charging problems

Hello,

Starting late last summer the original battery in my '91 Miata would run down after a week or so of use, so I got in the habit of leaving it hooked up to my trickle charger when I wasn't using it. I replaced the battery, but ran that one down probably three or four times (after the

2nd time or so it was obvious that I had a charging problem). What's kind of strange is that I tested the output of the alternator using a voltmeter across the battery terminals and it seemed pretty good, I can't recall exactly, but think that it was at around 14 V.

I had the alternator checked at my local auto parts store and it wasn't up to snuff. He said that the alternator could be generating 14 VDC, but not putting out enough current to charge the battery, which seemed plausible. I bought a rebuilt alternator and installed it. Then after several months of use my battery was dead again. I put it on my trickle charger for a couple days and according to the lights on the charger it seemed okay.

I had the same autoparts store that I bought the alternator from check out the output and it was good. The device that he had purported to measure the output in watts from the battery, so it should be a better test than just measuring the voltage.

The dude at the autoparts store noticed that my battery terminal was a bit loose on the post, and felt that that might be my problem. You can twist it back and forth by hand, but you can't pull it straight off by hand, not ideal, but I'd think adequate as far as electrical contact goes.

I ran a couple wires from the battery terminals into the passenger compartment and then drove around with the multimeter hooked into these. The battery checked out at 12.5 V with everything turned off. When the engine was running it wa s over 14 V, generally at or above 14.2 V.

I went for a 200 mile trip and it was staying at at or above 14.2 V, when I stopped it was up to 12.75 V. The next morning it was around

12.75 V. When I drove the 200 miles home the voltage was staying at at or above 14.2 V, when I stopped it was up to 12.75 V again. I thought that maybe by letting the car sit for weeks at a time and driving it for short trips mostly (I use a motorcycle as my main transportation), I had run down the battery.

I then let my girlfriend use my car for a week and the battery ran down again. Is my battery toast from having been run down several times and that's the reason that it won't hold a charge.

The way that I have the battery charger and voltmeter hooked up is that I have a couple ring connectors under the nuts on the battery terminal clamp bolts and a short wire crimped to the ring connector. On the other end of the short length of wire I have a female bullet connector crimped onto. So when I connect my voltmeter, trickle charger, etc. I'm going through the battery terminals.

Any ideas?

-Eric

Reply to
Eric Goforth
Loading thread data ...

Volts don't charge the battery, amps do. Years ago they used to put ammeters in cars which was a useful gauge. Then they put in voltmeters which are pretty much worthless. Same with testing a battery - if you don't put a draw on it, the voltage means nothing. Charging a Miata battery incorrectly pretty much toasts it forever. It needs to be slow charged (2 amps) back to full capacity. As to why it ran down, that could certainly be the alternator. Do you have an aftermarket alarm system on the car - they usually create drawdown problems with dark current. Also, your lose terminal is creating problems you can't readily see. If it's lose enough to twish, then it's lose enough to allow corrosion to build up between it and the terminal. Sounds like you should invest in some new terminal ends, clean the posts well and put the connectors on so that you can't twist them. At this point, you may also need yet another battery.

Tom

92 Red

Eric Goforth wrote:

Reply to
Tom Howlin

Can I ask you why in the world you would leave it like that? This might or might not be causing the problem, but the 1 minute that would take to fix it would rule it out as the cause.

Having said that, my bet is that you have something draining the battery. It's not a charging problem.

Running a battery down several times can definitely kill a battery.

Reply to
Marcio Watanabe

Killing even a new battery "flat" is not good, and, in my experience, causes permanent damage - it will never work as good as it should.

Sounds to me like you are familiar with using a DVM - voltages all sound normal. My guess is you have something draining the battery. Perhaps your girlfriend is doing something different like leaving the radio on and the motor off. Another thing to try is pull one wire off the battery and measure the current draw with an amp meter while the vehicle is just sitting there, everything turned off. There should be almost zero miliamps. DO NOT try starting your car with the amp meter (DVM?) hooked up as that will fry the meter.

Reply to
Mike

MotorsForum website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.