Maxell v. Amperex Technology: Federal Circuit Affirms Battery Patent Unpatentable
Maxell, Ltd. challenged Amperex Technology, Ltd. over US9166251B2, a patent covering battery separator and nonaqueous electrolyte battery technology. After 518 days before the Federal Circuit, the court affirmed the lower unpatentability ruling, leaving Maxell’s patent invalidated and Amperex’s freedom to operate intact.
Federal Circuit closes the door on Maxell’s battery separator patent
Maxell, Ltd., a Japanese electronics and IP licensing company, filed appeal Case No. 23-2285 before the United States Court of Appeals for the Federal Circuit on August 16, 2023. The dispute centred on US9166251B2, a patent covering battery separator technology and nonaqueous electrolyte battery systems — a technically sensitive area in modern lithium-ion cell design. The appellee, Amperex Technology, Ltd., a major lithium-ion battery manufacturer, had successfully challenged the patent’s validity at the tribunal level.
The Federal Circuit issued its judgment on January 15, 2025, affirming the prior unpatentability determination. The court’s order — ‘AFFIRMED’ — confirms that it found no reversible error in the underlying ruling cancelling US9166251B2. For Maxell, the affirmance extinguishes this patent as an enforcement asset. For Amperex, it conclusively resolves the validity cloud over the battery separator technology it manufactures.
The 518-day appellate duration is consistent with cases requiring substantive technical review of patentability grounds, suggesting the Federal Circuit engaged meaningfully with the underlying record. The public case record does not disclose the precise grounds of unpatentability — whether obviousness, anticipation, or another basis — nor whether a written opinion accompanied the order, both of which would materially affect the precedential weight of this outcome.
Filing to Unpatentable in 518 days
518 days at the Federal Circuit — above the median for single-patent patentability appeals
Federal Circuit affirms: what the ruling means for both parties
What ‘AFFIRMED’ means at the Federal Circuit level
An affirmance by the Federal Circuit means the appellate panel reviewed the record and found no reversible legal error in the lower tribunal’s unpatentability determination. The prior ruling cancelling US9166251B2 now stands as final. Maxell exhausted its appellate rights at this level; further review would require a petition for certiorari to the US Supreme Court, an exceptionally rare path in patentability matters.
No reversible error foundMaxell loses US9166251B2 as an enforceable asset
The affirmance is a definitive adverse outcome for Maxell. US9166251B2 is cancelled and cannot be enforced against any party in US proceedings. For a company whose business model is substantially built on IP licensing, the loss of a battery separator patent — a technically crowded but commercially vital space — reduces the breadth of its licensing portfolio and eliminates leverage in any ongoing or future licensing negotiations referencing this patent.
Patent cancelled — unenforceableAmperex secures clean freedom to operate on this technology
For Amperex Technology, the Federal Circuit’s affirmance is a complete victory. The unpatentability of US9166251B2 eliminates the risk of infringement liability tied to this patent across Amperex’s battery separator manufacturing operations. The ruling also raises the bar for any future Maxell enforcement effort on related battery technology, as the cancelled claims can no longer be reasserted. Amperex’s appellate counsel at Alston & Bird, LLP successfully defended the lower ruling.
FTO confirmed on separator techBattery separator IP landscape shifts after Federal Circuit ruling
The cancellation of a Maxell battery separator patent at the Federal Circuit level signals that invalidation challenges against legacy battery IP portfolios can succeed even through full appellate review. Competitors and new entrants in nonaqueous electrolyte battery technology — including EV cell suppliers and consumer electronics manufacturers — may find this outcome encouraging for FTO clearance efforts or as a reference point in licensing negotiations involving similar Maxell-held battery patents.
Heightened FTO confidence for sectorFull party and counsel information
| Role | Name | Type | Detail |
|---|---|---|---|
| Plaintiff | Maxell, Ltd. | Company | Japanese IP licensing company — holder of US9166251B2 battery separator patentSearch in Eureka ↗ |
| Defendant | Amperex Technology, Ltd. | Company | Amperex Technology, Ltd. — major lithium-ion battery manufacturer and appelleeSearch in Eureka ↗ |
| Plaintiff counsel | Corbin Cessna | Attorney | Counsel for Maxell, Ltd.Search in Eureka ↗ |
| Plaintiff counsel | Eric Joseph Klein | Attorney | Counsel for Maxell, Ltd.Search in Eureka ↗ |
| Plaintiff counsel | Erik Shallman | Attorney | Counsel for Maxell, Ltd.Search in Eureka ↗ |
| Plaintiff counsel | Hilary L. Preston | Attorney | Counsel for Maxell, Ltd.Search in Eureka ↗ |
| Plaintiff counsel | Jeffrey TaHwa Han | Attorney | Counsel for Maxell, Ltd.Search in Eureka ↗ |
| Plaintiff counsel | Paige Holland Wright | Attorney | Counsel for Maxell, Ltd.Search in Eureka ↗ |
| Plaintiff law firm | Vinson & Elkins LLP | Law Firm | Representing Maxell, Ltd.Search in Eureka ↗ |
| Defendant counsel | Brady Cox | Attorney | Counsel for Amperex Technology, Ltd.Search in Eureka ↗ |
| Defendant counsel | Christopher Timothy Lawn Douglas | Attorney | Counsel for Amperex Technology, Ltd.Search in Eureka ↗ |
| Defendant counsel | Kirk T. Bradley | Attorney | Counsel for Amperex Technology, Ltd.Search in Eureka ↗ |
| Defendant counsel | Nicholas Christopher Marais | Attorney | Counsel for Amperex Technology, Ltd.Search in Eureka ↗ |
| Defendant law firm | Alston & Bird, LLP | Law Firm | Representing Amperex Technology, Ltd.Search in Eureka ↗ |
| Presiding judge | Judge N/A | Judge | Court of Appeals for the Federal CircuitSearch in Eureka ↗ |
Official order — verbatim text
The Federal Circuit’s order — ‘AFFIRMED’ — is terse but unambiguous: the panel found no reversible error in the underlying unpatentability determination against US9166251B2. At the appellate level, factual findings supporting unpatentability are reviewed under a deferential standard, meaning the challenger’s burden to overturn the lower ruling was substantial. The brevity of the order may suggest a Rule 36 affirmance without written opinion, which would carry no precedential weight but leaves the cancellation of Maxell’s battery separator patent fully intact and final.
