Ampere-hours and Watt-hours — How Much It Holds, How Fast It Gives It Back

Two Numbers on the Box

On the back of a phone box, you will find a number like "5,000mAh." In an electric vehicle brochure, numbers appear with "kWh" attached. People call both numbers "capacity," but the units are different. You cannot calculate how many phone batteries it takes to make one electric vehicle battery using just these two numbers alone. There must be one more number.

There are other numbers attached to batteries as well. These are numbers that tell you how much is packed into the same weight, how quickly it can be drawn out, and how much is left right now. This article reads these numbers one by one, and at the end, puts them all to work together on an electric vehicle on a winter morning. What we cover here are the numbers read from outside the battery, while what happens one step inside is covered by other parts of this series.

Ampere-Hours Are the Amount Sent Through, Watt-Hours Are the Work Done

Current is the flow of electricity moving along a wire, measured in a unit called the amperes, indicating how much passes a single point in one second. The "A" in mAh stands for this ampere, "h" stands for hour, and the preceding "m" means one-thousandth.

Lithium ions are particles with a positive electrical charge created when lithium atoms give up an electron. While discharging, lithium ions cross the inside of the battery, and their partner electrons travel around the external wire to make the phone work. The flow thus completes a full loop connecting the inside and outside of the battery. How these two paths split is covered by another part of this series that deals with the structure inside a single battery cell. What is needed for this article is just one thing: the mAh on the box counts the amount of flow that can be sent out into the external wire.

Ampere-hours are the amount found by multiplying the current by the time that current flowed; 1 ampere flowing for 1 hour is 1 ampere-hour. 5,000 mAh is 5 ampere-hours, so mathematically, it can send 1 ampere for 5 hours, or 0.5 amperes for 10 hours. The reason "mathematically" is added here is that the actual amount produced varies depending on the speed of extraction and the temperature.

Energy does not come from ampere-hours alone. Even with the exact same flow, the work done changes depending on how large a difference pushes it along. Voltage is the difference that makes electricity flow between two points, measured in a unit called volts.

Power is the amount of energy electricity moves in one second, measured in a unit called watts. Power is the value obtained by multiplying voltage by current. Watt-hours are the amount of energy found by multiplying power by the time it was used; using 1 watt for 1 hour is 1 watt-hour. The "Watts and Watt-Hours" piece separated these two numbers into a speedometer and an odometer; watt-hours belong on the odometer side.

Thus, multiplying ampere-hours by voltage yields watt-hours. The catch is that the battery voltage does not stay at a single value. It is high when full and gets lower as it empties. It also shifts depending on the intensity of extraction—the harder you pull, the lower the voltage turns out. Nominal voltage is a single representative value written for the battery voltage that drops while discharging. Numbers like "3.7V" written on batteries are this.

Let us calculate using the numbers from this article's example. Assume a 5,000 mAh phone battery consists of a single cell, and take the nominal voltage as 3.7 volts. 5,000 mAh is 5 ampere-hours, and 5 ampere-hours × 3.7 volts = 18.5 watt-hours. We can calculate it another way too. 5,000 milliampere-hours × 3.7 volts = 18,500 milliwatt-hours, and dividing by 1,000 gives 18.5 watt-hours. Both calculations are correct. However, this 18.5 is an approximation multiplied using the representative value called nominal voltage, and if you set the voltage differently, the watt-hours will also change by that proportion.

mAh counts the amount of flow that can be sent to the external wire, and Wh counts the energy found by multiplying that by voltage. The two numbers need different names. Battery capacity is the amount of energy that can be stored in a battery and drawn out for use, written in watt-hours. In this article, "battery capacity" is used exclusively to mean this, and the mAh on the box is called "ampere-hours." In everyday life, mAh is also called "capacity," and in research literature, a cell's "capacity" is frequently written in ampere-hours. One engineering document describes a cell as a 70 Ah one with "high capacity" and a pack as having an "energy capacity of 25 kWh" within the same text (DOI: 10.4271/2012-01-2017). If the unit next to the word "capacity" is mAh or Ah, it is the amount of flow; if it is Wh, it is energy. The "capacity loss" mentioned in another part of this series—which covers why batteries age—also refers to the ampere-hour side.

