The Stovebolt Site Forums The Shop Area The Electrical Bay Battery not holding charge?

With nothing running and everything off, the battery seems to be discharging. I put it on the charger, crank the truck. 30 minutes later the starter motor won't roll over unless I charge the battery more. The battery came with the truck. It is years old. How can I test the goodness of a battery?

Try a new voltage regulator and a new battery may not hurt either. And if you do all this don't forget to polarize the generator with a smaller jump wire between the left and middle terminals. Hold the jumper there for a sec or two, you should see a spark and you should be good to go.

There are two indicators of battery condition, the "specific gravity", or strength of the acid concentration, and load amperage, the current the battery can produce under starting load. A totally discharged battery would have the same specific gravity as plain water, 1.000. A fully-charged battery will have a specific gravity of 1.270 or so, (1.270 times as heavy as an equal volume of water). Buy a good-quality battery hydrometer, one that reads out in numbers, not a series of floating balls. Put the battery on a slow charge, about 10 amps or less, and check the specific gravity frequently. When the S.G. remains the same for several checks, (probably overnight) the battery has taken as much charge as it can. Further charging will simply boil the water out of it. It should top out about 1.265 or higher, and all cells should be approximately equal. One low cell means the battery is junk.

Load test- - - - -If you have access to a battery load tester, put a current load on the battery of 1/2 the cold cranking amp rating for 15 seconds, then read the voltage at the terminals with the load still applied. 12 volt batteries should read a minimum of 9.6 volts, 6 volt batteries should maintain 4.5 volts minimum after a 15-second load test. A pretty good load test can be done with the starter. Crank the engine for 15 seconds, with high beam headlights and any other high-draw accessory such as the cigar lighter also in use, and check battery voltage at the end of the 15-second test. 12 volt batteries- - -10 volts or above, while still cranking. 6 volt systems, 4.5 volts or more while cranking. Short boost-charging sessions are almost worthless, particularly with 6-volt batteries.
Jerry

Hotrod Lincoln,

Thanks for the "Battery Education" lesson it is a pleasure to learn something that is written to be easily understood.

Let me piggie back on this one. I've noticed the last few days that my ammeter shows fairly strong charge like midway between midpoint and the + for maybe 10 miles of fwy driving before coming back to midpoint. It used to charge to about that for a minute or 2 after startup. Starts fine though and no other indications of something wrong. Is this an indication the battery is getting pretty weak?

When you say years old. Just how old? Battery
is good for about 4yrs. If its that old or older
I'd just replace it. Cecil.....

Dennis, as long as it's starting well, I wouldn't get too concerned. Really, I'd rather see a little more time at a heavy charge than one which recovers quickly, because the really quick recovery might indicate a "sulfated" battery which has very little reserve. Some batteries will work fine for a start or two, and then fail to start the engine under a flooded condition, or whenever several seconds of cranking are required. Watch the water level, if possible, and if one cell starts to need more water added than the others, that one's got a problem.

P.S.: Betcha you couldn't tell I teach Auto Mechanics in a high school!
Jerry

Dennis,
I'd like to add to what Hotrod said. Take a small 10-15 amp home battery charger and two batteries. One is perfectly good but discharged and one "sulfated" battery. The good battery will take overnight to recharge("0" reading on gauge). The bad battery may show a full charge in a couple of hours or less. I.E., the more you put in the more you get out.
In your case, you said you noticed a change in the last couple of days, but yet the ammeter returns to "0" as before. In most cases with a "sulfated" battery the ammeter can't return to "0", because the battery can't stay above the constant drain and the alternator is always charging it.
This is why I prefer a ammeter in a car as opposed to a volt meter. The volt meter will generally have the same reading regardless of which of the two batteries I mentioned above are in the car.
Back to your problem. I'd check the charge voltage at the battery. It should be 14v plus. Also check the voltage from batt post(post not cable end) to ground and also to engine block. Should be "0" or a few tenths above. Positive batt post to alternator output post. Should also be only a few tenths. Anything above .5 volts is a voltage drop that you should try to locate and clean up.

Find your battery reserve capcity. Sometimes it's printed right on the battery. If not, it may be listed in a manual somewhere. This is just an example of Hotrod's reference to "battey reserve".
Divide this number by 4. Example: 90/4 = 22.5
Compare this to the ammeter reading.
The current drain in milliamps should not exceed this number.
Example: if a battery has a reserve capacity of 90 minutes , the current drain should not exceed 22 milliamps .
This formula is used when checking for "parasitic" draw.