The proANTs are equipped with modern battery technology, lithium ion secondary batteries (lithium iron phosphate LiFeYPo4) are used. The lithium iron phosphate accumulator is a further development of the lithium ion accumulator. LiFe-PO4 is used as cathode material and a doping with yttrium improves the technical properties (performance and lifetime). This battery is a dry battery.
Properties of LiFeYPo4
Lithium iron phosphate is non-toxic and non-flammable. The LiFeYPo4 cells are high current capable, cycle resistant and hardly self-discharging. In contrast to conventional Li-ion cells, no metallic lithium is deposited in case of overcharging and no oxygen is released. With these batteries there is no memory effect (like NiCd or NiMH rechargeable batteries), so that the cells have a very long service life when used properly.
They are therefore also used millions of times in the consumer sector, e.g. in eBikes, mobile phones and notebooks. The largest cell blocks up to 30,000 Ah are used in submarines.
LiFeYPo4 batteries allow short charging times, depending on the charging current the battery can be fully charged in a few minutes.
LiFeYPo4 batteries deliver almost their full nominal voltage until shortly after deep discharge, but then the voltage collapses abruptly.
Advantages and Disadvantages LiFeYPo4
|Advantages LiFeYPo4||Disadvantages LiFeYPo4|
|• high discharge currents
• high safety
• high cycle stability
• short charging times
• less negative environmental impact than LiMn accumulators
|• cells are slightly heavier than other secondary batteries of equal capacity
• high price if the quality is good
• Battery management and balancer circuits necessary
Exemplary calculation: 40 Ah battery pack at an average current consumption of 5.7 A
Using the battery monitoring information, the AGV is sent to a stationary charging station at 80% DOD (depth of discharge) at the latest. In this case the AGV can be charged at least 5000 times according to the data sheet of the battery manufacturer. This results in a minimum lifetime of 5000 x 6 h = 30,000 h = 1,250 days = 3.42 years. Due to the so-called “opportunity charging” described in the following chapter, however, the battery packs of the AGVs are almost never discharge til 80%. In the data sheet of the battery cell you can see that the life cycle increases by 40 % with only 10 % less DOD. This should result in a realistic life cycle of 7000 x 6 h = 42,000 h = 1,750 days = 4.79 years.
Charging the battery pack
The most effective, but most costly solution is charging each individual cell, but this is not feasible when used in an AGV. Therefore the cells are connected to a block and protected by a balancer circuit.
A balancer board is connected to each cell, which ensures that all cells are at the same level. It balances the charge current of the cells when one cell is already full but the other cells still need to be charged.
All balancer boards communicate via a bus system with the Battery Management System BMS and thus ultimately with the charger.
On each balancer board is a temperature sensor which monitors the temperature of the cell.
The AGV has charging contact surfaces which are spring-loaded at the charging station. The charging current only flows when the AGV is docked and confirms the contact of the charging station by data transmission. As soon as the battery management system reports to the AGV that the battery is fully charged or is too warm, the charging process is terminated.
Due to the short charging times that lithium-iron technology allows without negative effects on the capacity and lifetime of the battery, the AGV can be continuously recharged during operations, e.g. when waiting for a load transfer. The charging time when the AGV is standing at the transfer station is approx. 40-60 s.
During normal operations, the charging station will only be used to restore the battery of the AGV to a state of charge of 80-20% in case of a deep discharge. It is located near the maintenance station.
Conditions for maximum lifetime
The lifetime of the LiFeYPo4 can be increased with the following measures:
- – Using the battery in the range 80-20% of the state of charge
- – Installation of a balancer circuit
- – Use of a battery management system which monitors voltage, state of charge (SOC), temperature and status
- – lowest possible charging current (0.5 CA)
No oxygen is released during the charging procedure of this battery. Furthermore, in contrast to conventional Li-ion cells, no metallic lithium is deposited in the event of overcharging. The deposition of oxygen leads in older types of Li-ion accumulators to thermal damage, which under unfavorable conditions can cause the explosion of the cell. In the case of a lithium-iron-phosphate accumulator, this is not possible if the battery is used properly.
In addition, the temperature of each individual cell is monitored by the balancer board.
Hazards and measures
A thermal reaction can occur with the following defects:
- Short circuit/polarity reversal
- mechanical defect
- too high external temperature
- The battery cell pack is operated with a battery management system which detects a defect and reports it to the control system.
- Overcharging or deep discharging does not create a fire hazard with this type of battery.
- The battery pack is protected against mechanical damage in the AGV by a metal plate.
- In case of fire (production) the battery can be extinguished with water or a CO2 fire extinguisher.