Auto Service World
News   July 28, 2010   by CARS Magazine

Resistance is Futile (July 28, 2010)

Electric cars are all the rage in the media these days, but professional techs have known that gas and diesel ...

Electric cars are all the rage in the media these days, but professional techs have known that gas and diesel cars and light trucks have been getting more “electric” every year. Almost every one of the increasing number of accessories in modern vehicles is electrically powered. Even suspension and steering are getting wired, making electrical system troubleshooting more important than ever. At the core of it all is the battery, the source for the current that drives the system…except that it’s more than a holding tank for the alternator. What do we need to know?

Current and voltage are not the same thing

It’s surprising how many successful techs get by nicely without a clear understanding of what electricity is. For our purposes, it’s like water flowing in a pipe…except voltage is pressure and current is flow. The important thing to know is that it’s the current that does work, whether it’s firing a spark plug or stopping your heart when you stick that screwdriver into the coil pack. To use the water analogy, if you need to fill your pool, you need a certain number of litres of water and to speed the process up you need the hose to flow more litres per minute. You do it by opening the valve all the way, increasing the pressure, or more accurately the pressure differential between the water main and the end of your hose.

The battery’s job is to deliver current to the vehicle accessories so they can do work. The voltage is the “pressure” the battery uses to drive the current. Ever measure the supply voltage across the supply and ground of an accessory like a wiper motor while it was running? A lot less than 12 volts! That’s because the 12 volt magic number is measured open circuit which in our technology means with one battery terminal disconnected. But how can we be sure that a battery that shows 12 or 13 volts open circuit can still deliver the current needed for heavy duty accessories, like the starter? We can’t, at least not for sure.

The battery has its own resistance

We all know that there’s resistance in every circuit. Drop a wrench across the battery terminals and you can see instantly what happens when you reduce the resistance to current flow in that very short circuit. If you use Ohm’s Law you can guesstimate the flow. My favourite Proto 13/16-inch combination wrench is perfect for shorting out a top post battery because its surface resistance is a little under 0.1 ohms by the multimeter. According to Ohm’s Law, that works out to 120 amps at a nominal 12 volts for the battery, which is solidly into arc welder territory. That wrench, however, is a chrome-vanadium steel alloy. Put an 18-gauge copper wire with very little resistance across the battery and in theory you should be seeing five or six hundred amps through the wire. Obviously, in the real world this is impossible. Another way to think about it is to look at the battery’s rating. A 100 amp-hour battery theoretically could deliver 100 amps for an hour before going “dead.” The real world is a lot different; try to draw 100 amps out of a battery and it won’t last an hour. Why? Because the battery has its own internal resistance, in part because serious draw changes the chemistry inside the cells…and even then charge has to travel across the plates through the electrolyte, which takes time. The electrolyte is also heating as the current flows.

Battery amp-hour ratings are typically measured at current draws low enough to avoid these internal issues, typically 3.5 amps. In our 100 amp-hour example, the baseline measurement would be a 3.5 hour load draining over 29 hours. It’s a good test for comparison, but a 3.5 amp draw doesn’t even come close to the current needed to spin a starter…which is the essential point: Batteries need to be seriously loaded to truly test their capacity. If you don’t use a purpose-built battery tester, you will likely miss a weak battery, especially with modern high torque starters and ignition/fuel system that can fire an engine with a single spin of the crankshaft. Can the ones that “fall through the cracks” affect modules and CAN buses? For sure. A 7-series BMW has 50 controllers and it’s not unusual…why not make sure that the battery is solid before the mind-bending troubleshooting process?

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