Reference

Hybrid and EV Battery Glossary

Plain meanings for 80 terms you will meet on this site, in a battery report and at the dealer. Underlined words across the site show the short version when you point at them.

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12 Volt Battery: Every hybrid and electric car still has a low voltage auxiliary battery. It powers the computers and accessories and starts the car before the traction battery is engaged. A weak one causes many false alarms that get blamed on the big pack. Read more. Source: afdc.energy.gov

A

Amp Hour (Ah): An amp hour measures how much charge a battery holds. Tools like LeafSpy report pack capacity in amp hours. Comparing amp hours now to amp hours when new is one way to work out state of health. Read more.
Arc Flash: An arc flash happens when a short circuit lets current jump through the air. It can cause severe burns even without touching anything. Electrical safety rules like NFPA 70E exist largely because of it. Read more. Source: nfpa.org
ASE L3: The L3 is ASE's Light Duty Hybrid/Electric Vehicle Specialist Certification Test. To register, a technician must already have passed the A6 electrical and A8 engine performance tests. It covers the battery system, drive system, power electronics and supporting systems. Read more. Source: ase.com

B

Battery Cooling Intake: Many hybrids cool the pack with cabin air drawn through a vent. A vent blocked by dust, pet hair or bags lets the pack run hot. Looking at the vent is a safe check. Removing panels behind it is not an owner job. Read more.
Battery Electric Vehicle (BEV): A battery electric vehicle runs only on its battery and electric motor. It charges from a plug and from regenerative braking. Source: afdc.energy.gov
Battery Management System (BMS): The battery management system monitors cell or block voltages, temperatures and current. It balances the cells and sets fault codes like P0A80. Its state of health number is an estimate. Read more.
Battery Pack: The pack is the complete high voltage battery that drives the car. It stores energy for the electric traction motor. It is usually the most expensive single part on a hybrid or electric car. Read more. Source: afdc.energy.gov
Buffer Capacity: Most packs hold back some capacity at the top and bottom to protect the cells. The car does not show it, so gross and net numbers differ. A software update can change how much is held back. Read more.

C

Calendar Aging: Calendar aging is the capacity a battery loses just by existing. Heat and sitting at a high state of charge make it worse. It is why a low mileage car can still have an aged pack. Read more.
Capacity: Capacity is the amount of energy the pack can store and give back. It is usually stated in kilowatt hours or amp hours. Lost capacity is what shows up as lost range. Read more.
CCS (Combined Charging System): CCS lets drivers use the same charge port with AC Level 1, Level 2 and DC fast charging equipment, according to the Department of Energy. Source: afdc.energy.gov
Cell: A cell is one battery unit. Cells come as cylinders, flat prismatic cans or soft pouches. Electric car cells are packed tightly together to produce the voltage, power and energy the car needs. Source: ntsb.gov
Cell Balancing: Balancing keeps cells at a similar state of charge. On a lithium pack it is automatic and continuous, so there is nothing for an owner to start. On older nickel hybrids, a shop can run a balancing charge. Read more.
Cell Block: Many hybrids read voltage in blocks, often two modules at a time. Code P0A80 means the battery computer found a weak block. It does not mean every module is finished. Read more. Source: obd-codes.com
CHAdeMO: CHAdeMO is a DC fast charging connector. The Department of Energy describes it as common among Japanese automakers. Source: afdc.energy.gov
Check Hybrid System Warning: On Toyota hybrids a master warning light, often a red triangle, comes on with a Check Hybrid System message. Toyota's guide says it appears when the system detects high voltage leakage or another fault. A scan tells you why. Read more. Source: toyotaonthepark.ca
Class 0 Insulating Gloves: OSHA's table rates Class 0 rubber insulating gloves for a maximum use voltage of 1,000 volts AC. They must be electrically tested before first issue and every six months after. Techs wear leather protectors over them. Read more. Source: osha.gov
Contactors: Contactors are the switches between the pack and the car. On the Prius v they are 12 volt normally open relays that stop current leaving the pack when the car is shut off. If the 12 volt battery is dead, they cannot close. Read more. Source: toyotaonthepark.ca
Cycle Aging: Cycle aging is the wear that comes from use. Each charge and discharge adds a little. How deep the cycles are and how hot the pack gets both matter. Read more.