When 96 Cells Are Connected in a Single Row

A cell is the smallest single compartment making up a battery, containing a positive electrode, a negative electrode, an electrolyte filling the space between them, and a separator dividing the two inside a single container. The positive electrode is the + side electrode of the cell, where lithium ions return and settle when discharging. The negative electrode is the − side electrode of the cell, where lithium ions cross over and stay when charging.

The electrolyte is a substance filling the space between the two electrodes that lets lithium ions pass through while blocking electrons from crossing. The separator is a thin membrane dividing the positive and negative electrodes so they do not touch each other, with many tiny holes through which lithium ions pass. How each of the four parts works is covered in the article dealing with the structure inside a single battery compartment.

Electric vehicle batteries connect hundreds of cells in a single row and side by side to produce the required power and energy (DOI: 10.1109/access.2021.3068776). There are layers between the cells and the finished battery (DOI: 10.3390/wevj11040077). A module is a bundle made by grouping several cells and putting them in a frame, and a pack is a finished product containing the modules gathered together with wiring, a cooling device, and a management device in a single box. Here, the management device refers to the battery management system covered in the article dealing with why batteries age.

Two battery packs designed for a plug-in hybrid vehicle show the actual numbers. Both packs have a nominal voltage of 355 volts and an energy of 25 kilowatt-hours. The first pack connected 96 cells of 70 ampere-hours in a single row. The second pack bundled 192 cells of 35 ampere-hours two by two side by side, and connected those 96 bundles in a single row (DOI: 10.4271/2012-01-2017).

Connecting cells in a single row adds up the voltage. Dividing 355 volts by 96 gives roughly 3.7 volts, meaning the nominal voltage of a single cell matches the value set in the previous section. Bundling them side by side adds up the ampere-hours. One bundle of two 35 ampere-hours side by side is 70 ampere-hours, which is the same as a single cell in the first pack. That is why the energy of the two packs is the same.

We calculate it in two ways. Looking at the pack as a whole, it is 355 volts × 70 ampere-hours = 24,850 watt-hours. Looking at the cells one by one, the first pack has 96 cells of 3.7 volts × 70 ampere-hours = 259 watt-hours, giving 24,864 watt-hours, and the second pack has 192 cells of 3.7 volts × 35 ampere-hours = 129.5 watt-hours, also giving 24,864 watt-hours. Either way, it is roughly 25 kilowatt-hours, matching the value written in the design document. The small difference between the two calculations arises because 3.7 volts is a rounded value of 355 ÷ 96.

Now we can answer the initial question. Dividing the 24,850 watt-hours of this pack, calculated as 355 volts × 70 ampere-hours, by the 18.5 watt-hours of the cell phone battery from the previous section gives roughly 1,340 cells. Comparing them by ampere-hours yields a completely different answer. Even when cells are connected in a single row, the ampere-hours do not add up, so the ampere-hours of the entire pack are 70, which is merely 14 times the 5 ampere-hours of the cell phone. What separates 14 times and roughly 1,340 times is the voltage. Multiplying 14 by 355 ÷ 3.7, which is roughly 96, yields roughly 1,340 again. Both numbers are comparisons between representative values calculated using nominal voltage, and what becomes equal when you gather 1,340 of them is the energy. The number used when comparing batteries is the watt-hour.

By Weight, By Volume — Energy Density

The weight and space that can be loaded into a car are fixed. Therefore, the standard for comparing batteries is how much they pack into the same weight and the same volume (DOI: 10.1002/aesr.202400377). Energy density is the energy a battery packs per 1 kilogram of weight or 1 liter of volume, written as Wh/kg for the weight side and Wh/L for the volume side. This energy density is decisive for how far an electric car can go on a single charge (DOI: 10.1016/j.sajce.2024.09.008).

Current lithium-ion batteries pack 250 to 693 watt-hours into 1 liter of volume. Still, they fall short of gasoline (DOI: 10.1016/j.sajce.2024.09.008). Review papers write the upper end slightly differently, and abstracts do not clarify whether the numbers are for cells or packs. On the weight side, commercial cells exceeding 200 watt-hours per 1 kilogram are counted as high in energy (DOI: 10.1038/s41467-023-38823-9).

Let us feel the weight using the calculation given as an example in this article. If we assume the 25 kilowatt-hours from the previous section are filled entirely with cells rated at 200 Wh/kg, 25,000 watt-hours ÷ 200 = cell weight alone is 125 kilograms. Conversely, 125 kilograms × 200 Wh/kg = 25,000 watt-hours, which matches. This 125 kilograms does not include the weight of the frame, wiring, and cooling system. This also does not mean the pack in the previous section actually used such cells.

