Views: 0 Author: ZERNE Battery Technical Content Team Publish Time: 2026-09-15 Origin: Site
A LiPo battery specification may show voltage, capacity and discharge current, but these values do not fully explain how the battery will behave inside an OEM device.
The discharge curve provides more useful information. It shows how battery voltage changes as the battery delivers energy under a defined load.
OEM engineers can use the curve to evaluate:
voltage stability;
voltage sag under load;
device minimum input voltage;
battery or BMS cutoff;
practical usable capacity;
discharge-rate effects;
temperature effects;
remaining energy near the end of discharge.
A battery may have enough nominal capacity but still cause the device to shut down early if its loaded voltage falls below the device’s minimum input voltage.
This is why the discharge curve should be reviewed together with the device load, converter input range, battery protection system and operating temperature.
To read a LiPo discharge curve:
Check the test current, temperature, cutoff condition and battery sample.
Identify the voltage axis and delivered-capacity or state-of-charge axis.
Compare resting voltage with voltage under load.
Locate the plateau and the end-of-discharge knee.
Mark the host device’s minimum input voltage.
Check whether the loaded battery voltage crosses that limit too early.
Separate cell cutoff, BMS cutoff and device shutdown.
Estimate usable capacity under the actual device load.
Validate the result with the final battery and device.
The key principle is:
Usable Capacity
= Capacity Delivered Before the Relevant System Limit Is ReachedThe relevant limit may be:
the device’s minimum operating voltage;
the converter’s minimum input voltage;
the BMS cutoff;
the cell’s specified discharge cutoff;
a project-specific reserve limit.
A discharge curve should never be interpreted without its test conditions.
A typical LiPo discharge curve plots voltage against one of the following:
delivered capacity;
state of charge;
discharge time;
remaining capacity.
The horizontal axis may show:
Ah;
mAh;
percentage of capacity;
minutes or hours.
The vertical axis normally shows battery voltage.
A curve usually includes three general regions:
The voltage may fall quickly after the load is applied. This initial change can include:
internal resistance;
connection resistance;
polarization effects;
measurement conditions;
the difference between open-circuit and loaded voltage.
The battery voltage may remain within a relatively stable range while most of the usable capacity is delivered.
The plateau is important because the host device may operate normally during this period. However, the plateau voltage can shift lower when the discharge current increases or the battery temperature falls.
Near the end of discharge, voltage may fall more quickly.
This region is important for cutoff design. If the device continues operating too far into the knee, it may experience:
unstable operation;
converter shutdown;
inaccurate battery-level reporting;
sudden loss of runtime;
protection activation.
Two discharge curves should not be compared directly unless their test conditions are reasonably similar.
Check:
Test Condition | Why It Matters |
|---|---|
Discharge current | Higher current usually creates more voltage sag |
C-rate | Normalizes current against cell capacity |
Temperature | Cold conditions can increase resistance and reduce usable capacity |
Starting state of charge | Different starting conditions affect the result |
Cutoff voltage | A lower cutoff may produce more delivered capacity |
Rest time | Rest allows voltage recovery and changes interpretation |
Battery age | Aged cells may show greater sag and lower capacity |
Test equipment | Accuracy and wiring affect the measured curve |
Sample quantity | One sample may not represent production variation |
A curve without test conditions is incomplete evidence.
For OEM comparison, request:
battery model;
cell chemistry;
capacity;
discharge current;
cutoff voltage;
temperature;
sample condition;
measurement method;
whether the curve shows resting or loaded voltage.
A LiPo battery can show different voltages depending on whether current is flowing.
Resting voltage is measured after the load is removed and the battery has been allowed to stabilize.
It is useful for understanding the battery’s approximate electrochemical state, but it may not represent the voltage available to the device during operation.
Loaded voltage is measured while the battery is supplying current.
This is the voltage the device actually receives during operation. It is therefore more important when evaluating:
device stability;
converter input;
processor operation;
motor or radio activity;
low-voltage shutdown;
peak-current events.
A simplified relationship is:
Loaded Voltage
≈ Resting Voltage − Current × Internal ResistanceThis is only a simplified model. Actual voltage behavior also depends on cell chemistry, state of charge, temperature, dynamic polarization and connection resistance.
The difference between resting and loaded voltage is commonly referred to as voltage sag.
