Power can be generated in many ways: burning fuel in a combustion engine, converting mechanical movement in a generator, using photosynthesis to store solar energy chemically, or producing electrical energy directly through electrochemical reactions. A battery belongs to the last category. It stores chemical energy and releases it as electrical energy when a voltage potential drives current through an external load.
The distinction between how much energy a battery stores and how quickly it can deliver that energy is central to battery selection. A small cell may run a low-current device for a long time but collapse under a high-current load. Another cell may supply a large current burst but have less total stored energy. The Ragone plot is a practical way to visualize this tradeoff.
Power, Energy, and Capacity in Battery Performance
A battery is commonly described by voltage, current, capacity, energy, and power. These terms are related, but they do not mean the same thing.
Voltage is the electrical potential difference produced by the cell chemistry. Current is the flow of charge through the circuit. When a load is connected, the battery attempts to maintain voltage while supplying current. The product of voltage and current is power, measured in watts:
Power (W) = Voltage (V) × Current (A)
The watt is the SI unit of power and is named after James Watt, whose work on steam engines helped establish practical ways to compare the rate at which machines do work. In battery engineering, power describes the rate of electrical energy delivery, not the total amount of energy stored.
Energy is power integrated over time. For batteries, it is often expressed in watt-hours:
Energy (Wh) = Voltage (V) × Capacity (Ah)
This is why ampere-hour capacity alone can be misleading. A 2 Ah cell at one voltage does not store the same energy as a 2 Ah battery at a different voltage. Even at the same nominal voltage, usable energy depends on discharge current, cutoff voltage, temperature, cell condition, and the manufacturer’s rating method.
A simple analogy is a bicycle rider. The rider’s stored energy is the amount of work available before exhaustion. The rider’s power is how quickly that energy can be delivered. On level ground, the rider may travel for a long time at moderate power. On a steep climb or during a sprint, the rider delivers higher power but for a shorter duration. The total available energy and the instantaneous power capability are connected, but one does not fully define the other.
A fluid-container analogy is also useful:
- The amount of liquid in the container represents stored energy or capacity.
- The size of the outlet and the pressure behind it represent power capability.
- A large container with a narrow outlet can store much liquid but deliver it slowly.
- A smaller container with a large outlet may empty quickly but supply a high flow rate.
This helps explain why battery capacity ratings in ampere-hours should be interpreted with care. A cell rated at 2 Ah can theoretically deliver 2 A for one hour, 1 A for two hours, or 0.5 A for four hours under idealized conditions. Real batteries do not behave perfectly this way. Higher currents increase internal losses, voltage sag, heating, and sometimes reduce usable capacity.
Primary alkaline cells illustrate the difference between energy and power. In low-drain service, alkaline cells can offer relatively high specific energy for their size and cost. However, they are generally poor high-power sources compared with many rechargeable systems. Rechargeable lithium-ion, nickel-based, and lead-acid batteries are often designed for higher load currents, such as power tools, robotics, engine starting, and backup power. Their usable capacity may be lower than a primary chemistry under light-drain conditions, but their power delivery is much stronger.
How a Ragone Plot Shows the Energy-Power Tradeoff
A Ragone plot is a graph used to compare energy storage or energy conversion devices by their energy and power characteristics. It is named after David V. Ragone and was first associated with battery performance comparison, though the same type of plot is now used more widely.
The core idea is simple: place energy on one axis and power on the other. The position of a device or cell then shows whether it is better suited to long-duration energy delivery, short high-power bursts, or a compromise between the two.
The Battery University reference describes the traditional layout as:
- energy in watt-hours on the horizontal x-axis
- power in watts on the vertical y-axis
Many modern Ragone plots reverse this convention or use specific values instead:
- specific energy in Wh/kg
- specific power in W/kg
- energy density in Wh/L
- power density in W/L
Because conventions vary, the safest rule is: always read the axis labels before interpreting a Ragone plot. Do not assume that energy is vertical or horizontal. Also check whether the plotted values are absolute values for a particular cell, gravimetric values normalized by mass, or volumetric values normalized by volume.
Ragone plots commonly use logarithmic scales. This matters because energy storage devices can differ by orders of magnitude. A supercapacitor, a lithium-ion battery, a flywheel, and a fuel cell system may occupy very different ranges of energy and power. A logarithmic scale allows those ranges to appear on one graph without compressing the lower values into an unreadable corner.