The numbers also vary depending on the cell materials. In the series using nickel, manganese, and cobalt, nickel is the side that pulls up the energy density. The iron-phosphate series prioritizes safety and stability instead of having a lower energy density (DOI: 10.1016/j.sajce.2024.09.008). How the two series differ inside a cell is covered in the installment dealing with the inside structure of a cell. What to take away from this section is one line. The number per weight tells you how heavy the car gets, and the number per volume tells you how much fits into the car floor.

When You Go from Cell to Pack, the Numbers Go Down

Every time you move up a floor—from cell to module, and module to pack—the energy per weight and per volume tends to drop compared to the floor below. That was true for the batteries of 25 electric vehicles released between 2010 and 2019 (DOI: 10.3390/wevj11040077). A pack contains frames, wiring, cooling systems, and management systems in addition to cells, and these take up weight and space without storing energy.

Designs for safety also exact a price. The degree to which heat starting in one cell spreads to the neighboring cell depends on the gap between cells, the material placed between them, and the way the cells are stacked. Because these safety devices trim the pack's energy density, safety and energy density become competing goals (DOI: 10.1016/j.etran.2023.100305). The process of heat spreading is covered in another piece of this series on Thermal Runaway — A Staircase Climbed on Its Own Heat.

In that same analysis of 25 vehicles, the numbers at the pack stage were improving, and the numbers per weight were improving faster than the numbers per volume. The authors saw room for the volume side to improve further by refining the packaging method. There was also a trend toward cells changing into larger, more angular shapes (DOI: 10.3390/wevj11040077).

The way the floors are stacked also narrows the differences between cells. At the cell stage, iron-phosphate chemistries have much lower energy density than nickel-based ones. Yet when calculating the retention ratio from cell to pack using the average of both chemistries, the disadvantage per weight at the pack stage shrank to 16%, and the disadvantage per volume could be entirely made up. The authors add that this question needs more study to account for safety designs tailored to cell types (DOI: 10.1016/j.etran.2023.100305). The numbers for the cell and the numbers for the pack loaded onto the car are numbers on different floors, and when comparing them, you compare the same floors.

How Much It Holds and How Fast You Can Take It Out

Lithium-ion batteries can hold as much as 705 watt-hours in 1 liter of volume, and separately, some can deliver as high as 10,000 watts per liter (DOI: 10.1016/j.pnsc.2018.11.002). These two numbers are their respective maximum values. The first number is how much it holds, and the second number is how fast you can take it out. Battery output is the limit of the electric power a battery can deliver, written in watts. Much like energy density, output is also written per weight or per volume for comparison (DOI: 10.1002/aesr.202400377).

There is a way to look at both numbers placed on a single graph. It is a chart that plots energy on one axis and output on the other while mapping various storage devices, and it is a widely used framework for comparing the relationship between energy and output (DOI: 10.1016/j.est.2023.109097). In lithium-ion batteries, the two numbers have a trade-off relationship. You cannot freely increase the ability to hold a lot and the ability to charge quickly at the same time (DOI: 10.1002/aenm.201902523).

One study finds the root of this trade-off in the limits of movement inside the electrolyte (DOI: 10.1002/aenm.201902523). As seen in the cell section, the electrolyte is the path where lithium ions cross between the two electrodes. An electrode is a lump full of tiny gaps, and the slow movement of lithium ions through the electrolyte filling those gaps is one cause that blocks fast charging (DOI: 10.1002/aenm.202101126).

Let us compare this to a scene where an audience moves across the street to take seats in a building opposite after a performance ends. The audience members are lithium ions, the two buildings are the two electrodes inside the cell, and the road between the buildings and the aisles between the seats are the electrolyte. The audience members do not go outside the two buildings, so even when they move their seats, the head count stays the same. If you add more rows of seats in the opposite building, more people can sit, but to reach the inner rows, they have to walk farther. If you rush them by saying, "Everyone sit down within 10 minutes," the aisles get jammed and the inner rows are left empty. There are no electrons running through the external wires in this picture. The path of electrons is covered in the primer dealing with the internal structure of a single battery cell.

