Synopsis: The benefits of a larger battery go far beyond range.
The benefits of a larger battery are clear when comparing two battery options of one model of vehicle, although it does become a little more complex when battery options use a different battery chemistry. Even more complex is comparing different vehicles from different brands, but a starting point is to understand the impact of battery size alone, before then taking into account vehicle efficiency or other factors.
So, the first step is to consider the impact of changing only battery size, and then separately consider cases when all else is not equal.
Coming from an ICE vehicle, EV battery size can misleadingly seem like a direct equivalent to fuel tank size, with increased range as the only benefit of a bigger battery. In reality, a better comparison is that a bigger battery more closely resembles an upgrade to both engine performance, durability, regenerative braking, and vehicle life-expectancy, in addition to that increase range similar to the effect of a longer-range fuel tank in an ICE vehicle.
Battery size matters because:
- Battery size determines vehicle/battery life in terms of the total distance that can be driven over the life of the vehicle before battery degradation.
- Double the battery size to double the years before a replacement battery is needed.
- Increased battery size reduces required “charge time” for any given trip.
- The same % charge will take the same time or less with a larger battery, and that same % will enable driving further.
- The battery determines potential vehicle performance.
- Just as the engine in an ICE vehicle converts chemical potential energy into heat, the battery in an EV converts chemical potential energy into electricity.
- A larger battery provides for increased regenerative braking.
- Regenerative braking is limited by the ability of the battery to store the energy.
Battery size and battery life and degradation.
Battery life for the same power is directly proportional to battery size.
The simple rule is: “double the battery size to double the battery life under the same conditions“.
Conversely, halving the battery size will half the battery life… etc.
The battery life for any given battery technology is expressed in terms of the number of “full-charge cycles”, where one charge cycle is from fully charged to discharged, and then again recharged.
Note that two “half-charge cycles” would only make at most one “full-charge cycle”, as explained in more detail in “EV Battery Lifetime, Care And Management“.
This means to get the same life from half the battery capacity, the only path to maintain battery life is to halve the usage of the battery. In an EV, this means with half the battery size, the vehicle can only be driven half the distance on battery power during the life of the vehicle.
The total distance a battery can power a vehicle over the life of the battery can be approximately calculated by:
- charge cycles x battery powered range.
This means for any given battery technology, if you halve the battery powered range of a vehicle, you half the distance the vehicle can be driven on battery power, since then same total difference would require double the number of full battery charge cycles. Find a battery chemistry that provides a lifetime of double the number of charge cycles, and then the battery powered range can be halved with no impact on battery life. Overall, the range of a vehicle together with battery chemistry and measures to maximize battery life determine more than just range alone.
Vehicles with smaller batteries don’t save more emissions per battery resource.
The implications for hybrids and plug in hybrids, is that while two plug-in hybrids with half sized batteries can be built instead of one EV with a full-size battery, each of the plug-in hybrids could only be driven half the distance on battery power as the EV before the batteries must be replaced.
Thus, the argument that, for example “hybrids could enable building 5x the number of vehicles for a given battery production capability”, is really only valid if those hybrids vehicles will be powered by fossil fuels 80% of the time as otherwise, as otherwise they could need to have their batteries replaced during their lifetime.
Battery life also changes with battery type.
Manufacturers can also address the smaller lifespan problem for smaller batteries problem by changing battery chemistries. For example, while prior to 2023 almost all EVs other than BYD vehicles and some Tesla vehicles used Lithium batteries with chemistry based on Nickel and Cobalt (sometimes called “ternary” batteries), more vehicles from 2023 onwards are being fitted with LFP batteries which typically have over 2x or 3x the lifetime recharge cycles of ternary batteries.
This means a Tesla with a lower-range LFP battery may have a longer total lifespan than a Tesla with a longer-range ternary battery. More significantly, since so far Teslas ass have quite long-range batteries, is the fact that the use of LFP batteries can avoid the problem of PHEVs (plug-in-hybrids) that are driving mostly on electric power could be at risk of a short battery life.
When comparing batteries, the simple rules only apply if battery chemistries are the same.
Charging speeds and battery size.
Increasing battery size enables at least a similar increase in charging speed.
Generally, all batteries of the same exact type will have the same total charge, which can be confusing as makes it sound like charging speed doesn’t change, but taking the same time to charge larger battery means in fact means faster charging, as with as larger battery, more energy can be added in the same time, and the same energy in less time.
Consider that if both a 100kWh battery and 50kWh battery take one hour to fully charge, then on average 100kWh battery is accepting charge at 100kW and the 50kWh battery is accepting charge at 50kW.
Specifications such as “from 10% to 80% in 20 minutes”, apply independently of battery size, so a 100kWh battery will be able charge from 10% to 80% in the same 20 minutes time as a 50kWh battery charges from 10% to 80%, despite 70kWh of charge being added to the 100kWh battery and only 35kWh of charge being added to the 70kWh battery in those 20 minutes.
