Until you open the hood to take it apart. Warning labels with lightning bolts, orange "DANGER" placards and strange-looking boxes full of electrical gadgets threaten to turn your fingers into kielbasa 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 specific 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 crispy critter. Anytime you are working on one of these vehicles, whether it's on the engine or 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 energized only when the electric motor is powered. So your first line of defense is to be sure the vehicle is truly turned off, which is not always obvious. Since one of the gas-saving features of the hybrid is the ability to cut the engine when the car isn't moving, your auto could be ready to go even if the gas 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've got 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 work underhood. 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—10 feet or more—to be sure you haven't made a mistake. Similar precautions apply for pure EVs, although those vehicles are still rare at this point.
As always, the factory service manual specific to your vehicle is the definitive resource for proper repair procedures.
Unhooking the Beast Within
Internal servicing of the battery pack, the motor controller and the traction motor is still specialized enough that it's best left to qualified professionals. 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 out a pack or controller if necessary. To do so, the battery pack needs to be disabled before twisting any wrenches. Just as important, it's essential to 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.
Insulated GlovesWorking near the battery pack should be a low-risk operation if you follow procedures and wear insulated gloves. We bought ours at Grainger, but any electrical-supply house will have an assortment. You need gloves rated for 500 volts, but we generally prefer more headroom—ours are rated for 1000 volts. The thicker rubber is less prone to damage but more difficult to work in. Reserve these gloves for electrical work—any surface contamination might degrade them, and the smallest pinhole could allow a current leak.
Every time you reach for these gloves, perform this simple test: Cuff the gauntlet of the glove, roll it up a few inches and squeeze it to inflate the fingers and palm area. Any air escaping from a pinhole should relegate these gloves to dishwashing duty.
Pulling the Plug
Remove any interior or access panels over the high-voltage battery pack. Pull on your gloves (see below). 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.
Disabling the battery pack is the first step to working on any hybrid or EV. While the full battery voltage, anywhere from 144 to nearly 300 volts DC, is available at the output terminals, battery manufacturers have left us a simple way to deactivate the contacts and safely work on or remove the pack. The battery pack is a long series of individual cells. Somewhere near the electrical center of this string is a service switch or removable plug. Opening this connection breaks the circuit so there's no voltage across the pack's main connection. Regardless, take normal precautions when working around any electrical device with this potential.
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. These cells can sometimes be refurbished by zapping them with a large, high-voltage capacitor, vaporizing 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 swapped out. But identifying which cells are bad requires sophisticated test equipment. So what's the DIYer to do? 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 $400 to $1200, depending on the problem. That's still far cheaper than the $1800 to $5000 for a new one at the dealer.
Why not a junkyard pack? 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 snowbirds or anyone else whose needs for a car are episodic, not daily.
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