JC in XTC5

I'm thinking my battery is starting to go. I put a voltmeter on the battery with everything off, and it was only 12.2 and as low as 11.8. I believe a fully charged battery should be at about 13.

I've been having some hard starting problems where the car cranks a bit long before firing, or fires right up on the second try, and I'm thinking the lower than normal voltage is the reason since it could affect the sensor readings and PCM operation during cranking.

I plan to get an Optima Red Top this weekend, but wanted to check if anyone could confirm my suspicions.

HollywoodFRC

Get a Volt Ohm meter. Drive the car to reach normal operating temperature. Park as normal. Hook up the meter and note voltage. Put meter up on lower windshield and gently close the hood to turn off underhood light. Check meter right before bed and in the morning. Voltage should not drop appreciably. I have an Optima Red top and it typically registers 12.54V when I park it and sometimes drops to 12.51 or 12.52 overnight, not much. For three months I've had the Red Top and no problems. Good luck.

Oldvetter

From batterytender.com:

1. How can I tell if my battery is charged or not?

Lead acid batteries are made up of cells. Each cell is approximately 2 volts, so a 12-volt battery has 6 individual cells. It turns out that a fully charged 2-volt cell has a voltage of approximately 2.15 volts. Oddly enough, a fully discharged 2-volt cell has a voltage of 1.9 volts. That’s only a difference of 0.25 volts on each cell from fully charged to fully discharged. So a 12-volt battery will measure at about 12.9 volts when it’s fully charged and about 11.4 volts when it is fully discharged. That’s a total of 1.5 volts that represents the full range of charge on a 12-volt battery. To make a good guess at how much charge your battery has left, you can assign a percentage of charge remaining that is directly proportional to the battery voltage. Let’s see how we can do that. If the battery voltage is 12.15 volts, how much charge is left? Beginning with 11.4 volts representing no charge or 0% charge available, subtract 11.4 volts from the voltage that you read. So 12.15 – 11.4 = 0.75 volts. Since there are only 1.5 volts above 11.4 volts that represents the full range of charge, we can divide the difference that we just calculated by 1.5 volts to get the percentage of charge remaining. 0.75 volts / 1.5 volts = 0.5 or when expressed as a percentage, multiply by 100 and get 50%. Here’s the procedure written as a formula that is applicable to 12 Volt Batteries:

OPEN CIRCUIT BATTERY STATE OF CHARGE CALCULATION

% Charge = SOC

% Charge = ((Measured Battery Voltage – 11.4 volts) / 1.5 volts) x 100

Equation 1
That seems easy enough. So what’s the catch? In order for this formula to work, the battery must be in a rest state. In other words, the battery should not be supplying power to any type of load. The experts say that the battery should remain at rest for at least 24 hours to get an accurate measurement, but in a pinch a couple of hours are good enough to make a reasonable guess. Even if the battery is connected to a load, as long as the load current is less than 1% of the battery capacity in amp-hours, then this method is probably good enough in most cases. It’s all a matter of how accurate you want to be. If you’re a scientist or engineer trying to develop a battery powered product, then you probably want a more accurate measurement than if you’re going fishing for the weekend and you just want to know if you need to take the time to charge your battery before you use it. There is one more thing to keep in mind. The only way to be absolutely sure that your battery is fully charged is to do a load test. It is best to have the battery dealer do this for you. We only mention it here because it is possible for a battery to indicate a good voltage, but then immediately when you try to use it, it acts like it’s dead. This doesn’t happen very often, but it’s good to know that it is a possibility.

CraZee ZO6

Ive seen mine vary from 13.8 to 14.1.......

JC in XTC5

That's the alternator voltage with the car running. I'm looking for the battery voltage with the car off.

I'll re-measure tomorrow to see if it drops below 12 overnight as I suspect.

GrayC5

Taking a voltage reading at the battery terminals will only tell you the state of charge of the battery. In order to get an accurate voltage reading, you need to use an accurate digital volt-ohm meter (DVOM). Too, you will first need to remove any surface charge of the battery and the temperature of the electrolyte will need to be taken into consideration. To completely determine the condition of the battery, other tests are required. The following link contains some very good information on the proper way to test an automotive battery. A State-of-Charge chart is also included there:

CAR BATTERY FREQUENTLY ASKED QUESTIONS

HTH

JC in XTC5

Yes, I've been using a DVOM to take my readings. This morning it was at 12.14V after sitting overnight and leaving the hood unlatched so I wouldn't activate any load (keyless) before the reading, which is less than 50% from the chart on the link you posted. I drove to work this morning (~ 20 minutes) and now 3 hours later I'm taking a reading (90F outdoor temp). Battery reads 12.30V, which is still barely above 50%.

I must mention that this battery as the AC Delco AGM (78P-7YR) that was installed about 5 years ago. The sides of the case are bulging a bit which I understand is normal for AGMs due to the positive pressure they operate under.

I had put my battery tender on it earlier in the week to see if it would fully charge, and the next morning my tender was completely dead (don't know if it's just coincidence or another indication that the battery is going). Later that day the battery was at 12.50V which is still not great assuming the tender was able to get some charging done before it died.

I guess the safest thing to do is just replace it at this point.

patton

5 yrs old ? that batt was going down hill 2 yrs ago may be over kill but i pop a new batt in ever 3 to 4 yrs . i like top of the line interstates

Tact

I haven't seen many batteries lasting much longer than yours. Also with your readings it appears the battery is on it's last leg. I've always tested my batteries and felt assured they were OK with a reading near 12.6. My battery is only a couple of years old and only shows a reading of 12.3, but it sits a long time between drives. I'm thinking of switching to a jell battery , mainly for piece of mind.

JC in XTC5

Thanks Guys.

Well I just bought an Exide Orbital battery today. They're supposed to be great AGM batteries that can withstand deep cycle use. $129 at NTB.