An older battery can reach 100% state of charge and still provide less runtime than it did when it was new. The battery could be fully charged, but “full” no longer represents the same amount of stored energy.
This gradual loss of capacity is one part of battery state of health, commonly abbreviated as SOH. State of health describes how a battery’s present condition compares with its condition when it was new. It can help explain why an aging battery runs out sooner, experiences more voltage drop under load, or shuts down earlier than expected.
Unlike voltage, current, or temperature, SOH is not something a meter can directly measure. Much like the state of charge (SOC), it must be calculated or estimated using measurable battery characteristics.
Understanding battery state of health can help you estimate runtime more accurately, configure a battery monitor correctly, identify abnormal degradation, and decide when a battery actually needs replacement.
Table of Contents
- What Is Battery State of Health?
- Why SOH Changes the Meaning of SOC
- Battery State of Health vs. State of Charge
- Why Knowing Battery SOH Matters
- What Causes Lithium Battery State of Health to Decline?
- What Causes Lead-Acid Battery State of Health to Decline?
- How Is Battery State of Health Determined?
- How to Measure a Lithium Battery’s State of Health
- How to Check the State of Health of a Lead-Acid Battery
- How to Interpret a Battery SOH Percentage
- Updating Your Battery Monitor as SOH Declines
- When Should You Test Battery State of Health?
- Understanding SOH Helps You Get More Dependable Battery Runtime
- Frequently Asked Questions
What Is Battery State of Health?
Battery state of health describes the present performance and condition of a battery compared with a new battery of the same type. It is usually expressed as a percentage.
A new battery typically begins at or near 100% SOH. As it ages, its state of health declines. A battery at 80% SOH retains approximately 80% of its original capacity, power capability, or another performance characteristic being used to define health.
That final distinction is important because there is no single measurement that completely describes battery health.
Depending on the battery and its application, SOH may refer to:
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Remaining energy-storage capacity (most important in storage applications)
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Ability to deliver high current (important in electric vehicles and engine starting)
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Changes in internal resistance (important for rapid charging)
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Voltage behavior under load
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Overall degradation compared with a new battery
For a deep-cycle battery powering an RV, boat, off-grid property, or backup system, usable capacity is often the most meaningful measure. For a starting battery or electric vehicle, the ability to deliver power without excessive voltage sag may be just as important.
Because different applications prioritize different characteristics, two battery-testing systems may report SOH differently even when evaluating the same battery.
A Simple Capacity-Based SOH Formula
One of the most understandable ways to calculate battery state of health is to compare its present usable capacity with its original capacity:
For example, suppose a 100Ah battery delivers 85Ah during a controlled capacity test (we explain this later):
Current measured capacity ÷ Original rated or baseline capacity × 100
SOH = 85Ah ÷ 100Ah × 100
SOH = 85%
Based on capacity, the battery has approximately 85% state of health.
This calculation is only meaningful when the test conditions are controlled. Temperature, discharge rate, starting charge level, cutoff voltage, and test equipment can all affect the result. A test performed at a high load or in cold weather may show less available capacity than the same battery tested under its manufacturer’s rated conditions.
Whenever possible, compare the battery with a capacity test performed when it was new. This beginning-of-life baseline accounts for normal manufacturing tolerance and provides a more accurate reference than the nameplate rating alone.
⚡️ Learn more: Energy, Power, and Charge: Why They Matter In Batteries
Battery State of Health vs. State of Charge
State of charge and state of health describe two different things:
State of charge (SOC) answers the question, “How full is the battery right now?”
State of health (SOH) answers, “How much performance does the battery retain compared with when it was new?”
A battery can be at 100% state of charge while having a reduced state of health. It has accepted all the energy it can currently store, but that amount may be lower than its original capacity.
Think of an older phone that shows a full battery icon but no longer lasts all day. The phone may be at 100% SOC, but its battery has lost some of its original capacity. The same principle applies to electric vehicles, deep-cycle battery banks, backup systems, industrial equipment, and other battery-powered applications.
⚡️ For a closer look at how SOC is calculated and monitored, read our guide to battery state of charge.
Why SOH Changes the Meaning of SOC
State-of-charge estimates are most useful when they are based on the battery’s current usable capacity, not simply the capacity printed on its label.
Consider a 100Ah battery that has degraded to 80Ah of usable capacity. At 50% SOC, that battery has approximately 40Ah remaining under equivalent conditions, not 50Ah.
If a shunt-based battery monitor is still configured for a 100Ah battery, it may overestimate how much energy remains. The battery could reach its low-voltage cutoff or BMS shutdown before the monitor reaches 0%.
