How does SOC non-linearly affect lithium-ion pulse power? Key trends for discharge & charge

The key result is that SOC shifts pulse power in opposite directions for discharge and charge: as SOC rises, peak discharge power generally increases non-linearly, while peak charge power generally decreases non-linearly. In an 8 Ah lithium-manganese-oxide cell, for example, discharge capability increased from approximately 222 W at 10% SOC to 693 W at 90% SOC, while charge capability fell from approximately 675 W to 300 W over the same range.

SOC determines both the cell’s available lithium inventory and its distance from voltage limits. Low SOC restricts discharge because the cell voltage collapses more rapidly under load, while high SOC restricts charging because the terminal voltage reaches its upper limit quickly.

Why SOC Changes Pulse Power Non-Linearly

Pulse power is voltage-constrained

A pulse-power test does not measure current capability alone. It determines the largest current the cell can accept or deliver before reaching a specified lower or upper voltage limit.

A simplified relationship is:

[ P_{\text{pulse}} \approx V_{\text{terminal}} I_{\text{pulse}} ]

The terminal voltage depends on open-circuit voltage, internal resistance, charge-transfer behavior, and concentration polarization. Because each of these changes with SOC, pulse power is not proportional to SOC.

Discharge capability rises with SOC

At low SOC, the cell has limited electrochemical margin for delivering current. A discharge pulse causes a larger voltage drop, and the terminal voltage can reach the lower cutoff before the cell can deliver substantial power.

As SOC increases, the cell begins at a higher voltage and generally has lower pulse resistance. It can therefore sustain a larger discharge current while remaining above its cutoff voltage.

The increase is usually steep near the low-SOC region rather than uniform across the entire SOC range. This is why a modest SOC change near empty can produce a much larger change in available discharge power than the same SOC change in the middle of the operating range.

Charge capability falls with SOC

During charging, the limiting condition is usually the upper voltage boundary. At high SOC, the cell’s open-circuit voltage is already close to that limit, leaving less voltage headroom for a high-current pulse.

The charging current creates additional ohmic and electrochemical overpotential. Even if the cell still has remaining nominal capacity, this overpotential can drive the terminal voltage to the safety limit quickly.

Charging power therefore decreases as SOC rises. The decline is often particularly pronounced near the upper end of the SOC range, where small increases in SOC can sharply reduce the allowable current.

The Electrochemical Reasons Behind the Curve

Lithium availability changes with electrode state

During discharge, lithium moves from the negative electrode toward the positive electrode. At low SOC, the negative electrode has less readily available lithium to support a strong discharge pulse, and concentration gradients develop more quickly under high current.

During charge, lithium is driven back toward the negative electrode. At high SOC, the cell is closer to the condition where further lithium insertion is limited, while the positive electrode has less lithium remaining to release. These conditions increase polarization and make the voltage limit binding sooner.

The exact behavior depends on the cell chemistry, electrode design, temperature, aging condition, and pulse duration. The direction of the trend is common, but the shape and magnitude of the curve are cell-specific.

Internal resistance changes with SOC

Pulse power is strongly influenced by direct-current internal resistance. A useful approximation for a discharge pulse is:

[ P_{\text{max}} \approx V_{\text{OCV}} I_{\text{max}} ]

with the current constrained by:

[ I_{\text{max}} \approx \frac{V_{\text{OCV}}-V_{\text{cutoff}}}{R_{\text{pulse}}} ]

As SOC falls toward the lower end of the range, pulse resistance commonly rises. For one lithium-manganese spinel/lithium-titanate example, 10-second pulse impedance increased from approximately 0.0342 ohm at 100% SOC to 0.0652 ohm at 0% SOC.

That resistance increase amplifies the discharge voltage drop and reduces the available power. It also explains why power can fall sharply near the ends of the SOC range rather than changing linearly.

Temperature modifies the SOC relationship

Temperature affects ion transport and internal resistance. Elevated temperature can temporarily reduce resistance and increase measured pulse power, while low temperature generally increases polarization and reduces both charge acceptance and discharge capability.

SOC and temperature must therefore be controlled together during characterization. A power-versus-SOC curve measured at one temperature cannot be treated as universally valid across the battery’s operating range.

How Pulse Testing Reveals the Usable Power Envelope

Charge and discharge tests use different limits

A discharge pulse is normally limited by the minimum permitted terminal voltage. A charge pulse is normally limited by the maximum permitted terminal voltage.

