What Are Signs Of A Bad Cell In An EV Battery Pack

A bad cell in an EV battery pack doesn’t announce itself all at once. It degrades gradually, showing specific warning signs that get worse over time. Catching a bad cell early means the difference between a module replacement and a stranded car. This guide covers every sign of a failing cell and exactly how to confirm your suspicion.

This guide belongs to our battery health section. For the full overview, see Why an EV Battery SOH Reading Can Be Incorrect.

In Short

The most reliable signs of a bad cell are: a battery warning light that appears at a consistent mid-range SOC (not at low battery), range that drops suddenly rather than gradually, a battery warning that appears during high-demand events (hard acceleration, DC fast charging), and individual cell voltage readings showing one group well below the pack average. An OBD2 scan with a tool that reads individual cell voltages is the definitive diagnostic step. For the full step-by-step scanning process, see our cell imbalance diagnosis guide.

What a “Bad Cell” Actually Means

EV batteries contain hundreds to thousands of individual lithium cells grouped into modules. A “bad cell” is one that has degraded faster than the others, lower capacity, higher internal resistance, or both. The BMS monitors every cell group and limits the entire pack’s performance to protect the weakest member. One noticeably degraded cell group can limit the entire battery’s usable capacity and trigger warnings even when the overall pack appears healthy.

Warning Signs of a Bad Cell

Sign 1: Battery warning light at a consistent mid-range SOC

A warning that appears reliably at 35–50% charge, not at 10% where low-battery warnings normally trigger, means the BMS detected a voltage excursion in a specific cell group when overall pack SOC reached that level. The weak cell group depletes to its minimum before the overall pack does. This pattern is one of the clearest indicators of cell-level imbalance from a bad cell group.

Sign 2: Range drop that is sudden rather than gradual

Normal battery degradation produces a slow, gradual range reduction over months and years, maybe 1–2% per year. A bad cell causes a more abrupt drop: losing 20–30 miles of range over a few weeks, or a sudden step-change in displayed range after a particular event (a deep discharge, a hot DC fast charge, or a very cold charging session). Sudden range drops that don’t recover after a BMS recalibration cycle are more consistent with cell failure than normal degradation. See our guide on why EV battery capacity suddenly drops for more on distinguishing this from a measurement artifact.

Sign 3: Warning that appears during hard acceleration or fast charging

A weak cell group that stays within tolerable parameters during normal driving may briefly drop below minimum voltage during maximum-demand events, full-throttle acceleration or DC fast charging at high power. The BMS detects this transient voltage excursion and logs a fault. If battery warnings consistently appear only during high-demand events and clear afterward, a cell group that can’t handle peak current demands is the cause.

Sign 4: Power reduction without low battery

The BMS implements power limits when it detects a cell group approaching its minimum voltage, even if overall SOC is still moderate. If you’re experiencing “Battery Power Reduced” or reduced acceleration at 40–60% SOC, the BMS is protecting a weak cell group from over-discharge stress. This is distinct from cold-weather power reduction (which affects the whole pack at cold temperatures).

Sign 5: BMS fault code specific to a cell group

An OBD2 scan that reveals a cell voltage fault code, rather than a generic battery code, is the most specific indicator of a bad cell group. Codes like P0A80 (Replace Hybrid/EV Battery), P0A7A (Cell Voltage Low), or manufacturer-specific codes pointing to a specific cell group position confirm the BMS identified a problem cell. The cell group number in the fault code data tells the technician exactly where in the pack the problem is located.

