Dead battery, alternator or both?
Discussion
My mx5 has a dead battery. I drive it home from work on Thursday night and noticed that I was getting severe interference on AM on the radio. High pitched tone rising with engine speed.
The next day the battery was completely dead. I charged the battery today and the car fired up okay but high pitched interference still there. I cleaned the battery terminals and alternator connections and the interference went away.
The car now drives fine. No odd noises, but the battery simply won't hold a charge. I can go out for a drive and then leave the car for 2 hours and the battery will be flat as a pancake.
With engine running, voltage across battery terminals is around 13v.
Time for a new battery?
The next day the battery was completely dead. I charged the battery today and the car fired up okay but high pitched interference still there. I cleaned the battery terminals and alternator connections and the interference went away.
The car now drives fine. No odd noises, but the battery simply won't hold a charge. I can go out for a drive and then leave the car for 2 hours and the battery will be flat as a pancake.
With engine running, voltage across battery terminals is around 13v.
Time for a new battery?
It is your alternator in the first instance if what you say above is correct.
With the engine running at tickover you should get around 14 volts, 13 will not fully charge your battery.
Start the engine & have an assistant hold the revs at a fast tickover (1500 for arguments sake) then CAREFULLY (so that you do not short something out that you shouldn't) remove one leg off the battery & check the voltage at the leds, not the battery. You should get 14 to 14.5 volts, any less is a stuffed alternator.
While you are in the area check ALL HIGH POWERED CONNECTIONS and earths to battery/chassis/alternator, as this can also cause your problem.
With the engine running at tickover you should get around 14 volts, 13 will not fully charge your battery.
Start the engine & have an assistant hold the revs at a fast tickover (1500 for arguments sake) then CAREFULLY (so that you do not short something out that you shouldn't) remove one leg off the battery & check the voltage at the leds, not the battery. You should get 14 to 14.5 volts, any less is a stuffed alternator.
While you are in the area check ALL HIGH POWERED CONNECTIONS and earths to battery/chassis/alternator, as this can also cause your problem.
andyiley said:
It is your alternator in the first instance if what you say above is correct.
With the engine running at tickover you should get around 14 volts, 13 will not fully charge your battery.
Start the engine & have an assistant hold the revs at a fast tickover (1500 for arguments sake) then CAREFULLY (so that you do not short something out that you shouldn't) remove one leg off the battery & check the voltage at the leds, not the battery. You should get 14 to 14.5 volts, any less is a stuffed alternator.
While you are in the area check ALL HIGH POWERED CONNECTIONS and earths to battery/chassis/alternator, as this can also cause your problem.
Does not running the engine with the battery disconnected stuff the alternator anyway?With the engine running at tickover you should get around 14 volts, 13 will not fully charge your battery.
Start the engine & have an assistant hold the revs at a fast tickover (1500 for arguments sake) then CAREFULLY (so that you do not short something out that you shouldn't) remove one leg off the battery & check the voltage at the leds, not the battery. You should get 14 to 14.5 volts, any less is a stuffed alternator.
While you are in the area check ALL HIGH POWERED CONNECTIONS and earths to battery/chassis/alternator, as this can also cause your problem.
andyiley said:
Start the engine & have an assistant hold the revs at a fast tickover (1500 for arguments sake) then CAREFULLY (so that you do not short something out that you shouldn't) remove one leg off the battery & check the voltage at the leds, not the battery. You should get 14 to 14.5 volts, any less is a stuffed alternator.
Sorry but I don't agree with this idea - you risk a massive voltage spike in the car's electrical system as the alternator goes from a load to no load. Simply measure the voltage at the battery with the engine off and then with the engine on. A healthy and charged battery should show over 12 volts and when the engine's running a healthy alternator should bring the system voltage up to 14 volts or thereabouts and be able to maintain that voltage as you turn on more electrical loads such as the main beams and heating fan.And where does this voltage spike come from when you disconnect the scource of electrical power that is the alternator?
The laws of physics apply!
Also, without more than a voltmeter, how do you reckon to remove the potential of the unknown battery condition affecting the results?
I have done exactly the test suggested on MANY dozens of vehicles when presented with this situation and not the desired test equipment available.
On top of that, how do you think the alternator would provide this MASSIVE SPIKE anyway? (when it is disconnected)
The laws of physics apply!
Also, without more than a voltmeter, how do you reckon to remove the potential of the unknown battery condition affecting the results?
I have done exactly the test suggested on MANY dozens of vehicles when presented with this situation and not the desired test equipment available.