The same thing happens at the electrodes. Making the electrodes thick can pack more energy into the pack and also lower the cost. However, if you discharge past a set speed, the movement through the electrolyte cannot keep up, meaning you cannot fully use the cathode, which is the side that receives lithium ions. On the charging side, this same limit leads to lithium plating as metal on the surface of the negative electrode, a story told in the article on why batteries age. Because this limitation was not well understood, as of 2015, most cells going into cars were built with moderately thick electrodes to ensure long life (DOI: 10.1149/2.0321602jes). Conversely, design changes that boost the fast-discharge side reduce the amount they can hold (DOI: 10.1002/aenm.201902523).

In actual design, it is not just the two of them that end up on the scale. Alongside energy and output, lifespan, cost, safety, and environmental impact are all traded off depending on the intended use (DOI: 10.1021/acscentsci.7b00288). Designs that hold a lot and designs that take it out quickly usually undercut each other.

C-Rate — The Same Battery at Different Speeds

The C-rate is a value expressing the strength of the current in relation to the battery's amp-hours, where 1C is enough current to empty a full battery in 1 hour. 2C is the strength to fill it in 30 minutes, and 3C in mathematically 20 minutes (DOI: 10.1002/aenm.202100884). Whether that much actually goes in and comes out depends on the thresholds we will look at below.

Let us calculate with the 70 amp-hour cell from earlier. 1C is 70 amps, and 4C is 4 × 70 = 280 amps. At 4C, mathematically, 70 amp-hours ÷ 280 amps = 0.25 hours, or 15 minutes to empty it completely. Dividing 60 minutes by 4 also gives 15 minutes. The pack's design study modeled the cell's temperature changes by setting the most punishing use case as continuously discharging at 4C from a full state until 90% is extracted (DOI: 10.4271/2012-01-2017).

Speed changes the amount you can extract. When the C-rate crosses the threshold allowed by movement within the electrolyte, the extractable amount drops sharply (DOI: 10.1002/aenm.201902523). It is like rushing an audience—the shorter the time given, the more inner rows are left empty. Large currents passed to charge quickly lower energy efficiency, and are known to accelerate the aging of the battery, which reduces the extractable amount and output (DOI: 10.1016/j.etran.2019.100011). Under what conditions and to what extent this aging occurs is covered in the later section and the installment dealing with why batteries age.

In the graphite electrode, the speeds for inserting and extracting are also not equal. In a study comparing various graphites in laboratory test cells, all cells were fast on discharge and relatively slow on charge, and one of those graphites extracted over 96% even when discharged at 20C (DOI: 10.1149/1.1851055). The decisive step that slows down fast charging is the speed at which lithium moves inside the electrode (DOI: 10.1002/aenm.202101126). Even for the same battery, how quickly you pull the energy out changes how much it delivers.

The Amount You Can Draw Decreases When It Is Cold

When the temperature drops to minus 20 degrees Celsius, current lithium-ion batteries lose a significant amount of the extractable capacity and output they can deliver, failing to operate normally. One review points to the main reason for this: it becomes difficult for lithium ions to move through the electrolyte and pass through the boundary layer that forms within the electrolyte and coats the electrode surface (DOI: 10.1016/j.esci.2023.100170; DOI: 10.1002/anie.202205967). This is like the audience members themselves slowing down their walking pace. Because this boundary layer also acts as a membrane that protects the cell, the installment covering why batteries age discusses it in detail.

The effects of cold on electric vehicle batteries include losses in extractable capacity and output, shortened lifespan, safety risks, imbalances between cells, difficulty in charging, and the complexity of estimating the state of charge, all of which are intertwined (DOI: 10.1109/tvt.2019.2906487). There are also attempts to reduce this loss by changing the electrolyte. A laboratory cell containing a modified electrolyte maintained 82% of the capacity it would deliver at room temperature when tested at minus 20 degrees Celsius and 1/3C (DOI: 10.1002/anie.202205967, laboratory cell and single study).

Charging on a cold day is even trickier. A 2018 study notes that none of the electric vehicles at the time could withstand fast charging in cold or cool temperatures, citing the risk of lithium depositing as metal on the negative electrode surface as the reason. The same study demonstrated a 9.5-ampere-hour, 170 Wh/kg laboratory cell that avoided this risk and charged to 80% in just 15 minutes even at minus 50 degrees Celsius (DOI: 10.1073/pnas.1807115115, laboratory cell and single study). A review covering fast charging highlights thermal management—cooling the battery during charging and pre-heating it on cold days—as a crucial technology (DOI: 10.1016/j.etran.2019.100011).

