Charging System: Quick Check or Complete Checkup?

charging system DSC07199 200x153Gear case (3)When a vehicle is in for a major service, a pre purchase inspection or battery replacement, we perform a quick charging system test. The equipment first logs the labelled battery capacity and checks the battery temperature. It then checks the battery capacity and graphs it according to the labelled capacity. Next a graph of the battery voltage is made while the engine is started, and and a reading is given of the starting voltage after the initial spike from starter actuation. The charging system voltage at idle is measured, then the charging system voltage at 1500 rpm with loads such as the headlights and heater fan turned on. The results confirm that the alternator and starting systems are working correctly, and if any cables or other issues are afoot, we get the clues to do further testing as required.

Cars more than 10 years old may have voltage losses on their battery cables or charging cable. When your records, or ours, show a battery did not last the minimum average of 5 years, we may want to test for some electrical component that is not shutting down correctly when you turn off the car. Typically, we find the culprit to be an aftermarket stereo or alarm that is incorrectly installed. Occasionally, we find a door latch microswitch that is misbehaving, causing one of the car’s computers to wait forever for the door to open as the driver gets out, or for the door to close. Some microswitch may be keeping the light on inside the trunk. We call that a draw or a parasitic draw.

Complete Charging System Tests Include:

  • Hydrometer test of electrolyte: The specific gravity, or density, of the battery acid indicates the state of charge. First the readings of the 6 cells are taken to get a picture of the current state of charge. Then the battery must be fully charged by hopefully an overnight slow charge. Charging can be done more quickly, but the time required depends mostly on the size and capacity of the battery, and how far it might be discharged. After charging, the electrolyte specific gravity indicates the condition of the battery: 1250 – 1275 is a usable battery. 1225 is too low, and maybe has very little life left. 1200 is just too old. More than 50 points difference between cells indicates a failure.

  • Standing voltage of the battery: First the surface charge, or residual charge is removed by turning on the headlights for 30 seconds, and turned back off. Then after 2 minutes, the voltage is checked. The scale is as follows: 12.7 V = great battery. 12.6 V = good battery. 12.5 V = midlife, but OK battery. 12.4 v = old battery. 12.3 v = battery is too old. 12.2 V = battery life way past over.

  • Load test the battery: A battery must be able to stay at a high enough voltage during cranking, or the fuel and ignition system simply won’t work. We load test with a carbon pile variable resistance load tester to about 1/3 the CCA or Cold Cranking Amps stated on the battery label, generally loading to 200 to 250 Amps. The voltage under load must stay above 9.6 Volts, most good batteries are over 10 volts during this test.

  • Battery Positive and Negative cables: While cranking the engine, voltmeter readings are taken from end to end on each of the the two big battery cables. Maximum voltage drop is .5 Volts.

  • Charging system cable from the alternator to the battery: Voltage readings are taken between the alternator charging post and the battery positive terminal with the engine at 1500 rpms. All the accessories are turned on, meaning headlights, heater blower, and air conditioner. Alternatively, the load tester can be attached to the battery and set to 30-40 amps, imitating the accessories. The voltage loss across the charging system cable should not exceed .5 Volts.

  • Alternator test: At idle, the alternator should have 13.8 to 14.2 Volts minimum output. At 1500 rpms with all the accessories turned on or a load tester at 30 amps, the voltage should not fall below 13.8 Volts. Also at 1500 rpms, and with the load tester applying enough load to lower the output voltage down to 12 Volts, the alternator output must be within 10% of the stated output, which is usually 90 or 120 Amps.

  • Parasitic draw test: We prefer to use a clamp-on amperage probe, but sometimes connect an inline amp meter. Once the vehicle has shut off, and sometimes locked up, we have to wait for the sleep cycle of the alarm and/or convenience systems to finish. Then the amperage draw should be between 16 to 40 milliamps, depending on the vehicle. If the parasitic draw exceeds those specifications, then further testing is required to determine the cause.

 Gear case (3)