The main selling point of lithium Ion technology is its energy density, which is typically 3 times that of Lead Acid and 40% more than NiMH. This energy density advantage is further increased in large format battery cells. Larger battery cells mean that proportionally less of the cell mass is casing and in terms of the battery pack construction, less of the total pack weight is connections and fusing. Additionally, the large format cells are typically of a regular shape allowing better packaging when compared to cylindrical cells. Modern pouch cells such as those supplied by Kokam™ and EiG further benefit from having large connection terminal surfaces, allowing low impedance connections and an efficient means of conductive cell cooling.
Safety is the single biggest challenge for large format Lithium Ion cells. Smaller cells such as the typical 18650 size cells can be equipped with built-in, passive, safety features. In advent of a mishap, a single cell with a capacity of 2.2AH (9Wh) is of manageable impact compared with a 50AH or even 400AH (1.6kWH) unit.
Minimizing risk when working with large format batteries falls to the design of the pack, the switching circuitry and most importantly, the battery management system.
Lithium Ion batteries can exhibit thermal runaway. This is where a rapid, exothermic reaction occurs typically triggered by localized overheating within the battery cell. The resultant reaction leads to venting of noxious gases, fire or an explosion.
Much research has gone into the causes of thermal runaway, but this has mainly focused on Laptop and cell phone batteries which are typically LiCoO2. The weak point of this chemistry is that when heated sufficiently, the LiCoO2cathode decomposes releasing oxygen, to further accelerate the thermal runaway reaction. By substituting the cathode material of LiFePO4 which is more temperature stable, there was the mistaken belief that Li Ion batteries had suddenly become safe, this is wrong. Experiments have shown that LiFePO4 batteries will also exhibit thermal runaway; a simple overcharging experiment produced a violent explosion at just 130% SOC!
A battery management system needs to detect and log problems within the battery system, to give notification of potential errors and to manage these problems safely.
Lithium Ion batteries can show instability (reduced performance, poor longevity and potentially, thermal runaway) due to poor quality or abuse. Poor quality can mean contamination of the materials used, poor physical construction of the cells and use of poor separators. Abuse can involve overcharging, charging at low temperatures, discharging above the recommended rate, shorting the terminals, mechanical abuse.
Whereas the dangers presented by poor quality cells are difficult to counter, the battery management system (BMS) can be used to prevent most types of abuse. A good quality BMS should control the charger and provide options for controlling contactors, relays and auxiliary systems used in the battery pack. To prevent overcharging, a BMS should have control of the charger as well as having control of the charger relay – providing a failsafe physical disconnect, should the charger not shut down when required.
Thermal management is a crucial aspect of battery management, whereby the BMS should prevent excessive temperatures occurring during charging or discharge, and in some cases, heating when the battery system is used in cold conditions.
In case of short circuits and excessive current draw, the BMS should provide a warning followed by the opening of the main contactors, should current or temperature exceed safe levels. The BMS should also provide an isolation check whereby the insulation between ground and the positive and negative leg is measured and matched against a target in ohms/volt, typically 250 ohms/volt or 500 ohms/volt. This check ensures that there are no shorts to earth or poorly isolated connections within the system or indeed moisture inside the battery system.
The battery system needs to be able to communicate with other systems providing status information, giving warnings or controlling other devices such as chargers. Information from other systems such as crash sensors or smoke detectors needs to be read by BMS. The battery system also needs to “remember” its usage history by logging status and events to an onboard memory that can be read during maintenance procedures or in the event of a failure.
Safe failure modes also need to be in place, whereby if the BMS fails or is disconnected from its power supply, the pack should shut down in a safe manner and remain isolated both from the associated connected system and from chassis or ground.
In the case of an EV, simply opening the contactors during an emergency situation may endanger the vehicle occupants by stranding them in a dangerous situation. A two stage process is required here, whereby a warning is issued over CAN giving the engine controller an opportunity to throttle demand, and only when there is imminent danger of fire are the contactors opened.
Good pack design and construction provides a further defense against mishap. The battery pack needs to be a single box construction containing the BMS, the switching electronics and the battery cells. This allows the whole battery pack can be rendered safe by opening the main contactors. The cell interconnections need to be of a type and quality that minimizes impedance avoiding dangerous hot spots with the high currents the battery is capable of providing. The hotspots can easily melt a battery cell and initiate a fire. In the event of a mishap, the pack casing should provide some degree of protection against breach, loss of integrity, exposure to high voltages and possible creation of short circuits. The cells themselves, need packaging in a way that prevents loading on the terminals, provides the necessary constraining or packing forces to maintain correct cell geometry, while still providing room for controlled thermal expansion and contraction during operation.
In conclusion, the safe use of large format Lithium Ion batteries requires the use of a high end battery management system built into a professionally made pack. The larger format cells may improve energy density but it comes at a price as more care needs to be taken.