Second-Life EV Batteries as Home Energy Storage

Why retired EV packs attract interest

An EV traction battery is a large, well-built energy store. According to UL (opens an external site), a pack that reaches the end of its automotive life typically still holds about 80 percent of its original capacity, which is often more than a dedicated home battery offers. Packs from written-off vehicles reach the salvage market well below the price of new stationary batteries.

The obstacle has always been communication. Each pack has a battery management system (BMS) (opens an external site), the electronics that measure cell voltages and temperatures and decide what the pack may safely do, and it reports in the vehicle maker's own messages over a CAN bus (opens an external site), the wired network vehicles use between controllers. A home hybrid inverter expects a different battery protocol, usually that of a battery brand it has approved. Bridging the two is the job of projects such as Battery-Emulator, described in the next section.

We have also seen a homeowner in Florida integrate a battery from a Nissan Leaf with a PV inverter. That was not our project, and we cannot speak to its design, permits or insurance. It shows that a capable builder can make such a system run; it does not show that the result meets the codes discussed below.

The wider idea has merit. Batteries that already exist, whether in a parked car or retired from one, could serve as flexible storage that powers homes in outages, stores solar energy for later use and reduces peak demand, which in turn could reduce the generating capacity utilities build for peak hours. Overviews of vehicle-to-grid services (opens an external site) list these among the potential benefits. The main article of this series sets out that case with its costs and open questions. Our position is that those benefits arrive through standards and listed products that make such storage routine and safe, not through improvised installations.

What Battery-Emulator does, and what its authors warn

Battery-Emulator (opens an external site) is an open-source project licensed under GPL-3.0. In its own words it enables EV battery packs to be repurposed for stationary storage and acts as a translation layer between the EV battery and the home inverter. Running on small microcontroller boards, it reads the pack BMS over CAN and presents the battery to the inverter as a battery type the inverter supports.

The project is candid about its limits, and its warnings are the best summary of the risk. The following statements come from the Battery-Emulator wiki (opens an external site) and its installation guidelines (opens an external site).

• The project requires qualified electrician skills along with general IT knowledge, and working with high voltage is dangerous.
• Almost all safety features rely on communication data, so a physical error such as damaged cell casings, ruptured or leaking cells or corrosion will not be detectable through software; fuses and periodic visual inspections are needed for that.
• For a leak from the high-voltage side to the casing, the system relies on the inverter's insulation-resistance measurement.
• Almost all salvage batteries come from crashed vehicles with an unknown history, and any used EV pack should be operated where a potential fire would not put human life at risk.
• EV packs are generally splash-proof but cannot cope with large amounts of water or rain, and many lithium chemistries stop charging or discharging above about 50 °C.
• Ultimately, the owner alone is responsible for the system.

On a fault, the emulator stops charging and discharging and, where it controls the pack contactors directly, opens them. The protection therefore depends on the pack's BMS reporting truthfully and on the inverter check described in our guide to insulation monitoring and other inverter protections. This guide explains the concept only; it does not cover wiring or configuration.

What the car provided that a garage does not

A traction pack is designed as one component of a vehicle. Several protective layers live outside the pack, in the vehicle around it, and they do not come along when the pack is removed. The layers below are typical of current EVs; each design differs, so the table describes common practice rather than every pack.

The contactor layer deserves particular attention. A contactor (opens an external site) is the heavy-duty relay that connects the pack's high-voltage terminals to the outside; in a vehicle, it closes only after a precharge sequence and a set of checks that the vehicle controller supervises. A second-life system replaces that supervisor with a new one, and the new one has never been evaluated with that pack.

Capacity is also not a safety screen. UL notes that each retired battery must be evaluated individually because each was exposed to different charging and discharging conditions in the vehicle. A pack that reads a healthy capacity can still carry cell damage from a collision or water intrusion, and lithium-ion cells that are damaged or abused can enter thermal runaway (opens an external site), a self-heating reaction that is difficult to stop once it starts.

The standards that apply to reused EV batteries

Fire codes now address reuse directly. The International Fire Code section 1207.3.9, as adopted by Seattle, states that storage batteries previously used in other applications, such as electric vehicle propulsion, shall not be reused unless refurbished (opens an external site) by a battery refurbishing company approved in accordance with UL 1974. Austin applies the same section in plan review. Commentary on NFPA 855 (2023 edition) reports equivalent requirements that repurposing be carried out by a UL 1974-compliant company and that reused equipment be reconditioned, tested and approved; section numbers may differ in the 2026 edition, so confirm them with the authority having jurisdiction.

UL is explicit that grading alone is not enough: repurposed batteries for stationary storage (opens an external site) must also be certified to UL 1973, with the entire ESS certified to UL 9540.

The residential rules in the 2021 IRC section R328 require UL 9540 listing, a maximum of 20 kWh per unit (opens an external site), 3 feet between units unless UL 9540A data supports less, and an aggregate of 40 kWh in utility closets, basements and storage spaces or 80 kWh in garages, accessory structures, on exterior walls and outdoors. A typical EV pack holds well over 20 kWh in a single enclosure.

