I've been having an issue with an '03 Toyota Echo. The alternator died (at least that is what I thought) as the battery kept going flat. I found an alternator at a wrecker and fitted it, and I replaced the battery with a new one. Over the past month the battery has completely died again. It's over a year old now, but I'm getting concerned there could be a problem with the voltage regulator causing batteries to overcharge and boil dry.

I've tested it today with a known good battery that was charged up. I drove it for close to an hour just to give it a chance to put some charge in if it needed to and then checked it with a meter. It was still reading 14 volts as though it was still trying to charge.

I am not really much of an electrician so maybe I'm missing something. Once fully charged, shouldn't the alternator stop providing a higher voltage? If it is constantly charging would that kill a battery?

2 Answers 2


The output of an alternator is typically between 13.8-14.2vdc. If you were in the 15vdc or higher range, I'd consider that the alternator is overcharging. It runs continuously at the ~14vdc range to continue to provide all the power the car needs as well as keep the battery charged.

It sounds like to me your new-ish battery may have gone bad. A cell in it could have gone bad, which won't allow it to recharge correctly. This should be under warranty (though I'm assuming warranties in AU work the same as they do here in the US), so should be able to get it replaced, or at the very least load tested to see if this is the case. I am also going to assume most of the major vehicle parts stores in AU are also going to be able to test your alternator for free.

I'd also check to ensure all of your connections are good. These connections are things like to ensure there is no corrosion at the battery terminals, ensure the connections are tight, ensure your ground (earth) at the body is in good shape (tight as well), ensure the connection at the starter and alternator are tight.

The only other area of concern might be if you have some type of energy drain when the ignition is off. If you put any type of aftermarket stereo in the vehicle which may be grounding power in some way, or any other type of non-standard equipment in the vehicle, there might be a problem there as well. This type of problem is a lot harder to diagnose as you have to find the culprit which is causing the issue.

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    I just recently had a 9 month old expensive AGM battery lose a cell. It had never seemed to hold a charge well from the beginning. Getting it warrantied was a major hassle as the manufacturer said it was impossible. Yet, I threw an cheapie battery from the corner shop in there and it's been perfect for a couple months... These things happen sometimes... Apr 27, 2014 at 22:20

Did you try checking the voltage at the battery terminals/alternator while revving the engine? I had a similar problem, did this and found that the volts output by the alternator went up to 18v (at around 3000rpm on a Diesel where redline was 4000rpm). The regulator should keep the voltage at less than 14.2v to prevent cooking the battery.

So all the time I was driving the battery would have been getting 18v.

Then you will know if it is the voltage regulator.

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