Dead battery, alternator or both?

[Gad-Westy]

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?

[Gad-Westy]

bearman68 said:

Disconnect the battery, and see if it holds a charge then.
Also test voltage across disconnected battery, see if you get a good 12v.

Good plan. Will try that tomorrow. Cheers.

[Gad-Westy]

finlo said:

Does the battery warning light stay on with the ignition off?

No. What would that imply? Cheers

[Gad-Westy]

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.

[Gad-Westy]

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.

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?....

[Gad-Westy]

A little baffled here. I left the battery disconnected for 5 hours this morning. Came back to the car, checked voltage across battery and it had barely dropped from 12 something. Drove the car for 30 mins or so, checked voltage with engine off and its still 12 something. Left the car for a further 5 hours but with battery connected and then checked voltage across battery and it was down to 2.5v.

So it looks like I have something draining the battery rather than the battery itself being an issue.

I'd noticed that when I check voltage across the battery when its disconnected it reads about 0.5v higher than when it is connected and I've verified that by then disconnecting it again and it jumps back up 0.5v.

So I started unplugging fuses 1 by 1 to see if any changed that. None of the normal circuits had any affect but when I removed the main 80a fuse for the car, the voltage went up 0.5v.

So then I dug a little deeper. If I disconnect the alternator multi plug, nothing happens but if I disconnect the alternators thick single cable, the battery voltage climbs up 0.5v.

So am I reading this right? The alternator itself is draining the battery when the car isn't running...

Edited by Gad-Westy on Monday 2nd March 21:25

[Gad-Westy]

Cheers chaps, very much appreciated. With a failed diode, would you expect to see a charge light on the dash on with ignition on but engine not started. Mine isn't illuminating.

[Gad-Westy]

Thanks folks. Will perform some of these tests today once I get a chance.

Another little observation is that the alternator seems to be whining a little.