The Current Conundrum

We all learned about automotive electrical systems the same way, whether we started in the era of the Studebaker or the Subaru. DC electrical current flow has always been taught as water moving in pipes, and for most purposes, that analogy is close enough to the truth to get the job done. It’s not, however, the whole story.

Back to the battery basics

Current automotive batteries are superior to the units made a generation ago, yet many techs and motorists believe that battery life has shortened due to cheaper materials or low build quality. In fact, higher vehicle electrical loads, higher compression ratios and the need for physically smaller batteries to fit into smaller engine compartments all play a part in battery longevity. Another is consumer perception. With longer and longer maintenance intervals, motorists don’t realize that batteries are consumables, like brake pads or tires, and can’t be discharged and charged indefinitely.

Most techs know that lead-acid automotive batteries operate through chemical reactions between lead and lead oxide plates in a sulfuric acid bath. The reaction is reversible, which is why automotive batteries are rechargeable, but the process isn’t perfect, so batteries age. Most shops use electronic battery testers for a fast status report. They generally work by superimposing an AC signal on a small DC current and analyzing the amplitude and waveform of the AC component. It’s a good technique for a fast, non-invasive check that doesn’t deplete the battery appreciably. Load testing simulates cranking or near cranking conditions and measures voltage drop and recovery voltage. A standard 20-30 seconds of testing generally simulates most cranking events. However, it does discharge the battery and it is something which should be considered if the unit is tested further.

Which testing procedure is better? Both work, but the load test can reveal even hairline cracks in a battery’s plates and cell-connecting straps. In either case, the ideal unit generates a graph or printout that can be shown to the customer.

Charging myths and reality

Failed or faulty alternators often arrive with the vehicle “on the hook.” If the charging system died suddenly and recently, the battery might be savable by charging. Unfortunately, charging a lead-acid battery isn’t just about ramming current in at the fastest possible speed. The ideal charger uses a current level at about 10 per cent of the battery’s nominal rating, i.e. 6 amps for a 60 A/h unit. The charger should operate at a maximum voltage of 14.4V. High capacity “fast” chargers can deliver a “surface charge” adequate for initial starting and can be brought up to about 80 per cent of a full charge if performed carefully. They can also introduce killing heat, capable of buckling plates and separators. Cold reduces car cranking amps, but heat shortens battery life, as does neglect and mechanical damage. Even “shock proof” conventional lead acid units are typically designed to withstand a 6 “g” acceleration, which can be easily exceeded in a frontal collision. If the vehicle is just back from the body shop, did anyone load test the battery?

The charging “profile” also matters. Ideally, the charger should sense voltage and taper current to zero as the battery fully charges. Common household and garage manual units taper their delivered current somewhat due to their transformer/rectifier design, but can still push 10-25 per cent of their rated current into a “full” battery. The consequence is water loss, which is easy to replace in a “low maintenance” unit, but can be a killer in a sealed “maintenance free” battery.

Under the hood, it’s the alternator’s job to keep the battery healthy, but unlike the relatively inefficient and over-spec’d units from the 70s and 80s, modern units assume a functioning battery and are designed to “top up” and maintain the battery’s charge under normal operating loads. The battery, in fact, is an integral part of the alternator circuit, and attempting to run a modern engine without the battery in place can generate high peak voltages which can damage the alternator and control electronics. Integral regulators fail more often than the rotating portion of most alternators, and the critical rectifier diodes that convert AC to DC can fry. Heavy-duty rectifiers are one defense, but over-voltage protection circuits that chop excitation current to prevent it running away as B+ voltages increase, allow a more compact unit with additional protection for sensitive vehicle electronics. Aftermarket alternators don’t necessarily need “heavy duty” rectifiers if a multifunction regulator package protects the unit. The rectifier diodes are still one-way valves, however, so switching polarity, even for a moment, can kill a good alternator.

Cranky starters

Starter motors are the lowest technology part in the rotating electrical system, and in principal have changed little in the last eighty years. Rare earth permanent magnet motors and reduction gears have made units smaller and lighter, but starters are still awkward to service D.C. motors. Load and no-load testing is straightforward, but simple as starters are, many techs still ignore the crucial small items like cables, battery terminals and ground straps. High current/low voltage circuits tolerate a lot of abuse under hood, but connections which are corroded and/or loose can not only degrade performance, but can arc or flash over to ground causing more damage. To adequately install a starter, every other part of the system must be good, from alternator to battery to solenoid. Unlike modern alternators, however, starters don’t have hybrid thick film integrated circuits to monitor their current draw. As a result, installing a heavy-duty oversize battery in a hard-starting car can overheat or over-torque a starter, causing damage. At the other extreme, a weak battery that can’t pull in the starter pinion with authority can prematurely wear the pinion, ring gear or both. A few high end applications may use start-locking and start repeating relays to save the overrunning clutch and solenoid windings in case of failures, but in general, a good alternator, the correct battery and good electrical connections, especially between battery, solenoid and starter solenoid/relay are critical to reliable operation.

Fast test light checks

Every tech owns a test light, but in this age of OBD II and computerized diagnostics, it spends a lot of time in the toolbox. There are a few checks, however, that still make sense with this simple tool. The first and most obvious is back probing dead circuits for power, but they’re easy to use when tracking down the parasitic key-off drains that can kill good batteries. Disconnect a terminal and put the test light in series with the battery and cable. If it lights, there’s a drain. Pull fuses one at a time, and when the light goes out, you have your suspect circuit. Because the test light works regardless of polarity, either battery terminal will work for this test.

Bad grounds can also be tested easily. Attach the clip to the battery positive or a known good positive source and probe the ground side of the suspect circuit from the switch or actuator back to the chassis or engine ground. When it lights, you have a good ground. Holding the probe upstream of the chassis or engine ground and wiggling the harness can sometimes weed out “intermittents.” Ever find a slow cranking starter motor even though the battery and charging system is O.K.? Its circuit may not be complete; test lights are great for quick checks of the engine ground “pigtail” or ground strap. Clip the lead to a good body ground and touch the probe tip to the engine block and then crank the engine. If the lamp lights, then the engine isn’t properly grounded to the chassis. Why does this test work? Because a properly grounded engine acts with the test light to form a parallel circuit with the ground strap forming a very low resistance branch. If it’s good,
there should not be enough current passing through the relatively high resistance test light bulb filament to make the lamp glow. That same filament resistance, however, can make the test light a misleading or even damaging tool if used indiscriminately. Sensors and computer circuits can be loaded down by test lights, causing code sets or erratic operation and maybe even damage, so restrict this simple tool to simple troubleshooting jobs. For applications in the starting and charging systems, however, it’s tough to beat at twice the price, but beware the pocket-sized LED types. They’re handy, but LED stands for Light Emitting Diode, and that one-way diode action means that polarity is critical, so many of the tests described above won’t work. With the cool-running and compact LED’s taking over automotive lighting from the taillights forward, the test light might be the glass bulb’s last stand.