On top of that, how do you think the alternator would provide this MASSIVE SPIKE anyway? (when it is disconnected)
Edited by andyiley on Sunday 1st March 15:58
Cheers for all the additional, and comprehensive relies folks. In between bouts of ripping our house apart, I have done a little further diagnosis. Charged the battery up for a couple of hours then started the car. Alternator pulling a nice healthy 14v measured at the battery. That was fast idle, around 1500 rpm.
Killed the ignition and disconnected the battery positive. Measure voltage across battery at 12.44v. I cleaned a few connectors around the battery and also disconnected the knackered electric aerial.
Left the car for around 5 hours. Checked voltage across battery, 12.42v.
Reconnected the battery and now it only reads 12.15v across terminals. Difference comes just from reconnecting it, 2 mintutes between measurements? Is that normal?
I did fire the car up again after that and could see a steady 14v even at normal idle speed.
I'll leave it overnight now and see if the battery is dead in the morning. A bit strange.
Killed the ignition and disconnected the battery positive. Measure voltage across battery at 12.44v. I cleaned a few connectors around the battery and also disconnected the knackered electric aerial.
Left the car for around 5 hours. Checked voltage across battery, 12.42v.
Reconnected the battery and now it only reads 12.15v across terminals. Difference comes just from reconnecting it, 2 mintutes between measurements? Is that normal?
I did fire the car up again after that and could see a steady 14v even at normal idle speed.
I'll leave it overnight now and see if the battery is dead in the morning. A bit strange.
Cliftonite said:
Does not running the engine with the battery disconnected stuff the alternator anyway?
See:
http://www.troubleshooters.com/dont_disconnect_bat...
andyiley said:
And where does this voltage spike come from when you disconnect the scource of electrical power that is the alternator?
The laws of physics apply!
Also, without more than a voltmeter, how do you reckon to remove the potential of the unknown battery condition affecting the results?
I have done exactly the test suggested on MANY dozens of vehicles when presented with this situation and not the desired test equipment available.
On top of that, how do you think the alternator would provide this MASSIVE SPIKE anyway? (when it is disconnected)
But as I understand it you're suggesting disconnecting the battery from the car's electrical system and the alternator is still connected and being spun by the engine and attempting to regulate the overall electrical system voltage? In most cars the alternator and battery are connected in parallel.The laws of physics apply!
Also, without more than a voltmeter, how do you reckon to remove the potential of the unknown battery condition affecting the results?
I have done exactly the test suggested on MANY dozens of vehicles when presented with this situation and not the desired test equipment available.
On top of that, how do you think the alternator would provide this MASSIVE SPIKE anyway? (when it is disconnected)
It's the same logic that says when starting one car from another to connect the jump leads for a few minutes and then turn off the donor car's engine. Else the donor's car alternator will see a massive voltage drop when the other car is cranked over and respond accordingly. So far so good. But then when the other car starts and the starter is shut off the donor car's alternator may not be able to reduce its output fast enough and both cars will see a voltage spike.
It may work 99 times out of 100 for you but the 1 time it doesn't the cost of fried electrics will make you wince. Your cars - your choice.
As far as removing the unknown influence of the battery a simple voltage test of the battery with the engine off should give a good insight into its health.
AW10 said:
But as I understand it you're suggesting disconnecting the battery from the car's electrical system and the alternator is still connected and being spun by the engine and attempting to regulate the overall electrical system voltage? In most cars the alternator and battery are connected in parallel.
It's the same logic that says when starting one car from another to connect the jump leads for a few minutes and then turn off the donor car's engine. Else the donor's car alternator will see a massive voltage drop when the other car is cranked over and respond accordingly. So far so good. But then when the other car starts and the starter is shut off the donor car's alternator may not be able to reduce its output fast enough and both cars will see a voltage spike.
It may work 99 times out of 100 for you but the 1 time it doesn't the cost of fried electrics will make you wince. Your cars - your choice.
As far as removing the unknown influence of the battery a simple voltage test of the battery with the engine off should give a good insight into its health.
Cheers. I'll not be disconnecting the battery with the engine running. Hadn't occurred to me that was even possible however I've read enough now to put me off the idea.It's the same logic that says when starting one car from another to connect the jump leads for a few minutes and then turn off the donor car's engine. Else the donor's car alternator will see a massive voltage drop when the other car is cranked over and respond accordingly. So far so good. But then when the other car starts and the starter is shut off the donor car's alternator may not be able to reduce its output fast enough and both cars will see a voltage spike.
It may work 99 times out of 100 for you but the 1 time it doesn't the cost of fried electrics will make you wince. Your cars - your choice.
As far as removing the unknown influence of the battery a simple voltage test of the battery with the engine off should give a good insight into its health.
