Batteries and multimeters

Gotta disagree with that. A five quid mutleymeter and a grasp of basic DC electrics not extending beyond GCSE Electronics can get you a bloody long way.

Did you try a motor factor or your local battery specialist?

Not a lover of Haynes, but their Automotive Electrics Manual is one of the best around - covers basics as well as reasonably advanced stuff, and is an easy read.

That's an interesting way of looking at it! Are you sure it's only

That's the maximum the battery can supply under good conditions. The actual current depends on the temperature of the oil, etc. But rarely more than 300 amps. And it's the current and time it's delivered for that determines how long it will take to re-charge the battery. Most modern cars start pretty quickly.

There are two problems with that:

Firstly it won't diagnose most charge problems. You can measure the output voltage, but measuring the high currents involved needs access to test gear that represents more investment than most average drivers will pay for a lifetime of quick tests.

Secondly, car electrics aren't digital. Most faults are intermittent, partial and damned annoying! I've been caught myself - grabbed the nearest meter (a Fluke digital) and measured that the relevant circuit was "live", then spent 3 days (and a lot of upholstery stripping) trying to find a fault that I'd already found. Sticking a two-quid test lamp on it instead of a DVM showed me that the might be volts present, but there weren't any milliamps! The fault was were I'd looked in the first place all along, but it was a high impedance fault, not an open circuit.

A voltmeter and an ohmmeter are great, especially if the ohmmeter does buzz tests and diode buzzes too. A meter with a 10A range is good too (and they're cheaper than fuses for the Fluke). Beyond that though, there's not much a DVM offers for car electrics that is really useful. I've got all this test kit, but the stuff I still use most of the time on cars is a test bulb, an old GPO continuity buzzer and a "hold it nearby" 20 / 100A ammeter.

(and yes, I know digital electronics aren't digital either)

Give the battery a overnight charge. It won't make up for lost charge on a short journey like that. Not even a long journey. Battery needs a good 10 hours to get it back to its best capacity.

-- bucket

Lead acid batteries don't respond like that. They need time to get the electro-chemical reaction going.

-- bucket

One place I work at is less than 4 miles away - through town. And in the winter I'll go and come back in the dark. 5 days a week. Never had a problem with a flat battery due to not enough charge on either car.

Err, what?

Yes, they may be slow to respond to load demand under certain conditions, but if it starts, they have responded. And in any case whatsoever, the battery, if it has started the car, is in a condition to accept charge, at least at a low rate.

Charging.

-- bucket

Eggplant.

Then you have a good battery, alternator and everything is in tip top condition and there is no need to charge it up. However, the op does appear to have a minor problem that is concerning him and would surely benefit from a full charge before he starts pissing about with making measurements.

-- bucket

I read in various threads that a car needs to be run at a fast idle for optimum charging, but whenever I've checked mine at idle I get 14.5v (no load) and drops to about 14v with lights, blower & heated rear window. But when I increase the revs it doesn't seem to make any difference to the battery volts, in fact increasing the revs actually drops the voltage slightly? Does that mean my charging system works just as well when stationary in heavy traffic than it would driven at speed on a motorway?

ROFLMAO. I have never heard so much bollocks.

They're both and mostly digital now.

No, the fault was in the person using the equipment. A DVM in the hands of someone with half a clue would have given you the answer.

Clue genius, Multimeters can do all three. Try buying decent stuff.

Only because you lack the knowledge of how to use it properly.

That's because you're thick as shit. See below.

ROFLMFAO. Priceless.

As well as that, Ian is forgetting that although his alternator may chuck out 40 AMPs, a great deal of that will be running the fuel injection, fuel pump and ignition system.

Especially when the ignition, engine management and fuel pump will be drawing more than that.

No, he's not.

Notice the charging fugure I assumed.

In fact, a 4 mile trip, taking 6 minutes, the average charging current needed is 1.5A.

Neglecting the fact that one of these trips a day is likely to not have the headlights on, which dwarves the amount needed to recharge the battery from cold start.

Generally, batteries and alternators either work or they don't. There's not much in between these days.