Batteries: the 12 V one, the hybrid one and the EV one
Every car has a 12 volt battery, electric cars included. Hybrids and electric cars also have another, huge one, which is the one that moves the car. Both wear out, and both can be measured.
The 12 V battery
It is the usual one in the engine bay, in yellow in illustration 1 (on some cars it lives in the boot or under a seat). It starts the engine and powers all the electronics while the car is off. In a hybrid or an electric car it does not start any engine, but without it the car will not even wake up.
After the car has been parked for a good while, a charged battery reads 12.6 or 12.7 volts. At 12.4 it is about three quarters full; below 12.2 it is flat and suffering. With the engine running it should rise to between 13.8 and 14.7, which is the alternator charging it. Many modern cars have a smart alternator that stops charging when it can to save fuel, so seeing 12.8 for a while with the engine running is not necessarily a fault.
They last between four and six years, less with heat and lots of short trips. Cars with automatic stop-start use special batteries, EFB or AGM. If you replace it, fit the same technology, and bear in mind that on many cars the engine control unit has to be told the battery is new so it charges it properly.
The hybrid battery
A self-charging hybrid has a small battery of around 200 to 300 volts, made of many blocks or cells in a row (in green, in illustration 2, with the electric motor and the cables joining them). It works all the time: it charges when you brake and lift off, and helps the engine when pulling away and accelerating. Toyota’s nickel batteries last a very long time, it is not unusual to see them go past 300,000 kilometres (about 186,000 miles), but they do tire: when one block falls behind the others, the whole battery performs worse.
The plug-in hybrid and EV battery
An electric car’s battery fills the whole floor, as you can see in illustration 3. They are lithium, from 300 to 800 volts, and what matters is their capacity: how many kilowatt hours fit in today compared with when it was new. That is its state of health. It is lost little by little, faster with heat, with lots of rapid charging or if it is left for long periods at one hundred per cent or nearly empty. It is normal to lose a few points in the first years and then much more slowly. Almost every brand guarantees it for eight years or 160,000 kilometres (100,000 miles) above 70 %.
If you are buying a used one, the battery’s health is the first thing to check: it is by far the most expensive part of the car.
The most common faults
- Flat or worn 12 V battery
Slow starts, strange warnings on the dashboard, stop-start that stops working. In hybrids and electric cars, a car that will not wake up.
Typical codes: P0562, P0560 - Alternator not charging or overcharging
With the engine running the voltage stays below 13 volts all the time or goes above 15.
Typical codes: P0562, P0563 - Drain with the car switched off
Something stays awake and empties it in a few days: a module, a light, a badly fitted accessory.
- Unbalanced blocks in a hybrid
One or more blocks with lower voltage than the rest when current is drawn. The car uses the electric side less and ends up warning you.
Typical codes: P0A80, P0A7F - Blocked hybrid battery cooling
Many are cooled with air from the cabin. If the filter or the grille clogs with dust or dog hair, the battery runs hot and ages sooner.
What you can see with Final OBD
You can see the 12 V battery on any car: its voltage, whether it is charging and how much and, from Pro, its history, so you can tell whether it is fading over the weeks. On the screen, a Peugeot Traveller’s charging, with the current and how hard the alternator is working.
On hybrids and electric cars that provide it, you see inside the big battery: the voltage of each block or each cell, their temperatures, the highest and the lowest, and the state of health the car calculates itself. On the screen, the 80 cells of a Mitsubishi Outlander plug-in hybrid, one by one.
Open the bonnetthe in-depth explanation
Why a 12 V battery dies
Inside there are lead plates in acid. As it discharges, lead sulphate forms on the plates; as it charges, it dissolves again. If the battery sits discharged for a while, that sulphate hardens into crystals that no longer dissolve, and the battery loses capacity for good. That is why it is so harmful to leave a car parked for weeks without a maintainer, or to only ever do five minute trips that never give back what the start used.
The other big enemy is heat, which speeds up corrosion of the plates. In hot climates, or in cars with the battery right next to the engine, they last less.
Volts, current and internal resistance
Resting voltage tells you how much charge it has, not how healthy it is. An old battery can read 12.6 volts and collapse when you try to start. What really tells you its health is how far the voltage drops when current is drawn, and that depends on its internal resistance. That is why garage testers apply a load or measure that resistance, rather than just reading the volts.
There is one clue you can check yourself: the lowest voltage while cranking. With a healthy, charged battery it stays above roughly 9.6 to 10 volts. If it drops much further, the battery is asking to be replaced.
Smart alternators, EFB and AGM
Modern cars manage charging to save fuel: the alternator charges hard when you brake and lift off, when the energy is free, and rests when you accelerate. The battery therefore works partly charged and cycles a lot, which a normal battery cannot take. That is why stop-start cars use EFB (a reinforced lead battery) or AGM (with the acid soaked into glass fibre, which survives far more cycles).
Many also have a sensor on the negative terminal that measures the current going in and out and the temperature, and uses that to work out the battery’s condition. When you replace the battery, the engine control unit needs to know it is new, what type it is and its capacity; otherwise it will keep charging it as if it were the old one.
Inside a hybrid battery
A hybrid battery is a lot of cells in series. Toyota’s nickel metal hydride batteries have 1.2 volt cells grouped in modules of six; a Prius has 28 of those modules, about 200 volts in total.
The battery management system does not measure each cell: it measures blocks, usually of two modules. What says most about its health is the difference between the highest and the lowest block, especially under load, when accelerating or braking hard. If they are all almost equal at rest and one drops away when current is drawn, that block has high resistance and is the one that will cause trouble.
The charge the car shows is not the real one either. A hybrid keeps its battery roughly between 40 and 80 % to look after it, and the dashboard gauge shows that range as if it went from empty to full.
The health of an EV or plug-in hybrid
State of health is the capacity that fits today divided by what fitted when the battery was new. Almost every electric car calculates its own and stores it in the battery management system, and that is what Final OBD shows you when the car provides it, as on the MG4. But be careful: it is the car’s own calculation, and some keep it fixed or update it very rarely.
What ages a lithium battery most
Heat, especially while charging. Spending a long time at one hundred per cent or below ten. Back to back rapid charges, which heat it up. And the years, even if the car does not move. What treats it best: charging at home, slowly, between 20 and 80 % for everyday use, and only filling it right up before a long trip.
Regeneration: when the electric motor brakes
In a hybrid or an electric car, when you lift off or brake gently, the electric motor works in reverse, as a generator, and puts energy back into the battery. On one of our testers’ Eclipse Cross plug-in hybrid we have seen 59 amps going in during one stop, about 18 kilowatts at 310 volts. That is why the brake pads on a hybrid last so long: a good share of the braking is done by the electric motor.
All sections of the mechanics guide
See it in your own car
Almost everything in this guide can be seen with Final OBD and an adapter plugged in: faults explained, live data and the details of each system.
