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Battery question
On the way to an endodontist for a root canal this morning, I stopped at my dentist to pick up an x-ray. When I left, the 85 TD failed to start. The initial start this morning had seemed a little anemic. I got a jump, drove home, picked up another car and continued on my way. When I returned, I checked the voltage on the battery--12.66. I tried to start it. Just a click again. Checked the voltage again--10.58. I'm virtually certain it is a bad battery, but precisely what is wrong with a battery that shows good voltage but after one start attempt, drops so drastically?
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I cannot tell you precisely how it functions internally, but, a battery that won't hold a full charge will exhibit rapidly declining voltage after the slightest load is placed upon it.
Many times folks will get by with a battery that is 60% deteriorated simply because the engine will start in the first three seconds of cranking. They only have about six seconds.........but...........they don't know it. A battery starts to deteriorate immediately after it's placed in service. Depending on the service it encounters, the same battery may last 2 years or 10 years. |
Using a VOM to gage the condition of the battery is bordering on worthless, Kerry. You need to have it "load tested" to get some determination of condition. OTOH, if you know the battery is old, get a fresh one. Cold weather is coming and we all know how critical it is to maintain a diesel's starting system in TOP condition.
Good luck and buy a big battery! :D Even the late-model gassers have HUGE batteries. |
As Brian implied, sometimes an old battery deteriorates until it takes only a "surface charge." The voltage may measure something that looks reasonable but the capacity of the battery to deliver the high current required of a starter (hundreds of Amperes) is gone. It might as well be a flashlight battery for all the good it does you. Auto service places and places that sell batteries usually have a load tester, as uberwagon said, that simulates the conditions a battery finds itself when you turn the key.
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Many SEARS auto stores have a fancy load tester. I think they'll check for free. The test takes about 10 mins and checks a number of parameters. Most gas stations (with service bays) also have a simple load tester, it should be capable of giving a good indication of condition.
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dont mess with those new fangdangled electronic load testers-
i brought in a battery at my local parts house-6yr warr w/ 2 yr free replacement- it was under 2 yrs old, and this one kid says it tests fine- i watched him hook up this little box- looked like a multi tester, with little wimpy wires coming off it. he pushes a couple bottons, and said that it put a simulated load on it and was fine. i put it in my F-150 300 6 cyl, and it would not start it. this was in the summer. bring it back in, and get one of the other guys to check it out- used a real battery load tester- big (some as big as a shoe box) box with heavy wires coming out- fan cooled, big knob you turn to apply load, you can smell the battery cooking. tested bad. way bad. new battery for me:) just love it when the new kid thinks his high tech state of the art tools are the best, and are wrong. |
I've had enough bad batteries over the years as to be virtually assured that the battery was bad completely independent of the voltage test. I am going to get a new battery. I only did the voltage test to get a quick read on whether the battery was getting any charge at all. I was just curious as to what the voltage drop meant. It's accepting a charge but not accepting it very deeply. Why has it lost its appetitite for electrons?
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Quote:
Clearly he does not understand P=I*E or P=I*I*R |
When batteries get old, their internal resistance rises. So when you load it down, the battery drops voltage internally, and cannot deliver it to the starter. Simply Ohm's law, V=IR.
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One of the best tools for testing a battery's preformance is a hydrometer. You can get one at Wally World for 99 cents. Put an over night charge on the battery and then test all the cells. If each cell doesn't come up to at least 4 balls the chances are that they are not working correctly. The battery will probably start your car if your engine is in very good condition, but the battery does not have any reserve, i.e., you can't crank it for very long.
It is also a good way to tell you that you better be putting a bit of money for a new battery. IF the readings are less than perfect you will be needing a new one, esp. if winter is coming. |
I've got an hydrometer, but I was in my office clothes and I didn't want to be drawing up acid so I grabbed the multimeter and did the quick test.
I'm actually surprised the battery has lasted this long. It was in the car when I purchased it and I spent a weekend last winter at 35 below zero with #1 fuel in the car (unbeknownst to me) and the battery took a vicious beating as a result. It has earned its retirement. |
The battery is likely suffering from one or more dead or weak cells. If you have an old test lead set, you can dip into the electrolyte and measure the voltage for each of the 6 cells. You will very likely find one or more that do not have the voltage level of the rest. Those have usually been chemically damaged inside the plates, often called sulphated plates. When this happens it is the end of the line for that battery.
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A simple volt meter and the car is the only test equipment needed to test a battery. Start by checking the battery voltage with everything in the car turned off. Should be 12.5 or higher. If not, get the car started to charge the battery (or use a battery charger) and make sure the voltage is now 13.5 to 14.5 with the engine running. If not, alternator time. If all is OK so far, turn the engine off and check the voltage. Should be somewhere above 12.5. Now turn the headlights on and watch the meter. If it stays at or above 12.5 for 10 or 15 min then look elsewhere for the problem. If it drops to 10.5 or less very quickly, it is battery time. Another test you can do while you are at it is with the engine running and a good known battery, turn on the headlights and A/C. check the voltage and make sure it stays at atleast 12.5 or higher. Then increase the RPM and make sure the voltage goes up to 13.5 or more. If not, check the alternator belt and make sure it is tight. If the voltage seems to be low but working, a new regulator may help.(Just had to do this to my 84 500SEL.)
The best tester for automotive electrical devices is the car itself. You can check batterys, alternators, starters, etc all you want but if it won't do what is needed for the car it is going on, it is bad. Now I agree and know from experience the above test is not perfect but if it doesn't show the problem, specialized equipment not normally found in even the best equiped home shops will be needed. |
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