A more detailed explanation is available here, but the bottom line is, all else being equal, the bigger the battery, the faster it can be charged with the power required to reach any one destination.
Battery size is not the only charging speed factor: C-Rates and wiring/pack limits.
The reason batteries typically take the same amount of time to charge despite even they are different sizes, is that batteries are made of a number of battery cells which call all be charged simultaneously, so the charge time of the entire battery is determined by the charge time those cells within the battery.
The simplest way to produce batteries of different capacities is to use a different number of the same cells. Since charge time of the entire battery is determined by charge time of the individual cells, just using a different number of the same cells clear will not change charging time. So, both a 50kWh and 100kWh battery using the same cells will take the same time to charge 100%. If that charge time is 1 hour, then the average charge rate of the 50kWh battery would be 50kW, and of the 100kWh battery would be 100kW.
Other than the number of cells or size of cells in use, the other factor of the battery affecting charging rates is the rate at which the individual cells can accept charge, often described as the “C-rate” of the cells, which is determined by factors including:
- The overall battery cell chemistry.
- Details of battery construction and materials.
- The balance between fastest charging rates and battery life chosen by the manufacturer.
Then then are the current limits of the wiring and components connecting the charger to the cells of the battery as the final factor limiting charging speeds.
Battery size can seem like engine size in an ICEV.
EV batteries as the power unit: More like an ICE vehicle engine than fuel tank?
Another, and often overlooked factor, is that batteries are not only limited in their charging speed, but also their discharging speed, which dictates the performance potential under electric power.
The maximum discharge rate of a battery is the rate at which it can convert the chemical potential energy inside the battery into electrical power.
For an EV to have maximum power of 300kW, the battery must be capable of delivering 300kW. While the maximum power of an EV can be higher than the maximum charge rate, this is because the maximum charge rate will be sustained longer than maximum acceleration can occur. The maximum sustained power over long time is typically around the same as the maximum charge rate.
The battery of an EV is many ways the equivalent to the engine of combustion vehicles.
Battery size can also improve regenerative power and thus vehicle efficiency.
In the absence of making uses of a more advanced battery technology, a larger battery will normally weigh more, and that extra weight will reduce vehicle efficiency, and more energy is required to accelerate a heavier vehicle, and rolling resistance is increased.
Accelerations times will usually improve, because while a 2x larger battery could in a 2x heavier battery, it does not make the entire vehicle anywhere near 2x heavier, but it does make 2x more power available to the motor(s). While that faster acceleration will require more power the reality can be something a 1600kg vehicle becoming 80kg or 5% heavier vehicle, and with 25% more battery power. The vehicle increases in weight being much greater than increase in power and that increase in power also translates to more deceleration power when using regenerative braking. More powerful regenerative braking can result in more efficient urban driving and less wear on friction brakes, but will help combat any increased rolling resistance in highway driving at higher speeds and with less use of regenerative braking.
In practice, the better regen will more reduce or to some extent negate efficiency losses from the extra weight, rather than actually result in overall efficiency gains.
Other considerations: What happens when all else is not equal?
Battery size as the only difference is the simplest case.
There are also cases when vehicles are different in more ways than just battery size.
While most often when the same make and model is offered in two battery sizes, the only difference between option is the number and/or size of the cells in the two batteries.
In this simplest case, the vehicle with the larger battery will:
- Have a longer battery lifespan/lifetime.
- Be able to be charged at a higher charge rate.
- Have the potential for greater motor power, and higher maximum regenerative braking.
- Have very similar overall efficiency.
But it is not always that simple, and there are other times when the value of a larger battery is to be considered.
More complex cases range from two versions of the same vehicle but with batteries with different chemistries through to comparisons between different brands of vehicles that can be considered competitors or alternative buying choices but are offered with different battery sizes.
Battery lifespan: battery size, chemistry and battery management.
By using an alternative battery chemistry, a smaller battery could outlast a larger battery. For example, in 2023 the Tesla Model 3 and Model Y smaller battery size models use are available with an LFP battery that would be expected to outlast longer range models with NCA batteries of the longer-range models.
Even with the same battery chemistry, a larger battery may not outlast a smaller battery when fitted to a less efficient vehicle. In 2022, the Tesla Model 3 and Polestar 2 could be considered vehicles competing in the same segment, but the Tesla was more efficient, and provided a longer range even when fitted with a smaller battery. This means for the same number of battery cycles, the Tesla could be driven further despite having a smaller battery, so there for could provide a longer battery life in terms of distance travelled, despite the smaller battery.
Effective charging is about battery size, “c-rates” and efficiency.
Yes, charging speeds are directly impacted by battery size, and while this is the main factor when all else is equal, all else is not always equal. Different generations of battery, makes and constructions result in different charging speeds for batteries of the same size: effectively different “C-Rates”. The rule about “bigger is better” does not necessarily apply when comparing batteries that differ in ways beyond their size, and thus have different “C-Rates”.
