940: Battery test

[writer100]

My wife and i are going on a long trip. Being able to start the car is essential my domestic happiness.

Can I use a multimeter to test battery strength, and what are the numbers I should get?

Thanks.

[billofdurham]

You can test a battery with a multimeter. Set it to the 20 volt range. When testing with a multimeter you should try to touch the battery post rather than the lead as this gives greater accuracy. It is not necessary to disconnect the battery from the car first.

The first test is measuring open circuit voltage which determines the state of charge of your battery. To allow surface charge to dissipate and enable a more accurate reading open circuit voltage is measured 12 hours after stopping your car and is carried out with the engine not running. The figures you should get are:

For standard or low maintenance 12V car battery:

Open circuit voltage State of charge
12.65V 100% (fully charged)
12.45V 75%
12.24V 50%
12.06V 25%
11.89V 0% (dead car battery)

For maintenance free or Gel Cell 12V car battery:

Open circuit voltage State of charge
12.77V 100% (fully charged)
12.54V 75%
12.29V 50%
11.99V 25%
11.75V 0% (dead car battery)

Car battery voltages above are for measurements with battery temperature 70°F (21°C). When the temperature is lower, voltages will also be slightly lower. For example, at 30°F (-1°C) for a maintenance free car battery 50% charge is at 12.24V.

A more accurate check on a non-sealed battery can be made with a hydrometer which measures the concentration of the sulphuric acid in the battery cell by cell. The figures working this way should read:

Specific gravity State of charge
1.269 100% (fully charged)
1.229 75%
1.194 50%
1.159 25%
1.124 0% (dead car battery)

These numbers are for measurements with battery temperature 70°F (21°C). When the temperature is lower, specific gravity will be slightly higher. For example, at 30°F (-1°C) 50% charge is at 1.210.

If two or more cells are lower than the others your battery is slowly dying.

Some sealed maintenance free car batteries have a small built-in hydrometer called Magic Eye with colored readings indicating a good condition (green), a condition when charge is needed and when it is time to replace the battery.

After testing the battery for open circuit voltage test the output of the alternator. Using the multimeter on the 20 volt scale start the engine, touch the meter's leads to the battery posts, if possible, and you should get a reading about the 14 volts mark. Lower than 13 volts, the alternator may not be charging properly. Over 15 volts, the alternator is over-charging and will kill the battery.

Bill.

[writer100]

12 hours after stopping your car

In other words, when the engine is cold, correct? 12 hours or more?

Also, I assume I conduct the test from battery positive post to negative post? Thanks.

[billofdurham]

After 12 hours the engine should be cold. If not you've got a problem. However, the wait is not for the engine to go cold. When the battery has been charged, by alternator or by an external battery charger a 'surface charge' builds up and this can take some time to dissipate. Testing with a multimeter before it does dissipate will give you a false reading. Some shops can test batteries immediately the car comes in but they use a purpose made battery tester not a multimeter.

When I test mine I do it after the car has been standing all night. It only takes a couple of minutes so you wouldn't need to get out of bed that much earlier. The test on the alternator can be done at any time.

You do use the battery posts as testing through the clamps can give a false reading.

Bill.

[writer100]

Am I correct in assuming that at rest the battery should be at or close to 100% of its charge?

I performed the overnight battery test. Unfortunately, my multimeter is analog, so it is not precise. But it did read close to 13 volts, which I take to be a 100% charge.

Can you generalize the battery numbers you provide above to all cars? Thanks, Bill.

[electech]

Here is my two cents...If this makes sense, great. If not, just go with what Bill said

Yes, the battery should be fully charged at rest, from 12 hours up to around 2 weeks. However, not all batteries will dissipate surface charge in 12 hours. (see below for more)

(As an aside: If the car is left longer than 2 weeks, it can slowly drain. Cars that rest for a month or more should generally be recharged. Some cars drain faster than others, but if it drains faster than 2 weeks it is a problem).

An analog meter can do OK if it is all you have. You do need to check it's accuracy, and make certain it is kept still while making measurements. (Sitting in the engine compartment of a running car can cause enough vibration to give false readings on a low-end meter). You also need to be confident that you can read at least 0.1 or 0.2 volt increments on the meter. If you can't trust the meter, pick up a digital meter...you can usually find them for as little as $15. If the local auto parts store wants $40 for the cheapest, shop around. Try computer parts stores (Micro Center?) or department stores.

The battery numbers Bill gave are general enough to apply to nearly all cars and light trucks. However, you have to be sure you have eliminated the surface charge for the measurement to be valid. A car battery at 13.0 V at rest is not accurate, it still has surface charge.

I suggest turning on the parking lights for 30 seconds to a minute to take off the charge. To see how this works, take measurements before you turn on the lights, then while they are on, then after they go off. You should see something like this:

before: 12.9 volts
during: 12.4 or 12.5 volts
after: 12.6 or 12.7 volts.

If your meter reads 13V after the lights have been turned on, don't trust it. As Bill said, if you read less than 12.6 volts when it was above that before turning on the lights, then the battery is weak or failing.

The important part for your trip is checking the alternator. With the car running, with the A/C and lights off, the car battery should be in the ballpark of 13.5 volts. Some cars will read up as high as 14.4 or so, although I am not used to Volvos doing that. With the lights and the A/C on, the battery should still maintain around 13 volts. If it is around 12.7 to 12.9, it's probably OK...kind of a gray area. If it is less than 12.6 with just lights and A/C on, it is probably heading for trouble.

Hope this helps!
Bobby

[writer100]

Bobby, thanks for the thorough response. For the record, it didn't show 13 at rest, it was close to 13. And the car had been resting for a couple of days. I still have to do the alternator test.

No way I can read .1 or .2 increments on my multimeter. I guess a digital one is in my future.

Thanks again.

[electech]

One thing I should throw out about digital multimeters, do be careful what setting you use. You are pretty safe in the DC volts category. If you are using the resistance (Ohms, with the Greek Omega sign) or connectivity setting and you run current through it (like testing a battery) you can fry the meter pretty fast. This goes for analog ones too, although some analog meters seem to be more resilient. The digital meter you will know very fast if it gets fried.

Also, be wary of the current (amps) settings, especially with cheaper meters. If you cross a battery in the current setting, it will either blow the internal fuse or spark really bad and melt the lead wires. Cheap meter have cheap fuses and bad things happen when using the amp setting at high current circuits (like testing the battery).

Basically, as long as you stick with the DC volts (or AC volts for household current) you are pretty safe.

By the way, if this is a fun project for you, I wouldn't want to deprive you of it. If it isn't, a lot of auto parts stores will test your battery for free, and some will even charge it for free. (Advance does here)

Stay safe, and have fun on your road trip!

[writer100]

Thanks, Bobby. I have two multimeters: one is a Gardner Bender, and the other is a Suntune from the "80's. I thought that each one was the cat's meow when I got it, but now... I see their shortcomings in their inablity to read voltage accurately.

Yes, I could bring this over to Sears, but a. I get nervous when their personnel are aound my car, and, b., this is a learning experience for me.

I will try to do the alternator test this weekend when I have some time. Thanks for your expertise.

Alex

[writer100]

Okay, I finally had time to do this test. With lights off, engine running, it came in at 14 volts, which from the above, I take to be normal. Thanks.