Sign 6: Individual cell group voltage deviation on a scanner

This is the definitive confirmation. Using an OBD2 tool that reads individual cell group voltages (LeafSpy for Nissan Leaf, ScanMyTesla for Tesla, Car Scanner with VW/Hyundai PIDs, OBDeleven for Audi/VW), check voltages at approximately 50% SOC:

Normal: All cell groups within 20–30 mV of each other
Mild imbalance: One group 50–100 mV below the average, monitor closely
Significant fault: One group 100–200 mV below average, dealer inspection needed
Failed cell group: One group 200+ mV below average or reading near 0V, module replacement needed

Sign 7: Unusual heat in the battery during charging

A cell with elevated internal resistance generates more heat for the same current flow than healthy cells. A battery with a bad cell may run noticeably hotter during charging, particularly visible in the temperature readings on a monitoring tool. If battery temperature during Level 2 charging consistently reaches 40°C+ (104°F) in mild weather, a high-resistance cell group contributing disproportionate heat is possible.

Sign 8: Faster-than-normal SOC drop in the first 10% of discharge

A common pattern with a failing cell group: the battery drops from 100% to 90% much faster than expected, then slows to a more normal rate for the middle range, then drops quickly again near the end. This “sagging” at the top and bottom of the discharge curve indicates a cell group that has higher resistance at full and low charge states, a characteristic of lithium cells with significant capacity degradation.

How to Confirm a Bad Cell

Step 1: OBD2 scan for fault codes

Connect an OBD2 scanner compatible with your EV and read all stored fault codes. Any cell voltage code confirms the BMS detected a cell-level issue. Record all codes before clearing, the specific code type tells you whether this is a one-time transient or a confirmed ongoing fault.

For a related problem, see Why Does My EV Battery Health Show N/A.

Step 2: Read individual cell group voltages

At approximately 50% SOC, connect a scanner capable of reading individual cell group voltages and record all readings. Look for the outlier. A single group reading noticeably lower than the rest at the same SOC confirms a bad cell group. Note the group number, this tells the dealer exactly which module to inspect.

Step 3: Watch the outlier during charging and discharging

A truly bad cell group shows disproportionate voltage drop during discharge and slower voltage recovery during charging compared to healthy groups. Monitoring the suspect group across a charge cycle confirms whether its behavior deviates from the pack average consistently, the hallmark of a genuinely degraded cell group rather than a temporary imbalance.

Step 4: Dealer inspection with OEM diagnostic tools

Bring your OBD2 cell voltage data and fault codes to the dealer. OEM diagnostic tools (CONSULT for Nissan, GDS2 for GM, ISTA for BMW, etc.) perform more detailed cell characterization tests than aftermarket scanners, including internal resistance measurements that confirm whether the identified cell group’s resistance is above spec. This OEM data is what authorizes a warranty module replacement. If the dealer confirms a failed cell group, see our guide on how to tell if you need a battery module replacement for what happens next.

FAQs

Can one bad cell destroy an entire battery pack?

In most cases, no, the BMS limits power delivery and charging rates when it detects a bad cell group, protecting the rest of the pack. A bad cell that’s caught and the module replaced doesn’t damage the remaining cells. However, a bad cell that reaches thermal runaway (extremely rare with good BMS protection) can potentially cascade. The BMS exists specifically to prevent this, don’t disable or bypass BMS warnings.

Is a bad cell covered under the 8-year battery warranty?

Yes, a confirmed cell voltage fault resulting in capacity loss below the warranty minimum, or a cell that fails outright, is covered under the manufacturer’s 8-year/100,000-mile battery warranty. Document the OBD2 fault codes, cell voltage readings, and the date/mileage when symptoms began. The dealer uses OEM diagnostic data to authorize the warranty claim.

Bottom Line

Bad cells in an EV battery pack show consistent patterns: mid-range SOC warnings, sudden range drops, high-demand power limitations, and measurable voltage deviation on a scanner. The most important diagnostic step is reading individual cell group voltages with a compatible OBD2 tool, a group reading 100+ mV below the pack average at 50% SOC is the clearest evidence of a bad cell group. Bring this data to a dealer for OEM-level diagnosis and warranty assessment within the 8-year/100,000-mile coverage period.

What Are Signs of a Bad Cell in an EV Battery Pack