While lithium-ion (Li-ion) batteries promise exciting new possibilities, lead-acid technology has also continued to develop and has plenty of innovations to offer. Leading materials handling equipment manufacturers like Cat® Lift Trucks study the available technologies and choose optimum solutions for users today. Li-ion may be specified more frequently in future Cat® products, but at present lead-acid is considered the best option in most cases.

The contenders

The choice is not just between Li-ion and lead-acid. Within each of those broad categories there are many different products to consider, with widely varying capabilities. A simplified summary follows.

Flooded lead-acid batteries

This is the traditional type which powers most electric lift trucks. Its lead plates, which act as electrodes, are suspended in a sulphuric acid electrolyte.

Sealed lead-acid batteries

These are sometimes known as valve-regulated lead-acid (VRLA) batteries. To avoid the need for topping up with water, and the risk of acid leaks, the electrolyte is securely contained. There are two main types:

• Gel batteries – with electrolyte forming a gel
• Absorbent glass mat (AGM) batteries – with electrolyte held in a fibreglass mesh between the plates

The AGM category of sealed batteries has seen many recent advances. Developments have included using purer lead for the plates, making the plates thinner or enhancing them with carbon or other substances. To save time, we will simply refer to these as ‘advanced AGM batteries’. Importantly, some of them can match or improve upon certain advantages previously considered unique to Li-ion.

Li-ion batteries

This battery type, benefiting from the high electrochemical potential of lithium, is also sealed. Its positive electrodes are made from lithium compounds such as lithium iron phosphate or lithium nickel manganese cobalt oxide. You will see these in the names and descriptions of Li-ion batteries. The specific compound chosen has a large influence on each battery’s cost, performance and lifespan. A key feature of all Li-ion batteries is their electronic battery management system (BMS). This is essential to prevent overheating, overloading, excessive discharge and other potentially damaging or dangerous conditions.

How they compare

Power and energy

Energy density, in watt-hours per kilogram or per litre, is a measure of how much energy a battery of a specific weight or size can hold. A battery with high energy density can hold a lot of energy and give a long runtime. Battery capacities can also be measured in amp-hours.

Power density, in watts per kilogram or per litre, is a measure of how rapidly the energy in a battery of a specific weight or size can be delivered. A battery with high power density can reliably drive and maintain the high currents needed for heavy duties.

Li-ion batteries have very high energy and power density, so they can store and deliver a lot of energy while taking up little space. Along with runtime and performance advantages, this may allow greater design freedom as the truck’s shape need not be so heavily influenced by that of the battery. For counterweighting, heavy materials may need to be added to compensate for the lighter battery.

With their bulky construction, traditional flooded lead-acid batteries have a relatively low energy density. However, they do have a high power density, enabling rapid supply of high current for heavy-duty applications. Gel and standard AGM batteries have less capacity, but there are advanced AGM products which are equal or superior to flooded lead-acid in storing and delivering energy.

“I don’t favour one technology type over another, but rather feel that each has applications it is better suited to than the other. However, I am a little concerned that Li-ion has been somewhat oversold by some people recently. It’s important to look beyond the hype and find the battery that’s right for you.”

Terry Kendrew, Managing Director of UK Cat lift truck distributor Impact Handling

Charging

Flooded lead-acid batteries need to be charged for several hours at a time, while gel and standard AGM types may take a little longer. For forklifts in continuous operation, this means changing the battery after each shift. Timing of recharging is critical for most lead-acid batteries. If they are recharged before discharging to about 20% of capacity, their lifespan will be shortened. Note that lead-acid battery performance can be improved by using intelligent chargers which avoid under- or overcharging, lower consumption, reduce gassing and prolong lifetime.

Li-ion batteries can be opportunity-charged at any time – during a lunch break, for instance – without losing performance or longevity. The same is true for some advanced AGM products – but note that in their case it is a convenient option rather than a necessity. With most Li-ion batteries, you absolutely must make sure those regular short recharges happen.

