EV and hybrid safety

Many independent garages refuse to work on hybrids and EVS, running scared of their lethal voltages. Rob Marshall evaluates their safety, by attending the Imiaccredited Level 2 & 3 Hybrid/ev Repair & Replacement course.

We attend a professional Repair & Replacement course to learn to deal with high-voltage batteries.

Despite questions raised about their environmental credentials, hybrid and electric vehicles (EVS) have the political will behind them to supplement petrol and diesel internal combustion engines (ICES). Currently, powertrains that range from combustion engine hybrid/plugin hybrids through to fuel cell and pure EVS all involve a high-voltage system.

Independent garages have been very slow to embrace this technology, with many of them turning away servicing and repair work, because they either do not understand it or are busy enough repairing conventional combustionengined vehicles. This situation cannot last forever. Just as the motor trade must come to terms with high-voltage circuitry, so must the practical-minded motorist, whether you plan to service/ repair your own vehicle, choose to install accessories, or drive one as part of your work.

The components

While various layouts of hybrids exist (for example, parallel, series, power-split and dual), all of them employ a high-voltage system with the same basic components. Unlike Ice-powered vehicles, the components’ locations can vary, depending on the make/model, so researching your particular vehicle’s layout is essential prior to thinking about performing DIY work.

First, a low-voltage circuit is powered by a conventional 12-volt flooded leadacid battery that serves not only the vehicle’s ancillaries but also activates the high-voltage side. Should the 12V battery be discharged, the high-voltage system will not power up. Many people do not realise that you can connect jumpleads in the conventional manner to a hybrid/ev’s 12-volt battery, although a power-pack is preferable, but check the instructions provided in the owner’s handbook. Should your specific hybrid lack an alternator (an EV will certainly not have one), the 12V battery’s state of charge is monitored and recharged by the high-voltage circuit, using a DC/DC stepdown converter, which tends to be located near the high-voltage side’s inverter.

The high-voltage circuit consists of another much larger battery, which is switched on and off by a set of relays. High-voltage direct current (DC) cables, with bright orange insulation, connect the battery terminals to an inverter. This converts DC into the required alternating current (AC) as electrical energy flows to

and from the combined motor/generator unit, which is wired to the inverter using three-phase high-voltage cabling. The electrically-operated air-conditioning compressor has its speed controlled by an AC driver unit, which is also connected to the high-voltage DC battery supply. Crucially, as the battery may not be able to deliver sufficient current when moving off from a standstill, for example, capacitors act as electrical accumulators to store and rapidly release current. Due to the high currents involved, certain components become hot. This is why the inverter possesses either a water (typical with Toyota) or air (typical with Honda) cooling circuit, while the high-voltage battery usually has its own integral cooling fan to blow air over the cells.

Safety considerations

As it takes just 40-60 volts to overcome the natural resistance of unbroken human skin and 0.08 amps to induce cardiac arrest, safety must be your priority, when working on the highvoltage system, where between 100-600 volts and up to 150 amps of direct current are present. You must buy appropriate safety equipment, prior to learning how to disable the high-voltage system.

Aside from wearing overalls that do not expose bare skin and goggles, do not think that domestic rubber washing-up gloves will protect you. Special insulated gloves that offer protection against a minimum of 1000V (Class Zero category) and comply with IEC/EN60903, must be used, but they must never be considered after their expiry dates have passed, or if they have been damaged.

When measuring such high voltages, a basic multimeter is unlikely to protect you against an electric shock. You need to buy one that complies with IEC/ EN60900 and is insulated to a minimum of Category (CAT) III/IV; do not forget to use leads and probes that offer the same insulation properties.

You need to consider the safety of those around you, so that a curious observer cannot be at put risk – for example, do not take out and leave a high-voltage battery unattended on your driveway while you go indoors for a cuppa. Professional workshops are recommended to carry insulated safety rescue hooks, so your body can be pulled away from the vehicle should you be electrocuted, without harming your rescuer. You may wish to buy one for your home workshop.

Making safe

Wearing the necessary protective equipment and using suitably-insulated tools, you will be able to isolate the highvoltage battery from the vehicle and take appropriate voltage measurements. When a hybrid is started up, the capacitors are powered-up via a pre-start battery relay and DC current is available at the inverter. When switched-off, the power supply is cut but the capacitors remain live, until they are discharged. This happens automatically by shedding the current through one of two methods: either through the motor windings, or a discharge resistor, but be aware that this can take up to five minutes on some models, meaning that the high-voltage system can remain live for a length of time after the start/ stop button has been depressed.

Before you start any work, familiarise yourself with any specific manufacturer power-down instructions. Should none be obtainable, you can follow this general advice: Ensure that the ignition is switched off. It has been known for Diyers (and even garages) to forget this, or to not understand the auto-stop function on certain hybrids, only for the engine to start itself unexpectedly. This is hardly desirable when you are halfway through an oil change, for example! Check that any ‘ready’ dashboard symbols are not present and that the key-fob is outside the vehicle but in an accessible place, so the ignition cannot reinitialise. Disconnect the 12V battery’s negative terminal and, wearing your protective clothing, gloves and suitable eye protection, remove the service connector/switch (if fitted) from the high-voltage battery. This isolates the high-voltage supply from the vehicle’s electrical system. Store the connector in a safe place and wait for five minutes, to ensure that the capacitors have discharged. After this, your DC voltage checks at the high-voltage terminals can then be carried-out, using your CAT III/ IV specification multimeter. A voltage reading of very close to zero should be expected before you can conclude that the vehicle is electrically safe, so that further maintenance, diagnostics or repair work can be carried out.

The inverter is a solidstate semi-conductor that converts DC from the battery to AC for the motor/generator. It also converts AC to DC to recharge the battery from the motor/generator during regenerative braking. The inverter is controlled by the hybrid control unit’s ECU to regulate speeds/current of the motor/generator. Note its separate liquid cooling system reservoir.

The 12-volt battery in the boot of this 2009 Toyota Prius powers the ancillary electrics and provides the trigger signal to activate the separate high-voltage circuit. Without it, the car will not ‘start’.

Provided its service connector is removed and you have checked that the vehicle has powered down, you can remove the entire high-voltage battery from the vehicle safely, detach its covers and compare the voltage readings at each cell. However, be extremely wary of the high voltages present. Also visible here is the cooling fan, which can be checked for integrity should the battery overheat regularly.

The high-voltage battery on this Prius is positioned beneath the rear seats and comprises not only the battery pack but also HV relays, fuses and a cooling system. Its temperature and voltage is monitored by a battery control unit that can be built into the hybrid control unit’s ECU. The service connector, which you remove to isolate the battery from the vehicle, is arrowed.

Using your insulated CAT III/IV multimeter, you can test for faulty relays, by assessing the voltage readings taken at the appropriate parts of the circuit.