Voltage sag is the temporary or load-related drop in voltage that occurs when current increases.
It may become larger when:
discharge current increases;
the battery is cold;
the battery is aged;
internal resistance is high;
the cable or connector adds resistance;
the battery is near the end of discharge;
the load changes quickly.
Voltage sag can create a system problem even when the battery still contains significant capacity.
For example, an OEM device may require at least 3.3 V at its input. The battery may show a higher voltage at rest, but the loaded voltage may temporarily fall below 3.3 V during a peak current event.
The result could be:
device reset;
display interruption;
communication failure;
premature shutdown;
BMS protection activation.
The correct evaluation must therefore compare the device’s minimum input voltage with the battery voltage under the actual load.
The host device should have a defined voltage range:
maximum input voltage;
nominal input voltage;
normal operating range;
minimum operating voltage;
shutdown threshold;
converter input range.
Place the minimum input voltage on the discharge-curve analysis.
Then ask:
At what delivered capacity does the loaded voltage cross the minimum input?
Does the crossing occur during a normal load or only during a peak event?
Does the voltage recover after the load is removed?
Does the device shut down immediately or continue operating?
Does the battery protection system activate before the device limit?
The usable capacity for the application should be based on the first relevant system limit, not simply the point where the cell reaches its theoretical final voltage.
Several different cutoff values may exist in one battery system.
This is the lower voltage limit used in the cell’s discharge specification.
The protection system may disconnect the battery when it detects:
low cell voltage;
overcurrent;
short circuit;
abnormal temperature;
cell imbalance;
another fault condition.
The host device or its power converter may stop operating when the input voltage becomes too low.
These limits may not be the same.
A device may shut down before the BMS cutoff because its converter needs a higher minimum input voltage. Conversely, the protection board may disconnect the battery before the device reaches its intended operating limit.
A correct design must map all three limits together.
Nominal capacity is measured under a defined test method. Usable capacity is the portion that the actual device can access before reaching its relevant voltage or protection limit.
A practical analysis should identify:
starting voltage;
discharge current;
device minimum input;
converter efficiency;
battery cutoff;
test temperature;
delivered capacity at the relevant limit.
The following values are illustrative only.
Assume:
device minimum input voltage: 3.3 V;
battery current: constant during the test;
loaded voltage crosses 3.3 V at 82% of delivered capacity;
battery protection cutoff occurs at 95% of the rated test capacity.
In this case, the device may stop operating at approximately 82% of the measured capacity, even though the protection system has not yet reached its own cutoff.
The practical usable capacity is therefore closer to:
Rated Capacity × 82%This does not mean the remaining capacity has disappeared. It means the battery voltage is no longer suitable for that device under the specified load.
If the device uses a suitable converter or the load is reduced, more of the battery capacity may become usable. That change must be validated rather than assumed.
A LiPo battery may produce different curves at different discharge rates.
At a lower discharge rate, the curve may show:
less voltage sag;
higher loaded voltage;
longer time before the voltage knee;
more delivered capacity.
At a higher discharge rate, the curve may show:
larger initial voltage drop;
lower plateau voltage;
earlier cutoff;
greater temperature rise;
reduced usable capacity.
For this reason, a curve measured at a low current may not represent a device with short high-current pulses.
When reviewing supplier data, compare the curve with:
average device current;
continuous current;
peak current;
peak duration;
pulse frequency;
expected temperature.
For variable-load devices, use the actual load profile whenever possible.
Temperature changes the relationship between voltage, current and usable capacity.
Low temperatures may cause:
higher internal resistance;
larger voltage sag;
lower available capacity;
earlier device cutoff;
slower voltage recovery;
reduced peak-current performance.
High temperatures may change:
battery aging rate;
protection behavior;
enclosure temperature;
long-term capacity retention;
charger and device thermal conditions.
A discharge curve measured at room temperature should not automatically be used to predict cold-weather performance.
If the device operates across a wide temperature range, request or perform tests at:
normal room temperature;
minimum operating temperature;
maximum operating temperature;
relevant charging temperatures.
The discharge curve can help engineers evaluate:
single-cell versus multi-cell configuration;
required nominal voltage;
converter input range;
series-cell count;
parallel capacity;
current capability;
protection settings;
reserve capacity.
