Yes, it could. However, I didn't have a poor earth contact. Just connecting a 2A charger for a few seconds was enough to make the engine start. After that it was trouble-free for at least a year. Something else was going on.
John
Not quite - tungsten take a lot more current at start up.
Ahem, it has been my experience with lead acid batteries that a lot of the deadness is due to corrosion on earths, terminals and connections. if the guy who tested it has made sure all of these were good, which I'd hope he would as a first test before testing the battery, then it amy well be it will be fine until things work loose or corrode again. I also note that according to people I know most modern cars have a smaller battery than they would expect, so it could easily be that using ordinary lights would run it down. I'm assuming the hazards are the usual filament bulbs and bimetal strip switcher of yore. Brian
Weatherlawyer has brought this to us :
Some people have a tendency to make it up as they go along.
Extra resistance in a circuit will REDUCE the current drawn by the circuit.
A running alternator will put an initial large lump of charge into a low battery, but that will rapidly tail off to a very low value. It takes a long time running, to put a full charge back into a battery. Bringing a battery back to a full charge, is best done with a 24 hour session on a modern charger.
Assuming a 50/50 duty cycle on the flashers and 21w per lamp, that works out at a steady discharge of 42w, or roughly 3.5amps or 3.5aH - left on for 1 hour. Call it 3.0aH discharge over 3/4 hour.
Call it a fully charged 60aH battery, suggests it should be able to cope with that load for 20 hours to complete discharge.
A worn out battery will have much less aH capacity.
snipped-for-privacy@gmail.com was thinking very hard :
The more likely scenario is that the charge current going into the battery, warmed the battery up, allowing it to produce a higher voltage.
OG wrote on 12/01/2015 :
Whilst that would be possible, the battery saver circuits on modern cars, only turn off interior lights if they are left on. The rest of the electronics gradually fall away in current demand anyway as time passes and the car is left undisturbed. Some take upto 30 minutes others just a few minutes.
My interior lights switch off after 20 minutes, the rest of the car goes to sleep over the first couple of minutes.
I have never tried it, but I suspect the exterior lighting module will switch off the lights as the voltage falls anyway, but I doubt the car would then have enough in the battery to start the engine.
The switching circuit draws almost nothing, the function of switching the lamps on with a (relatively) cold filament, will cause a larger current draw than normal, but only for a fraction of a second until the filament comes up to temperature.
Martin Brown formulated on Monday :
Isn't the whole point of parking with your hazard lights on, one of being able to leave the car parked on double yellow lines with the engine off?
In message , Harry Bloomfield writes
I doubt if a few seconds 2A charge would have time to warm the battery. However, it might possibly 'heal' an internal high impedance bad connection. There are also reports of a similar effect by switching all the vehicle lights on for a few seconds.
Andrew Gabriel has brought this to us :
Fitting an ammeter suggests it had a dynamo rather than an alternator.
Alternators used a battery voltage meter, because the currents involved were so large.
A modern alternator can produce current in the 110 to 150 amp range - They have to be able to do that, to provide for all of the necessities of modern cars, like heated wind-screens, heated seats, electric cooling fans and a/c systems.
They are voltage limited and battery voltages when on charge rise rapidly from flat. Hence the charge current will fall off rapidly in the first few minutes, which is what you saw on the ammeter. It doesn't mean the battery is fully recharged, rather that the voltage has risen. Some suggest it can take 8 to 24 hours engine running, to bring a flat battery back up from flat, to a fully charged state. A fully charged battery, which has just started the engine, can take 20 minutes to put that used charge back in. The best way as always, is to make use of a battery charger, than burn fuel.
Ian Jackson expressed precisely :
Any charge or discharge, will produce localised heating.
You can get the same effect by cranking it over briefly, letting it rest, then trying again.
Discrete ammeters went out of fashion as high output alternators appeared
- you'd need some pretty expensive and bulky cable to supply one. But you can use electronics to measure the voltage drop across say the battery earth lead to produce a pretty accurate indication of current flow.
But an accurate voltmeter is a decent thing to have. You can buy an LED chassis type on Ebay specifically for this sort of job for a few quid. Of course fitting it neatly to the dash may not be easy.
Not nowadays, hall effect current sensors are cheap and don't require a shunt or anything, just a little toriod round the wire in which the current is being measured and some electronics to drive a meter.
Yes - there are ways of doing it now. But in general, cars dispensed with ammeters before such electronics became cheap. Last car I had with one was a '74 Rover. The next model had a voltmeter - the one after that nothing, just the warning light.
... oh dear, 'toroid' !! :-)
and with 2 wheels on the pavement so that baby-buggies have to go into the road to get past.
or they used a current shunt and a low current meter.
Testing the battery is easy and worth doing before looking at the joints and cables. It only takes a minute or two.
I don't agree, modern cars have much bigger capacity batteries than the old cars I had. It should be fine to leave the hazards on for a few hours and still start the car. If it isn't something needs fixing.
I repeat. Tell me of any car which used this. For the meter to be even reasonably accurate the shunt has to be a known value. Car wiring harnesses ain't made to the micron.
So it's not fitted with an ammeter.
I'm talking about how they were done when they were common. Not how it could be done today.
It is, the computer uses it to diagnose faults, etc.
They were never common, I only ever saw one car with an ammeter and I took it out the day after I bought it.
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