Working on hybrid high-voltage, high-amperage circuits.

The used-car dealer’s offer is just too tempting: a cleanlooking hybrid sedan for a great price. The petrol-electric hybrid system works perfectly, and the previous owner has thoughtfully provided fuel receipts that prove the little car is consistently capable of 5,8 litres/100 km fuel economy.

The only problem is that the internal combustion engine has blown a head gasket, but against that, the price asked by the dealer is so attractive that you reckon you can easily fix it yourself and still be ahead of the curve. Limping the wounded creature home from the dealer is undramatic, the cost of a new gasket set is only slightly scary, and the manual says the job can be done in one day.

You calculate it will take four days to complete a DIY job – one afternoon to pull the head, a couple of days in the engineering works getting the head rebuilt, and another day to reassemble. Then you’ll be ready to drive in gas-sipping style while thumbing your nose at the gas-guzzlers. Until you open the bonnet to take it apart.

Warning labels with lightning bolts, orange “DANGER” stickers and strangelooking boxes full of electrical gadgets threaten to turn your fingers into burned sausages and your children into orphans. Maybe trying to work on your own hybrid car isn’t such a good idea.

Barbecue, anyone?
Hybrids, and the upcoming round of plug-in hybrids, have one thing in common, regardless of the type of hybrid power system they use: a big-ass DC battery pack – as much as 280 volts – that’s capable of delivering enough current to instantly turn you into a burnt offering. Whenever you work on one of these vehicles, whether it’s on the engine, the body or anywhere near the battery pack or cables, you need to take extra precautions.

The DC battery pack is only one source of high-voltage danger. There are also multiple fat cables that run from the pack to the motor controller, and more cables that carry even higher AC voltage – as high as 600 volts – from the controller to the electric traction motor.

Fortunately, the AC side of the circuit is energised only when the electric motor is powered. So your first line of defence is to be sure the vehicle is truly turned off, which is not always obvious. Since one of the fuel-saving features of the hybrid is its ability to shut off the engine when the car isn’t moving, your car could be ready to go even if the petrol engine isn’t running.

Imagine this scenario: you’ve parked your car and left the key in the ignition (or even in your pocket, if you have a smart key) while you tinker. Now – did you remember to turn the car well and truly off? Or is it still in “ready” mode, which is the equivalent of a conventional car idling patiently away? Aside from a discreet light on the dash, there’s no audible or tactile confirmation that your hybrid is completely shut off. The engine or the cooling fan could suddenly start – without warning – while you’re working under the bonnet. That’s an easy way to lose a finger, or worse.

Make a habit of removing the key from the ignition or, if your car uses a smart key, of keeping it a safe distance away – 3 metres or more – to be sure you haven’t made a mistake. Similar precautions apply for pure EVs, although these vehicles are not yet on the South African horizon – optimistic forecasts notwithstanding. As always, the factory service manual specific to your vehicle is the definitive resource for proper repair procedures.

Unhooking the beast
Internal servicing of the battery pack, the motor controller and the traction motor is still sufficiently specialised for it to be left to qualified professionals. However, that doesn’t mean that a careful Saturday Mechanic armed with a factory service manual (and a few hand tools) couldn’t or shouldn’t be able to swap a pack or controller if necessary. To do so, the battery pack needs to be disabled before twisting any wrenches. It’s equally essential that you disable the battery pack before doing any work, whether it’s powertrain-related or not, anywhere near the pack or any high-voltage AC components or wiring.

Start at the bottom
The first step to disabling the vehicle is to disconnect the low-voltage battery. Start by writing down the radio presets for later, because they’ll evaporate within a few seconds after disconnecting the battery. Also, if your radio has a security code, be sure you have it written down, or you may be forced to visit the dealer to get it working again.

Now you can simply remove the negative clamp from the battery. Do not remove the positive clamp, which could potentially short to some nearby ground and strike a pretty good high-amperage spark. The negative clamp is already grounded, so it can’t arc to anything except the positive terminal.

Let’s talk safety
I’m going to assume that you have taken the most basic precautions and have your hybrid or EV parked indoors or at least out of the rain, and that the work area and the car are reasonably dry. The battery pack is isolated electrically from the vehicle ground, so the only potential difference is between the two fat orange wires leading from the pack forward to the motor controller. So, unlike working on household wiring, there’s no need to work on a dry floor or wear insulated boots. In addition, the pack has its own ground fault device.

Pulling the plug
Remove any interior or access panels over the high-voltage battery pack. Pull on your gloves (see right). Now you can unlock the clamp on the service plug and remove the plug from its socket. You’ve just disconnected the battery pack from itself internally, somewhere near the middle of the stack of individual cells. This means the voltage that you could measure at the pack’s output connectors is zero. If it’s necessary to remove the pack, you could stick your fingers into the output connectors safely (although I recommend against it).

Our photo car is a Toyota Prius PHEV, which has three battery packs: one supporting the normal hybrid function of capturing energy from braking, and two providing the Prius’s short-range EV operation. Consequently, there are three service plugs. Other hybrids use a simple switch on the battery pack, hidden from view, to deactivate and make the battery pack safe. Again, check your service manual.

When you’re done, simply put your gloves back on and insert the service plug back into its socket. You’ll need to be sure the plug is fully seated and its locking bail is thrown completely shut. There are additional contacts in the socket that tell the onboard computer the plug is installed and the lock is fully seated. Now you can reinstall any access panels or trim and reconnect the 12-volt battery’s negative clamp. Key in any radio security code and the station presets.

Digging in
Owners of older hybrid cars may experience a loss of fuel economy as the battery pack ages, leaving many to wonder if anything can be done to restore performance. Usually, only a few of the pack’s individual cells – the total number of cells can vary from a dozen to well over 100 – are to blame. These bad cells can be victims of dendrite-growth internal shorts, caused by metal-salt crystals bridging the electrodes inside the cells.

The cells can sometimes be refurbished by zapping them with a large, high-voltage capacitor, vaporising the dendrites. The onboard charging circuitry does not have this capability, which is okay since the work is best left to the pros. More often, subassemblies of six to eight individual cells can simply be exchanged.

But identifying which cells are bad requires sophisticated test equipment. So what’s the DIYer to do? If you’re in the US, your best bet might be to remove the battery pack, crate it and ship it to a company that refurbishes battery packs. Prices for a rebuilt or refurbished battery pack vary, from about R300 to about R9 000, depending on the problem. (At the launch of the new Prius, Toyota South Africa quoted about R20 000 to replace the car’s battery with a factorysupplied unit. To put that into perspective, they said the original item was guaranteed for eight years – and 10 years on, they hadn’t had a warranty claim.)

If hybrids take off in South Africa, there might be the option, as there is in the US, of a scrapyard battery pack. However, caveat emptor: experience in the field reveals that partially charged battery packs that have been sidelined for a few months have a short life span when returned to service. The weakest cells discharge faster and can even reverse their voltage. That consideration also makes hybrids a poor choice for anyone who say, leaves the car standing idle for months, perhaps at a holiday destination, or for anyone else whose needs for a car are episodic, not daily.