"BOOTSTRAP STARTING"

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AND OTHER COLD WEATHER BATTERY LORE

"In a couple of minutes, my car was running, too . . . but I wasn't about to let him get away without an explanation."

by Andy Turn bull

The temperature was about 40 below zero, and my car wouldn't start. I cast a hopeful glance at a farmer getting into an old pickup parked next to me . . . he might well be, I thought, the sort of guy who would carry jumper cables.

But his vehicle also failed to start. The engine groaned for a few minutes, and then he simply quit trying. I watched as he climbed out, booster cables in hand, and I cranked down my window when he lifted the hood of his truck.

"If you get someone to boost you," I asked, "can you start me, too?"

"Sure can," he said, "but I won't need a boost."

He proceeded to clip one end of each cable to the appropriate battery terminal and then backed away from the truck, holding the free ends, one in each hand. For about ten seconds, he touched the clamps together. Then he jerked them apart . . . pulled the cables off the battery . . . hopped back into his truck . . . and pressed the starter button. The engine turned and caught easily.

In a couple of minutes, my car was running, too-after a conventional jump from the gentleman's truck-but I wasn't about to let him get away without an explanation. So, while the two vehicles warmed up, I dragged him into a nearby diner for a cup o1 coffee.

ALL YOU HAVE TO DO IS . . .

"It's just a little trick I picked up in the logging camps," he told me. "All you have to do is short the battery out for five to ten seconds. That heats the cells up a bit, and the battery will put out more power when it's warm.

"The heat doesn't last very long, though . . . you've got to get the warmed engine started within a minute or so, or you'll be worse off than you were before.

"Some people call it `bootstrapping', because the battery sort of picks itself up by the bootstraps. Others call it the `screwdriver trick', because you can do it with a screwdriver if you don't have cables.

"I wouldn't use a screwdriver, though you can blow up the battery that way. There was a cook in one of the camps up by Winnipegosis who tried it with a screwdriver . . . and he spent about six months in the hospital getting his face put back together.

"One guy out where I worked had it down to an art: He rigged a set of cables to an old starter switch, so he could short out his battery without getting out of his truck. But I wouldn't try that, either. You don't want to bootstrap any more than you have to."

Ron Jamison-an engineer with Exide Canada Inc--doesn't think people should bootstrap at all, but he can explain how it works:

"The technique uses the battery's internal resistance," says Jamison, "to make it into a heater for a few seconds. On a dead short, the output probably drops to around 2 volts, but you get about 2,000 amps out of a heavy-duty battery. And that translates into about 4,000 watts of heat.

"And all that warmth is right where it counts the most . . . where the electrolyte touches the plates. You can actually raise the temperature of the whole battery by about 2°F, and the temperature at the plates might go up by 10 or 20 °.

"But bootstrapping is dangerous, and we don't recommend it. Short a battery with a screwdriver, and you risk an explosion. There may be free hydrogen around the battery vents, and the spark you get when you make contact could ignite it. A battery explosion is no joke!

"It's safer with booster cables-because you don't strike a spark near the battery but you're still taking a chance. You should never deliberately short a battery."

Maybe you shouldn't, but a 10 to 20° temperature rise at the battery's plates could make the difference between starting a dead car back in the woods, and taking a long, chilly walk. As shown in the top chart on the facing page, lead-acid batteries lose much of their power when they're cold. A battery that puts out full power at 76 °F has about half its rated capacity at five below zero . . . and only about 15% at -35°F!

Let's say your car's been sitting out on a night that hit 10 below, and it won't start because the battery's down to 45% of its rated power. If you can warm the plates 20 ° by bootstrapping, you'll get them up to 10 above . . . where the battery will be back up to about 70% of its rated power. That's a gain of more than half the output available at - 10°F.

COLD TURKEYS

And, believe me, during the winter you need every bit of juice you can get. A modern auto in good condition that requires only 100 amps to get going in the summer would need about twice that amount at 0°F. At 40 below, there'd better be 400 amps on tap . . . even with winter-weight oil in the crankcase.

The car and battery manufacturers know this, of course, and they try to compensate by installing bigger batteries in autos sold in cold climates. The same vehicle that's delivered with a 40 amp-hour battery in middle latitudes may come with a 75 amp-hour unit up North. And even bigger ones are used in northern Canada and Alaska.

