Thin-plate pure lead (TPPL) and lithium are two of the most advanced batteries today for forklifts. However, at their current state of development, TPPL batteries have a few drawbacks compared to lithium-iron-phosphate (LFP) batteries, including a lower energy density, lower maximum depth of discharge, and less-than-stellar performance at sub-zero temperatures.
Typically, LFPs are the lithium battery of choice for forklift providers that can afford it, offering a greater ROI in terms of service life and lower energy costs. But LFPs come with a heavy price tag for smaller forklift fleets.
Looking ahead, the cost of lithium-ion (Li-on) batteries is expected to decrease with scale as advances in TPPL batteries become more expensive. Eventually, Li-ion batteries’ share of motive power is expected to grow to nearly 50% of the market by 2030.
Understanding lead-acid battery technology
TPPL technology shares the same engineering principle as all lead-acid batteries: lead plates are suspended in an electrolyte consisting of a water (H2O) and sulfuric acid (H2SO4) solution within a casing. Positive (cathode) and negative (anode) plates are made of lead (Pb) alloy grid supports (current collectors) with dissimilar lead dioxide (PbO2) and lead coatings.
As current flows between the plates due to the chemical reaction, lead sulfate forms on the positive and negative plates. As the formation of lead sulfate increases, voltage begins to decrease. The lead sulfate crystallizes upon the plates if a battery is not immediately connected to a charger and a charging current is applied.
Figure 1. Working of a lead-acid battery during (a) Discharging (b) Charging phases (c) Schematic of a valve regulated lead-acid battery structure. (Source: “Impact of carbon additives on lead-acid battery electrodes: A review“)
Developments in lead-acid batteries
As lead-acid technology has advanced, new designs were launched. The table below describes the various improvements in the technology, placing TPPL within the context of these lead-acid battery innovations.
Table 1. The lead-acid battery sub-technologies are currently available on the market.
There are significant fundamental restrictions on further lead-acid technology improvements because of the chemical processes inside the battery.
Figure 2. Constant temperature curves of conductivity versus concentration (in %).
Considerations include:
- The electrical conductivity of a sulfuric-acid (H2SO4) solution is highest at a concentration of 30-33%, but it decreases sharply at temperatures above +32° C and below +15° C. Concentrated sulfuric acid has a lower electrical conductivity than dilute sulfuric acid. This is due to the presence of fewer H+ and SO42- ions in concentrated form.
- The preferred depth of discharge (DOD) is only 50%. When discharging a battery, the formation of PbSO4 sulfate on the electrodes is a natural process. As the DOD increases, sulfation also increases. A 50% DOD is preferable, but an 80% DOD is the maximum safe discharge. Increased sulfation is extremely harmful because instead of loose, fine sulfate crystals, a continuous dense layer of large crystals is formed on the electrode surface. With deep discharge, the volume of plates increases greatly, which can lead to deformation and destruction of the plates.
Comparing TPPL technology to Li-ion
Essentially, TPPL battery technology is an improved version of the AGM, offering several advantages over a regular flooded lead-acid battery (FLA):
- TPPL plates are thinner, allowing for more plates and a larger reactive surface area and resulting in lower internal resistance and lower energy losses. This enables quicker recharging and higher current delivery with lower voltage drop.
- There’s no need to top-up the water daily.
- Gas emissions are lower.
- The cycle life is longer, particularly when subjected to repeated micro-cycles of discharging followed by partial opportunity charging.
However, despite recent improvements, TPPL batteries remain inferior to LFP lithium batteries:
- A lag in energy density. Currently, the lead plates account for an estimated 40 to 60% of the weight of an average lead-acid battery. (A detailed comparison of AGM and LFP can be found here). This means TPPL batteries cannot compete with lithium for demanding applications, especially when more energy is needed to keep multi-shift operations running smoothly with one battery per truck.
- Lower performance at sub-zero temperatures. The maximum conductivity of a TPPL battery at -6° F is half the conductivity when the electrolyte temperature is 60° F. Lithium batteries withstand harsher conditions and offer special insulation with built-in thermo-regulation (see Figure 3).
Future TPPL developments
There are several ways in which the design of TPPL batteries can be improved to enhance performance.
1. Thinner electrodes and increasing grid perforation (adding hole area to the total surface area). Changes to the electrode plate design and using powders increase the electrolyte-electrode contact surface, increasing the battery’s electrical capacity.
Plates are made by pasting lead dioxide (PbO2) on the positive electrode (cathode) and the lead (Pb) mass on the negative electrode (anode) — while both electrodes have a perforated grid shape. The higher the degree of perforation, the more powder can be added. But an extremely thin grid structure is likely too fragile. Other metals in small quantities would need to be alloyed with lead for added strength and improved electrical properties.
Figure 3. The FROST Series OneCharge battery features an inbuilt heater and temperature sensors to maintain an optimum working temperature for its LFP cells. The enhanced IP protection in this series protects the battery against condensation, which often forms when a lift truck goes in and out of a cold-storage facility.
2. For collector grids, using lighter materials may significantly decrease plate weight and increase specific energy (Wh/kg). Such materials must not react with sulfuric acid and must bond to lead dioxide and lead coatings tightly.
Titanium is 2.5 times lighter than lead and could be used for plate production — and some data is available for titanium-based positive grids. According to The Elements: CRC Handbook of Chemistry and Physics (103rd Edition, 2022-2023), titanium’s specific gravity at 20° C is 4.54 tons/m3, while the same parameter for lead is 11.35 tons/m3.
3. Carbon as an additive for plate coatings may be used for positive and negative electrodes to help prevent the formation of a passive lead-sulfate (PbSO4) layer and increase the proportion of electrode mass involved in reactions.
The primary constraint for all three opportunities is the cost of production. Adding complex and costly components into the least expensive technology will make TPPL products unaffordable. What’s more: they’ll be competing at a similar price point with more efficient lithium and newer battery technologies, such as sodium-ion.
Conclusion
The next generation of TPPL batteries will directly compete with the newest lithium-ion and sodium-ion batteries, and the cost of more complex manufacturing processes will inevitably fall on end users.
As TPPL technology is undergoing increased manufacturing expenses, with a lower starting point, lithium batteries are become less expensive with increasing scale and competition.
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