This does not always mean the battery is damaged. Early shutdown can also result from high loads, cold temperatures, cell imbalance, poor connections, undersized wiring, or incorrect monitor settings. However, declining capacity is one possibility that should be considered when the displayed SOC no longer matches actual runtime.
Why Knowing Battery SOH Matters
For many users, the most immediate benefit of knowing battery state of health is a more accurate estimate of runtime. If a battery bank now stores 15% less energy than it did when new, your appliances, electronics, or other loads will also run for less time.
A reliable SOH estimate can also help you:
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Enter a more accurate capacity setting in a battery monitor
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Identify abnormal degradation before it causes an unexpected shutdown
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Plan maintenance or replacement around actual performance
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Evaluate the condition of a used battery
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Compare batteries within a larger bank
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Improve electric-vehicle range estimates
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Maintain dependable backup and off-grid power
SOH becomes especially important in systems where running out of power creates more than a minor inconvenience. An unexpected battery shutdown can interrupt refrigeration, communications, medical equipment, industrial processes, critical electrical loads, or leave an electric car stranded.
What Causes Lithium Battery State of Health to Decline?
All rechargeable batteries degrade because the chemical and physical structures inside them slowly change with use, time, and environmental exposure.
In a lithium-ion battery, lithium ions move back and forth between the positive (cathode) and negative (anode) electrodes during charging and discharging. That movement is reversible, but it is not perfectly lossless. As ions move in and out of the electrode materials, those materials expand and contract slightly. Over time, that repeated movement can create mechanical stress, small cracks, or loss of contact between the active material and the conductive structure around it.
Other forms of degradation are chemical. A protective layer forms on the negative electrode early in a lithium battery’s life, and while that layer is necessary, it can continue to grow slowly over time. As it grows, it consumes lithium and increases resistance. In the wrong conditions, such as charging too fast in cold temperatures or charging outside the manufacturer’s specifications, metallic lithium can also plate onto the negative electrode instead of moving cleanly into the electrode material. This plating can permanently reduce capacity and, in severe cases, create internal safety risks. Electrolyte breakdown, gas generation, corrosion of current collectors, and damage from heat can also contribute to a lower state of health.
This is why temperature, charge rate, depth of discharge, and proper charging equipment matter. A well-designed lithium battery management system helps protect the cells from damaging conditions like overcharge, over-discharge, excessive current, and unsafe charging temperatures. But even when a battery is used correctly, normal chemical aging still occurs gradually over thousands of cycles.
What Causes Lead-Acid Battery State of Health to Decline?
Lead-acid batteries face their own degradation problems. And in many deep-cycle RV, marine, and off-grid applications, those problems can cause capacity to fall much faster.
When a lead-acid battery discharges, lead sulfate forms on the plates. When the battery charges properly, much of that reaction reverses. But if the battery sits partially discharged, is repeatedly undercharged, or is deeply discharged too often, that lead sulfate can harden into crystals that no longer convert back efficiently. This is sulfation, and it is one of the most common reasons that lead-acid batteries lose capacity.
Lead-acid batteries can also lose water through gassing, suffer corrosion of the positive plates, shed active material from the plates, and experience electrolyte stratification, where the acid concentration becomes uneven inside the battery. Flooded lead-acid batteries require regular maintenance because water loss and electrolyte condition directly affect performance. AGM and gel batteries reduce some maintenance needs, but they can still suffer from sulfation, heat damage, overcharging, and permanent capacity loss.
In short, battery state of health declines because the battery’s internal materials become less able to store and deliver energy. Lithium batteries tend to age more slowly in deep-cycle applications when properly managed, while lead-acid batteries are more sensitive to incomplete charging, deep cycling, and maintenance conditions.
Either way, the result is the same: less usable capacity, more voltage sag, and shorter runtime than the battery provided when it was new.
^ Lead-acid batteries can lose state of health more quickly when sulfation, corrosion, water loss, and repeated deep discharges reduce their usable capacity.
How Is Battery State of Health Determined?
SOH calculations generally use one or more measurable battery characteristics and compare them with a reference value or battery model.
The best method depends on what kind of performance matters most. A deep-cycle battery may be evaluated primarily by capacity, while a starting battery may be evaluated by its ability to provide a large burst of current.
1. Capacity-Based SOH
Capacity-based SOH compares the amount of energy a battery can currently deliver with the amount it could deliver when new.
For deep-cycle users, capacity-based SOH often provides the most practical answer because it relates directly to runtime. If a battery stores less energy, it cannot run the same loads for as long.
However, measuring capacity requires a controlled charge and discharge test. That takes time and temporarily removes the battery from normal service. In integrated systems, capacity estimates may be performed by the BMS based on actual usage, but are not as accurate as controlled discharge.
