Dying Battery Issues
#22
06-10-2015, 10:22 AM
Veteran Member
I just put the positive lead to the DVOM on the positive post, touch the negative lead to the negative post and read the voltage (?)
My wife's Maxima was hard starting last week: voltage was 11.5DCV after a short drive; but the running voltage was 13.8, which suggested the alternator was functioning. Sent her over to Advance Auto to have the battery load tested (I think I can sort of do that with a meter, but not really reliable and would probably drain the battery completely). Load test on their machine confirmed a bad battery.
My wife's Maxima was hard starting last week: voltage was 11.5DCV after a short drive; but the running voltage was 13.8, which suggested the alternator was functioning. Sent her over to Advance Auto to have the battery load tested (I think I can sort of do that with a meter, but not really reliable and would probably drain the battery completely). Load test on their machine confirmed a bad battery.
Last edited by Jhartz; 06-10-2015 at 10:27 AM.
#23
06-10-2015, 01:38 PM
The load testers are kind of useless. The trouble is a bad battery can have an intermittent internal short, resulting in randomly going flat.
They also only check for one thing : short term ability to put out a lot of current. They don't check for ability to hold a charge, or how much charge capacity is being held.
As a battery ages, its charge capacity drops, due to sulfation of the plates. There are many people out there who think their battery is fine, even though the capacity is down to 10%, as it still starts the car. Guess what happens when it drops from 10% to nothing.
On my boat I test the batteries every 6 months by applying a load of capacity /20, in this case 170 Ah / 20 = about 8.5 amps, and measuring how many amp hours I get out before the voltage drops to a given threshold. The threshold I use is 50% charged, or about 12.1 V.
You should get 85 Ah. As the battery ages, this drops.
This is the only accurate measure of battery condition.
They also only check for one thing : short term ability to put out a lot of current. They don't check for ability to hold a charge, or how much charge capacity is being held.
As a battery ages, its charge capacity drops, due to sulfation of the plates. There are many people out there who think their battery is fine, even though the capacity is down to 10%, as it still starts the car. Guess what happens when it drops from 10% to nothing.
On my boat I test the batteries every 6 months by applying a load of capacity /20, in this case 170 Ah / 20 = about 8.5 amps, and measuring how many amp hours I get out before the voltage drops to a given threshold. The threshold I use is 50% charged, or about 12.1 V.
You should get 85 Ah. As the battery ages, this drops.
This is the only accurate measure of battery condition.
Last edited by Mark SF; 06-10-2015 at 01:44 PM.
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Mark Murphy (11-02-2016)
#24
06-10-2015, 01:48 PM
You can measure amps, but you have to put the meter in series. A typical meter will measure up to 10 amps. This means you can't measure the kind of current a lead acid battery will put out, with a meter. It'll either melt, or the fuse will blow.
Large currents are usually measured with a shunt, a very low resistance resistor in line, across which you measure volts, and calculate the current by ohms law.
On a car, the best way to measure the battery current is with the starter motor. That's what the battery is for, after all. Place the meter across the battery terminals in volt mode, and measure the battery voltage while cranking. A healthy battery will maintain 10V, even 11, while cranking. I've seen a dead one drop to 6V. This is what the store battery testers do, effectively.
Last edited by Mark SF; 06-10-2015 at 01:51 PM.
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Mark Murphy (11-02-2016)
#25
06-10-2015, 03:45 PM
Veteran Member
Mark said: "On a car, the best way to measure the battery current is with the starter motor. That's what the battery is for, after all. Place the meter across the battery terminals in volt mode, and measure the battery voltage while cranking. A healthy battery will maintain 10V, even 11, while cranking. I've seen a dead one drop to 6V. This is what the store battery testers do, effectively."
That's what I would have done: loaded lights, wiper, blinkers, radio, HVAC motor on high and had the wife try to start it while I read the voltage at the battery. But I was pretty positive it was the battery and was afraid that test would have completely drained it to the point I would have to run cables or have it towed. Took this easy route: "Hon, you need a new battery, run over to Advance Auto Parts, let them confirm, and buy and get installed. Unless you would rather have AAA come out and do it. Whichever, Now!"