D

DC Fast Charging (DCFC): DC fast charging sends direct current straight to the pack. The Department of Energy puts it at about 100 to 200 or more miles of range per 30 minutes. How often a car fast charges is a contested factor in battery aging. Read more. Source: afdc.energy.gov
DC-DC Converter: The DC to DC converter takes high voltage power from the traction pack and turns it into the lower voltage the car's accessories need. It also recharges the 12 volt auxiliary battery. It does the job an alternator does in a gas car. Read more. Source: afdc.energy.gov
Degradation: Degradation is the gradual loss of capacity as a battery ages. It comes from time, heat, and use. Gradual fade is the normal case, and sudden failure is much rarer. Read more.
Diagnostic Trouble Code (DTC): A diagnostic trouble code is a short code the car stores when a system fails a check. Codes can be stored, pending, or permanent. A scan of every module, not just the engine, gives the full picture. Read more.
Discharge Test: A physical discharge test measures energy delivered between a full charge and a low state of charge. It is different in kind from a dashboard reading or an app estimate. It takes time and the right equipment, so it is a job for a trained technician. Read more.

E

eCVT: An electronically controlled continuously variable transmission is used in many hybrids. It uses electric motors and gears to blend engine and motor power. Grinding, whine or clunk under power or regen is worth noting. Read more.
Emergency Response Guide (ERG): Carmakers publish emergency response guides and shorter rescue sheets for their electric and hybrid models. NHTSA now hosts them, searchable by make and model. They cover disabling, fire, submersion, towing and storage. Read more. Source: nhtsa.gov
EV Mode: Most hybrids can move short distances at low speed with the engine off. If EV mode no longer engages when it used to, that can be an early sign of a weak pack. Read more.

H

Heat Pump: A heat pump moves heat rather than making it, so it uses less battery energy in cold weather. It recovers some, not all, of the range lost in winter. Read more.
High Voltage (HV): The NTSB describes electric car batteries as 300 to 400 volts or more, with 800 volt cars on the market. It gives 50 or 60 volts DC and 30 volts AC as the maximum considered safe for humans. Hybrid packs are lower but still dangerous. Read more. Source: ntsb.gov
High Voltage Interlock: An interlock is a low voltage loop that runs through high voltage connectors and covers. If the loop is broken, the system opens the contactors. It is a safeguard, not a reason to trust a pack is dead. Read more.
Hybrid (HEV): A hybrid electric vehicle combines an engine with an electric motor and a traction battery. The battery is charged by the engine and by regenerative braking. It never plugs in. Read more. Source: afdc.energy.gov

I

Internal Resistance: Internal resistance is the battery's own opposition to current. A cell with high resistance heats up more and its voltage sags more under load. Comparing modules under load is one way a technician finds a weak one. Read more.
Inverter: The inverter changes direct current from the pack into alternating current for the motors, and the reverse during regen. In many hybrids it also boosts voltage. The Prius v boosts 201.6 volts DC up to 650 volts. Read more. Source: toyotaonthepark.ca

J

J1772: J1772 is the standard connector for AC Level 1 and Level 2 charging. It is an SAE standard. Source: sae.org

K

Kilowatt Hour (kWh): A kilowatt hour is a unit of energy. Battery capacity is measured in kilowatt hours. The NTSB gives the 2011 Chevrolet Volt pack as 16 kWh and the 2019 Tesla Model S as 60 to 100 kWh. Source: ntsb.gov

