Is there any way to diagnose a faulty battery using a multimeter.

[Sharky]

I think that my current battery is about to die and perhaps it's about time since it's almost 5 years old. The other day it was raining cats and dogs so I had the heater, both windshield wipers, lights and rear defogger and the radio on. I got home and didn't use the car for 36 hours. When I tried to start it the battery was dead. I managed to get it started by pushing it and I've haven't had any problems since then (but then again I haven't demanded so much from the battery since that day).

Now, I'm almost sure that it's the battery but I want to be absolutely sure as I've already had occasions in which I was about to change it and it turned out to be a problem with the alternator. So I want to be sure as I'm on a very tight budget at the moment. I have a multimeter and I was thinking that perhaps there was a way to diagnose the battery that way.

BTW, the battery has enough water. I've also been having the following problem. When starting the car with the lights on nothing happens as I turn the switch. Not even does the engine try to start. I have to turn everything off and the start it. Hasn't happened always but many times.

Help will be greatly appreciated

[thebear]

Originally posted by Sharky
I think that my current battery is about to die and perhaps it's about time since it's almost 5 years old.
BTW, the battery has enough water. I've also been having the following problem. When starting the car with the lights on nothing happens as I turn the switch. Not even does the engine try to start. I have to turn everything off and the start it. Hasn't happened always but many times.

Help will be greatly appreciated

A fully charged battery should read at least 14vdc. You can check this with your voltmeter (assuming it is digital or has a scale that will allow reading at less than 1v graduations.) Let the car run at a fast idle with radio/lights/heater off for at least two minutes before shutting down for the day/night and check that the voltage exceeds 14vdc. If not, you could have a loose alternator belt, bad alternator or bad cell(s) in the battery. Another way to diagnose the battery is with a Hygrometer (looks like an eye dropper with little balls that float and indicate the charge state of each cell/local garage should do that for free). hrug: Shut down and immeditely check the voltage. If it has dropped to 12-13vdc you have a bad battery OR another electrical malady that is draining the battery while not in use. Check the voltage again in the morning before starting the car as it should be the same as the night before (assuming that the temperature has not gone down more than 3 or 4°C). Once again, there should be no less than ½v loss. Another item to evaluate is the condition of the the cables at the battery terminals (should be free of corrosion). If they are `fuzzy' or have `crumbly' deposits, carefully (so as not to touch the wrench on another part of the car body) undo them one at a time, scrape (old toothbrush/sandpaper?)the crud off the post and the terminal, coat the whole business with a drop of any kind of oil (from dipstick will do) and re-assemble.

Sorry that I can't look over your shoulder, but the Hygrometer will probably tell the tale.

[Dave Brand]

Simple voltage checks will tell part of the story. A fully-charged battery should give a voltage reading around 12.7 (NOT 14!!!!). Anything below about 12.2 indicates a flat battery. Voltage radings will be lower in cold weather. With the engine running, you should see around 14 volts across the battery terminals, which indicates that the alternator is producing the required voltage -current's another matter! You do also need to check the battery under load - battery suppliers have testers which will do this.

A hydrometer (NOT hygrometer ) is a useful means of checking state of charge & will also show up defective cells.

In your situation, the first thing to do is charge the battery - if it's been completely flat the alternator may not charge it fully; your problem may be nothing more than that - on the other hand, five years is a reasonable life for a battery.

[HiRich]

From practical experience (good old Fiat!), I agree with Dave:

- A well charged battery should show a "high-twelves" voltage with no draw. However, it may still be knackered with the voltage dropping severely when under load.
- The turkey-baster hydrometer gives an indication, though I'm a little suspicious of how much they really tell you.

However, does anyone know what voltage a good battery will deliver when under starting load (so you can check it accurately while a mate tries to start the car)?

Whilst it could just be that your battery has reached the end of the road, as the bear says it could also be a weak alternator (check voltage with the engine running), corroded cables/connections, or your winter usage isn't giving enough time for the battery to recover. Or any combination.
It's also possible that you have a sluggish starter motor (although your symptoms don't fit)

Solutions:
- As Dave says, give it a charge straight away. Note how long it takes to reach fully charged, and recheck the voltage afterwards (immediately and a few hours later). If it still hasn't recovered to 12.7-12.9V, it's probably dying.
- Can you switch off any draw (e.g. the alarm)?
- Granville BatAid tablets (couple of quid from Halfords) appear to help, if the battery is recoverable.
- Keep the water topped up, connections waterproofed, etc.
- If the car's outside (and in the sun), I've found a solar charger very helpful.

