Used EV pack or new LFP?
Is a used EV pack cheaper or better than new LFP?
What LFP is. LFP stands for lithium iron phosphate (LiFePO4). It uses no cobalt or nickel, so it costs less to make, it is harder to set on fire, and it lasts many more charge cycles than the nickel-based cells in most older EVs. It is common in budget EVs and in home batteries.
What is being compared. A used EV pack is a battery taken out of a car at the end of its life, sold as it is or refurbished. A new LFP pack is a battery bought new for home storage, usually built for 48 V inverters.
The answer. A used pack is usually cheaper up front, but whether it is better depends on the job. Its remaining capacity (state of health) and history are often unknown, it rarely comes with a warranty, and it usually needs extra parts such as a BMS or a CAN translator before a home inverter will use it. New LFP costs more, but its capacity is known, it comes with a warranty and it plugs straight into common inverters. On the forums the usual verdict is that cheap new LFP has made used EV packs not worth the hassle unless the pack is very cheap. The UK can be the exception, where people report used packs under £30 per kWh against about £60 per kWh for new LFP.
Source: diysolarforum.com · diysolarforum.com · Reddit r/SolarDIY
Why aren't more people using old Leafs as home batteries?
A Leaf pack is 96S, around 360 V. That voltage can kill you, and it sits outside the window a 48 V inverter accepts. The car's BMS does not speak the inverter's messages, so you add a translator or take the pack apart. After that work the price is often no lower than new LFP.
Source: Reddit r/SolarDIY
Voltage: keep it high or rebuild to 48V
Should I keep the pack at 400V with Battery-Emulator, or rebuild it to 48V with a JK BMS?
A whole Leaf pack is about 360 V. Leaving it whole with Battery-Emulator keeps the car's BMS, and you need an inverter built for that voltage. Rewiring the modules to 48 V with a JK BMS is much safer to work on.
Source: diysolarforum.com
How do I make 48V from Leaf modules?
Seven modules in series gives 14S, because each module is 2S2P internally: two cells of voltage, with a pair side by side at each step, so one module is only about 7-8 V. Fourteen cells in that chain are about 42 V empty and about 57 V if you stop at 4.1 V per cell, which fits a 48 V inverter. Add parallel groups to add capacity at the same voltage.
Source: diysolarforum.com · diysolarforum.com
Why don't EV voltages suit 48V inverters?
12S drops too low for most 48 V inverters and 16S goes too high; 13S-14S is the sweet spot. Twelve nickel cells in series fall to about 36 V, under where many of those inverters shut off, and sixteen rise to about 67 V when full. A 16S LFP battery still fits, because those cells only reach about 3.6 V.
Source: diysolarforum.com
Can I use Tesla Model S modules for 24V or 48V, and in parallel?
A Model S module is 6S, about 22 V. Two in series gives 12S "48V" (roughly 36-50 V). That empty end is about 36 V, so a 48 V inverter may shut off while the cells still have charge. Give each parallel string its own BMS, so the higher string cannot dump current into the lower one.
Source: diysolarforum.com · diysolarforum.com · secondlifestorage.com
BMS, CAN and inverter compatibility
Which BMS, and what settings, for Leaf modules?
Use a BMS that can do 14S, one sensor for each cell in the Leaf chain. Set it to about 3.0-3.2 V minimum and 4.1 V maximum per cell, a little inside the cell's absolute limits.
Source: diysolarforum.com
Do Tesla modules need a BMS controller?
Yes. The small boards on the modules only balance the cells when a master computer asks them to. Out of the car that master is gone, so people use SimpBMS or similar.
Source: teslamotorsclub.com · teslamotorsclub.com
Can I use a whole Model S pack intact, or the whole car?
The cells are usable. The pack stays disconnected until the right computer, on the CAN wiring, tells the BMS to close the contactors.
Source: teslamotorsclub.com · teslamotorsclub.com
How do I wire the battery and inverter CAN to Battery-Emulator (e.g. FoxESS with Leaf)?
Some inverters need their own CAN channel, or the Leaf and the inverter talk over each other on one pair of wires. A LilyGo T-2CAN board does this, with the inverter on CAN-A and the battery on CAN-B. Battery-Emulator runs on that board and translates.
Source: GitHub discussion #1907
Can one battery feed several inverters?
