Lead-acid batteries have existed for more than 100 years, but they should not be dismissed as outdated technology. Modern advancements combine their proven benefits with features that are increasingly demanded for deployment in intralogistics: high power density, longer operating times, flexible charging, zero maintenance and cost effectiveness.
Anssi Laitinen, Senior Director Marketing EMEA, EnerSys Motive Power.
The number of electrically powered vehicles is increasing in the logistics sector. Easier handling, higher flexibility for outdoor and indoor deployment and lower maintenance costs are key selling points. Lithiumion batteries are commonly regarded as the technology of the future for onboard power in industrial trucks. They are assumed to have significant advantages over conventional leadacid batteries.
The main reason given for choosing lithium-ion batteries is high energy density, which means the ability to store a lot of energy in a small space. Another reason is they require virtually zero maintenance, since lithium-ion batteries (unlike their lead-acid counterparts) do not require refilling with water. A further major plus point is flexible charging capability: lithium-ion batteries can be recharged at any time. This does not cause any outgassing as with conventional lead-acid batteries, and there is no memory effect (long-term loss of storage capacity due to frequent partial discharge). Although these reasons are all valid, they often fall short of the mark. To properly compare lithium-ion and lead-acid batteries for use in intralogistics, you have to delve deeper into the technology.
Lithium-ion: one term, many varieties
The term ‘lithium-ion’ covers a wide range of battery systems with different chemical compositions. For example, the negative electrode can be made from graphite, titanium or another material, while the positive electrode consists of a lithium metal oxide based on cobalt (LiCoO2), nickel (LiNiO2), manganese (LiMn2O4), or another metal. The electrical characteristics of the cell depend on the specific combination in each case – the nominal cell voltage of current batteries usually ranges from 2.2 to 4.2 V, depending on the material combination. This means that if you want to replace a conventional leadacid battery by a lithium-ion battery, the battery configuration depends on the chosen chemistry. For instance, to replace a 24 V lead-acid battery you would need six or seven cells if you opt for lithium nickel cobalt oxide (NCA) with a cell voltage of 3.6 V, or nine cells if you choose lithium titanium oxide (LTO) as the anode material, with a cell voltage of 2.4 V. If the cell count is not adapted, you will have different charge and discharge voltages every time you change to a different material combination. The chosen chemistry also determines the energy density, capacity, cycle lifetime, storage and ageing characteristics. In addition, it also affects sensitivity to high and low temperatures, and sensitivity to deep discharge and overcharging.
Close integration in industrial trucks
A common aspect of all types of lithium-ion cells is that they are more sensitive to certain operating conditions and external factors than lead-acid batteries. To ensure proper operation of these cells, it is therefore necessary to use an electronic battery management system equipped with sensors for current, voltage and temperature. This is in order to monitor charging and discharging at the cell and system levels. This battery management system usually has a direct interface to the electric vehicle. A consequence of this close integration of lithium-ion batteries into the vehicle electronics is that most industrial truck manufacturers make and install their own lithiumion battery systems. Manufacturers are reluctant to give external parties access to the CAN bus of their forklift trucks – a prerequisite for integrating batteries from “independent” suppliers. For operators of industrial trucks, this means that they have to buy their lithium-ion batteries from the equipment manufacturer. They do not have the option of purchasing batteries for their various forklifts, lift trucks or cleaning machines from the battery manufacturer of their choice. By contrast, lead-acid batteries from different manufacturers are mutually compatible, so a wide variety of vehicles can be equipped with batteries from a single supplier. The electronics required with lithium-ion systems also present a certain amount of potential residual risk of faults and outages – experience shows that most battery system failures are due to faulty electronic components. This should not be understood to mean that the lithium-ion batteries currently available for industrial applications are unreliable, but simply that this reliability comes at the cost of higher technical complexity.
