Used full hybrid: battery, cooling, transmission

A full hybrid of the common design lacks several of the parts that drive the bills on a high-mileage combustion car. There is no starter motor, because the electric machine spins the engine up, no alternator, and no clutch to wear out. On some models the air-conditioning compressor and the water pump are electric too, so the accessory belt disappears as well.

The engine itself has an easier life than the mileage suggests. It stops at every traffic light, often sits out traffic jams altogether, and is relieved in exactly the load ranges where an engine working alone suffers most. Some of the distance covered is therefore distance the engine never ran. The brakes last longer too, because much of the deceleration goes through the electric machine.

The downside lies in the same characteristics. An engine that keeps starting and stopping warms up more slowly on short trips in the cold, and its oil sees more condensation. Brakes that are rarely used tend to rust rather than wear, and callipers can seize. On top of that come the parts only a hybrid has: the hybrid battery, its cooling, the inverter and the inverter's own cooling circuit.

Many full hybrids with very high mileage served as taxis or fleet cars. That is no argument against them: such cars are often serviced strictly by the book, and regular use generally suits a hybrid battery better than long periods standing still. It simply shifts the focus — to the interior, the suspension and whether the paperwork is complete.

A full hybrid's battery is small compared with an electric car's, and the control system uses only the middle part of its capacity: it is never run completely full or completely empty. That is kind to it, which is why many of these batteries reach a great age. Depending on model and year they use nickel-metal hydride or lithium-ion cells, grouped into blocks and connected in series.

The battery ages mainly through heat. On many models it is cooled with air from the cabin: a fan draws it in through a grille, usually near the rear seat or in a side trim panel, and passes it through the battery casing. If the grille is blocked by a jacket, a bag or a dog blanket, or the filter behind it — where there is one — is clogged with dust and pet hair, the battery runs hot. The control system then limits electric power, the engine runs more often, consumption rises and the battery ages faster.

So the intake grille is one of the first things to look at. Dust in the slats, hair in the grille or a cabin that has obviously carried a dog are not a disaster, but they are a reason to ask when the fan and filter were last cleaned. On some models that cleaning is in the service schedule, on others it is not.

An ageing battery gives itself away less through outright failure than through its behaviour: the charge display jumps quickly between full and empty, the engine runs almost all the time even when the system is warm, and the electric boost under acceleration feels weak. Usually not all the cells fade at once; individual blocks drift away from the others in voltage. That is exactly what can be measured — more on that below.

The inverter sits between the battery and the electric machines. It turns the battery's direct current into alternating current for the motors, and does the reverse under braking; on some systems it also steps up the voltage. That produces heat, which on most full hybrids is carried away by a separate cooling circuit — independent of the engine's, with its own electric pump, its own radiator section and its own expansion tank.

This circuit is the one most often overlooked at viewings, because its reservoir sits next to the familiar one for engine coolant and looks much the same. Check both on a cold car: the level belongs between the marks, the fluid should be clean, and there should be no dried residue around the cap. Nothing gets opened on a warm system.

If this circuit's pump fails or coolant is missing, the inverter overheats and the control system limits power or shuts down, usually with a warning about the hybrid system. The coolant in this circuit often has its own change interval in the service schedule, and it is easily missed because it is not part of a routine service. The invoices tell you whether it was done.

The orange cables carry high voltage. Work on them and on the inverter belongs in a workshop with staff trained for it. At a viewing you look, but you do not touch.

In the common design a planetary gearset links the combustion engine, one electric machine that mainly works as a generator, and a second one that drives the wheels. There are no gears to shift, no torque converter and no clutch to wear; the ratio follows from how fast the generator spins. It is sometimes called continuously variable, but it has nothing to do with a belt-and-pulley gearbox.

Because little rubs against anything, the design is robust. What fades over the years is the bearings, the transmission oil and the torsional damper between engine and transmission that absorbs the engine's pulses. Then there are the engine mounts, which work harder in a hybrid than in a conventional car: every start and every stop is a small jolt, and that happens many times on every journey.

Whether the transmission oil needs changing is set by the manufacturer's service schedule — on some models only under severe conditions such as frequent towing. A change recorded in the paperwork is a good sign on a high-mileage car, even where it was not required.

Not every full hybrid is built this way. Other manufacturers put the electric motor into a conventional stepped gearbox, automatic or dual-clutch, and separate the engine with a clutch of its own; others again run mostly as a series hybrid and couple the engine directly to the wheels only at higher speeds. The technical data tell you which design your candidate has. If it has a stepped gearbox, that gearbox's warning signs apply as well: a jolt on shifting, judder when pulling away, a moment's hesitation.

Start with a cold car. When switched on, the system must reach its ready state without any warning message. The engine starting shortly afterwards while you are still stationary is normal: it is warming the catalytic converter and the cabin. If the system hesitates, shows odd messages or does nothing at all, the culprit is often the small 12-volt battery rather than the hybrid battery — a comparatively harmless finding, but one you should know about.

Once the system is warm and the charge display is not at the bottom, the car should creep away electrically under light throttle and keep running without the engine at town speeds. In the cold, with the heater turned right up or with a nearly empty battery, the engine runs more; that is the operating strategy, not a fault. But if it runs almost continuously in mild weather after a longer drive, ask why.

Watch the charge display throughout the drive. It should move at a leisurely pace: rising a little when braking and coasting, falling a little when accelerating. If it drops from full to nearly empty under moderate throttle and climbs straight back on the next coast, the battery has little usable capacity left. And pay attention to the switch between electric driving and engine running: it should be barely noticeable. A distinct thump when the engine starts or stops points to the engine mounts or the damper.

In a full hybrid's service book you are looking for more than oil changes. Good signs are entries for the hybrid system: a battery check, cleaning of the fan or filter, the inverter circuit's coolant, and on a high-mileage car the transmission oil. Some manufacturers extend the hybrid battery warranty as long as a hybrid check is carried out regularly within the brand's network; the terms are in the warranty documents, and a franchised workshop can tell from the VIN whether it is still running. That is also where you find out whether any recalls or software campaigns are outstanding.

The most telling check is a diagnosis that goes beyond a generic scan. A basic code reader shows the engine's emissions-related faults but often not those of the hybrid system. A workshop with the right equipment can compare the voltages of the individual battery blocks under load, read stored faults in the hybrid system and look at battery and inverter temperatures. If some blocks differ clearly from the rest, that is the finding that moves the price.

Ask for the result as a printout, not a verbal opinion. A weak battery does not mean the car is worthless: there are new, reconditioned and used replacement batteries, and on some models individual blocks can be swapped. It does mean the item belongs in your calculation before you commit, not afterwards.

If you enquire through Carvexia, you can send questions from the seller's statements catalogue along with your message — about the last service with a receipt, say, or a warning light on the dashboard. The answers appear on the car's page as the seller's statements, not checked, and whether and when the seller answers is up to them. They do not replace a diagnosis; they tell you where to look first at the viewing.