The Volkswagen Beetle in simpler days had minimal primary battery problems.The only management system was ensuring that the battery was being charged while driving.Onboard electronics for safety,convenience,comfort and pleasure have greatly added to the demands on the battery in modern cars since then.For the accessories to function reliably,the state-of-charge of the battery must be known at all times.This is especially critical with start-stop technologies,a mandated requirement on new European cars to improve fuel economy.
When the engine stops at a red light,the battery draws 25–50 amperes of current to feed the lights,ventilators,windshield wipers and other accessories.When the light changes,the battery must have enough charge to crank the engine,which requires an additional 350A.With the engine started again and accelerating to the posted speed limit,the battery begins charging after a 10-second delay.Realizing the importance of battery monitoring,car manufacturers have added battery sensors that measure voltage,current and temperature.
Packaged in a small housing that forms part of the positive clamp,the electronic battery monitor(EBM)provides useful information about the battery and provides an accuracy of about +/–15 percent when the battery is new.As the battery ages,the EBM begins drifting and the accuracy drops to 20-30 percent.The model used for monitoring the battery is simply not able to adjust.To solve this problem,EBM would need to know the state-of-health of the 18650 battery,and that includes the all-important capacity.No method exists today that is fully satisfactory,and some mechanics disconnect the battery management system to stop the false warning messages.
A typical start-stop vehicle goes through about 2,000 micro cycles per year.Test data obtained from automakers and the Cadex laboratories indicate that with normal usage in a start-stop configuration,the battery capacity drops to approximately 60 percent in two years.Field use reveals that the standard flooded lead acid lacks robustness,and carmakers are reverting to a modified version lead acid battery.Automakers want to ensure that no driver gets stuck in traffic with a dead battery.To conserve energy,modern cars automatically turn off unnecessary accessories when the battery is low and the motor stays running at a stoplight.Even with this measure,state-of-charge can remain low if commuting in gridlock conditions because motor idling does not provide much charge to the battery,and with essential accessories like lights and windshield wipers on,the net effect could be a small discharge.
Battery monitoring is also important on hybrid vehicles to optimize charge levels.Intelligent charge management prevents stressful overcharge and avoids deep discharges.On a fully charged battery,the IC engine turns off and the car moves on the electrical motor in slow traffic.Improved battery management is of special interest to the manufacturers of the electric vehicle.In terms of state-of-charge,a discerning driver expects similar accuracies in energy reserve as are possible with a fuel-powered vehicle,and current technologies do not yet allow this.Furthermore,the driver of an EV anticipates a fully charged lithium battery will power the vehicle for the same distance as the car ages.This is not the case and the drivable distance will get shorter with each passing year.Distances will also be shorter when driving in cold temperatures because of reduced battery performance.
