Low battery voltage

My 95 Riviera is acting weird again. A couple of times a day my voltage will drop to 10 or 11 volts with the car running sitting at a light. If I rev the engine a little, the voltage will increase. By the time I stop again, the voltage is back to normal. My question is, could a bad battery cause this issue? My battery is not the original. It is a 60 month battery and the 60th month is this coming August. I plan on having the alternator and battery tested but my concern is since it's intermittent, they may test normal. Please help!

10-11v running suggests the alternator/regulator is bad. Should be 13-14.5v running. A bad battery can contribute to a failing alternator, but it sounds like in your case the alternator will soon cause death to the battery. The low voltage suggests the battery is not getting charged. Is the car having any trouble starting? If not, the battery is likely still good.

I have no trouble starting it. Will the alternator test at Auto Zone detect a bad alternator even though the problem is intermittent?

artlock wrote:

My 95 Riviera is acting weird again. A couple of times a day my voltage will drop to 10 or 11 volts with the car running sitting at a light. If I rev the engine a little, the voltage will increase. By the time I stop again, the voltage is back to normal. My question is, could a bad battery cause this issue? My battery is not the original. It is a 60 month battery and the 60th month is this coming August. I plan on having the alternator and battery tested but my concern is since it's intermittent, they may test normal. Please help!

Is your battery the proper vented Group 79?

Yes it is. The original vent tube broke but I have since replaced it.

artlock wrote:

Yes it is. The original vent tube broke but I have since replaced it.

I suggest that you pull the positive cable off the battery and check the voltage. If it is not at least 12.4 try charging the battery. If the voltage does not return to normal (definitely above 12V no load) after charging, then chances are very strong that your battery has a bad cell. Each cell in a wet cell battery generates about 2.1 volts, a little less per cell as the battery wears.

The Delco group 79 batteries were made quite well, not necessarily so for some of the aftermarket ones.

The problem that you are having sounds like due to possibly an internal short one of your cells in the battery (there are 6) is not working right. One dead cell = 10-11 volts on the meter no load. Two dead cells = 8-9 volts on the meter no load and the car also will start to have anomalous electrical problems due to low voltage such as phantom chiming, retained accessory power not working right, etc. During the short condition voltage drops. A little vibration and it temporarily corrects, the voltage goes back up.

This could be in conjunction with a dying alternator. If the alternator does not work properly all the time the battery will discharge too deeply and the cells will suffer internal damage one by one. Does not work properly means alternator does not put out power, or puts out pulsing/wavering power or AC. You can make a quick check of this by measuring voltage at idel. With a well-charged battery your voltage at idle, at the battery or at the power point under the hood should be in the neighborhood of 13.2 or so (call it 12 to 13 v).

Thanks for the info. I will check the voltage this evening. One question, I thought the voltage should be around 14 volts and not 12 - 13 volts?

artlock wrote:

Thanks for the info. I will check the voltage this evening. One question, I thought the voltage should be around 14 volts and not 12 - 13 volts?

The cells in a good battery will show 2.1 to 2.3 volts depending on the specific battery's chemistry. Calcium batteries (such as the delco) run a little high. standard lead acid a little low. Each cell in a wet cell battery generates a little less power per cell as the battery wears.

Voltage from alternators depends on their construction. Most generate more than 12v but have "ripple" meaning the voltage output won't be constant (if you look at the output using a scope), it will sag a bit under load too. You can use electronics to condition the power. From an alternator with conditioned power you can get a solid 12v under load by generating 14v, permitting some sag, and filtering out the ripple.

Remember to pull the cable off before you check voltage, whether at power point or under the backseat. If at the powerpoint voltage is too low then check under backseat, as it means that there may be an issue with some of the car's basic electronics (like the thermistor in the + cable I think) and you won't tell what's what until it's all disconnected.

Ok, I disconnected the battery and the voltage was 12.2 volts. So I guess I will have to order a new alternator.

Maybe... you can remove the alternator (look at it, not a tough job) and test it at a parts store.

Before doing that might want to look at rockauto.com and see what new and reman alts are going for.

Also, if you have a pick-pull-it-yourself near you, you might want to go there and pull an alt. People do not junk their cars for bad starters or alts, so the odds you get a good one are very high. Actually if you get 2 or if the pull is warranted it's about risk free. If you get 2 you can break em downs clean them and build one from the best of the parts. Also, the pick-pull may have a tester-use it.

Thanks for all of the help. The alternator was bad. I replaced it and went ahead and replaced the battery too. It's been three days and no low voltage issues! I love this site!