Lead acid batteries have a history of over 150 years since the invention of Plant in 1859. Lead acid batteries have always had an absolute advantage in chemical power sources since their invention. This is because of its low price, easy availability of raw materials, sufficient reliability in use, suitability for high current discharge, and a wide range of environmental temperatures. The power battery of electric vehicles is currently commonly used as a rich liquid open lead-acid battery. Compared to the advantages of lead-acid batteries, the disadvantages of lead-acid batteries are also prominent, because there is water loss and acid overflow during the use of rich liquid open lead-acid batteries, which require frequent supplementation of distilled water, long charging time, short lifespan, and low energy density. Especially in recent years, the frequent occurrence of blood lead incidents has led us to pay more attention to environmental factors in the production and use of lead-acid batteries.
Due to the many unfavorable factors of lead-acid batteries as power batteries, the development of electric vehicles is seriously restricted. In order to obtain more efficient and environmentally friendly battery technologies, the country strongly supports new energy technologies, and various battery technologies have sprung up like mushrooms after rain, making people dizzy, such as various types of lithium batteries, fuel cells, supercapacitors, etc. The application of lithium batteries in forklifts is still in its early stages. This article analyzes the performance of lithium batteries on forklifts through the selection of lithium batteries, lithium battery pack solutions, and comparative experiments with traditional rich liquid open lead-acid batteries. It is hoped that this can provide reference for the reasonable application of lithium batteries on forklifts.
Types and Selection of Lithium Batteries
The commonly used lithium-ion batteries include lithium cobalt oxide, lithium manganese oxide ternary materials, and lithium iron phosphate. Lithium iron phosphate battery refers to a lithium-ion battery that uses lithium iron phosphate as the positive electrode material. Lithium iron phosphate batteries have high safety and are suitable for high current discharge, making them particularly suitable for use as power batteries. In addition, there are many domestic manufacturers of lithium iron phosphate batteries with mature technology and a wide range of choices. In the absence of significant breakthroughs in the safety of other types of lithium batteries, choosing lithium iron phosphate batteries as forklift power batteries is more suitable
Solution for 3 lithium iron phosphate battery pack
In order to fully understand the discharge characteristics of lead-acid and lithium batteries, Hangyou Group Co., Ltd. (hereinafter referred to as „Hangcha“) selected domestically produced 500Ah lead-acid batteries and 100Ah lithium battery monomers for discharge testing.
After analyzing the discharge curves of lead-acid and lithium batteries, it can be concluded that: (1) Compared with lead-acid, lithium iron phosphate batteries have a small voltage drop in the early stage of discharge, but a rapid decrease in voltage after discharge to 80% is important for the treatment of lithium batteries in the final stage of discharge. (2) At 1C discharge, the capacity of lithium iron phosphate batteries is equivalent to twice that of lead-acid batteries, indicating that lithium iron phosphate batteries are more suitable for use on electric forklifts that frequently discharge high currents than lead-acid batteries.
When forming a DC 80V battery pack, lead-acid batteries use a 40 cell series connection method. Influenced by the domestic electric vehicle industry, DC 80V lithium iron phosphate battery packs generally use a 24 cell series connection method, which means the rated voltage of the lithium battery pack is 24×3.2V=76.8V. From the discharge curve, it can be seen that at 0.2C discharge, the operating voltage of the 24 cell series connected lithium iron phosphate battery pack is basically equivalent to that of the 40 cell series connected lead-acid battery pack. But after discharging more than 80% of the battery, the voltage of the lithium battery pack will be significantly lower than that of the lead-acid battery pack. Therefore, when implementing the 24 series scheme, it is advisable to charge before the remaining 20% of the battery.
At present, there are no foreign companies producing 80V lithium battery packs. The highest current voltage for lithium battery packs is 48V, using a 16 cell series connection method. When converted to 80V, the lithium battery pack should be connected in series with 26 units. The maximum voltage of the lithium battery pack using the 26 series connection method will reach around 88.4V, but it is still within the safe working voltage range of 80V electric forklift electrical components. The biggest advantage of using the 26 string method is that it can control the remaining power of the battery pack to around 5%, without worrying about the motor and electronic control overheating during the end of discharge.
Lithium iron phosphate batteries are the safest among all lithium batteries, but to prevent extreme situations and increase cycle life, lithium battery packs require a BMS system. The lithium battery pack is equipped with a built-in main contact and fuse, ensuring self-protection in extreme situations such as external short circuits and overcharging. Reserved space for electric heating, suitable for charging conditions below 0 ℃. Due to the much lighter weight of lithium battery packs compared to lead-acid battery packs, corresponding counterweight blocks have been added to the lithium battery packs to achieve the same weight as lead-acid battery packs. In addition, balanced forklifts also need to supplement their own weight to ensure the stability of the entire vehicle.
Conclusion
- (1)After using lithium batteries, the capacity can be smaller than that of lead-acid batteries under the same usage time. The 26 series solution can reduce the capacity by about 40% compared to lead-acid batteries, which means that 26 series lithium batteries can achieve a usage time of 500Ah of lead-acid batteries as long as 300Ah. When lithium iron phosphate batteries have the same capacity as lead-acid, they can have a longer usage time, reduce forklift charging times, and therefore have a longer service life.
- (2) Due to the high temperature rise of components such as the motor and fuel tank of the balanced forklift, and the usage temperature of lithium batteries cannot exceed 65 ℃, it is important to avoid the heat source of the forklift when using lithium battery packs.
- (3) Due to the small internal resistance of lithium battery packs, their heat generation is lower than that of lead-acid, and with the BMS management system, safety is also higher.
- (4) Due to the relatively light weight of lithium iron phosphate batteries, their application in vehicle models such as pallet trucks that are not sensitive to battery weight is more convenient. The next step that needs to be made is to redesign a balance weight forklift specifically designed for lithium batteries, taking advantage of the small size and light weight of lithium iron phosphate batteries
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