Extended maintenance intervals
The subject of outages leads directly to the required maintenance expense for various battery systems. Here the main advantage of lithium-ion batteries over lead-acid batteries is their claim to deliver virtually zero maintenance. That is true to the extent that conventional lead-acid batteries use diluted sulphuric acid as the electrolyte. As a rule, these batteries do indeed require servicing at least once a week, which amounts to topping up the cells with water. However, lead-acid battery technology has also progressed. For example, the Hawker® Water Less® Battery from EnerSys®, based on PzS technology, allows refilling intervals of four, eight, or even thirteen weeks, depending on the charging technology and configuration.
Virtually zero maintenance with high power density
Thin Plate Pure Lead (TPPL) technology can even dispense entirely with water refilling. It is based on traditional lead-acid batteries, but with a number of significant added features. The electrolyte is absorbed in micro-porous glass mat separators, eliminating the need for topping up with water. Particularly with regard to power density, TPPL has advantages over conventional lead-acid batteries because the positive and negative electrodes are only 1 mm thick, compared to 9 mm electrode thickness in typical conventional lead-acid batteries. This allows many more electrodes to be fitted in the same space, increasing battery capacity and boosting power density. The TPPL batteries in the NexSys® portfolio from EnerSys®, for example, offer the same or higher ampere-hour capacities as conventional liquidelectrolyte batteries. Another key feature of TPPL batteries is that pure lead is used for the grids. The grain structure of this pure lead makes the grids much less susceptible to corrosion and allows them to easily handle relatively long storage times. However, the main thing is that the chemical behaviour of the batteries is significantly more stable, which offers advantages with regard to charging characteristics and lifetime. For example, the TPPL batteries in the Hawker® NexSys® series have a cycle life of about 1450 cycles at 60% end discharge depth, depending on the version. In addition, TPPL batteries can be operated under shallow discharge cycle conditions with no memory effect, have a very low gassing rate, enable short charging times, and have high energy throughput over the working day.
Long service life for heavy-duty applications
Where power for heavy-duty applications is required, lead-acid batteries with square tube design can be an attractive option. In this design developed by EnerSys®, the tubes of the positive plates are square instead of round as in conventional batteries. The EnerSys® batteries sold under the Ironclad® brand name can be operated with higher specific acid density, which not only increases performance and operating time, but also extends the lifetime due to the lower specific load on the positive electrodes. They have a design lifetime of 1800 cycles, which means an additional year of service in many applications. In addition, these batteries can maintain higher voltages for a longer period during discharge, enabling longer operating times. When industrial trucks are used in shift operation, this means less frequent exchange of spare batteries, and thus higher productivity.
Don’t ignore sustainability
The sustainability of the battery system should also be considered, because lead-acid and lithium-ion batteries differ significantly in this regard. Lead-acid batteries are composed of relatively few materials (essentially lead, sulphuric acid and plastic) and are therefore easily and virtually completely recyclable. The materials can not only pay for the cost of collection and processing, but also potentially yield a profit, depending on market conditions. By contrast, recycling of lithium-ion batteries presents a greater challenge. A chemical analysis is first necessary to determine whether the battery actually contains any valuable materials. In the case of lithium iron phosphate (LFP) batteries, recycling is currently hardly worthwhile, while with lithium cobalt oxide (LCO) batteries the cobalt can be recovered and corresponds to about 10 per cent of the new value of the cell. The difficulty of recycling is also indicated by the target stated in the EU Battery Directive (2006/66/EC) – one of the most advanced recycling directives in the world – which sets a goal of just 50 per cent by weight for lithium-ion batteries.
Check the details when comparing systems
In order to take a truly informed decision on which battery system is the best for a specific application, you should carefully examine the various systems and technologies. Each system has its own particular advantages. With lithium-ion batteries, the basic advantages are high power density and flexible charging. However, this technology is still in its infancy – the market share for industrial trucks is less than 2 per cent. This share will rise in future, especially when the price of lithium-ion batteries drops as a result of increasing use in electric cars. The share of lithium-ion technology is likely to rise in particular with small manually operated industrial trucks with little space available in the equipment. By contrast, lead-acid batteries – with their track record of robust and reliable technology – will continue to score well in the heavy-duty sector. Modern advancements, such as TPPL batteries or the square tube design, can enable lead-acid batteries to meet more stringent demands in the intralogistics sector for productivity, flexible charging and performance.
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