Anyway, today's update. Left car with battery connected overnight having shown just over 12v the last time I checked later yesterday. This morning it was reading 2.5v
Before I go out and buy a new battery (it's looking that way). I have the car at work today having given it a boost charge this morning. When I arrived at work I disconnected the battery as I want to see if it's something else draining it or whether the battery just can't hold a charge. Have the battery charger/starter with me just in case.
Jumping forward a step though, the alternator noise/interference that I heard the other night is bothering me somewhat. I seemed to cure it by cleaning up a load of connectors but could a bad connection have lead to the battery being fried? The battery seemed to go from being healthy to seriously unhealthy in no time. Or could the alternator noise have been a sign of the alternator straining to charge a dying battery?....
AW10 said:
But as I understand it you're suggesting disconnecting the battery from the car's electrical system and the alternator is still connected and being spun by the engine and attempting to regulate the overall electrical system voltage? In most cars the alternator and battery are connected in parallel.
It's the same logic that says when starting one car from another to connect the jump leads for a few minutes and then turn off the donor car's engine. Else the donor's car alternator will see a massive voltage drop when the other car is cranked over and respond accordingly. So far so good. But then when the other car starts and the starter is shut off the donor car's alternator may not be able to reduce its output fast enough and both cars will see a voltage spike.
It may work 99 times out of 100 for you but the 1 time it doesn't the cost of fried electrics will make you wince. Your cars - your choice.
As far as removing the unknown influence of the battery a simple voltage test of the battery with the engine off should give a good insight into its health.
Of course the battery & alternator are effectively connected in parallel, and the rest of the electrical system is a pretty much constant load, whereas the battery will not be a constant load if it is flat & trying to charge. Also it would be drawing a very high current at that point, and what you are trying to test is the alternator output into a static load to see its nominal load output.It's the same logic that says when starting one car from another to connect the jump leads for a few minutes and then turn off the donor car's engine. Else the donor's car alternator will see a massive voltage drop when the other car is cranked over and respond accordingly. So far so good. But then when the other car starts and the starter is shut off the donor car's alternator may not be able to reduce its output fast enough and both cars will see a voltage spike.
It may work 99 times out of 100 for you but the 1 time it doesn't the cost of fried electrics will make you wince. Your cars - your choice.
As far as removing the unknown influence of the battery a simple voltage test of the battery with the engine off should give a good insight into its health.
I don't see how much difference you are expecting in the jump start situation you mention as compared to the same situation under normal (non-jump-start) conditions on a marginal battery anyway? The system is designed to cope with it.
Yes, I agree with you in most cases unless the battery in question has either a high or low resistance cell, the only way to check this is by disconnecting it.
andyiley said:
I don't see how much difference you are expecting in the jump start situation you mention as compared to the same situation under normal (non-jump-start) conditions on a marginal battery anyway? The system is designed to cope with it.
The big difference is that in a normal engine start sequence the alternator is turning too slowly to have any affect whereas in the jump start sequence I mentioned the donor car's alternator is fully engaged in trying to maintain the voltage level.AW10 said:
andyiley said:
I don't see how much difference you are expecting in the jump start situation you mention as compared to the same situation under normal (non-jump-start) conditions on a marginal battery anyway? The system is designed to cope with it.
The big difference is that in a normal engine start sequence the alternator is turning too slowly to have any affect whereas in the jump start sequence I mentioned the donor car's alternator is fully engaged in trying to maintain the voltage level.andyiley said:
It is your alternator in the first instance if what you say above is correct.
With the engine running at tickover you should get around 14 volts, 13 will not fully charge your battery.
Start the engine & have an assistant hold the revs at a fast tickover (1500 for arguments sake) then CAREFULLY (so that you do not short something out that you shouldn't) remove one leg off the battery & check the voltage at the leds, not the battery. You should get 14 to 14.5 volts, any less is a stuffed alternator.
DO NOT DO THISWith the engine running at tickover you should get around 14 volts, 13 will not fully charge your battery.
Start the engine & have an assistant hold the revs at a fast tickover (1500 for arguments sake) then CAREFULLY (so that you do not short something out that you shouldn't) remove one leg off the battery & check the voltage at the leds, not the battery. You should get 14 to 14.5 volts, any less is a stuffed alternator.
Removing the battery whilst the engine is running creates a condition called "load dump". It takes a finite time for the current through the field coils in an alternator to reduce in response to a reduction in load, and if the alternator is delivering a high current into the battery when it's disconnected (which it will be after an engine start) then you get a very large voltage spike, up to 100v in some cases and it can be quite lengthy in duration, so it's a lot of energy.
Whilst modern OEM automotive electronics are supposed to provide protection against load dump, deliberately invoking this situation is like rolling dice. Sooner or later an ECU's number is going to come up.
Gassing Station | Home Mechanics | Top of Page | What's New | My Stuff