Higher temperatures bring a different set of problems. An increase in battery temperature above room temperature is the single biggest factor accelerating aging (DOI: 10.3390/en14051248). In the same literature review, the impact of the state of charge during use and the C-rate on aging varied from cell to cell, and a significant portion of the results showing reduced lifespan at high C-rates could be attributed to rising cell temperatures, particularly when high-energy cells were tested with inadequate cooling (DOI: 10.3390/en14051248). The deposition of lithium and aging are covered in another installment of this series, Battery Aging — Fewer Marbles, Cracked Shelves.

The Remaining Percentage is a Calculated Value

A 「37%」 sits in the corner of a phone screen. The state of charge is the amount currently left in the cell compared to the amount when full, written as a percentage. The remaining amount is counted as the amount that can be sent further out through the external wires in the future, namely in ampere-hours (DOI: 10.1109/access.2019.2942213).

Calculating this percentage is the job of the management system that watches over the battery. The management system is responsible for measuring the battery's condition, keeping it operating safely, and extending its lifespan (DOI: 10.3390/en12030446). However, accurately estimating the state of charge is difficult. This is because lithium-ion batteries change their properties over time and form a complex electrochemical system that cannot be drawn as a simple proportion (DOI: 10.3390/en12030446). The estimation also varies depending on the battery's age and the outside temperature (DOI: 10.1109/access.2019.2942213).

The simplest calculation is to keep accumulating the ampere-hour counts from the second section. If you add up the flowing-out current over time, you get how much has been taken out since it was fully charged. Actual estimation methods are broader than this. One review article organizes this into two branches: one that translates the battery's operation into a mathematical model for calculation, and one that has it learn the battery's behavior using a large amount of measurement data (DOI: 10.1109/access.2019.2942213). Pros and cons and errors are reported separately for each method (DOI: 10.3390/en12030446).

It is more difficult in a pack. Hundreds of cells inevitably differ slightly in ampere-hours, resistance, and voltage (DOI: 10.1109/access.2017.2780258), and the conditions they are placed in while in use also differ from cell to cell. Therefore, the management system is also responsible for balancing the differences among cells (DOI: 10.1109/access.2021.3068776). Estimating the pack's state of charge robustly is the basis for accurately informing drivers of their remaining driving range (DOI: 10.1109/access.2021.3068776). This estimation is more difficult in heavily aged batteries. One study attempting to solve that difficulty tested its method with an aged iron-phosphate pack made of 120 cells connected in a single line (DOI: 10.3390/en8065217, single study).

The percentage on the screen is an estimate calculated from values the management system can measure. The accuracy of that estimate varies depending on the battery's age and temperature.

A Winter Morning, Reading the Same Battery Again

Let us imagine a car carrying the 25-kilowatt-hour plug-in hybrid pack we saw earlier spent a night outdoors at minus 20 degrees Celsius, cooling the battery down to minus 20 degrees Celsius as well. Now we can read this car's numbers one by one.

25 kilowatt-hours is a representative value that matches a nominal voltage of 355 volts multiplied by 70 amp-hours. The amount this car can actually draw and use that morning may be less than that. At minus 20 degrees Celsius, lithium-ion batteries lose a significant amount of their extractable capacity and output (DOI: 10.1016/j.esci.2023.100170). There is also a limit on the speed side if you keep drawing a large current. If the C-rate crosses the threshold while drawing at that speed, the extractable capacity drops sharply (DOI: 10.1002/aenm.201902523). Whether this car's usage crosses that threshold is not stated by the evidence in this article.

The percentage and remaining driving range shown on the instrument cluster are estimates by the management system. Those estimates vary depending on the outside temperature (DOI: 10.1109/access.2019.2942213), and the cold makes state estimation more complex (DOI: 10.1109/tvt.2019.2906487). If you try to charge while the battery is cold, the cold adds yet another problem: the difficulty of charging (DOI: 10.1109/tvt.2019.2906487). This is why a review paper covering fast charging considers thermal management that pre-heats the battery on cold days to be decisive (DOI: 10.1016/j.etran.2019.100011).