Two inverters on one battery can each try to charge or discharge on their own, so one can push power in while the other pulls it out. Keeping them in sync means one of them leads. The usual advice is one battery per inverter.
Source: GitHub discussion #1675
Can I use the Leaf (car or pack) as a home battery?
The car's 12 V battery only runs lights, locks and computers. The traction pack is about 360 V. Leave it at that voltage and translate its CAN messages, or rebuild modules to 48 V. V2H keeps the car whole and runs the house from a charger.
Source: mynissanleaf.com · mynissanleaf.com
Can other EV packs (Zoe, Volvo) talk to a hybrid inverter?
Yes. A hybrid inverter expects a particular set of CAN messages. Zoe and Volvo packs use CAN too, but a different set, so people record the messages on a Pi or use an emulator that already knows the car.
Source: openinverter.org · openinverter.org
Safety and chemistry
Is reusing an EV pack safe?
NMC is riskier than LFP: a crushed, shorted or overcharged cell can go into a fire that keeps itself going. You need to know why the pack came out of the car, because crash damage can crack a cell that looks fine until it is charged at home. Some people have lived off-grid on one with no problems.
Source: diysolarforum.com
Contactors, precharge and fault codes
Why won't my i3 contactors close ("Precharge blocked")?
The battery's own BMS is refusing to close the contactors, so the high voltage stays disconnected. Precharge is the step it blocked. Check the HVIL interlock, isolation and CAN timing, and clear fault codes (Battery-Emulator firmware 9.3.0 and later can do this).
Source: GitHub discussion #2180 · GitHub issue #994 · GitHub issue #1047
My contactors click on then off and nothing happens. Why?
The click is the contactor closing, then opening again because the BMS still sees a fault. One pack in the thread came from a bus project, so its BMS firmware was not ordinary i3 software and Battery-Emulator's messages did not match.
Source: GitHub discussion #1295 · GitHub issue #2313
Which pack or module to buy
Which used EV pack is best for DIY?
i3 (bolted busbars, easy to undo and regroup into a 14S string), Ioniq 5 and Kia Niro are favoured. Tesla modules are seen as the worst because they're glued and potted, so you are stuck with a 6S block. BYD Blade is LFP, but its 126 cells don't divide neatly into 16S or 15S. Those are the usual counts for an LFP 48 V battery, so equal strings leave cells over.
Source: diysolarforum.com
How do I set up a large off-grid system on Model S modules?
See this example build on the Tesla Motors Club forum.
Source: teslamotorsclub.com
Checking health and matching modules
How do I test capacity or state of health and match modules?
State of health is capacity left compared with new. Test it with a full charge, then a discharge into a known load, and measure the kWh that come out. Sort modules so each parallel group has equal total capacity: in a series chain the weakest group hits empty first and stops the string. Don't mix modules with unknown histories (crash, heat, or years left empty). A voltage check will not show that.
Source: diysolarforum.com · Reddit r/SolarDIY
How close do the voltages need to be before connecting a second bank in parallel?
Match the voltages closely before connecting the banks in parallel. A fuller bank tied to an emptier one dumps a large current into the emptier one until they match, and that current can damage cells and cables. The thread gave no agreed figure.
Source: diysolarforum.com
Insurance, permits and regulations
Does a battery affect my home insurance (UK)?
Tell your insurer, ideally before installing. If a battery fire happens and the insurer was never told, they can refuse the claim for the house. A DIY or undisclosed system risks the whole policy.
Source: Reddit r/SolarUK
UK rules (from guides)
On UK legal and certification rules: I didn't find forum threads on this, only guides. These say a householder can't self-certify under Part P, so the work needs building control or a registered electrician. The network operator must be notified (G98/G99), and PAS 63100 restricts where a battery can go.
Source: GOV.UK guidance · iwantsolar.co.uk
Cold weather, heating and cooling
How cold can they get, and do I need heating?
Don't charge below about 0-2°C. Charging there can plate lithium metal on the anode, which permanently loses capacity and can later short the cell. A heated garage, or a BMS that refuses to charge until the cells warm up, avoids that. Cold cells also show voltage sag: they look emptier than they are.
Source: diysolarforum.com · diysolarforum.com
Words used on this page
Short definitions for words the answers above use as if they were already familiar. The first use of each one links here.