Charging times are complex, as batteries do not accept charge at a constant speed, and specification for peak charge rate can be misleading, as that peak charge rate can apply for only a very brief period during charging. When comparing different vehicles, the best guides are if there is a similar specification between vehicles such as charge time “from 10% to 80%”.
The, again efficiency is a critical factor. One vehicle being able to add 10% more kW every 15 minutes than a rival sounds impressive, but if the rival travels further on those 10% less kW, then rival adds the charge needed to reach a destination faster, and because it needs less energy, both the time and cost of charging are reduced.
Charging rate and Regenerative braking: battery size and c-rate.
The c-rate determines the time needed to charge a battery to a given percentage, the battery size determines the amount of energy corresponding to that given percentage. Combine c-rate and battery size and you get the maximum charge rate in kW for a given battery.
What can be easily overlooked is that the maximum charge rate is also the maximum available regenerative braking power.
The Hybrid and Plugin Hybrid Battery Life Limitation.
(Note, the plan is to add links to claims by manufacturers)
The appeal of the hybrid.
Consider the appeal of the plug-in hybrid, or small battery EV designed for commuting: instead of “lugging” around a heavy battery that could last an entire week in the city, why not a less expensive lighter battery that will still be sufficient for just over one full day? Or in the case of a plug-in hybrid, a why not only battery that is only sufficient for a typical day, given there is the “back-up” of an Internal Combustion Engine for any occasion when the battery is not enough?
One problem with these smaller batteries, is that these batteries could experience a full cycle every 1 or 2 days, resulting in battery life similar to that of a mobile phone of around 5 years, or if there is a full cycle every day, even just half the life of the battery of a mobile phone!
The potential for hybrid battery life problems.
Taking into account the fact that battery life is proportional to battery size, then if an EV with a 60kWh battery has a battery size that means the battery will last the full vehicle lifetime, then a hybrid with a 2kWh battery can only be driven 1/30th of the distance during its lifetime as the EV with the 60kWh battery. While this is possible, the goal of the hybrid is to use the battery to reduce fuel consumption by more than 1/30th! Battery chemistries like LFP provide longer battery life with an increased number of recharge cycles, which could mean the EV battery would last 3x longer than the vehicle lifetime, and small hybrid battery be able to in use for 1/10th of the time, which is more realistic.
Two strategies are needed with hybrids and plug-in hybrids:
- Limit the use of driving on battery power in line with battery lifetime estimates.
- Choose a battery chemistry that provides for a long battery life.
Hybrid battery live rule 1: limit the use of driving on battery power.
To maximise battery life of a small battery, the easiest solution is to limit how much power is sourced from the battery per year. These means limiting the number of kilometers/miles driven under battery power and/or partially under battery. In the ideal world battery power comes either from regenerative braking and clean renewable energy, but the smaller the battery, the less time the vehicle can be driven from these sources of power.
Hybrids or “self-charging hybrids”, mostly only make use of the battery when accelerating or decelerating, and with a battery usually typically around 5% of the size of a full EV, if the hybrid obtains more than 5% of its power from the battery, then that battery will have a shorter life than the battery of the full EV.
This can be an even more significant limitation with PHEVs, which do have larger batteries, but in theory offer the promise of only needing ICE power when on longer road trips, when the EV power only range is insufficient. However, with a battery of anywhere between 20% and 50% of than range of a typical full EV, a very short battery life could result if the owner actually succeeds in doing the bulk of their driving on battery power alone.
Hybrid battery live rule 2: Choose a long-life battery chemistry.
Consider the steps taken by GM for the Chevrolet Volt battery:
The carmaker evaluated about 25 battery cell chemistries and constructions from around two dozen lithium-ion battery makers around the world.[6] Due to their more promising cell technologies, two companies were selected in June 2007, Compact Power (CPI), which uses a lithium manganese oxide (LiMn2O4) cell made by its parent company, LG Chemical; and Continental Automotive Systems, which uses lithium iron phosphate based cylindrical cells made by A123Systems.[6][7][8] By the end of October 2007 CPI (LG Chem) delivered their finished battery pack prototypes, and A123 delivered theirs by January 2008. GM’s testing process was conducted at the laboratory the carmaker had created for the GM EV1 program. The battery packs included monitoring systems designed to keep the batteries cool and operating at optimum capacity despite a wide range of ambient temperatures.[6][8] To ensure the battery pack would last 10 years and 150,000 miles (240,000 km) expected for the battery warranty, the Volt team decided to use only half of the 16 kWh capacity to reduce the rate of capacity degradation, limiting the state of charge (SOC) up to 80% of capacity and never depleting the battery below 30%.[8][9] GM also expected the battery to withstand 5,000 full discharges without losing more than 10% of its charge capacity.[8]
https://en.wikipedia.org/wiki/Chevrolet_Volt
Updates.
More details and explanations to be added.
- 2023 November 1 st: full page revision.
- 2023 April 13 th: preliminary version.