The tester adjusts pulse current until one of these limits is reached within the specified pulse duration. The resulting power is then recorded against the cell’s measured SOC.

Pulse duration changes the result

Short pulses emphasize ohmic resistance and immediate voltage response. Longer pulses allow concentration gradients, diffusion limitations, and thermal effects to develop, usually reducing the measured power capability.

A 10-second pulse-power curve should therefore not be used interchangeably with a 1-second or 30-second curve. Pulse duration, rest period, temperature, voltage limits, and direction of current must be reported with the result.

The crossover point matters

Plotting charge and discharge power on the same SOC axis identifies the region where the two capabilities are approximately equal. This crossover helps define a practical operating window for applications that require both regenerative charging and high-power discharge.

The best operating window is usually inside the extreme SOC regions. It balances available discharge power, charge acceptance, safety margin, and long-term degradation.

Why Cell Imbalance Reduces Pack Power

The weakest cell sets the discharge limit

In a series-connected pack, the lowest-SOC or highest-resistance cell can reach the discharge cutoff first. The entire pack must then reduce or stop discharge current, even if other cells still have substantial capability.

This makes pack discharge power lower than the sum suggested by testing a representative cell alone.

The highest-SOC cell sets the charge limit

During charging, the highest-SOC cell typically reaches the upper voltage limit first. The battery-management system must limit charging power for the entire series group to protect that cell.

At extreme SOC values, even moderate SOC mismatch can cause a large difference in cell-level pulse capability because the power-SOC curve is steep. Accurate SOC estimation, balancing, and cell matching are therefore essential for pack-level power prediction.

Understanding the Trade-offs

Maximum power is not the same as maximum usable energy

A fully charged cell often offers the highest discharge power because its voltage is high and its resistance may be relatively low. However, operating continuously near full SOC can accelerate degradation and leaves little room for accepting regenerative or charging power.

Likewise, a low-SOC cell may still contain usable energy, but its ability to deliver high pulse power can be severely restricted by voltage sag and increased resistance.

Narrow SOC windows improve durability

Operating between approximately 25% and 85% SOC, rather than across the full 0% to 100% range, can reduce exposure to the conditions associated with accelerated degradation. Very low SOC is associated with increased resistance and stress, while prolonged operation near full SOC can accelerate chemical aging.

The appropriate window remains chemistry- and application-dependent. It should be selected from measured power, thermal, safety, and life data rather than from nominal capacity alone.

Nominal SOC is not a complete test variable

SOC estimation errors, capacity variation with temperature, current-measurement error, and cell aging can all shift the apparent position of a power curve. High-rate pulses also make coulomb-counted SOC less reliable if the initial SOC and rest conditions are not well controlled.

Testing should therefore use controlled initial conditions, calibrated current and voltage measurement, defined rest periods, and temperature monitoring.

Avoid extrapolating from one cell design

The numerical values from one 8 Ah lithium-manganese-oxide cell do not represent all lithium-ion cells. Different chemistries, electrode loadings, formats, aging states, and voltage limits can produce substantially different charge and discharge curves.

The reliable conclusion is the trend: discharge capability generally increases with SOC, charge capability generally decreases with SOC, and both relationships become strongly nonlinear near voltage and transport limits.

How to Apply This to Your Project

The following recommendations depend on the performance objective:

• If your primary focus is maximum discharge power: Characterize the low-SOC region with short and application-relevant pulses, because voltage sag and rising pulse resistance can sharply restrict power near empty.
• If your primary focus is fast charging or regenerative power: Measure the high-SOC region carefully, where limited voltage headroom causes charging power to decline rapidly.
• If your primary focus is battery-management control: Build separate charge- and discharge-power maps indexed by SOC, temperature, pulse duration, and aging state.
• If your primary focus is pack performance: Use cell-level variation and SOC imbalance in the model, because the highest-SOC cell limits charging and the lowest-SOC cell limits discharging.
• If your primary focus is service life: Avoid relying on the full SOC range continuously; select an operating window that preserves adequate power margin without prolonged exposure to extreme SOC conditions.

A properly controlled pulse-power map turns SOC from a simple fuel-gauge value into a practical limit on what the cell can safely accept or deliver.

Summary Table:

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