A key feature of many Ragone plots is the diagonal constant-time line. The operating time can be estimated from the ratio of energy to power:
Time (h) ≈ Energy (Wh) ÷ Power (W)
For specific quantities, the same relationship applies:
Time (h) ≈ Specific energy (Wh/kg) ÷ Specific power (W/kg)
These diagonal lines are sometimes called iso-time lines or constant-discharge-time lines. They show whether a battery can support a given power level for seconds, minutes, or hours. A device far toward the high-power region may be excellent for pulses but unsuitable for long runtime. A device far toward the high-energy region may run longer but may be unable to safely supply a high current.
A Ragone plot is therefore not just a ranking chart. It is a map of compromise.
Comparing Lithium-Ion Cells on a Ragone Plot
Lithium-ion cells are a useful example because the same cylindrical 18650 package can contain very different internal designs. A cell optimized for power tools is not the same as a cell optimized for laptop runtime, even if both are called lithium-ion and both use an 18650 format.
Different lithium-ion chemistries and electrode designs trade specific energy against specific power. In simplified terms:
| Cell design priority | Typical strength | Typical limitation |
|---|---|---|
| High-power design | High discharge current, lower voltage sag, better short-duration power | Lower total energy than high-capacity cells |
| High-energy design | Longer runtime at moderate loads | Lower maximum safe discharge power |
| Balanced design | Moderate energy and power | Not best at either extreme |
The Battery University example compares four lithium-ion systems packaged as 18650 cells. It highlights the difference between a high-power cell such as an A123-type cell and a higher-capacity cell such as a Sanyo F-type cell.
At a very short discharge duration, the high-power cell is favored. On the 3.3 minute diagonal line, the A123 example can deliver up to about 40 W. The Sanyo F example is slightly lower at about 36 W. In this region, the main requirement is high discharge power for a short time, so the high-power cell is the stronger choice.
The conclusion changes when the required discharge time is longer. Along the 33 minute discharge line, the A123 example provides about 5.8 W before its energy is depleted. The higher-capacity Sanyo F example can provide roughly 17 W for the same time, although its short-duration power capability is lower.
This is the central lesson of the Ragone plot: the best cell depends on the load duration as well as the load power. A cell that wins at a few minutes may lose at half an hour. A cell that stores more total energy may be unsuitable for high-current pulses.
The plotted result also depends on how the test was performed. A meaningful comparison should specify, or at least consider:
- discharge current or constant-power profile
- cutoff voltage
- initial state of charge
- cell temperature
- cell age and cycle history
- allowed temperature rise
- manufacturer limits for continuous and pulse discharge
- whether values are measured, calculated, or taken from a specification sheet
This is especially important for lithium-ion cells because internal resistance, heat generation, and voltage limits strongly affect high-power operation. Two cells with similar nominal capacity may behave very differently under a high-current load.
Using the Ragone Plot to Choose a Battery
The practical value of a Ragone plot is that it helps match a battery to an application. Rather than asking only which cell has the highest capacity or which cell has the highest discharge rating, the plot asks a more useful question: what combination of power, energy, and discharge time does the application require?
High-power applications include:
- cordless power tools
- robotics actuators
- drones during acceleration or climb
- engine starting
- pulsed industrial loads
- backup systems with high surge demand
These applications need low internal resistance and safe delivery of high current. Runtime may still matter, but the first requirement is often avoiding voltage collapse, overheating, or current-limit shutdown under peak load.
High-energy applications include:
- portable electronics
- instruments with long standby time
- sensors and data loggers
- low-power communications devices
- long-runtime battery packs with moderate current draw
These applications often prioritize watt-hours per kilogram or watt-hours per liter. High peak current capability may be less important if the load is steady and modest.
Many real systems need a balance. A portable medical device, mobile robot, or electric mobility product may require both acceptable runtime and safe peak power. In these cases, the Ragone plot can help narrow the candidate cells, but it should not be the only design input.
A practical selection process is:
- Define the required load profile in watts, including continuous load, peak load, and pulse duration.
- Define the required runtime or mission time.
- Convert runtime and load into required energy, including conversion losses where applicable.
- Compare candidate cells or packs on a Ragone plot using the correct axis units.
- Use constant-time lines to check whether the required power can be sustained for the required duration.
- Apply engineering margin for aging, temperature, manufacturing variation, and safety limits.