Overly large batteries will do the job . . . at least for a while. Any modern car in good tune and with winter-weight oil should start at -40°F on a fully charged, heavy-duty battery. The hitch is that most batteries aren't fully charged in cold weather.

The reason for this is that lead-acid cells are reluctant to take a charge when they're very cold. According to a chart-not shown -from the Battery Council International (the group that sets standards for the industry), a "typical" half-charged battery that will accept 25 amps of charge at 76°F will absorb only about 2 amps at 0°F. On that same chart, there are no figures listed for temperatures lower than 0°F, and for a good reason: When a lead-acid battery's temperature drops below zero, it won't take a charge at all!

One of the rapid chargers offers about 18 volts, which is enough to force a battery to take a charge down to about 0 °F. Below that temperature, however, such a device isn't effective, and you can't exceed the 18volt input without risking damage to the battery. There is a way, though, to get an extremely frigid battery to charge.

THE CHILL IS GONE

A bit of heat makes a big difference, and-in fact-Jamison figures that a lot of perfectly good batteries are junked every winter simply because no one bothers to warm them up long enough to charge them. For example, many people resort to visiting a service station for an hour-long, 50-amp quick "charge" when their cars won't start on cold mornings. The shot of juice will get a car going, and things will usually be OK for the rest of the day. Next morning, though, when the engine refuses to crank again, most people assume that the battery won't hold a charge and is therefore worthless . . . but the fact is that it never was charged.

The battery was stone-cold when it got the 50-amp "push", so most of the power put in was converted to heat. And that "power pack" stayed warm for the rest of the day, simply because the car was run from time to time. The next morning, of course, the electrolyte and plates had cooled off again. On the other hand, if the 50-amp charge had been delivered for two hours one to warm the battery up and another to charge it-the unit would most likely have recovered fully.

NO CRYOGENIC IMMERSION

Actually, a battery that isn't warmed up and fully charged from time to time will eventually lose its ability to take a charge ...through a destructive process called sulfation. All lead-acid batteries produce lead sulfate when they discharge (it's the way they work), but the sulfate is normally reconverted to sponge lead on the negative plate and lead dioxide on the positive plate during recharge. If the battery sits too long without being recharged, though, the crystals of lead sulfate will grow together into bigger and bigger agglomerations of what engineers call "hard sulfate". This compound can't be broken down by charging, and it reduces the cells' capacities (or even shorts them out altogether).

If you leave a battery at half charge for too long, then, you'll wind up with half the capacity for which it was rated. And just how long that "too long" is will depend upon several variables . . . including the battery's construction, the charging current, and even the weather. Because of these factors, however, you should consider your battery safe for only a couple of days when it's partially charged.

The caretakers' answer to this problem is the temperature-sensitive voltage regulator. Current Ford cars, for example, have regulators that are set to charge at 14.0 to 14.4 volts in warm weather, 14.2 to 14.7 at 32°F, and 15.0 at 0°F. Now even 15 volts won't do any good at -20°F, and not much charging will take place even at 0°F, but you just can't put much more into a battery on a regular basis without corroding the plates. Still, cars that are used frequently enough particularly those equipped with rubber cased batteries-will be able to maintain a charge.

This is because the difference between the battery's open-circuit voltage (about 12.6 volts) and the charge rate is converted to heat. Thus, at a 15-volt charge, there are about 2.4 volts available to keep the battery warm. When this heat is combined with that from the engine, and held by the insulation provided by the case, the battery may not cool all the way down at night.

In one test, it took an average-sized car battery 12 hours to cool from 65 °F to 0 °F, with an air temperature of -40°F . . . and if the unit had been next to a warm engine, it probably would have taken longer. (Newer batteries that are equipped with plastic instead of rubber-cases do cool more quickly, though.)

However, even the best rubber-cased batteries don't give you much reserve in cold weather. The bottom chart on the preceding page that was prepared by Exide Canada Inc. shows that a 100-amp-hour unit at 76°F puts out 200 amps for about 11-1/2 minutes . . . with the voltage dropping from 11 to less than 8. When the cells were forced to put out 400 amps at 0 °F, the battery dropped from an 8volt start to 5-1/2 volts in just over two minutes.