That's what I would have done: loaded lights, wiper, blinkers, radio, HVAC motor on high and had the wife try to start it while I read the voltage at the battery. But I was pretty positive it was the battery and was afraid that test would have completely drained it to the point I would have to run cables or have it towed. Took this easy route: "Hon, you need a new battery, run over to Advance Auto Parts, let them confirm, and buy and get installed. Unless you would rather have AAA come out and do it. Whichever, Now!"
#26
06-14-2015, 04:11 PM
Tagging it on to a thread about battery problems is not going to get you many reads or answers.
Welcome to the forums too BTW!
Vector
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RJ237 (06-14-2015)
#27
10-29-2016, 07:21 PM
What voltage do you get across the battery when the engine is actually running? You should be getting very close to 14v. This test will confirm that the alternator is charging normally.
You might have excessive current drain from the battery when the car is locked up overnight-it's possible that not all the vehicle electrical systems are shutting down, and so the battery gradually loses charge overnight.
If you measured the battery voltage on the dead car at 0.5v, then the battery was obviously totally & completely flat as a pancake. It needs to be fully recharged as soon as possible, otherwise it could sulphate & cause plate shorts internally which will render the battery useless.
A fully charged battery in good condition left standing for several hours with no electrical load should show above 12.6v & close to 13v.
If you have residual current drain, you need to remove the battery earth lead & connect a multimeter set to the Amps range in series with the battery negative terminal & the earth lead connection. Then carefully close the boot, leaving the multimeter outside the car for observation. Lock up the vehicle normally & wait for at least 30 seconds for all the systems to shut down. Then check the current reading on the multimeter-it should be less than 0.05 Amps. If you get around 1 Amp or anything around that level, then you've got a problem with electrical circuits which are not shutting down properly & draining the battery overnight.
This is not an easy thing to track down, so patience & a full set of circuit diagrams will be required, so you can individually isolate each circuit & then test for residual current drain.
A likely suspect is the main diode pack in the alternator becoming 'leaky' with age & heat. The diode pack is meant to stop current flow in the reverse direction, but they can become slightly 'leaky' with age & drain the battery charge in a few days.
Either way, there's no quick fix & you'll need to carefully & methodically work your way through the problem-starting first with checking the main system voltage at the battery with the engine running.
You might have excessive current drain from the battery when the car is locked up overnight-it's possible that not all the vehicle electrical systems are shutting down, and so the battery gradually loses charge overnight.
If you measured the battery voltage on the dead car at 0.5v, then the battery was obviously totally & completely flat as a pancake. It needs to be fully recharged as soon as possible, otherwise it could sulphate & cause plate shorts internally which will render the battery useless.
A fully charged battery in good condition left standing for several hours with no electrical load should show above 12.6v & close to 13v.
If you have residual current drain, you need to remove the battery earth lead & connect a multimeter set to the Amps range in series with the battery negative terminal & the earth lead connection. Then carefully close the boot, leaving the multimeter outside the car for observation. Lock up the vehicle normally & wait for at least 30 seconds for all the systems to shut down. Then check the current reading on the multimeter-it should be less than 0.05 Amps. If you get around 1 Amp or anything around that level, then you've got a problem with electrical circuits which are not shutting down properly & draining the battery overnight.
This is not an easy thing to track down, so patience & a full set of circuit diagrams will be required, so you can individually isolate each circuit & then test for residual current drain.
A likely suspect is the main diode pack in the alternator becoming 'leaky' with age & heat. The diode pack is meant to stop current flow in the reverse direction, but they can become slightly 'leaky' with age & drain the battery charge in a few days.
Either way, there's no quick fix & you'll need to carefully & methodically work your way through the problem-starting first with checking the main system voltage at the battery with the engine running.
Great post. I have a 96 XJS and it has a constant battery drain. I tried to add Bluetooth by attaching a cheap receiver and installing in in the boot fuse panel. Interestingly, NONE of the fuses powered down when the car was turned off. From your post, I bet I have a bad main diode pack. At least that gives me something to investigate. Thanks!
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