Testing battery and charging system

Test Your Charging and Electrical System

Years ago I was a managing engineer at a company that manufactured automotive instrumentation and test equipment. We built some of the most popular aftermarket timing lights, tachometers, dwell meters, and gauges. Our products were marketed by large companies like Sears, Mac Tools, and other mechanics tool suppliers, tool distributors, and tool retailers.

A lead-acid battery has certain defined characteristics than make it easy to evaluate the health of the charging system without fancy test gear. Here's how we can check a charging system with a few simple pieces of equipment.

Alternator

The alternator converts mechanical energy into electrical energy. Other than very small parasitic losses, it only draws enough power from the crankshaft to supply the output power necessary to run the electrical system and charge the battery. Using an underdrive pulley will not reduce the horsepower consumed by the alternator while racing. When an alternator shaft is slowed, the voltage regulator simply turns up the field current until the alternator is back drawing exactly the same power it would have when turning at higher speed! As a matter of fact since efficiency drops with reduced rotor speeds, the alternator can draw more engine horsepower with an underdrive pulley than with a standard pulley!

The only way to significantly reduce alternator drag is to turn the alternator off while racing, although turning lights and electrical accessories off certainly helps. Remember that when the vehicle is running the alternator tries to supply all the load energy. At reasonable to maximum engine speeds, typically from 1000 RPM to redline with heavy loads and idle to reline with light electrical accessory loads, the battery just goes along for the ride. The battery will consume engine power only when the battery is low on charge and the missing charge is being replenished.

NEVER pull the battery cable to check the alternator. This very crude test method was marginally OK when we had vacuum tube car radios and point-type ignitions, but it is a very bad idea now. The battery stabilizes the electrical system and loads the alternator, preventing high peak voltages or voltage surges as the alternator adjusts magnetic flux to produce the same average voltage at different current demands. If you rev the engine up and pull a battery cable, the alternator voltage can spike up to 100 volts or higher before the alternator flux dies off enough to bring voltage back down to 14 volts or so. This can kill the car's computer and other expensive electrical components. I've seen headlights blow out when a guy opened a battery switch while an engine was revved up. If you hear anyone telling someone this is a way to check an alternator in a modern vehicle, stop them!

To charge the battery, the alternator voltage output has to exceed a minimum charging voltage. This minimum voltage is 13.8 volts dc across the battery terminals. A single lead-acid cell starts to charge at anything over 2.25 volts. Since a 12 volt battery has six cells, it needs at least 13.8 volts to start to charge. This voltage will be enough to fully charge or maintain the battery on a trickle charge, but charging time will be very long at 13.8 volts.

The alternator output  must be 14.2 V to 14.5 V (measured at the battery posts) to fully charge the battery in a reasonable time. The battery voltage must be below 14.7 volts to prevent excessive gassing, which would prematurely dry the battery or could cause risk of explosion. Above 14.5 volts charging voltage, batteries have a greatly increased tendency to release excessive acidic vapors and corrode things around the battery.

In this case the battery charging voltage is 14.61 volts with the engine at high idle. 14.4 volts is the gassing threshold. The battery will slightly gas, but not enough to be harmful, and the battery will get a quick full charge recovery after starting. 14.8 would start to be a worry (there might be liquid or corrosion on the battery) and 15 volts would be a real concern, but 14.6 is fine. Less than 14.3 would be a "weak" alternator or regulator. Much less than 14.2 at a fast idle is bad wiring, bad alternator or regulator, or a bad connection or fuse link. Voltage should stay over 14.3 even with full load, like lights, heater blower, and everything else running. If this system was in a restored 1966 GT coupe, I would probably change the regulator to reduce maximum alternator voltage. This would prevent deterioration of metal around the battery from excessive charging vapors.

Turn the motor off with no load (headlights, etc) and read the voltage.

The battery voltage should be 13.2 volts to 13.8 volts, depending on the battery, how quickly you read it, and the state of charge. This voltage is not too important because the battery will slowly and steadily settle to a new voltage that indicates the true state of battery charge.

Measuring electrical system leakage current

Remove the negative post and check current draw with all electrical loads off using a test light.

The dull glow in the light filament indicates a problem. At this point I do not want to connect a current meter to check leakage because the short could damage the test meter! If a small clear test lamp like this does not light, then it is generally safe to directly measure current drain.

Measuring Parasitic Current Drain

With all electrical loads off connect the meter, on a low amperes scale of about 1 ampere or so, in series with the battery negative post to ground. The positive meter lead connects to the car chassis, and negative meter lead to the negative post of the battery.

A good electrical system battery drain

This is measured on the 20mA scale. The mA scale reads in thousandths of an ampere. My 1989 Mustang LX, after I changed a bad alternator diode, now has about 1.73 mA of battery drain. This drain is all from the EEC-IV computer memory. Different radios might have different standby drains for the EEC IV, radio memory, and accessories like clocks, but in no case should leakage exceed 25 mA or so. 100 mA is like leaving a small interior light on! My Kenwood stereo draws 1.5 mA when connected. If you have a digital clock that stays on, an alarm, or some other load this current will be higher. At 75 mA leakage could compromise battery life of infrequently driven vehicles.

Bad battery parasitic Drain

If the test light lit, you will want to locate the wire loading the battery. First make sure all lights are off. You can do this by having someone open and shut things with lights, like the trunk, and watching for a define large load change. You should see a definite load change when shutting doors with lights, like the glove compartment.

Connect the test light in series with the negative post, and start pulling feed wires. The first to check is the heavy charging wire from the alternator.  A bad or leaky diode in an alternator is a very common source of overnight battery drain.

Connect wires one at a time to see what lead is drawing current. In my case it was the alternator lead! Even though the alternator was charging fine, it was also draining the battery. My problem was a bad alternator diode. There could be a variety of other problems, like a bad voltage regulator or a stuck relay contact.

Download a wiring diagram

I downloaded this from T. Moss's website, which I find much more useful than other sources. AutoZone and others have some free schematics up also.

What a helpful website. T.Moss's diagram linked above showed me the heavy dark green wire from my starter relay causing my problem went directly to my alternator output.  In my case one of the diodes ("arrows") in the alternator was bad. This also made my alternator slightly warm to the touch even when sitting for several hours.

Other Helpful Voltages

Any open terminal voltage below 12 volts is considered full discharge or a dead battery.

I hope this helps you troubleshoot charging  systems.