Our proANT aAGVs are equipped with state of the art battery technology. The batteries are made of Lithium Ion secondary batteries (Lithium-Ironphosphate LiFeYPo4). The Lithium-Ironphosphate accumulator is a improved version of the traditional Lithium-Ion accumulator. As cathode material, LiFe-PO4 is used. To enhance technical propierties such as power and durability, the cathode is doted with Yttrium.
This battery is a dry battery, without liquid components.
Properties of LiFeYPo4
Lithium-Iron phosphate is neither hazardous nor inflammable. The LiFeYPo4 cells support high charging currents and virtually don’t self-discharge. In comparison to conventional Li-Ion batteries, LiFeYPo4 does not segregate metallic lithium nor oxygen when overcharged. These batteries don’t possess a memory-effect, enabling them to have great longevity when used correctly.
This battery technology is widespread in consumer products such as eBikes, Mobile phones and Notebooks. The biggest cell blocks are of up to 30.000 Ah are used in submarines.
LiFeYPo4 Batteries enable short charging times. Depending on the charging current, the batteries can be charged in a few minutes.
LiFeYPo4 accumulators supply the entire nominal voltage until they are deeply discharged. In case of deep discharge, the Voltage breaks together in a steep slope.
Advantages and disadvantages of LiFeYPo4
|Advantages LiFeYPo4||Disadvantages LiFeYPo4|
|• high discharge currents
• high safety
• high cycle strength
• short charging times
• less negative environmental impact than LiMn accumulators
|• cells are slightly heavier than other secondary batteries of equal capacity
• high price
• Battery management and balancer circuits necessary
Exemplary calculation of lifespan: 40 Ah battery pack with an average discharge consumption of 5,7 A
Using the information of battery monitoring, the aAGV is sent to a charging station when at a depth of discharge (DOD) level of less than 80%. When this DOD-threshold is respected, the battery can be charged 5000 times. This equals a minimal lifespan of 5000 x 6 h = 30.000 h = 1.250 days = 3,42 years. Furthermore, due to the process called „opportunity charging“ explained in the next chapter, the batteries are almost never discharged up to 80%. A look into the battery’s data sheet reveals that the battery’s lifespan increases by 40% when the DOD level is reduced by 10%. Therefore, a realistic calculation of lifespan would be 7000 x 6 h = 42.000 h = 1.750 days = 4,79 years.
Charging the battery pack
The most effective, but complicated solution is charging each cell at a time. Unfortunately, this is not possible in an aAGV application. Therefore, the cells are merged into a block and fused with a balancer board.
A balancer board is connected to each cell. It is used to adjust the charging current to each cell, balancing all cells to the same charge state and voltage level.
All balancer boards communicate over a bus system with the Battery Management System (BMS) and therefore with the charging station.
Besides leveling the charging current, a temperature sensor monitors each cell’s temperature.
The aAGV is equipped with two spring-suspended charging contacts. Only if the aAGV is docked to the charging station and the contacts of station and vehicle communicate with each other will the charging current begin to flow. Once the vehicle’s Battery Management System signals that the battery is fully charged or overheated, the charging process is terminated.
Due to the short charging times that batteries with Lithium-Iron technology support without losing their capacity or durability, the aAGV can permanently be recharged in continuous operation i.e. when waiting on a station to be loaded. These charging times usually amount to 40-60s.
Sometimes opportunity charging is not sufficient. In this case, an additional charging station is integrated into the application. The charging station is used when the vehicle’s battery status falls bellow a threshold, with which it wouldn’t be able to absolve another transport. It will then be used to return the charging level of the vehicle’s battery to 80-20% and be positioned near the maintenance area.
Requirements for a long battery lifespan
The durability of the LiFeYPo4 is optimized with the following measures:
– Using the batteries in the range of 80-20% of maximum charge
– Installation of a balancing circuit
– Using a Battery Management System which monitors voltage, state of charging, temperature and status of the battery
– Using a low charging current (0,5 CA)
No oxygen is released when charging LiFeYPo4 batteries. Additionally, no metallic Lithium is secreted when the batteries are overcharged, which is an advantage to traditional Lithium-Ion-Cells. The secretion of oxygen in older Lithium-Ion-Cells lead to thermal damage that could eventually lead to the battery exploding. When a Lithium-Iron-Phosphate accumulator is used correctly, this effect is not possible.
Additionally, the temperature of each cell is monitored by the battery management system.
Dangers and countermeasures
A thermal reaction can occur on following failures:
- Electrical short / voltage reversal
- mechanical failure
- too high environment temperature
- The battery pack is used with a battery management system, which detects failures and alarms the aAGV system’s base station
- An overcharge or exhaustion of this battery does not lead to risk of fire
- In the aAGV, the battery pack is protected from mechanical damage via a metal plate
- In case of a fire in the production site, the battery can be extinguished using water or a CO2 extinguisher