The last step is critical. A new cell at room temperature does not represent the worst case over product life. Battery packs lose capacity with cycling and calendar aging. Their internal resistance usually rises, which reduces high-power capability and increases heat generation. Low temperature can further increase resistance and reduce available energy. High temperature can accelerate aging and create thermal management challenges.
For this reason, a battery selected directly from a best-case Ragone point may fail to meet requirements after months or years of use. Engineers should account for:
- capacity fade
- resistance rise
- cycle count and depth of discharge
- storage history
- operating temperature range
- pack imbalance
- battery management system limits
- required end-of-life performance
A Ragone plot is best used as an early comparison and sizing tool. Final battery selection still requires cell data sheets, validation testing, thermal analysis, safety review, and pack-level design verification.
Limits and Other Uses of Ragone Plots
Ragone plots can compare more than rechargeable batteries. They are often used for energy storage devices such as:
- capacitors
- supercapacitors
- flywheels
- flow batteries
- fuel cells
- rechargeable and primary batteries
They may also be used, with care, for energy conversion devices such as internal combustion engines, gas turbines, and other systems where energy is converted from fuel into work or electricity.
The difficulty is that not all devices store their usable energy internally in the same way. A self-contained battery has a defined amount of active material and stored chemical energy before it must be recharged. A fuel cell or combustion engine supplied by a refillable tank can continue operating as long as fuel is supplied. Increasing the tank size increases total deliverable energy without necessarily changing the energy conversion device itself.
This creates a comparison problem. If the tank is included, the system energy depends on how much fuel is carried. If the tank is excluded, the plot may not represent real operating duration. The same caution applies to systems with external reservoirs, replaceable fuel cartridges, or continuous energy input.
A proper Ragone comparison should also avoid mixing incompatible specification points. For example, a device’s maximum energy density and maximum continuous power may not occur under the same test condition. Plotting both as if they were simultaneous can make the device appear better than it is. A more rigorous Ragone curve uses multiple energy-power pairs measured under defined discharge conditions and operating limits.
Ragone plots also omit many factors that matter in engineering design. They usually do not show:
- cost
- cycle life
- calendar life
- safety behavior
- thermal limits
- charge acceptance
- self-discharge
- voltage range
- volumetric packaging constraints
- mechanical robustness
- battery management requirements
- regulatory or transport constraints
For this reason, a Ragone plot should be treated as a performance map, not a complete design decision. It is excellent for visualizing the tradeoff between stored energy and deliverable power, especially when discharge duration is important. It does not replace detailed electrical, thermal, mechanical, and safety analysis.
Used correctly, the Ragone plot helps clarify one of the most common battery-selection mistakes: choosing a cell for capacity when the application is power-limited, or choosing a high-power cell when the mission is energy-limited. The right battery is not simply the one with the largest ampere-hour rating or the highest current rating. It is the one whose energy, power, and runtime capability match the actual load profile with enough margin for real operating conditions.
References
- Battery University | BU-503: Determining Power Deliver by the Ragone…. (n.d.). https://batteryuniversity.com/article/bu-503-how-to-calculate-battery-runtime
- Battery University | BU-503: Determining Power Deliver by the Ragone…. (n.d.). http://www.batteryuniversity.com/article/bu-503-determining-power-deliver-by-the-ragone-plot
- Battery University | BU-503: Determining Power Deliver by the Ragone…. (n.d.). https://batteryuniversity.com/article/bu-503-determining-power-deliver-by-the-ragone-plot
- How to Determine Power Delivery Using the Ragone Plot. (n.d.). https://www.large-battery.com/blog/determining-power-delivery-by-the-ragone-plot-lithium-battery
- Is there a chart / table listing the UPS run-time for various SLA battery sizes? - Hardware & Infrastructure - Spiceworks Community. (n.d.). https://community.spiceworks.com/topic/2057317-is-there-a-chart-table-listing-the-ups-run-time-for-various-sla-battery-sizes
- Ragone plot - Wikipedia. (n.d.). https://en.wikipedia.org/wiki/Ragone_plot
- Ragone Plot - an overview. (n.d.). https://www.sciencedirect.com/topics/engineering/ragone-plot
- Examining the Ragone Plot. (n.d.). https://www.linkedin.com/pulse/examining-ragone-plot-muhammed-a-mahmoud-utr3f
- Ragone Plot - Battery Design. (n.d.). https://www.batterydesign.net/pack/sizing/ragone-plot
- Ragone plot Meaning | Goong.com. (n.d.). https://goong.com/word/ragone-plot-meaning