Opportunity charging can be delivered via strategically placed electric points, so no time is wasted driving to and from a charging room. If enough recharging time is allowed, there is no need for battery changes.

Maintenance

Maintenance of Li-ion and sealed lead-acid batteries is virtually zero, as they require no topping up with water. Flooded lead-acid batteries need to be checked and topped up regularly, although use of the right charger will lengthen the interval. Some battery products are specifically designed for longer intervals, while automatic top-up systems for use during charging are another option.

Maintaining back-up batteries is a key issue with flooded lead-acid cells, as they lose their charge quite quickly when not in use. They may need to be recharged every few months to avoid damagingly low charge levels and to ensure they are ready for action. A cool battery storage area is needed for this. The latest advanced AGM batteries can be stored for up to two years before recharging is needed. Charge storage life for Li-ion is even longer.

“ In a world where reducing carbon footprint has become a daily goal, recent developments in our electric forklift trucks offer a real alternative to customers. Nevertheless, good management of electrical energy is vital to the success of this energy transition. Opposing the use of either flooded lead-acid or lithium-ion batteries does not make sense. The key to success lies in the right combination of battery technologies. The right battery in the right application.”

Jean-Jacques Boulet, Marketing Manager of French Cat lift truck distributor Aprolis

Safety

Flooded lead-acid batteries are a potential source of acid spillage and of unhealthy acid aerosols in the air. They also produce explosive gases while charging, so a well-ventilated charging room and careful practices are vital. There are no such problems for Li-ion and sealed lead-acid batteries. Li-ion and advanced AGM technology also minimises the need for battery changes, with their associated risks, although investing in the latest rapid battery changing equipment can reduce those.

It is worth mentioning that Li-ion batteries can generate very high temperatures if their electronic controls fail. For safety, as well as damage avoidance, full discharge should be avoided, and no machine should be left unattended with a fully discharged Li-ion battery.

Lifespan

Figures for battery longevity – measured as the number of discharge and recharge cycles they can provide within their service life – vary greatly between products. In general, flooded lead-acid batteries live a bit longer than sealed lead-acid – but with advanced AGM products the difference is less. Li-ion batteries last a lot longer than others, although it seems like lifespan predictions for Li-ion are shortening. Difficult working conditions may also shorten their lives.

Recycling

Almost all materials in a lead-acid battery can be recycled – and sometimes even sold at a profit. Li-ion batteries are currently much more difficult to recycle. One possibility being explored is to give them a second life, in a less demanding application, when they are no longer fit for their original forklift role. There may be major issues when everyone is struggling to recycle hundreds or thousands of Li-ion forklift batteries.

Energy efficiency

Flooded lead-acid batteries need to be overcharged by 10 to 20 per cent to help maintain an even distribution of acid and reduce deposition of lead sulphate on their plates. Sealed lead-acid batteries require less overcharging, so their energy cost is lower. Li-ion batteries are even more efficient, as they have no such requirement.

Total cost of ownership (TCO)

Li-ion batteries have a much higher purchase price than others. You will also have to consider the cost of additional charging points, and you may need to invest in upgrading your electrical infrastructure to handle Li-ion chargers’ higher peak demands. Bear in mind, too, that a truck cannot simply be swapped from lead-acid to Li-ion: someone needs to pay for it to be adapted. The possible disposal cost for end-of-life Li-ion batteries is a further and as-yet unknown expense. Amongst lead-acid batteries, you will pay more for the most advanced products. Whether investing in these technologies makes economic sense depends on how much they will save you in the long term.

Li-ion offers a longer battery lifetime for your money, as well as energy efficiency savings. Li-ion batteries – and advanced AGM products which can be opportunity-charged – save on forklift downtime and the cost of buying and maintaining reserve battery supplies. Li-ion and sealed lead-acid batteries free up valuable space – as there is no need for a special ventilated charging room – while saving on maintenance time. At the moment, there are relatively few cases in which these savings justify the higher purchase cost of Li-ion batteries, but this could change as prices fall.