2. Power- or Resistance-Based SOH
A battery’s health can also be evaluated by how effectively it supplies current.
As internal resistance increases, the battery experiences more voltage drop under load. It may generate more heat, lose efficiency, or become unable to support high-current equipment even if it still retains much of its energy capacity.
This type of SOH measurement is particularly useful for engine-starting batteries, electric vehicles, large inverters, and other applications with demanding loads.
Internal resistance alone does not reveal how many amp-hours or watt-hours the battery can deliver. A battery can retain reasonable capacity while struggling under a high load. It can also deliver current well but has less total energy storage than it once did.
Capacity and resistance, therefore, describe different aspects of battery health.
3. Model-Based or Composite SOH
Advanced battery systems often estimate SOH using a mathematical model rather than relying on a single capacity or resistance measurement.
Manufacturers may develop these models using laboratory data collected from batteries of the same chemistry and design as they age under different temperatures, loads, charge rates, and cycling patterns. The BMS or monitoring system then compares real-world battery behavior with the expected behavior in that model.
Depending on the system, an SOH algorithm may consider:
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Capacity history
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Voltage and current
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Temperature
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Internal resistance or impedance
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Charge throughput
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Cycle count
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Charging patterns
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Operating history
There is no universal SOH algorithm used by every battery manufacturer. Some systems rely on relatively simple calculations, while others use equivalent-circuit models, statistical methods, or machine learning.
These approaches are common in electric vehicles, phones, and integrated energy-storage systems. They allow SOH to be updated without regularly performing a complete discharge test, but the result remains an estimate. Its accuracy depends on the quality of the model, sensors, data, and assumptions behind it, so they are mostly deployed in integrated systems where the battery and device using it are fully designed together and integrated.
How to Measure a Lithium Battery’s State of Health
No single test captures every aspect of lithium battery health. However, the following methods can provide useful information about capacity, power delivery, and overall performance.
1. Perform a Controlled Capacity Test
For most deep-cycle lithium batteries, a controlled capacity test provides the clearest measurement of remaining runtime.
The general process is:
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Fully charge the battery according to the manufacturer’s instructions.
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Allow the required charging and cell-balancing process to finish.
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Connect a known, controlled load.
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Record the amp-hours or watt-hours delivered during the test.
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Stop at the manufacturer’s specified discharge cutoff or when the BMS disconnects.
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Compare the delivered capacity with the original rating or a beginning-of-life test.
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Calculate the capacity-based SOH percentage.
For example, if a 100Ah lithium battery delivers 92Ah under the same conditions used for its original test, its capacity-based SOH is approximately 92%.
To produce a useful result, control as many variables as possible. Battery temperature, discharge current, cutoff voltage, parasitic loads, and test-equipment accuracy can all change the reported capacity.
The discharge rate should also match the battery manufacturer’s capacity rating or test procedure whenever possible. If the original rating was established at one discharge rate and you test the battery at a much higher rate, you are no longer making a direct comparison.
Testing an entire battery bank can show its total usable capacity, but it may not identify a weak battery within that bank. If bank performance has become uneven, individual battery testing may be necessary.
A full capacity test also takes the battery system out of service and intentionally brings it to a low state of charge. In large, high-voltage, commercial, or critical systems, this work may be better handled by a qualified technician.
2. Review BMS or Smart-Battery Data
Some battery management systems and smart batteries estimate SOH automatically.
However, the BMS does not directly sense “health.” Instead, it measures characteristics such as voltage, current, and temperature, then uses an algorithm to estimate how the battery’s capacity or power capability has changed.
An electric vehicle may use this information to adjust its estimated usable battery capacity and driving range. A smart energy-storage system may use it to predict maintenance needs or limit operation as the battery ages.
Not every BMS calculates or displays SOH. In many drop-in lithium batteries, the BMS primarily protects the cells from overcharge, over-discharge, excessive current, short circuits, and temperature extremes.
Even when an SOH value is available, remember that it is model-based. The displayed percentage may not exactly match the result of an independent capacity test, especially if the system has not recently observed a complete charge or discharge reference point.
3. Watch for Real-World Capacity Changes
You may notice a change in battery performance before you have a precise SOH calculation.
Possible warning signs include:
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Shorter runtime under similar loads
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Earlier low-voltage or BMS shutdown
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Greater voltage drop under load
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Unusual charging or discharging behavior
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Growing imbalance between batteries or cells
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One battery contributing less current than the others in a bank
These symptoms do not automatically prove that the battery has permanently lost capacity. Problems elsewhere in the system can create the same behavior.