NEC 706.20(B) adds a voltage limit: an ESS for dwelling units shall not exceed 100 volts (opens an external site) between conductors or to ground. EV packs commonly run at several hundred volts. The NEC provides an exception for equipment whose live parts are not accessible during routine maintenance, and listed high-voltage residential batteries rely on provisions of this kind; confirm the exact wording in the edition your jurisdiction has adopted. High-voltage home batteries are not prohibited; the code accepts them on the basis of listed, enclosed construction that an open salvage pack lacks.

New York City and Long Island

Since October 26, 2025, residential storage in New York City has been governed by 1 RCNY 3616-07 (opens an external site), which adopts NFPA 855 with city amendments. Batteries are allowed above grade only, in garages or outdoors and never in living areas, with no more than 80 kWh per lot. Permanently wired systems above 1 kWh must be listed to UL 9540. Under the companion rule 1 RCNY 101-19, each installation also requires a DOB permit and a separate electrical permit, and the product an FDNY Certificate of Approval where one is required. Our guide to adding a battery to an existing solar system sets out the permitted locations and limits in detail.

The rule also addresses reuse directly. Section 4.2.10.1 states that storage batteries previously used in other applications, such as electric vehicle propulsion, “shall not be permitted to be reused or repurposed.” Inside the city, a salvaged EV pack used as home storage is therefore prohibited outright, not merely unlisted.

Product approval would be a second barrier: under the FDNY guide (opens an external site), the applicant for a Certificate of Approval is the battery unit manufacturer or its authorized agent, and the review relies on test reports to UL 1741, UL 1973 and UL 9540, none of which exist for a home-built system. An annex note, A.15.7.3, states that batteries on electric vehicles should not be included in the household capacity limits; our reading, which the rule does not state, is that this covers a battery still in a vehicle and used as one, not a pack removed from it.

On Long Island and elsewhere in New York State, the Residential Code of New York State and the local building department govern, and some fire marshals add requirements of their own. The same listing logic applies; confirm the current edition and local amendments with the town or village.

Insurance is a question to ask, not something we can answer for you. We found no published insurer position on second-life home batteries. Ask your carrier in writing before installing any battery, and expect unpermitted work and unlisted equipment to be scrutinized in any claim.

What the incident data shows

We found no reliable statistics on fires in residential second-life systems, and we do not claim a rate in either direction. The data that does exist points to integration as the weak link. A 2024 root-cause study by EPRI, PNNL and TWAICE, covering stationary battery failures with enough information to analyze (26 of 81 events), attributed 65 percent to operation and integration and 11 percent to the cells or modules themselves, and found that 72 percent occurred during construction, commissioning or the first two years of operation. Those were mostly professionally built utility and commercial systems; a home-built second-life system is a custom integration in its first years, the profile where recorded failures concentrate.

Commercial second-life storage does exist in listed form. B2U operates a storage site in California built from retired EV packs that, according to DatacenterDynamics, has passed UL 9540 certification (opens an external site), with each battery monitored and automatically disconnected if it deviates from design limits. What separates it from a garage build is the engineering and testing, not the cells.

The European approach to battery data and second life

The EU Battery Regulation (EU) 2023/1542 treats second life as a regulated activity rather than an informal one. Under Article 14(1), from August 18, 2024 the battery management systems of stationary storage, LMT and EV batteries (opens an external site) must include up-to-date data on the parameters that determine state of health and expected lifetime. Summaries of the regulation describe read-only access to that data for the battery's purchaser, or a party acting for them, for purposes that include assessing residual value and repurposing; readers in the EU should confirm the details and the status of implementing acts in the official text.

From February 18, 2027, EV, light-transport and industrial batteries above 2 kWh placed on the EU market need a battery passport (opens an external site), an electronic record of the battery's characteristics and history; secondary sources report that a repurposed battery receives a new passport linked to the original.

Our recommendation

Leave the battery in the car. In the vehicle, the pack keeps its cooling, its structure, its supervising controller and its maker's support, and a listed bidirectional charger can draw on it for home backup where the vehicle and charger support it. The main article of this series explains how an EV battery can serve a home while it stays in the car, and our guides on EV backup power in New York and why automakers restrict V2H cover the approved equipment and its limits.

• For dedicated storage, use a hybrid inverter with a battery listed to UL 9540 as a system, sited where the NYC or local rules allow.
• For an existing microinverter array, AC coupling the array to a hybrid inverter adds a listed battery without replacing the panels.
• Outside New York City, a product built from repurposed cells is an option only if the finished system carries a UL 9540 listing and meets the local fire code; inside the city, section 4.2.10.1 rules out reused EV batteries.
• If local data access is what attracts you to open projects, choose equipment with documented local monitoring interfaces instead of an emulated battery.

Technically possible is not the same as permitted

A system that runs is not necessarily a system that is allowed. A custom or unlisted battery interface is generally not a listed or approved system under US codes, including the NEC, UL listing requirements, utility interconnection rules and, in New York City, the Fire Code and FDNY approval, and it is often not permitted at all. It can void vehicle, battery and inverter warranties and affect insurance coverage. Inverter makers say so in writing; Solis, for example, states in its approved battery list that installing batteries not on the list will void the inverter warranty (opens an external site). High-voltage DC can kill, and lithium-ion batteries can burn.

Before any work, consult the authority having jurisdiction, your utility, the equipment manufacturers and licensed professionals. This guide describes how these systems work and why the safety layers exist; it is not installation guidance.