US9166251B2 — Battery Separator and Nonaqueous Electrolyte Battery
US9166251B2, filed under application number US12/673961, covers battery separator technology and nonaqueous electrolyte battery systems — core components in lithium-ion cell architecture. Battery separators regulate ion flow between electrodes while preventing short circuits; advances in separator material and structure are central to improving energy density, safety, and cycle life. The patent’s application lineage suggests development during a period of intense lithium-ion innovation, when separator IP was actively being staked by major Japanese electronics firms.
For the nonaqueous electrolyte battery sector — spanning consumer electronics, electric vehicles, and energy storage — separator patents represent a structural chokepoint in the supply chain. Maxell’s enforcement of this patent against Amperex, one of the world’s largest lithium-ion cell manufacturers, signals the commercial stakes attached to separator IP. With US9166251B2 now cancelled, the competitive dynamic shifts: companies designing around or licensing similar Maxell separator patents should reassess their exposure, and the cancelled claims may serve as prior art reference points in future prosecution.
Should you run an FTO against US9166251B2 and related Maxell battery patents?
Any company manufacturing, importing, or selling battery separators or nonaqueous electrolyte battery cells in the United States should be aware that US9166251B2 has been cancelled following Federal Circuit affirmance. However, Maxell holds additional battery-related patents that may cover overlapping or adjacent technology. R&D teams developing separator materials, electrolyte formulations, or cell assembly processes should verify whether related in-force Maxell patents present residual risk before assuming blanket freedom to operate.
PatSnap Eureka’s FTO Search Agent can map Maxell’s active battery patent portfolio against your specific product configuration — separators, electrolyte chemistry, cell geometry — and flag claims that survived or were not implicated in the US9166251B2 proceedings. Eureka also surfaces prosecution history and related family members across jurisdictions, giving your legal and product teams a defensible clearance baseline before launch or licensing negotiation.
Run a freedom-to-operate analysis on US9166251B2 to assess your product’s exposure
Run FTO in Eureka →Similar Federal Circuit Battery Technology Patentability Appeals
Federal Circuit appeals affirming unpatentability in lithium-ion battery and separator technology, relevant to Maxell v. Amperex Case 23-2285.
What this case signals for the battery technology IP landscape
The Federal Circuit’s affirmance of unpatentability in Maxell v. Amperex has practical consequences for battery IP enforcement and licensing strategy.
Validity challenges on battery separator patents can survive full appellate review
This outcome confirms that inter partes or similar validity challenges targeting battery separator and electrolyte technology patents are viable through the Federal Circuit level. Companies holding similar patents should audit claim strength proactively, as affirmance on unpatentability grounds leaves no further US appellate recourse short of Supreme Court petition.
Amperex’s win expands precedent for challenging legacy battery IP
Amperex Technology’s successful defence at the Federal Circuit, represented by Alston & Bird, suggests that well-resourced invalidity challenges against established electronics IP holders like Maxell can prevail. Battery manufacturers and their counsel should monitor whether Maxell’s remaining portfolio patents face similar scrutiny in parallel proceedings.
Maxell v Amperex — key questions answered
The Federal Circuit affirmed the unpatentability of Maxell’s US9166251B2 on January 15, 2025. The order, entered as ‘AFFIRMED’, confirms the lower tribunal’s cancellation of the battery separator patent stands. No reversible error was found. The case was closed after 518 days on appeal.
US9166251B2, application number US12/673961, covers battery separator and nonaqueous electrolyte battery technology — critical components in lithium-ion cell design. Maxell asserted this patent against Amperex Technology, a major lithium-ion battery manufacturer. The validity of the patent was challenged and ultimately upheld as unpatentable, with that ruling affirmed by the Federal Circuit.
The affirmance means US9166251B2 is cancelled and unenforceable in any US proceeding. Maxell cannot assert these claims against Amperex or any other party. Further review would require a petition for certiorari to the US Supreme Court, which is rarely granted in patentability cases. The patent is effectively extinguished as an enforcement asset.
Maxell, Ltd. was represented by Vinson & Elkins LLP, with attorneys including Corbin Cessna, Eric Joseph Klein, Erik Shallman, Hilary L. Preston, Jeffrey TaHwa Han, and Paige Holland Wright. Amperex Technology, Ltd. was represented by Alston & Bird, LLP, with attorneys including Brady Cox, Christopher Timothy Lawn Douglas, Kirk T. Bradley, and Nicholas Christopher Marais.
The ruling’s precedential weight depends on whether the Federal Circuit issued a written opinion or a summary Rule 36 affirmance. A Rule 36 affirmance carries no precedential value but confirms the outcome. Regardless, the case demonstrates that unpatentability challenges against battery separator patents can succeed through full Federal Circuit review, which is commercially significant for lithium-ion manufacturers and IP holders in the sector.
PatSnap Eureka searches patents and litigation data to answer instantly.