L

LeafSpy: LeafSpy is a consumer app that works only on the Nissan Leaf. With a compatible adapter it reads battery data a generic code reader cannot. It is the best consumer battery tool there is, for that one car. Read more.
Level 1 Charging: Level 1 uses a standard 120 volt AC plug. The Department of Energy puts it at about 5 miles of range per hour of charging. Read more. Source: afdc.energy.gov
Level 2 Charging: Level 2 uses 240 volts at home or 208 volts in commercial settings. The Department of Energy puts it at about 25 miles of range per hour of charging. Read more. Source: afdc.energy.gov
LFP (Lithium Iron Phosphate): Lithium iron phosphate stores less energy for its weight but tolerates full charges well. It inverts the usual advice, and makers often ask owners to charge it to 100 percent from time to time so the car can calibrate. Read more.
Limp Mode: When the car finds a serious fault it may cut power to protect itself. A sudden loss of power on a hill or at merge speed is worth more than any parking lot check. Read more.
Lithium Ion: Most electric cars and plug in hybrids use lithium ion batteries. The exact chemistry varies, and the names below (NMC, NCA, LFP and others) describe what the cathode is made of. Read more. Source: afdc.energy.gov
LMO (Lithium Manganese Oxide): Lithium manganese oxide showed up in early electric cars, often blended with NMC. It is rarely used on its own in cars today. Read more.
Lockout: Lockout and tagout is the workplace practice of locking out an energy source and tagging it. On a hybrid or electric car it means the key is away from the car and the disconnect is secured before work starts. Read more. Source: osha.gov
LTO (Lithium Titanate): Lithium titanate uses a titanate anode. It handles many cycles and fast charging but stores less energy. It appeared in a few Japanese market cars and in buses. Read more.

M

MechanicRank EV and Hybrid Credential: The EV and Hybrid Systems track on MechanicRank is one of its specialty exams. It covers high voltage safety, hybrid system diagnosis and pack diagnosis at the module level. It is one credential to look for, alongside ASE and maker training. Read more. Source: mechanicrank.com
Meter CAT Rating: Measurement categories under IEC 61010 tell you what electrical environment a meter is built for. Higher categories withstand bigger voltage spikes. A tech measuring a high voltage pack should be using a meter rated for it. Read more. Source: hioki.com
Module: Cells are assembled into modules, and modules into a pack. The Prius v pack, for example, is 28 modules of 7.2 volts each. A hybrid pack usually fails as one weak module rather than all at once. Read more. Source: toyotaonthepark.ca

N

NACS (SAE J3400): NACS was developed by Tesla and is standardized as SAE J3400. The Department of Energy says it works for all charging levels, including Tesla Superchargers. Source: sae.org
NCA (Nickel Cobalt Aluminium): NCA is a nickel based lithium ion chemistry. It packs a lot of energy into a small space. Like NMC, it does best when it is not left full and hot. Read more.
NHTSA: NHTSA is the federal agency for vehicle safety. It runs the public recall lookup by VIN, collects owner complaints, and hosts emergency response guides for electric and hybrid cars. Read more. Source: nhtsa.gov
Nickel Metal Hydride (NiMH): Nickel metal hydride packs are used widely in hybrid cars. They are safe and abuse tolerant and last longer than lead acid, but they self discharge faster and generate heat at high temperatures. Read more. Source: afdc.energy.gov
NMC (Nickel Manganese Cobalt): NMC is a nickel based lithium ion chemistry used by many carmakers. Like NCA, it ages faster when it sits at a high state of charge in heat. The batteries in all four fires the NTSB studied were NMC. Read more. Source: ntsb.gov

O

OBD-II: OBD-II gives a scan tool access to fault codes and readiness monitors. It was written for emissions, so a generic tool cannot read an electric car's battery health. That takes manufacturer specific access. Read more. Source: sae.org
Orange Cables: Makers color high voltage cables orange so they stand out. Toyota's emergency guide warns against touching, cutting or breaching any orange cable or high voltage part. Read more. Source: toyotaonthepark.ca

P

P0A80: P0A80 means the battery management system found a weak cell block. It does not mean every module is finished. Test the 12 volt battery before authorizing work on the main pack. Read more. Source: obd-codes.com
Pending Code: A pending code means the car saw a problem but has not confirmed it on a second check. Pending codes do not always light the dash, so they only show up on a scan. Read more.
Plug In Hybrid (PHEV): A plug in hybrid has an engine and a larger battery that charges from the grid. It can drive some distance on electricity alone before the engine is needed. Source: afdc.energy.gov
Pre Purchase Inspection (PPI): A pre purchase inspection is a check of a used car before money changes hands. Many cover the battery with only the dashboard reading. Ask what was actually measured. Read more.
Preconditioning: Preconditioning heats or cools the cabin and pack before a drive. Doing it while plugged in saves range. Arriving at a fast charger with a cold pack means slow charging. Read more.
PZEV and AT-PZEV: PZEV and AT-PZEV are California emissions categories. Some hybrids were sold as AT-PZEV, which can come with a longer emissions warranty that may cover hybrid parts. Confirm the terms for your exact VIN with a dealer. Read more.

