Battery voltages - is mine dead?

[landspeed]

I've noticed for ages (since I've had my car) that if the engine is switched off,
the headlights go a lot dimmer, and the windscreen wipers move at a significantly reduced speed. Recently I drove another car, and it didn't do this (actually it did, just very slightly). I had originally assumed it was just the difference between the battery and alternator voltages, but now have started to wonder if the battery is dead.

Today I've just tested my car battery, and got the following results (noting this is a day after a long drive, and I haven't done anything to drain down the battery since then):

Resting voltage - 12.24 volts.
Voltage with fans on 50%, headlights on - 11.4 volts (after these had been on for only 10 seconds)
Voltage after the above had been on for 30 seconds, then switched off : 11.45v. It was recovering at about 0.01v every 15 seconds.

The liquid level in the battery is OK, a tiny bit low in one of the cells but the plates are still well submerged. The plates look like they have a fine light-grey powder on then. I've had the car for about 2 years, and done a lot of driving in that time. The battery wasn't new when I got the car, although it is a 4-year lifespan one.

I guess this is a really basic question, but I've never looked much at batteries, so...

(1) Does this mean my battery has had it? (I'm tempted to buy a new battery this weekend)
(2) Would this create much extra drag on the alternator, to worsen my fuel economy?

Hope someone can help

[JanGeo]

Measure the battery voltage again using the same technique you just used but this time put the meter contacts directly on both posts of the battery and not the wires or terminals. You could have a dead cell or a poor battery connection.

[landspeed]

I actually jabbed the meters into the two lead connectors on the battery, so that shiny lead could be seen when I removed them. The voltmeter itself works fine - I tested it on another lead-acid (AGM) battery which I have to make my electric bike this summer .

So, with those results it sounds like a dead cell? (One of the cells had 'bits' near the top of the water, although the plates didn't look like they had collapsed.

[GasSavers_Brock]

Most likely it is a dying battery, but it could mean your alternator isn't charging the battery fully either. After you drive it somewhere on a normal trip check the voltage right at the battery with the car idling. It should be 14.0v to 14.4v If it is I would put money on the battery being on it's last legs. How old is the battery?

[JanGeo]

I mean to make sure the meter was on the battery terminals - which is what you did - so that you didn't measure any battery terminal connector losses due to loads. Yeah is sounds like if some of the fluid levels are different then you may have a weak cell. Best to run a charger on it and get all the cells bubbling - this charges completely all the cells - a little over charging will not hurt the battery and is how you charge up the weaker cells. If a single cell or two does not bubble or one or two bubble a lot then you have some weak cells. And of course as Brock says check the battery voltage with the car running to make sure the alternator is putting out proper voltage to charge it in the first place.

[landspeed]

The alternator charges well - when the car is running it is 14.4 volts and when engine-off coasting, the lights go really dim. I think the battery is most likely dead/dying, and I'm just going to buy a new one. I might try to recondition the battery with the charging method described above - but I think it is a good few years old. If I can get some life out of it, it will make a nice backup, or a 'test' battery when I finally get the electric drive-assist motor in place

Edit : my alternator belt is a tiny bit loose, but, this is not a problem (as the voltages remain high, the lights bright, etc).

If I drive fast through a very deep flood across the road, it might slip for 2-3 seconds, and the lights go very dim. After engine-off coasting, it very often slips for quite a while, suggesting that the battery is putting quite a load on the alternator.

With the above said, I will again note that it doesn't normally slip, the lights remain bright, etc

[zpiloto]

I replaced my battery last week. Most auto parts places will check it free. Mine would do the same thing. After shutdown the voltage would go to 11.8-12 volts within 30 second. Now when I get in it the next morning it's sitting around 12.4 volts with the new battery. Driving it on a regular bases kept it charged enough to start ok and not give much indication of a problem. I did notice on the SG that the volts varied quite a bit between 13.1-13.6 while running. Had the battery tested since I was new to the car and it was toast. I wish that I would have turned the core in later to do some test runs but did not think about it. My worst FE on my commute after changing the battery has been better then the best before. Nothing scientific by any means but I would wag maybe 3-5% increase.

[JanGeo]

Yeah go for a new battery - any EOC loading will quickly kill the remaining capacity of the battery. Be good to put a slow charger on the new one also because after a good long trickle charge the battery will have a very very low self discharge rate.

[Lug_Nut]

Originally Posted by landspeed

Voltage after the above had been on for 30 seconds, then switched off : 11.45v.

A drop in static (resting) voltage from 12.2 to 11.5 with just 30 seconds of headlamp usage indicates the battery reserve capacity is too small. Age alone may have caused this reserve capacity drop.

Each cell of a lead/acid battery has a static voltage of about 2.2 volts, so a six cell battery should have had closer to 13.2 V, not the 12.2 you measured. Place a meter lead on one post and dip the other in the electrolyte. You should see about a 2.2 volt. Dip the meter's lead in the next cell's electrolyte. The meter should read about 4.4 V. There should be an additional 2.2 V meter increase as each additional cell in the series of six cells is added, culminating in the full voltage from post to post. Any cell not adding that 2.2 volts is a dead cell.

Note: These per cell voltages are valid for lead/acid batteries. Other materials have different per cell voltages.

[MetroMPG]

Lug Nut: that's an interesting technique for testing cells. Hadn't heard of it before. (Filed in my mental tool kit.)