Understanding Battery Abuse and Resultant Failures

Dustin Herte, CFEI, Energy Forensics Group

In the realm of product liability and subrogation, lithium-ion battery fires present a unique challenge. When a power tool or e-bike battery undergoes thermal runaway, the resulting damage is often catastrophic, leaving behind a charred narrative that seemingly points to a manufacturing defect.

However, for legal counsel, assuming a manufacturing defect as the sole cause can be a premature conclusion. As investigators, our role is to differentiate between an internal cell failure (such as separator contamination) and external stressors introduced by the operator.

While manufacturing defects certainly exist, physical evidence sometimes reveals that the battery was subjected to conditions—abuse, misuse, or modification—that exceeded its design limits. This post outlines the specific forensic indicators we look for to determine if a failure was potentially caused by owner interaction rather than product quality.

The Mechanism: Thermal Runaway

To understand causation, we must look past the result. "Thermal runaway" is the self-sustaining chemical reaction where heat generation exceeds heat dissipation. While this is the mechanism of the fire, the trigger varies.

A cell manufacturing defect typically triggers this from the inside out (e.g., debris that made it into the electrode layers, eventually piercing the separator). Abuse triggers this from the outside in (e.g., crushing the separator via impact or forcing over-voltage). Distinguishing between the two requires a close look at the debris.

Sector 1: Power Tool Batteries

Power tool batteries are ruggedized, but they are often subjected to extreme environmental stress. When investigating these claims, we analyze three specific vectors of potential abuse.

1. Mechanical Impact and "Witness Marks"

A common claim is that a battery "exploded" while sitting idle on a shelf. However, latent damage from a previous drop or other mechanical damage can manifest as a fire days or weeks later.

• The Investigation: While the external plastic housing often melts away, the steel cans of the individual 18650 or 21700 cells survive. We look for witness marks—specific creases or dents on the cell walls that align with the internal plastic ribs of the battery case.

• The Implication: These marks suggest the pack suffered a significant deceleration event (a drop or crush) prior to the fire. This impact can deform the internal layers of the cell, creating a slow-developing internal short circuit.

2. Moisture Ingress

Construction sites, workshops, and garages are not sterile environments. Water intrusion is a common cause of BMS (Battery Management System) failure.

• The Investigation: We examine the remains of the Printed Circuit Board (PCB). Fire causes carbonization (black soot), but moisture causes oxidation. We look for green copper oxide or white mineral deposits on the circuit traces.

• The Implication: If corrosion is found underneath the fire damage, it establishes that moisture entered the pack before the fire occurred. This conductive path can bridge safety components, leading to failure.

3. Cross-Platform Adapters

The market is flooded with aftermarket plastic adapters allowing Brand A batteries to power Brand B tools.

• The Risk: Protection protocols vary. Some manufacturers put the Low Voltage Cutoff (LVC) in the battery; others put it in the tool. Mixing them via an adapter can result in a system with no LVC.

• The Implication: This allows the user to drain the battery to 0V. This deep discharge chemically decomposes the internal electrolyte and breaks down the copper current collector foil. When the user attempts to recharge this "dead" battery, it can enter thermal runaway, immediately - or spontaneously, after delay. Unfortunately, the battery may successfully recharge and operate again before the thermal runaway occurs, so a battery returned to service may be misleading in its indications of safety.

Sector 2: E-Bikes and Scooters

E-bikes present an additional set of risks, often centered around intentional modification and charging hardware incompatibility.

1. The "Frankenstein" Pack: Cell Replacement Risks

Due to the high cost of replacement packs, a secondary market of "refurbished" batteries has emerged. This often involves replacing only the "dead" cells in a pack while keeping the remaining aged cells.

• The Investigation: During teardown, we look for inconsistent cell wrappers (e.g., a mix of green Samsung and brown LG cells in the same pack) or evidence of hand-soldering on the nickel strips, which indicates non-factory assembly.

• The Implication:  As the pack discharges, cells with different resistance heat up at different rates. The weaker (older) cells struggle to keep up, often dipping into dangerous voltage ranges or overheating before the BMS can detect the anomaly. Additionally, the cells may not be State-of-Charge balanced properly during reassembly, which may force some cells into an overcharged voltage range immediately on the first recharge.

2. The "Shunt" Modification (BMS Bypass)

Enthusiasts seeking higher speeds or torque may modify the battery’s safety systems.

• The Investigation: We perform visual and X-ray analysis of the BMS debris. Specifically, we look for non-factory soldering or thick wires bridged across the current sense resistor (shunt).

• The Implication: This modification blinds the BMS to over-current events. It allows the motor to draw power well beyond the battery's safety rating, generating heat and potentially other cell damage like dendrites, and this damage can trigger a fire.

3. Charger Incompatibility

One of the most significant factors in recent e-mobility fires is the mismatch between charger and battery voltage.

• The Investigation: We prioritize the recovery of the charger. E-bike systems typically run at 36V, 48V, or 52V, but often share identical connectors (XLR or barrel plugs). These plugs are also available separately online and make it easy for a buyer to modify a charger.

• The Implication: Plugging a 52V charger into a 36V battery, for example, may force the battery cells beyond their maximum voltage. While a high-quality BMS should stop this, many generic BMS units fail under significantly higher voltage than the pack is rated for. Additionally, a BMS may not fail right away.

4. The "Universal" Charger Warning

The risks associated with generic charging hardware are well-documented. In September 2024, CPSC Commissioner Richard Trumka Jr. issued a specific warning regarding "universal" chargers. The CPSC noted that these devices often fail to operate within the battery's expected safe charging limitations, leading to overcharging. The agency cited 156 reports of fire and thermal incidents involving these chargers in the first half of 2024 alone (1).

Summary: The Forensic Checklist

For counsel reviewing a file, the following artifacts—if identified by your expert—can serve as indicators that external factors contributed to the loss:

Artifact	Potential Indication
Fused/Welded Relay Contacts	Suggests an over-current event where the switch tried to open under heavy load (often seen in modified bikes).
Charger Plug Fused to Port	Indicates the device may have been energized and charging at the moment of failure.
Mismatched Internal Cells	In "refurbished" packs, finding different brands/ages of cells suggests dangerous cell voltage or internal resistance imbalances.
Debris in Housing Cracks	If dirt is found inside a housing crack but under the soot, the crack likely existed prior to the fire.

Conclusion

A lithium-ion battery fire is a complex chemical event. While manufacturing defects are a valid and frequent cause of failure, they are not the only cause.

By systematically excluding abuse, misuse, and modification, we build a more accurate understanding of the loss. For the legal professional, identifying these "red flags" early in the investigation is critical to determining the true liability exposure of a case.

Works Cited

(1) Trumka, Richard L., Jr. "Commissioner Trumka Urges Consumers Not to Use 'Universal' Chargers for E-bikes Because of Fire Hazard." U.S. Consumer Product Safety Commission, 5 Sept. 2024, https://www.cpsc.gov/About-CPSC/Commissioner/Richard-Trumka/Statement/Commissioner-Trumka-Urges-Consumers-Not-to-Use-%E2%80%9CUniversal%E2%80%9D-Chargers-for-E-bikes-Because-of-Fire-Hazard.