The broader lithium-ion battery pack design guide explains how voltage, capacity, current, BMS, charging and runtime requirements work together during pack design.
The discharge curve adds a more specific question:
Can the selected battery maintain the required voltage under the actual load until the target runtime is reached?
A larger capacity pack is not always the correct solution. The design may also require:
lower internal resistance;
a different cell format;
more parallel cells;
a different voltage platform;
a suitable converter;
revised cutoff settings;
improved cable and connector design.
The cutoff should not be set exactly at the point where the device stops working.
A practical design should allow margin for:
production variation;
temperature change;
battery aging;
internal resistance increase;
cable resistance;
peak current;
measurement error;
converter tolerance;
protection response time.
The margin should be established by the engineering team according to the device and battery requirements.
Avoid choosing a cutoff only to maximize the displayed capacity. A lower cutoff may increase measured capacity but may also create:
unstable operation;
reduced battery life;
greater voltage sag;
sudden shutdown;
higher stress on the system.
The selected cutoff should support stable device operation and the required battery service life.
A curve from a cell supplier is a useful reference, but the final battery pack should be validated in the actual system.
The test should include:
approved battery configuration;
final BMS or PCM;
final wires and connector;
production-intent charger;
actual device load;
relevant temperature;
normal and peak operation;
device cutoff behavior;
battery protection response.
Record:
battery voltage;
device input voltage;
battery current;
device current;
temperature;
delivered capacity;
runtime;
cutoff event;
voltage recovery after load removal.
ZERNE’s battery pack testing checklist covers continuous discharge, peak-load testing, voltage sag, cutoff behavior, usable capacity and device-level validation. This article should be used to interpret the discharge curve before those tests are finalized.
When comparing LiPo cells or battery packs from different suppliers, request equivalent data.
Check whether the curves use the same:
nominal capacity;
discharge current;
C-rate;
temperature;
cutoff voltage;
starting state of charge;
rest period;
equipment;
sample age;
measurement location.
A curve may look better simply because it was tested at a lower current or warmer temperature.
Also confirm whether the curve represents:
a single cell;
a battery pack;
a protected pack;
a pack with a BMS;
a device-side measurement;
a battery-terminal measurement.
The measurement point matters because cable, connector, converter and protection losses can change the voltage available to the device.
A 3.7 V LiPo battery does not remain at exactly 3.7 V during charging and discharging.
Resting voltage may look acceptable while loaded voltage falls below the device minimum.
A curve without discharge current cannot show how the battery will behave under the intended load.
BMS, wires, connectors, series configuration and temperature all affect pack-level performance.
Maximum delivered capacity is not always the same as stable device operation.
A room-temperature curve may overestimate usable capacity in cold conditions.
Capacity in Ah or mAh does not show the complete energy available unless voltage is also considered.
The final device may have a higher peak load, a narrower input range or greater converter loss than expected.
A LiPo discharge curve shows how battery voltage changes as the battery delivers capacity under defined conditions such as current, temperature and cutoff voltage.
Loaded voltage shows the voltage available to the device while current is flowing. It includes the effect of voltage sag and is therefore more useful for evaluating device stability and cutoff behavior.
Cutoff voltage is a battery, BMS or test limit. Minimum input voltage is the lowest voltage the host device or converter needs to operate correctly. The device may shut down before the battery reaches its protection cutoff.
It can. Higher current may increase voltage sag and cause the device or protection system to reach its limit earlier. The exact effect depends on the cell, temperature, cutoff and load profile.
They identify the first relevant system limit, such as device minimum input, converter cutoff or BMS protection, and record the capacity delivered before that limit is reached under the intended load.
No. A discharge curve is important evidence, but the battery should also be tested with the final charger, protection system, connector, enclosure and device load.
A LiPo discharge curve helps OEM engineers understand how battery voltage changes under defined operating conditions.
The most important steps are to check the test conditions, distinguish resting and loaded voltage, measure voltage sag, identify the device minimum input and separate cell, BMS and device cutoff limits.
Usable capacity should be based on the energy available before the first relevant system limit is reached. Temperature, discharge rate, aging, wiring and converter efficiency must also be considered.
The final decision should be validated with the complete battery pack and actual device. A curve is a design tool, not a substitute for system-level testing.
Need help selecting or developing a LiPo battery pack for an OEM device?