The watt-hours of the box are a number calculated using a representative value called nominal voltage. Even for the same battery, the amount it delivers varies depending on how quickly and in how cold a place you draw it from, and the screen estimates and shows that amount.

Where to Look Next

The difference between power and energy, and between watts and watt-hours, is covered in detail in "Watts and Watt-Hours — Reading Electricity's Two Numbers Like a Speedometer and an Odometer". The fact that current does not diminish even after making a full circuit, and therefore why a battery does not look like a vessel that stores current, continues in "Voltage, Current, Power — What Goes All the Way Around Undiminished".

Among the other parts of this series, "The Battery Cell — A Rocking Chair Between Two Electrodes", which covers the structure inside the cell, deals with how lithium ions and electrons travel different paths, and the differences between nickel-based and iron-phosphate-based chemistries. "Battery Aging — Fewer Marbles, Cracked Shelves", which covers why batteries age, addresses as far as the evidence states why the extractable amount decreases each year and how conditions like temperature, charging speed, and state of charge alter that pace. "Thermal Runaway — A Staircase Climbed on Its Own Heat", which covers thermal runaway, deals with what safety designs like the spacing between cells are meant to prevent.

Source Papers

• "Lithium-ion battery fundamentals and exploration of cathode materials: A review" (2024) — DOI: 10.1016/j.sajce.2024.09.008 — Energy density determines driving range, lithium-ion ranges from 250 to 693 Wh/L but falls short of gasoline, nickel trades for energy density and phosphate for safety and stability at the cost of lower density.
• "Fast charging of lithium-ion batteries at all temperatures" (2018) — DOI: 10.1073/pnas.1807115115 — Electric vehicles at the time could not withstand fast charging in cold or cool temperatures due to the risk of lithium plating. A 9.5 Ah, 170 Wh/kg laboratory cell achieved 80% charge in 15 minutes at −50 °C.
• "Extreme fast charging of commercial Li-ion batteries via combined thermal switching and self-heating approaches" (2023) — DOI: 10.1038/s41467-023-38823-9 — Defines commercial high-energy cells as exceeding 200 Wh/kg.
• "Ragone plots revisited: A review of methodology and application across energy storage technologies" (2023) — DOI: 10.1016/j.est.2023.109097 — A widely used comparison framework quantifying the energy-power relationship.
• "Electrolyte design principles for low-temperature lithium-ion batteries" (2023) — DOI: 10.1016/j.esci.2023.100170 — Capacity and power are heavily lost at −20 °C, with the primary cause being sluggish lithium-ion transport within the electrolyte and its derived boundary layers.
• "Fast Charging of Lithium‐Ion Batteries: A Review of Materials Aspects" (2021) — DOI: 10.1002/aenm.202101126 — The decisive rate-limiting step of fast charging is lithium transport inside the electrode, constrained further by slow ion migration through the electrolyte within the porous electrode.
• "Diffusion‐Limited C‐Rate: A Fundamental Principle Quantifying the Intrinsic Limits of Li‐Ion Batteries" (2019) — DOI: 10.1002/aenm.201902523 — The energy-power trade-off stems from electrolyte diffusion limits; exceeding that C-rate drastically reduces capacity, and modifications to increase it reduce the nominal specific capacity.
• "Design of Direct and Indirect Liquid Cooling Systems for High- Capacity, High-Power Lithium-Ion Battery Packs" (2012) — DOI: 10.4271/2012-01-2017 — 3D finite element thermal simulation of two 355 V, 25 kWh (「nominal energy capacity」) PHEV packs (70 Ah cell 96S1P and 35Ah cell 96S2P with 「large cell capacity」) under worst-case conditions of 4C continuous discharge and 90% depth of discharge.
• "From Cell to Battery System in BEVs: Analysis of System Packing Efficiency and Cell Types" (2020) — DOI: 10.3390/wevj11040077 — Density decreases at every step from cell to module to system across 25 models from 2010 to 2019, with per-weight improving faster than per-volume, leaving room for packing optimization and showing a trend toward large prismatic cells.
• "Thermal runaway propagation in automotive lithium-ion batteries with NMC-811 and LFP cathodes" (2023) — DOI: 10.1016/j.etran.2023.100305 — The conflicting goals of propagation-barrier design and energy density, with LFP's weight disadvantage narrowing to 16% at the pack stage on an average basis while its volume disadvantage is compensated, necessitating further research.
