dead battery or faulty alternator on my miata mx5 ?

As the title says is their any tell tell signs that the battery is on its last legs or the alternator has packed in?



A bit of back story will help...

So about 3 months ago I'm leaving my SO's house and the MX5 is flat. Stereo, lights and alarm all work but it just didn't have the juice to crank it and it seemed odd as the car was driven to her home the day before (some 30 miles) and we had only just come back from shopping. I just put it down to maybe leaving something on. We just jump started it and its been fine till about a week ago. The car had been sitting on the drive for 4 days while I waited for parts to repair a water leak. I just put this down to the car sitting while the alarm slowly drained the battery. The only thing that seemed odd was that it only took an hour to charge from totally empty to half full that seems really quick to me compared to other car battery's I've had to charge. Then today it drove fine to and from work but on the way home I had to turn it off and hop out to quickly to move another car then when I went to restart my car nothing, lights and stereo worked but just could not turn over.

I'm tempted to say its the battery due to how its gotten worse over time, if it had been the alt it would be a case of it never starting on its own after that first event.

I cant splash out on a replacement battery till next payday so I'm going to trickle charge it till then however if someone can chime in and tell me I'm spending money in the right area that would be great.

Using a volt meter, you should see ~13.8-14.4 v at the battery terminals while it is running. Should be around 12v when the car is off. The Miata batteries are small and can drain quickly. I don't know about the UK, but in the US, most autoparts stores can bench test your battery and your alternator (you have to remove them first). If you replace the battery, an AGM (absorbed glass mat) battery is recommended.

The other concern is if you are getting parasitic draw from something aftermarket, like your alarm, that may be killing the battery. My factory battery on the NA lasted about 10 years, the Westco I replaced it with about two months. The one from a large auto parts supplier sold me one exactly like the Westco, but $30 cheaper. It has been running on it fine for 4 years with the current owner. FWIW, the battery in my NB lasted about 5 years. NA alternators are less problematic than those in the NBs.

thanks for the tips. It seems that I've found out the battery is dead the easy way. I put it on charge for a few hours and plugged it in to see if I could get it to start with no luck. Lights, radio and alarms all work but it just cant get it to turn over.

I've ordered a gel Westco after deciding I can do without somethings for a few weeks. Fingers crossed I get it by sat as I'm on holiday from Monday.

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Originally Posted by BlitzWing

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I've ordered a gel Westco fingers crossed I get it by sat as I'm on holiday from Monday.

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good choice, Westco makes a great battery, that is all I have ever used in my past Miatas

thanks I've noticed they come highly recommended and the alternative was some cheap off brand. Fingers crossed this is the last issue for some time having the coolent leak and now a power issue has murdered my wallet. Plus it would be nice to drive it without fearing I might get stranded.

This reminds me of something related...I used to forget to turn off my lights (parking/fogs) when a few times and killed a new battery in short order. There was a mod I did on mine that helped me remember to turn off my lights. It was just a 12v buzzer, two wires. one went in the cabin fuse box and the other was somewhere else. If I shut the car off with the lights on, it would sound. I'll see if I can find that somewhere. and post it up.

That would be a good one.

Quote:

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Originally Posted by BlitzWing

thanks for the tips. It seems that I've found out the battery is dead the easy way. I put it on charge for a few hours and plugged it in to see if I could get it to start with no luck. Lights, radio and alarms all work but it just cant get it to turn over.

I've ordered a gel Westco after deciding I can do without somethings for a few weeks. Fingers crossed I get it by sat as I'm on holiday from Monday.

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I got a new Westco battery a month ago, and its definitely a good battery.

a bit of an update on my way out to work I tried firing it up... and it did. Not sure why over the space of a night it went from only working on aux stuff and a weak crank to seemingly fine. I didn't drive it as I didn't want to risk being stranded at work but in the afternoon when I got home I let it idle for 15mins. Maybe a combo of my short commute, the cold and its age finally did it in.

this seems to be what the car has now Link looks like a past owner went for the cheap option so I'm not shocked its become a door stop after what I'm guessing is about 4-5 years looking at its condition

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Originally Posted by kung fu jesus

This reminds me of something related...I used to forget to turn off my lights (parking/fogs) when a few times and killed a new battery in short order. There was a mod I did on mine that helped me remember to turn off my lights. It was just a 12v buzzer, two wires. one went in the cabin fuse box and the other was somewhere else. If I shut the car off with the lights on, it would sound. I'll see if I can find that somewhere. and post it up.

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I ran a wire from the back of my dimmer switch to the stock buzzer. It sounds if I open the door with the lights on. :)

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Originally Posted by Martin

I ran a wire from the back of my dimmer switch to the stock buzzer. It sounds if I open the door with the lights on. :)

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that I'd like to do just seems like such an easy mod.. just want to change the buzzer to something that does not make me want to scream.

oh a photo comparison of old VS new
http://i41.tinypic.com/2nbvrtj.jpg

I have a westco and it works fine!

If you still have problems after the new battery make sure to check your battery cables really well. A loose/dirty/damaged ground wire can cause havoc on an cranking system. Also make sure your power cable is in good condition and your battery ends are clean and tight fitting.

I've had the WestCo in for a few days and I've done around 100 miles all good so far. I did clean the contracts to be sure it wasn't them behind the problems. The wires looked fine from what I can see.