Battery jumpstart carelessness

It's been awhile since I had the BRG out and today seemed like a good day for the top down. The battery has a draw on it somewhere (likely from the phantom security system) so I grabbed the jumper cables and hooked it up to my truck. After jumpstarting untold number of vehicles since the 70s, I didn't pay attention and put them on reversed. Sparks and smoldering...

Here's the deal, the main fuse didn't blow and after I hooked up the cables the RIGHT way, it started right up. After a short drive around the neighborhood I parked it and attempted to restart. Nothing. Now I've got the charger on (polarity checked!)::Banana:: But I'm wondering what I managed to do. For one thing, the main fuse is 100A, not 80. Did I fry my battery? Alternator?

I won't even mention how yesterday I changed the oil on the 10AE and managed to pour almost a full quart through before I realized I hadn't put the drainplug back yet. Just not a good Miata maintenance weekend...::Rant::

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Originally Posted by cam76034

I won't even mention how yesterday I changed the oil on the 10AE and managed to pour almost a full quart through before I realized I hadn't put the drainplug back yet. Just not a good Miata maintenance weekend...::Rant::

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http://img.allposters.com/6/LRG/10/1066/ULOL000Z.jpg

If you can get it started again, measure the battery voltage with the car running. If it's ~14V, then the alternator should be good. I'm guessing you fried the battery since the car ran, then wouldn't restart...

Sounds like the battery is toast. The alternator is probably good if you didn't go dead while driving it.

I would bet the battery is bad and not the alternator. Lead acid batteries don't take kindly to being left dead too long. That and as Creek mentioned, the car would have died after a while of running because there would not be anything to power the coils for the spark. Unless you have a diesel model Miata. ;)

Battery is probably toast. Put it on a tester @ autozone and see if it has some dead cells.

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Originally Posted by Radio-Active

I would bet the battery is bad and not the alternator. Lead acid batteries don't take kindly to being left dead too long. That and as Creek mentioned, the car would have died after a while of running because there would not be anything to power the coils for the spark. Unless you have a diesel model Miata. ;)

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Miata's have glass-mat batteries, not lead acid. Don't know if that makes a difference. I know glass-mat take special care to recharge when dead.

If you haven't already, change the main fuse back to an 80 amp. I know first hand the 80 amp would have blown and saved you the trouble you're experiencing. Popped one myself when I accidentally grounded the power cable on the alternator once. I had a customer who popped that fuse when he crossed battery terminals. Strangely he also blew the radio fuse which also knocked out his OBDII port. We couldn't tell the radio fuse was blown until we actually pulled it a second time so you might want to pull each fuse and inspect closely.

I don't think the AGM batteries (which are just a special form of lead-acid battery) in the Miata take a charge very rapidly. I've had your experience (well, minus the reversed polarity thing) before - jump the dead battery, drive the car a short while, no start. With an overnight slow charge, all was well. Hope it works for you also...

After a trickle charge overnight it started right up. Drove around, turned it off, started right back up again. I'm not sure how long you can drive off of the battery charge if the alternator isn't working and short of taking it out for bench testing, I guess I'll just wait and see. On older cars one could disconnect the battery while running but I know that nowadays that isn't a good idea... Ditto on the radio not working, although the fuse box doesn't specify which one is for the radio. Guess I'll pull them one by one and hope at least one fuse did its job.

Still can't believe I did that.

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Originally Posted by trickyrix

If you can get it started again, measure the battery voltage with the car running. If it's ~14V, then the alternator should be good.

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Did you measure the voltage when it is running?

Depends on what you have running in the car. If you don't have anything running in the car and all the battery is powering is the ignition then it'll run for a while before dieing.

It shows 12 V whether its running or not, at least measured at the battery itself. Am I missing something on testing the alternator with the voltmeter, IOTW can I test its output while in the car running?

I guess I could start it up, turn on the heater and the lights and leave it running in the driveway.

::Banana::

Yea! Alternator is charging, just like in the video link. Thanks POS.

All fuses check out okay so I'm guessing the NA radio has an internal fuse ?

It's too bad Mazda didn't include a volt meter or ammeter with the rest of the instrument cluster. There are volt meters availalbe that plug into the cigarette lighter socket (or power outlet). I don't have my MX-5 with me today, so I don't know off hand the manufacturer that made mine. But it offers a numeric digital read out and 3 led's (red, yellow and green). It'll tell you the battery condition and alternator read out so that, hopefully, you won't be surprised.