A statement that EV owners are tired of hearing is that “you will have to replace the batteries in a few years costing you thousands of dollars that will negate all the savings that your EV has made cutting the fuel cost”.
This feels true? Right? Since the early 2000’s we have been using lithium-ion batteries in our portable devices. These batteries are known to lose their capacity to hold the charge as the years pass by. So is it relatable that the batteries in our beloved EV’s will give similar experience requiring to be replaced just after few years of use? also How Long Does a Tesla Battery Last?
Well, the good news is that correlation does not imply causation.
How Long Does a Tesla Battery Last
Let’s understand a few things about Tesla Batteries.
Tesla uses standard Panasonic lithium-ion batteries that we use in daily lives. When the battery is charging up, the lithium-cobalt oxide, positive electrode gives up some of its lithium ions which move through the electrolyte to the negative, graphite electrode and remain there. During this process, the battery takes in and stores the energy.
These batteries are connected in a combination of series and parallel that is very intrinsically calculated to give the balance of longer range and instant torque, performance, and longevity. These almost 7000 cells make up 16 modules that come together to make it easy for any replacement or service to be done.
The dimensions of these batteries govern the density at which the energy can be stored in a given volume of these cells and represents the ideal balance between the surface area for cooling, the amount of energy that can be stored in the given volume and most important “the price”.
The batteries used are 18650 and 2710 lithium-ion cell. These battery nomenclatures depict the dimensions of the battery cells where the cells stretched the length from 65mm to 70mm and increased the diameter of the cell from 18 mm to 21 mm.
Amongst other unique feats involved in these batteries a special epoxy seals the battery pack making removal, replacing and reusing individual cells difficult. In between cells plastic padding is used as a structural support preventing them from rattling around in the pack.
The anodes and cathodes of the cell have seen significant levels of improvements from the 18650s to the 2710s, hence improving the battery life and performance. The 2170 design is 46% larger in volume than 18650 and is able to hold 10-15% more energy as well.
Tesla has been able to cram a comparably larger surface area of battery material in its 2710 when compared to the 18650. This when adds to the improved design of the internal batteries equates to denser energy in the same volume at lower weight.
All of these superior ingredients and advanced construction of the battery pack, make up the Energy Storage System (ESS). There are a few very important systems incorporated to the ESS enabling it to perform as desired:
- The Battery Management System (BMS)
- The Cooling System
- The Charging Station
The Battery Management System
BMS is a dedicated control unit that observes each and every cell of the battery pack and works with each of them individually to maximize their efficiency and longevity so the degradation that happens over time is carefully controlled. The BMS controls the flow of energy to and fro in between the battery pack and the electric motors. The BMS can play a key role in increasing the life span of the Tesla Battery Pack.
Another amazing feat is charging behavior. Lithium-ion batteries are the happiest between 30% to 70% state of charge. The degradation of cells is rapid when they are charged down to 0% and up to 100% in every single charge cycle or when left plugged to charge at 100% for extra hours. The Tesla manuals recommend a daily charging amount. The AI in the car learns from the user’s usage pattern and suggest the user an appropriate charging percentage for their optimum drivability without overloading the battery. It suggests that charging over 90% is only to be done when planning for long-range drives and starting the trip immediately at reaching a 100% charge state. All this ensures good battery health.
An EV’s battery pack is given a buffer built on its top end and low end of the battery meaning that the 100% state of charge may not always means that all the battery cells are fully charged but the cells that are operational are charged at their 100 %. Similarly the lower end of the battery may be affected due to some cells that are inactive while holding some charge but is not usable. This wriggle room in the car’s battery helps in maintaining the car’s electrical system if the battery runs too low and the buffer system gives the BMS some leeway in estimating the total range of the car.
The Cooling System
An efficient cooling system, designed to prevent catastrophic cell failures from propagating to adjacent cells, even when the cooling system is off. The Cooling System and Battery Management System kept the temperatures and voltages under specific limits. As the battery packs get compact holding higher energy in smaller volumes their flammability temperature has significantly dropped from around 350 deg F to around 150 deg F. This should make them prone to catch fire easily and to overcome this serious issue the use of Glycol coolant in its cooling system has been shifted from Serpentine Cooling System to the much efficient Manifold Cooling system in the current lineup. These cells arrangement has passages in between them through which glycol coolant is passed effectively decapitating the heat generated which may reduce the efficiency as well may cause other issues such as overheating or catching fire.
The Charging Station
Since batteries are direct current (DC) devices and home electrical service is AC, charging at home generally use a 240-volt circuit supplying 40 amperes (10kW of power). The car equipped with built-in charging circuitry rectifies the AC, converting it to DC. Charging this way usually takes several hours so Tesla has installed Supercharger DC charging stations worldwide that supply up to about 135kW of power. The DC bypasses the car’s charging circuitry and charges the battery pack directly. This is much faster, requiring 20 to 40 minutes. Frequent use of supercharging may hasten degradation as it may rapidly damage the ions required to transfer the charge in the electrolyte making a part of the cells obsolete.
The life of a Tesla battery may vary on how well each battery is controlled to prevent its degradation which as a whole decides the life of the Battery Pack.
Talking in terms of battery degradation, data shows less than 10% degradation in the energy density after over 160000 miles on Tesla’s battery packs. The data gathered over a certain period of time shows that for the first 50000 miles the Tesla Battery Pack loses 5% of their capacity, but after the 50000-mile mark, the capacity levels off, and it is observed to be difficult to make a pack degrade by another 5%.
The data collected suggests that to come close to 90% capacity the average battery pack could cycle through over 186000 miles.
The warranty states that the Battery and Drive Unit in a Tesla Vehicles are covered for a period of –
- The 18650 battery pack: 8 years or 150000 miles, whichever comes first, with a minimum 70% retention of capacity over the warranty period.
- The 7210 battery pack(for standard range): 8 years or 100000miles, whichever comes first, with minimum 70% retention of capacity over the warranty period.
- The 7210 battery pack(for long-range): 8 years or 120000miles, whichever comes first, with minimum 70% retention of capacity over the warranty period.
According to the widely accepted data that an average driver drives around 12000 miles a year the life of a Tesla Battery can be assumed to be between 8 to 12 years under warranty coverage. Tesla Motors also claims that the current lifetime range of its Battery Packs varies from 300000 miles to 500000 miles. Taking into account standard conditions and the annual running of 12000 miles, a total life span of 25 to 40 years is to be expected.
Correlating the warranty details and using this data, again assuming standard conditions and an annual run of 12000 miles, keeping 70% retention of capacity as the limit after which the battery needs to be replaced hence to be replaced at this point it would take around 15-18 years.
So what is the problem with all the Tesla 85 packages? Is it the electronics in the BMS, the high voltage inverters/relays or the batteries that are getting defect? It broke down on my TMS 85 on 43k km.