Series (S) and parallel (P)
S means cells wired in a chain. Their voltages add. P means cells wired side by side. Capacity adds, and the voltage stays the same. A nickel EV cell, the kind in a Leaf, a Tesla module or an i3, is about 3.0 V empty and about 4.2 V full. Twelve in series (12S) are about 36 V empty, under where many 48 V inverters shut off. Sixteen (16S) are about 67 V full, over the top of many of those inverters. Fourteen (14S), stopped at 4.1 V per cell, is about 42-57 V. LFP cells only reach about 3.6 V, so a new home battery is often 16S on purpose and still stays under 60 V. 2S2P is two in series with a pair side by side at each step: four cells, and only two cells' worth of voltage.
48 V inverter
"48 V" is the name of a class of home inverter. The battery is allowed a window, often somewhere around 40-60 V. Below the bottom the inverter shuts off. Above the top it refuses to charge, or the extra voltage can damage it. The exact window is in that inverter's manual. A hybrid inverter is a solar inverter with a battery port.
Module
A module is a small group of cells already wired together, as it left the car. A Leaf module is four pouch cells in 2S2P, so it is only about 7-8 V. Seven of them in series are the 14S string that fits a 48 V inverter.
BMS
A battery management system is the board that watches every cell and disconnects the pack if one goes too high, too low, or too hot. The car's BMS often will not run once the pack is out of the car. A JK BMS is a common aftermarket board for a rebuilt 48 V string, including a 14S Leaf string. SimpBMS is an open project that replaces the master computer Tesla modules expect. Without it, the small balancing boards on those modules do nothing.
CAN
CAN (Controller Area Network) is the pair of data wires cars and inverters use to send messages such as voltage and how much power may be drawn. The thick cables that carry the pack current are separate. Brands do not speak the same messages. A CAN translator, such as Battery-Emulator, turns one set into another. Some inverters need their own pair of wires, separate from the battery's pair. A LilyGo T-2CAN is a small board with two CAN plugs for that. FoxESS is one brand of hybrid inverter people connect this way.
Battery-Emulator
Battery-Emulator is open-source software, usually run on a small LilyGo board. It talks to the car's battery computer on one side and pretends to be a battery brand the house inverter already understands on the other.
Contactor
A contactor is the large relay inside the pack. The high-voltage cables stay disconnected until the BMS agrees and this relay closes.
Precharge
The inverter has large capacitors. Connecting the pack straight onto empty capacitors is a huge inrush and can weld the contactor shut. Precharge connects through a resistor first, waits until the voltages match, then closes the main relay. "Precharge blocked" means the BMS refused to start that step.
HVIL
HVIL (high-voltage interlock loop) is a wire loop through the plugs and the lid. Unplug a connector or open the cover and the loop breaks. The BMS treats that as the cover being off and will not close the contactors.
Isolation
Isolation means the high voltage is staying off the metal case. The BMS measures that insulation. A crash-damaged pack often fails the test, and the contactors stay open.
State of health
State of health is the capacity left compared with when the pack was new. A 24 kWh pack at 80% state of health holds about 19 kWh.
Kilowatt-hour
A kilowatt-hour (kWh) is the energy a 1 kW load uses in one hour. A price per kWh is the price for each unit of storage.
Vehicle-to-home
Vehicle-to-home (V2H) means the car stays a car. A special charger runs the house from it. On a Leaf that is often the CHAdeMO port. The pack stays in the car.
Part P
Part P is the electrical part of the building regulations in England and Wales. Some of that work has to be signed off. A householder is not on the register of people who can sign their own work, so it needs a registered electrician, or building control told in advance.
G98 and G99
G98 and G99 are how you tell the local grid company about solar, or a battery that can export power. G98 is the short notification for a small system. G99 is the application you send before connecting a larger one.
PAS 63100
PAS 63100 is a published specification for where a home battery may sit, so a fire does not block the way out of the house. Insurers and building control treat it as the practice to follow. It is a standard, published for people to use, rather than an Act of Parliament.
LFP
LFP is lithium iron phosphate (LiFePO4). It uses no cobalt or nickel, so it costs less to make, it is harder to set on fire, and it lasts many more charge cycles than the nickel cells in most older EVs. The longer account is on the Chemistry page.
NMC
NMC (also written NCM) is nickel manganese cobalt, the chemistry in many older EVs. It holds more energy than LFP in the same space. A crushed, shorted or overcharged cell can go into a fire that keeps itself going. The longer account is on the Chemistry page.