It does sound like the existing battery has reached the end of the road (trying to start with lights on implies this). The nagging doubt is whether there are other problems as the root cause.

[GTRMagic]

I have seen the battery in my Magna hit high 13V when the engine in running, suggesting the alternator is pumping that much juice back into it.

I had noticed an issue with the power supply recently, when the headlights are clearly dim, and when the engine is revved, the lights get brighter.... the battery wont hold the charge nicely....

... its 4 and a bit years old, so will be recycled into something less useful when it is changed soon...

[graham bahr]

a properly charged battery will have a voltage of 12.5v off load and no less than 9.9 under cranking load, to check you'll have to disable the ignition system and meassure the voltage while you crank the starter motor.

[graham bahr]

dont forget guys if you check the battery voltage when the engines running like a lot of you seem to be suggesting, your actually checking the cars voltage which should be 13.5 to 14.5 volts as thats the charging voltage of most alternators.

[HiRich]

So to summarise, if you measure across the battery terminals:

13.5-14.5 Volts: With engine running. Indicates that the alternator & charging system are working OK.
12.5-12.7 Volts: With battery disconnected/absolutely no draw/engine not running. Indicates the battery is healthy, but not a guarantee.
9.9 Volts: Cranking (but with plugs disconnected so engine doesn't start). Indicates the battery is healthy.

Meet 2 & 3, and it looks like the battery is healthy. Is your starter sluggish or cables corroded?
Fail 2 & 3: try charging the battery, but it may be at death's door. If it passes after charging but then fails again after a few days, you may have an alternator problem, or the battery is duff (and unable to retain its charge)
Fail 1: you may have an alternator problem, or possibly earth/cable problems?

Is that agreed?

[Dave Brand]

Originally Posted by HiRich

Is that agreed?

Sounds OK to me.

[Sharky]

Thanks guys for the help. i haven't been able to try it out but I think that I'll do so no later than tomorrow. However, I hope my cheap multimeter works. It's analog and let's just say that for about 2 pound you don't get the best quality.

BTW, the other day I turned off the car while we unloaded some stuff. When I was about to start it again the cooling fan was running and the car failed to start. But once the fan stopped the car started.

Another question. I think that the current battery was pretty good. Most of the people I've known have to replace their batteries every 2 or 3 years but this one has lasted almost 5. It's a Varta and I'm planning on getting another one as soon as this one dies but it's a bit more expensive. My dad says that it doesn't matter because most batteries are pretty much the same so it wouldn't be worth spending more money on a "good brand". Is this true?

[graham bahr]

i'd agree with the sum up on voltages,

if you have starting problems its always worth doing a volt drop check, put your multi metter on volts attach one lead to a clean earth point on the engine, and the other to the neg terminal on the battery ( use the post itself not the termal connector) and check the voltage reading it should be just about nothing , but anything near 0.5 volt or above says you have a bad connection, by moving the lead that was cliped to the engine around, to say the earth strap then onto the terminal its self you can work out where the bad connection is, quite often on a modern car a bad connection causing a volt drop will drain a battery even if every thing is switched off ( this is beacause in most modern cars lots of ecu's are powered up even with the car turned off)

you can use the same volt drop technique on the positive side of things to check that your starter motor is getting its full battery voltage and your alternator has good connections.

as an employee of a major breakdown association, you would be amazed how many nearly new cars i go to that have bettery problems due to bad connections rather than actual faulty components.

most batteries will last 3 years after that.......

for what its worth if you have a calcium based battery and you run it dead flat, dont expect your car to recharge it, a totally flat calcium battery will require bench charging at 18 volts to bring it back to life.

[norman-normal]

I'm a retired mech&shop owner. It sounds like croded terminals: they will inhibit the flow of currernt much more during damp weather,much more, than in dry conditions. Also you will save your self some time and trouble if you mix a tea spoon of baking soda&8 o/z water and dip, or pour on the contacts; they will look like new in seconds.
You can also improve engine performance as well as starting, by running a couple of 10ga copper wires from the bat' neg term to the engine head and block.
Go to an electronics shop and buy a tube of anti-oxident grease, to coat all terminals.
You shoulden't have this problem again; if you do , it's the battery or alternator.