R

Readiness Monitors: The car runs a set of self tests as it is driven. Clearing codes or disconnecting the battery resets them to not ready. On a used car, monitors that are not complete can mean the codes were cleared before the sale. Read more.
READY Light: On many hybrids the READY light replaces the sound of a running engine. When it is lit the high voltage system is on and the car can drive silently. Never assume a hybrid is off just because it is quiet. Read more. Source: toyotaonthepark.ca
Recall Campaign Number: Every US safety recall gets a campaign number such as 20V-701. Some battery recalls were superseded by later campaigns, so a car showing the old one as done can still need the new remedy. Look up the VIN at NHTSA. Read more. Source: nhtsa.gov
Reconditioned or Rebuilt Pack: Reconditioned and rebuilt usually mean only the failed modules were replaced. These packs cost less and tend to last less. Reconditioning is real on older nickel hybrids and a myth on lithium packs. Read more.
Regenerative Braking: When you lift off or brake gently, the motor works as a generator and sends energy back to the pack. Because regen does much of the stopping, the friction brakes are used less and can rust. Read more. Source: afdc.energy.gov
Remanufactured Pack: A remanufactured pack is rebuilt to a stricter process than a reconditioned one. Every module is tested and matched. It is not the same product as a rebuilt pack, even when the words look alike. Read more.

S

Salvage Title: A salvage, flood or rebuilt title brand means the car was written off at some point. High voltage parts can be damaged by water or impact in ways that are not visible. Treat any branded title as a reason for a professional battery test. Read more.
Service Disconnect: Many packs have a service plug or disconnect that breaks the high voltage circuit. Removing it is part of a maker's disable procedure, done with insulated gloves and followed by a wait and a voltage check. It is not an owner task. Read more.
Sodium Ion (Na-ion): Sodium ion is the genuinely new chemistry on the list. It uses no lithium. It is just reaching customers, so there is no used market history yet. Read more.
State of Charge (SoC): State of charge is how full the pack is at this moment. A tired pack can still show 100 percent when charged. Do not confuse it with state of health, which is how much the pack can hold compared with new. Read more.
State of Health (SoH): State of health is capacity now divided by capacity when new. Most cars estimate it with a model rather than measuring it, so two tools can disagree on the same car. A good report says which quantity was measured. Read more.
Stranded Energy: When a high voltage pack is damaged, energy stays inside the undamaged cells with no path to discharge. The NTSB found this can cause shocks and repeated reignition after a fire is put out. Read more. Source: ntsb.gov

T

Technical Service Bulletin (TSB): A technical service bulletin tells technicians how to fix a known problem. Unlike a safety recall, the repair is not automatically free. Ask a dealer or specialist whether any bulletins apply to the battery or inverter on your VIN. Read more.
Telematics: Telematics is the data link built into many cars. Some battery services read the car's own estimates through it and compare them to a fleet. That is an estimate, not a physical test. Read more.
Thermal Management: Some packs are cooled by a fan, some by liquid coolant, and a few have no active cooling. Heat ages a battery faster than anything else. A car without active cooling is a different product from one with it. Read more.
Thermal Runaway: The NTSB defines thermal runaway as uncontrolled increases in temperature and pressure in damaged cells. It can lead to venting of toxic gases, cell rupture and fire, and a battery can reignite after it is put out. Read more. Source: ntsb.gov

V

Verify Zero Energy: After disabling the system and waiting the maker's stated time, a trained tech confirms with a meter that no voltage remains. Toyota's guide for the Prius v says the system may stay powered for up to 10 minutes after shut off. Read more. Source: toyotaonthepark.ca
VIN: A VIN is 17 letters and numbers with no I, O or Q. Recalls, warranties and history reports are all looked up by VIN. The VIN on the dash, door jamb and title should match. Read more. Source: vpic.nhtsa.dot.gov
Voltage Spread: Voltage spread, also called voltage delta, is the difference between the strongest and weakest module or cell block. An average can look fine while one module lags. That is why a good test compares the modules to each other and writes the spread down. Read more.