How to Check the State of Health of a Lead-Acid Battery
Lead-acid battery SOH testing uses some of the same principles as lithium testing, but the chemistry creates additional variables.
A lead-acid battery’s available capacity is heavily affected by discharge rate, temperature, charging history, and sulfation. The appropriate health test also depends on whether the battery is designed for deep-cycle use or engine starting.
Deep-Cycle Lead-Acid Capacity Testing
A controlled discharge test remains the best way to determine how much usable energy a deep-cycle lead-acid battery can provide.
Fully charge the battery, discharge it at the manufacturer’s specified test rate, and stop at the specified cutoff voltage, usually around 50% rated capacity. Compare the delivered amp-hours or watt-hours with the rated or original capacity.
Following the specified discharge rate is especially important with lead-acid batteries because of the Peukert effect. As discharge current increases, the battery may deliver substantially less apparent capacity before reaching its cutoff voltage.
A high-current test and a low-current test can therefore produce very different capacity results from the same battery.
⚡️ Read our guide to the Peukert effect to learn why discharge rate changes lead-acid battery runtime.
Flooded Lead-Acid Specific Gravity Testing
A hydrometer measures the specific gravity of the electrolyte inside a flooded lead-acid battery. The reading helps indicate the cell’s state of charge and can reveal differences between cells.
If one cell consistently has a lower specific gravity reading than the others after charging, that cell may be weak, sulfated, or damaged.
Specific gravity does not provide a complete SOH percentage by itself. It is most useful for checking charge condition, electrolyte consistency, and individual cell behavior.
Only flooded batteries with accessible cells can be tested this way. Always follow the battery manufacturer’s safety procedures when working around sulfuric acid.
Load and Conductance Testing
Load testers and electronic conductance testers are commonly used for automotive starting batteries.
These tools evaluate whether a battery can provide high current without its voltage falling too far. Some estimate cold-cranking performance or compare conductance with the expected value for a new battery.
This type of testing is useful when the battery’s main job is starting an engine. It is less useful for determining how many hours a deep-cycle battery can support a moderate load.
A battery may pass a starting test yet have reduced deep-cycle capacity. The opposite can also occur: it may retain energy capacity but no longer deliver the high current needed to start an engine reliably.
How to Interpret a Battery SOH Percentage
An SOH percentage is useful only when you understand what was measured and how the result was calculated.
An 85% capacity-based SOH does not necessarily mean the battery has 85% of its original power capability. Likewise, an SOH estimate based on internal resistance may not reveal exactly how much runtime remains.
Before comparing two SOH values, determine whether they represent capacity, resistance, power capability, or a composite model.
Is 100% SOH Always the Maximum?
A new battery may deliver slightly more or slightly less than its nameplate rating.
Battery ratings are established under specific test conditions, and normal manufacturing tolerance can affect individual results. Temperature, discharge rate, cutoff voltage, and test accuracy can also cause a new battery to test above or below 100% of its nominal capacity.
This is why a beginning-of-life capacity test is so valuable. If a new 100Ah battery initially delivers 104Ah and later delivers 88Ah under the same conditions, comparing 88Ah with the original 104Ah provides a more useful indication of degradation than comparing it only with the 100Ah label.
Some BMS systems may cap the displayed value at 100% even when measured capacity initially exceeds the rated capacity.
Is 80% SOH a Bad Battery?
Approximately 80% remaining capacity is a common end-of-life reference for many rechargeable batteries, but it is not a universal failure point.
Reaching 80% SOH does not mean the battery suddenly stops working. It means the battery no longer provides the same capacity or performance it did when new.
Whether the battery remains useful depends on the application. A battery with 80% of its original capacity may continue to serve a lightly loaded system for years. The same battery may be unacceptable in an electric vehicle, a critical backup system, or an off-grid installation that depends on nearly all of its original runtime.
Updating Your Battery Monitor as SOH Declines
A shunt-based battery monitor calculates state of charge by tracking current entering and leaving the battery bank. To estimate SOC accurately, it must know the bank’s usable capacity.
If a validated test shows that a 400Ah bank now provides 360Ah, continuing to configure the monitor as a 400Ah bank may cause it to overestimate the energy remaining.
After confirming the capacity change, update the programmed amp-hour capacity according to the monitor manufacturer’s instructions. Review other battery-specific settings, including charge efficiency, Peukert exponent where applicable, charged voltage, tail current, and synchronization requirements.
The monitor should then be synchronized at a confirmed full charge.
Do not lower the programmed capacity after one unexpected shutdown without further troubleshooting. An early cutoff could result from a large load, voltage drop, loose connection, low temperature, incomplete charging, cell imbalance, or a BMS protection event.
