My battery rating is 800 cca. My battery died after it was not used for 4 days in very cold weather. I gave it a boots over night with my charger. The gage reads amps 0 to 8amps. After charging it, it started fine and the gage on the charger reads 2.5amps. Is that good or is that low?
The gauge on the charger tells you how much current the charger is pumping into the battery. It does not tell you anything directly about the capacity of the battery.
To test a battery, use a voltmeter.
The "cold cranking amps" of a battery are not something you can measure straightforwardly, unless you are an engine :)
Indeed. There's just a limit to how much you can really tell for sure without a battery hygrometer and a load tester. The former is small and cheap. The latter is probably a nonstarter (no pun intended and not much of one achieved) unless you're a professional mechanic or are very very much into scientific car care. (Of course rubber gloves and safety glasses come into play at certain obvious points as well.) Some good information:
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A couple more things we need to know: whether the alternator is putting out enough voltage to keep the battery charged in normal operation, and how well that voltage is reaching the battery through dirty terminals, loose connections, dodgy grounds, etc. The place to measure voltage, for these purposes, is right on the actual battery terminal, not merely on the connector (though that measurement also tells you something important).
Note also that this is the winter of our electron debt -- nights of slow-and-go traffic with headlights and wipers and heater all at full roar can send a good battery into a slow decline even with a good alternator and connections. I've read an interesting and compelling electrochemistry based argument, perhaps in the above URL, that batteries are merely pronounced dead in winter after having been murdered in summer, but the point now is to figure out why the original poster's battery was flat and what to do about it....
Ok, I got my volt meter and it reads 12.5v engine off and 15.5volts engine running. Also, 29 AC amps with engine off (battery connected) I am not sure if this reading means anything. Does this say anything about the condition of my battery? thanks.
That's probably okay, indicating that your typical lead-acid six cell automotive battery has most, but not quite all, of its charge. As mentioned in my earlier posting, other problems can confound this simple logic, though.
Fifteen-five with the engine running will forever seem high to me, a symptom of a voltage regulator on the fritz; but I'm told that many late model GM's can charge at voltages as high as that, so it might be normal for your car. Is the battery swollen, or getting hot?
I think looking at a DC current with an AC instrument will mean not much, although a DC amperage can be useful to read when looking for mystery loads and leaks. Let's assume at first that you don't have any of them above and beyond the quiescent draw (few tens of mA at most) of cars today. The full current draw would zorch most volt-ohm- milliammeters anyway. Fortunately, current, voltage, and resistance have a simple relationship stated in Ohm's Law, so you can infer one from the other two unless you're looking for small sneaky current draws whose effect on voltage would be very subtle.
An optimally charged battery in good condition should have an open circuit voltage of 12.6-12.7 or so, assuming electrolyte is okay, etc. With low temperatures, etc, this can be lower even at full charge.
I dont know how (or why) you connected the ammeter to the battery in the AC mode with the engine off, but it probably means nothing (at best). If you want to measure the residual, you have to use the DC current function, in series with the battery, with the engine OFF. You have to wait until any active systems (burglar alarms, courtesy lights, etc) go dormant. Then you can measure residual current, and as previously mentioned it should be a few tens of milliamperes.
If you really have a multimeter that will measure AC amperes ONLY, you can use this as a test for shorted diodes in the alternator. This is done with the engine running and the AC ammeter in series with the battery.
Many (particularly older) multimeters do not really measure AC amperage exclusively, even when put on that scale. Some of them measure DC current through a halfwave rectifier, which is related to AC current. This can throw you for a loop (no pun unintended).
15.5V with the engine running is a little bit high, but it indicates that your alternator is running. 12.5V with the engine off is awfully low and indicates your battery isn't in such good shape.
I'd replace the battery, but I'd check the running voltage after doing that. If your alternator is running that hot, it could be overcharging the battery and damaging it.
Depends on the exact state of charge, the temperature, the phase of the moon, etc.
In some ways an ammeter is not as useful as a voltmeter. 2.5 amps seems a little high if the battery is fully charged and there is NO load on the battery. Is that in the car and the engine running?
The first question would be: "Is the car computer controlled"?
If so, you need to do a battery draw test, with an auto ranging multi-meter, set to the Amps scale, and test lead plugged into the amps socket.
Anything over 750 Milliamps, means the computer is not going into the dormant mode. Not that the computer is necessarily bad, but further diagnostics would be needed.
If the charger stayed between 0 to 1 Amp, I would say everything is OK, but I would do an amperage draw test.
If that is the external charger, that is indeed typical. I thought he meant he had the battery already charged and the car charging system was showing 2.5 amps.
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