The diy auto mechanic: Diagnosing battery problems

Winters up here in the beautiful North Country can be extremely hard on car batteries but when a battery fails to start our car or truck, it’s not always the battery’s fault. A couple of months ago, on one of the coldest mornings of the year, my wife’s car wouldn’t start. When she twisted the key in the ignition, the only sound that met her ear was a very faint clicking sound of some remotely located relay. She had spent the previous day visiting our grandkids and after spending a rough night in a drafty motel room she was anxious to get home. She called Triple-A, got a jump start and was on her way the hour. My wife loves fast foods so she stopped at Burger King for lunch and when she tried to start the car the battery was dead again. This time, after another jump start by Triple-A she drove to a nearby auto service center and had a new battery installed and then drove home. Twenty-four hours later the new battery was completely dead. What was the problem? It was time to get my hands dirty.

There can be many reasons for battery problems. Sometimes we are the cause of a dead battery because we leave our cars lights on or leave some other accessory on that draws large amounts of electrical energy from the battery. Sometimes there’s a problem with the charging system and the battery isn’t being charged when the engine is running. Sometimes there’s a parasitic key off drain that kills the battery because some relay is sticking close, some electronic module is malfunctioning, or there’s a shorted diode in the alternator. Sometimes the battery is simply old and in need of replacing but in my wife’s car’s case, I could rule that out unless they sold her a defective battery. That has been known to happen but it doesn’t happen all that often.

They make many specialized test instruments for diagnosing battery and charging system problems but you can do an adequate job with a simple Digital Multimeter. The Sears Craftsman model 82175 is a good choice because it’s an auto ranging meter that allows you to check capacitors, diodes and take temperature readings besides the usual voltage, resistance and current readings. It’s also priced within every diy mechanics budget range.

I rolled out my Sears Die Hard battery charger and set it up for a fast charge. Since the battery appeared to be completely dead it would take one to three hours to bring it to full charge with the charger set for a 40-Amp fast charge rate. After a leisurely country breakfast I kicked back and waited for the battery to charge. A fully charged 12-Volt battery reads 12.68 Volts, any battery reading 12.4 Volts or less is low and needs to be charged. The table below shows the relationship between battery voltage and the state of charge.

BATTERY VOLTAGE -Vs- PERCENT CHARGE

These readings are at 80 degrees F. Battery voltage readings will drop with temperature roughly 0.01 volts for every 10 degrees F. At 30 degrees F. a fully charged battery will measure about 12.588 volts, and at zero degrees F it will measure about 12.516 volts.

Going on the assumption that my wife could have been sold a defective battery, I started my troubleshooting by running a load test on the battery to make sure that it was capable of holding a charge. Clipping my Actron 130 amp Battery Load Tester across the battery and running a 10 second load test, the battery tested good. A battery load test requires that the battery be fully charged which is why I charged the battery before starting my troubleshooting procedure. Many auto parts stores, like Auto Zone and Advance Auto Parts will perform a load test on your battery for free but you need to have them charge the battery first or the test will not be accurate. On the other hand if you invest a few dollars in a load tester you will be able to perform the test yourself and know that it was performed correctly.Checking the alternator and charging system

With the engine running and my DMM (Digital Multimeter) set to read DC (Direct Current) Voltage connected across the battery I watched the voltage reading while the engine idled. The charging voltage of a good alternator will fall between 13.5 and 14.5 Volts and any readings above or below this range indicates a problem with the alternator or with it integral voltage regulator. More isn’t better. A charging voltage above 14.5 Volts will destroy an otherwise perfectly good battery. I had a 13.75 Volt reading which was close to the minimum which led me to think that I might have a shorted or leaking diode in the alternator’s rectifier bridge circuit. Checking rectifier diodes

Alternators actually produce AC (Alternating Current) which is then converted to DC by a rectifier Bridge Circuit in the alternator’s end housing. A good diode and in turn a good diode bridge will pass electric current in only one direction . You can check a rectifier bridge circuit using a DMM, a DSO (Digital Storage Oscilloscope) (You did read my factoid on DSOs, right?), or with a special digital alternator tester. If you set your DMM to read AC Voltage, any reading at all will indicate a shorted or leaking diode. A DSO is the best tool for this check because it will show a ripple voltage if there’s a shorted or leaking diode. I used both a DMM and a DSO on my wife’s car and they did indeed point to a problem with the rectifier bridge circuit.

I replaced the alternator with a rebuilt unit and everything worked fine. Alternators can be rebuilt but when you consider the time and special tools involved it’s easier and cheaper to simply replace the alternator and turn the old one in for a core charge savings. My wife’s car was fixed at this point but I want to go on and tell you how to check for the other problems that I mentioned earlier.Checking key-off battery drain

Back in the good old days, the days before digital dashboards and onboard computers there wasn’t any current drain on the battery once the ignition key was turned to off and removed unless your car was equipped with a dashboard clock. With today’s cars there’s always a drain on the battery even with the key turned off. This parasitic current drain can range from as little as 20 ma (milliamperes) or 0.020 A (Amperes) to as much as 400 ma. Some Ford products will pull a parasitic current of 850 ma for up to 20 minutes after the key is turned off. As a rule of thumb this parasitic current should fall to around 50 ma after one hour has passed. This key-off, parasitic current is used by onboard computers to preserve its KAM (Keep Alive Memory) settings. While we are on the subject of KAM I want to point out that with most late model car you need to keep a constant voltage applied to the onboard computers and you shouldn’t disconnect the battery without first plugging a backup voltage source into the cigarette lighter socket. My favorite is the CTA COMPUTER MEMORY SAVERS available from JC Whitney Auto Parts for less than $13.00. If you lose KAM you may be faced with some really big, expensive problems. I’ll cover the problems caused by loss of KAM in a future factoid. To measure the parasitic current using a DMM follow the following steps.

1. Set your DMM to the mA range. If you are using a different DMM and it isn’t an auto ranging meter set it to a 100 mA or 1000mA range.

2. Plug your CTA COMPUTER MEMORY SAVER into the cigarette lighter socket to preserve KAM.

3. Disconnect one of the battery cables and connect your DMM between the cable and the battery terminal.

4. Unplug the battery backup and note the reading on the DMM.

5. Plug the battery backup back in, disconnect the DMM and reconnect the battery cable to the battery post.If the DMM shows an excessive parasitic current draw during step 4

Isolating the offending circuit can be as easy as unplugging a relay or fuse. Using the fuse box layout and/or wiring diagram identify the fuses and relays. Then remove and replace one fuse or one relay at a time until the parasitic current drops to within the normal range. Leave the relays and fuses connected with PCM or other KAM-sensitive modules until you have checked all the others. Once you have found the offending circuit check all the switches, relays, electronic modules, etc. and replace as needed.

Once you have identified the offending component and replaced it, proceed to step 5 above.

Tool Sources

Actron 130 amp Battery Load Tester  http://www.sears.com/shc/s/p_10153_12605_00921019000P?keyword=battery+load+tester#desc

CTA COMPUTER MEMORY SAVERS  http://www.jcwhitney.com/Tools-and-Garage-Test-Diagnostic/600021471.jcw