A battery monitor can only calculate from the information it receives. Correct settings, proper shunt installation, and periodic synchronization are essential for an accurate SOC display.
⚡️ Read our guides to electrical shunts and battery monitoring for more information.
When Should You Test Battery State of Health?
Most users do not need to perform frequent full capacity tests. Testing makes the most sense when battery performance changes or when accurate capacity information is particularly important.
Consider checking SOH when:
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The battery has been deployed for 5+ years
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The battery has been cycled deeply or extensively
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A commercial maintenance schedule requires periodic testing
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Runtime has noticeably declined
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The battery reaches low-voltage shutdown earlier than expected
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SOC readings no longer match real-world performance
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You are evaluating a used battery
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You are troubleshooting one weak battery or cell
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The system is preparing for a demanding trip or deployment
There is no universal SOH testing interval for every battery. Follow the recommendations for the battery, monitoring equipment, and application. For guidance on SOH testing of Battle Born Batteries, we’d be happy to hop on a call and talk it through.
In critical systems, tracking test results over time is often more valuable than relying on a single measurement. A gradual trend can reveal whether the battery is aging normally or losing performance faster than expected.
Understanding SOH Helps You Get More Dependable Battery Runtime
Battery state of health helps turn battery aging into practical runtime information. It shows why a battery may not last as long as it used to, why a monitor may overestimate remaining power, and when a battery may need further testing or replacement.
For most deep-cycle systems, a controlled capacity test provides the clearest picture of usable runtime. Voltage sag, internal resistance, BMS estimates, and operating history can all add context, but SOH should always be interpreted based on battery chemistry, test conditions, and how the system is actually used.
If your battery bank no longer performs as expected, or you are designing a system for dependable RV, marine, or off-grid power, Battle Born’s technical specialists can help you evaluate your batteries, charging equipment, monitoring setup, and overall installation.
Frequently Asked Questions
Q: What is a good state of health for a battery?
A good SOH depends on the application and how the value was calculated. A battery above 90% of its original capacity is generally still close to its new condition. Approximately 80% capacity is a common end-of-life reference, but a battery at that level may remain useful when its reduced runtime still meets the application’s needs.
Q: How do I check my battery’s state of health?
For a deep-cycle battery, the most direct method is a controlled capacity test. Fully charge the battery, discharge it at a known rate to the manufacturer’s specified cutoff, record the delivered amp-hours or watt-hours, and compare the result with its original capacity.
Voltage behavior, resistance measurements, conductance tests, specific gravity, and BMS data may provide additional information depending on the battery chemistry and application.
Q: Can voltage tell you a battery’s state of health?
Voltage alone cannot provide a reliable SOH percentage. Resting and loaded voltage can reveal potential problems, but voltage also changes with SOC, temperature, load, chemistry, and connection quality.
A battery may show normal resting voltage while having significantly reduced capacity. Voltage should be combined with other tests.
Q: Can a battery be fully charged but have poor health?
Yes. A degraded battery can reach 100% SOC while storing less energy than it did when new. The battery is full relative to its current capacity, but its current capacity may be substantially lower than its original rating.
Q: Does a BMS measure battery state of health?
Some BMS systems estimate SOH, but they do not measure it directly. They use data such as voltage, current, temperature, resistance, charge throughput, and operating history to calculate an estimate.
Many BMS systems focus primarily on battery protection and do not provide a user-facing SOH value.
Q: What is the difference between SOC and SOH?
State of charge describes how full the battery is at the present moment. State of health describes how its current performance compares with when it was new.
SOC changes continually as the battery charges and discharges. SOH normally changes much more slowly as the battery ages.
Q: Can battery state of health improve?
A displayed SOH estimate may increase after recalibration, balancing, or more complete data becomes available. Available capacity can also improve when a cold battery warms up or when a charging problem is corrected.
However, permanent capacity loss caused by normal lithium battery degradation, battery misuse, or damage generally cannot be reversed.
Q: Why does my battery shut down before the monitor reaches 0%?
The monitor may be configured for more capacity than the battery can currently provide. It may also be out of synchronization.
Other possible causes include a large load, excessive voltage drop, cold temperatures, poor connections, cell imbalance, incomplete charging, or a BMS protection event. Inspect the entire system before assuming that declining SOH is the cause.
Q: How accurate are battery SOH estimates?
Accuracy depends on the testing method, available data, battery model, sensors, temperature, load profile, and calibration.
A properly performed capacity test can provide a strong capacity-based SOH measurement. Model-based BMS estimates are more convenient for continuous monitoring, but results vary by manufacturer and may not match an independent capacity test exactly.
