canadian automotive technician (November 01, 2008)

Batteries are the first thing taught about automotive electrical systems in every trade school or high school auto shop. Why, then, do so many apprentices and rookie techs consistently use improper testing techniques when batteries are suspect in the winter months? Here are a few basics to keep in mind as the colder weather brings on the “no starts.”

1 Use a true open circuit for “open circuit” testing. Older manuals dictated that techs turn everything off before performing a battery open circuit test. Modern vehicles have several key-off current draws which together can be significant, including any one of several ECMs as well as entertainment/information systems beyond the radio, like GPS. And don’t forget that a major cause of a “dead” battery is the draw you can’t see. Trunk or under hood lighting and power antennae are fault sources not easily seen. OBD will tell you the obvious, low voltage, but won’t help with the little-but-deadly draws. To test the battery, disconnect one of the terminals and clean the battery top to eliminate that leakage path.

2 Modern vehicles tolerate parasitic current draw. With the increase in battery run down protection built into newer electrical systems, a minor short or rogue load won’t conveniently advertise its presence by killing the battery overnight. Motorists will drive with the MIL on forever and never think twice about it.

3 The customer doesn’t always tell the truth. They’re not lying, but the fact is that modern engines will fire with very little rotation of the crankshaft, so it’s possible to limp along with a battery that would have been discarded in the days of carburettors or TBI. The customers says “it was fine yesterday,” but it may have been marginal for months. A cold night can easily put it over the edge.

4 There’s nothing more dangerous than an amateur with jumper cables. Multi-purpose household charger/boosters are cheaper than ever and like jumper cables, are a great way for amateurs to fry batteries/ECUs, etc. trying to get their vehicle started. They may or may not tell you that they tried the cables “both ways around,” so it’s best to assume nothing if there’s no cranking and/or no spark.

5 Electrolyte freezes. Every Canadian tech knows this, but many don’t know that the freezing point is dependent on acid concentration in the electrolyte, just like anti-freeze strength in the cooling system. Since automotive batteries have been “maintenance-free” for years, open circuit voltage is the only convenient quick check. The chart below shows how easy it is to freeze a discharged battery.

Voltage Room temperature Freezing temperature(open circuit) charge state 12.65 over 90% -67C 12.45 75% -37C 12.25 50% -23C 12.05 25% -9C 11.90 or lower dead -7C

6 Load testing isn’t good enough. Few shops have the time or bay space to heat soak a cold car for seven or eight hours, and outdoor testing is common. The only reliable way to test is with modern equipment with temperature compensation. The best method is conductance testing, requiring industrial strength equipment … forty bucks for a carbon pile in a fancy box isn’t a serious tool in our climate.

7 Fresher is better. Batteries self discharge as they sit and worse, sulphate. If you carry batteries in inventory, rotate the stock and consider trickle charging them periodically. And the surprisingly persistent belief that leaving batteries on a concrete floor kills them? It’s a myth left over from the pre-WW2 days of porous, electrolyte-soaked wooden battery cases. Moisture absorbing dirt and grease on the battery top, however, will provide a current path between the terminals. Go ahead and put the battery on concrete, but keep it clean and charged.

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