Light Electric Vehicles (LEVs) such as golf carts have been traditionally powered by lead-acid batteries. Original Equipment Manufacturers (OEMs) are transitioning to Lithium-ion (Li-ion) batteries as they offer several advantages over lead-acid batteries, such as higher power density, longer run time, and zero maintenance. However, a successful transition requires careful consideration of the differences in the cell chemistry and the battery pack behavior.
Li-ion batteries are designed with a Battery Management System (BMS) to provide safety and intelligence, which increases their overall complexity. This article discusses some of the key considerations that OEMs need to account for when replacing the lead-acid batteries with Li-ion batteries. The considerations are categorized into four sections: charging, regeneration, user interface, and power tripping.
The maximum charge voltage of a battery pack depends on the chemistry of the cells used. Comparing batteries with similar nominal voltages, the charge voltage for a lead-acid battery is different from that of a Li-ion battery. For example, for a vehicle with a nominal voltage of 24V, the lead-acid batteries can be charged to a maximum voltage of 29V-34V, whereas a 24V Li-ion battery pack with Lithium-Nickel-Manganese-Cobalt-Oxide cathode cells has a maximum charge voltage between 28 and 29V. Using a lead-acid charger to charge Li-ion batteries can trigger over voltage protection and result in reduced battery pack capacity.
Similar mismatch exists with the maximum charge current. The charge current for Li-ion batteries varies with the temperature and State of Charge (SOC). Overall, lead-acid batteries can support higher charge current. Using a lead-acid charger to charge a Li-ion battery can trigger an over current protection, or an over temperature fault due to the heat generated in cells with higher charge currents. This will also eventually degrade the lifecycle of the battery pack.
The presence of a BMS in Li-ion batteries brings additional levels of protection. During a severe cell under voltage condition, the BMS activates protection and to bring the battery pack out from this protection state, charging needs to be performed. A lead-acid charger may not be able to charge the Li-ion battery as it looks for a valid pack terminal voltage prior to starting the charge process. Since the pack is in a severe under voltage protection, the pack cannot provide a voltage at the terminals. This results in a condition where the pack does not get charged, and therefore, the user may not be able to drive the vehicle.
The solution is to replace the lead-acid charger with a Li-ion battery charger with a charge algorithm matching the chemistry of the cells used. Using a smart charger lets the BMS regulate the charge voltage and current over digital communication interface such as Controller Area Network (CAN). The charger also needs to account for the conditions during which the battery cannot provide a valid voltage on the terminals but requires a charge voltage and current to come out of that condition.
LEVs can generate regenerative current while braking and transferring that energy through the motor to be used as the main braking mechanism for the vehicle. The battery may absorb the energy that is transferred. Lead-acid batteries can do this under all operating conditions, but Li-ion batteries are limited in their capability to accept regen current at certain operating conditions.
These limitations include when the cells are cold, when they are fully charged, and when a charge protection is active, such as a cell over voltage or cell over temperature fault. In addition, the amplitude of regen current that the battery pack can absorb also varies with SOC and temperature. Typically, the BMS in the Li-ion battery calculates the allowable regen current in the battery pack and broadcasts this information over the supported digital communication interface. Anytime a battery cannot accept the regen current generated by the motor, multiple issues can occur. One of them is that the system voltage can spike causing damage to the components connected in the system power bus, including the battery. It also impacts the breaking power.
The battery installed in the vehicle needs to be capable of absorbing the generated regen current under all operating conditions. The battery or the system can add additional regen absorption components, such as a bleeding resistor, and sink the additional regen current that the battery pack cannot absorb. This requires precise monitoring of the regeneration current generated by the motor and the regen current absorption capability calculated by the BMS. Switches can be used to control the flow of the regen current and ensure smooth functioning of the braking and regeneration system.
LEVs have a battery gauge installed in the vehicle dashboard that displays the battery voltage or SOC. If displaying the SOC, it is typically calculated from the measured battery voltage. This works well for a lead-acid battery, but not very well for a Li-ion battery. The impedance of the Li-ion cells along with the impedance of BMS components impact the accuracy of SOC calculated by such gauges.
One of the main functions of a BMS in the Li-ion batteries is accurate gauging of SOC and measuring of the battery pack voltage. These values are available over a digital communication interface and can be read and displayed by gauges that have the capability to communicate with the battery packs.
A gauge with digital communication is typically referred to as a ‘smart gauge’. In addition, battery parameters such as time-to-empty, temperature, current, battery errors etc. are also available from the BMS, and can be displayed in the smart gauge. This feature enhances the user experience and helps the user to track battery parameters accurately. Figure 1 shows a smart gauge that can display battery parameters such as SOC, voltage, current, time-to-empty etc. over various screens. The figure has 100 percent as the SOC, 51.1V as the battery pack voltage and 10 hours and 01 minute as the time-to-empty.
Another option is to use the existing gauge in the vehicle that works with a lead-acid battery, without replacing it with a smart gauge. This avoids the additional cost of the smart gauge but requires a different interface between the Li-ion battery and the gauge.
The main functionality of a BMS in a Li-ion battery is protection. The protection includes cell over voltage, cell under voltage, over current and over/under temperature. The BMS monitors the current, cell voltages and cell temperature to identify a faulty situation and activates protection. This process may end up cutting the connection between the cell stack and the pack terminals, which would result in a tripping of power to the vehicle. This does not happen if using a lead-acid battery pack and presents a critical safety hazard as the user could get stuck in a busy intersection or lose braking capability while riding down a hill. Certain LEVs have a safety interlock that applies the full brake when the battery pack terminals lose power. This is a critical safety hazard for the riders in the vehicle.
While retrofitting an LEV with Li-ion battery pack, care must be taken to avoid sudden tripping of battery power. The battery pack needs to account for the safety hazards that could result from power tripping. There are multiple design considerations to be applied which solves the above problems and ensures a smooth and safe operation of the vehicle.
Li-ion batteries are enhancing the user experience of golf carts and other LEVs by providing longer run times, a maintenance-free solution, and a more reliable user interface. When choosing to replace lead-acid batteries with a higher performing, more sustainable, and intelligent battery technology like Li-ion, the key is to ensure the battery being installed has been designed, manufactured, and tested to perform to the application’s requirements. Consider asking your battery manufacturer how their batteries have been designed to handle charging, regeneration, a vehicle’s user interface, and power tripping.
This article was written by Anvin Joe Manadan, Technical Lead, Inventus Power (Woodridge, IL). For more information, visit here .