• "Temperature effect and thermal impact in lithium-ion batteries: A review" (2018) — DOI: 10.1016/j.pnsc.2018.11.002 — Lithium-ion energy density reaches up to 705 Wh/L, power density up to 10,000 W/L, with temperature heavily dictating performance.
• "Structured Electrodes for Lithium‐Ion Batteries and Their Impact on Cell Energy Density and Power Density: A Review" (2025) — DOI: 10.1002/aesr.202400377 — Defines energy and power density as storage capacity and delivery rate per given volume and mass, respectively.
• "Optimizing Areal Capacities through Understanding the Limitations of Lithium-Ion Electrodes" (2015) — DOI: 10.1149/2.0321602jes — Thick electrodes raise pack density and lower cost, but electrolyte transport limits utilization at critical C-rates, which is why most vehicle cells at the time had moderate thicknesses.
• "An Outlook on Lithium Ion Battery Technology" (2017) — DOI: 10.1021/acscentsci.7b00288 — Energy, power, lifespan, cost, safety, and environmental impact are traded off depending on the application.
• "Microstructure Engineered Ni‐Rich Layered Cathode for Electric Vehicle Batteries" (2021) — DOI: 10.1002/aenm.202100884 — Stable for 500 cycles at 3C charging (full charge in 20 minutes).
• "Lithium-ion battery fast charging: A review" (2019) — DOI: 10.1016/j.etran.2019.100011 — High currents in fast charging lower energy efficiency and accelerate capacity and power degradation, making thermal management through cooling during charge or preheating on cold days crucial.
• "High Rate Capability of Graphite Negative Electrodes for Lithium-Ion Batteries" (2005) — DOI: 10.1149/1.1851055 — Graphite half-cells exhibit high discharge rate performance and comparatively lower charge performance, maintaining over 96% at 20C discharge (for one graphite).
• "xEV Li-Ion Battery Low-Temperature Effects—Review" (2019) — DOI: 10.1109/tvt.2019.2906487 — Nine low-temperature effects (capacity and power loss, lifespan, safety, imbalance, charging difficulty, state estimation complexity, etc.) and their correlations.
• "Enhancing Li+ Transport in NMC811||Graphite Lithium‐Ion Batteries at Low Temperatures by Using Low‐Polarity‐Solvent Electrolytes" (2022) — DOI: 10.1002/anie.202205967 — Modified-electrolyte laboratory cells retain 82% of room-temperature capacity at −20 °C and 1/3C, with low-temperature performance governed by charge-transfer resistance at the electrolyte-electrode interface.
• "Temperature, Ageing and Thermal Management of Lithium-Ion Batteries" (2021) — DOI: 10.3390/en14051248 — Rising temperature is the single most important aging acceleration factor, SOC and C-rate impacts vary by cell, and a significant portion of high-C lifespan reduction may be attributable to temperature rises during testing.
• "State of Charge Estimation for Lithium-Ion Batteries Using Model-Based and Data-Driven Methods: A Review" (2019) — DOI: 10.1109/access.2019.2942213 — SOC is the remaining charge in the battery, and estimation is complicated by age and outside temperature, categorized into model-based and data-driven methods.
• "Towards a Smarter Battery Management System for Electric Vehicle Applications: A Critical Review of Lithium-Ion Battery State of Charge Estimation" (2019) — DOI: 10.3390/en12030446 — The BMS handles state measurement, safety, and lifespan extension, while SOC estimation for time-varying, nonlinear batteries remains difficult, complete with pros, cons, and errors of each method.
• "Critical Review on the Battery State of Charge Estimation Methods for Electric Vehicles" (2017) — DOI: 10.1109/access.2017.2780258 — Cells within a pack inevitably differ in capacity, resistance, and voltage.
• "Lithium-Ion Battery Pack Robust State of Charge Estimation, Cell Inconsistency, and Balancing: Review" (2021) — DOI: 10.1109/access.2021.3068776 — A pack consists of hundreds of cells in series and parallel, involving cell inconsistency and balancing, with pack SOC estimation serving as the foundation for remaining driving range displays.
• "Extended Kalman Filter with a Fuzzy Method for Accurate Battery Pack State of Charge Estimation" (2015) — DOI: 10.3390/en8065217 — SOC estimation becomes particularly difficult after aging has progressed, tested with a 120-cell series-aged LFP pack.