SOT: Car battery conundrum

[Another John]

I know this is the 'wrong' group, but a few of the blokes in the 'right' group are also in this group ...

My car battery is a few years old, but it seems to be managing OK. But the hazard lights seem to take an awful lot of juice out of it:

This morning it went dead after I had had the hazards on for about 3/4 hour when stopped by the roadside. Started again no problem when I got another car to connect jump leads to it. Got it home, and put it in the garage. 1/2 hour later I went out and just for the heck of it I thought I'd try starting the car -- thinking it would be dead until I had charged the battery up: but NO: it started first time, no problem.

Thing is: this happened a few months ago: had the hazards on for a while: car dead, and had to be jump-started. Took car to motor factors, to get a new battery ... guy kindly checked it first, and his machine said the battery was fine (as indeed it has been, until this morning's event).

Do hazard lights take more out of the battery than one would expect? OR: is the battery too old to be taking *any* load unless the engine is running?

Thanks for any opinions John

[Another John]

Should have mentioned (idiot) that the drive home was about 1/4 mile (400 yards) - so not a big run!

J.

[Dave Plowman (News)]

Indicators are generally 21 watts, so 4 x 21. Plus say 12w for a couple of side repeaters. Total approx 8 amps. So for 3/4hr will use approx 6 amp.hr.

Car batteries are generally 50 - 70 amp.hr, so 6 ain't going to matter much.

A decent battery place will test the battery for free.

[OG]

I think I've heard of battery saver circuits, that basically shut down all the electrics if the battery voltage drops below a certain value. This means the battery is kept safe from being damaged by being drained further until it gets a recharge.

I borrowed a car that seemed to have absolutely no life in the battery, but as soon as the jumper leads were connected, we seemed to have full power - even without the 'supplying' car engine running.

If on your car the hazards had dropped the voltage below the 'trigger' point, the circuit may have kicked in, even though the battery wasn't particularly low. Once it had been restarted, it would be just about back to normal, because the it had only ever been partially drained anyhow.

On the other hand, you may have a problem with your hazards (I recall reading somewhere that the switching circuit can be power hungry, but that would have been years ago).

[Weatherlawyer]

I am by no means expert but a short run in a modern car could well give a g ood battery plenty of juice to start a car in the right circumstances. Your s seems to have been cosseted with a garage and all. I wonder why you don't have a trickle charger to hand there though.

I read somewhere some years back that modern (even then) circuitry would fl ood the battery until it got up to scratch so all the generator was produci ng would fill the banks for you in 1/4 of a mile.

I wonder if there is too much resistance in the circuit to the flashers? A spot of rust/corrosion that might allow the circuit to work but at great cost. How old is the what sort of car?

[Martin Brown]

This time of year if you mostly drive short distances in the dark with the headlamps on in the morning and evening then the battery may not be getting charged well enough to replace the total starting current drawn.

If the battery is properly charged in the first place. It could be a fair proportion of the charge remaining in the poor battery if it is not being driven any great distance in daylight. Giving it a decent charge and then testing to see how long it can drive the hazards for from a full charge would be one way to proceed. Failing that keep the engine running when you have the hazard warning lamps on in winter!

In the bad old days of electromechanical flashers they would slow down amusingly as the battery faded out. Eventually going on and never going off. Batteries always seem to dislike damp cold winters in old cars.

Regards, Martin Brown

[Weatherlawyer]

Nice work. Ta.

But finding decnt ones can be a minefield if my experience with leaking tyr e rims is anything to go by. A lot of garages will rip you off over £300 quid when all you need is to take the tyres off and reseat them with some s ealer magic-smoke stuff thingie...

Had the same thing happen in several different garages doing MOTs and stuff . "Oh your treads are low do you want them replaced there is a special offe r." sort of thing.

I was broke by the time I realised what was going on so I had to ask around more carefully -and learned one hell of a lot. I would like to go back in time and administer due punishment but it is water under a bridge by now.

[jrwalliker]

I have had something like this with my Volvo V40 (around 15 years old). In very cold weather it would sometimes fail to start, even though the battery could turn over the engine. I found that connecting a 2A current limited 15V bench power supply to the battery for just a few seconds was enough to make it start.

This has only happened a few times and always in cold weather. In each case the battery seemed to be OK when tested. The terminals were definitely not loose.

This does suggest either a deliberate or accidental lockout, perhaps in the engine management or immobiliser circuits. I suspect it is accidental - perhaps a state machine stuck in a state which shouldn't exist - as the battery can almost completely discharge without this happening.

John

[bert]

Too much resistance reduces the current so battery would last longer.

[bert]

Failure to start can often indicate a poor earth contact as the current is very high when turning over the engine.

[Graham.]

And you can half that figure, 50% duty cycle.

[Andrew Gabriel]

I fitted battery ammeters to my first couple of cars, which was quite educational, if not particularly useful long-term.

Car batteries generally gradually reduce in capacity over time. This often goes completely unnoticed because, contrary to popular opinion, starting a car that starts easily doesn't use much of the battery's capacity. If you look at the ammeter, you can see that the battery is generally fully recharged from starting by the time you have driven a few hundred yards up the road - you don't need a long run to recharge after starting. Alternators can pump out 30A even at tickover, so you don't actually need to drive anywhere.

What's probably happened is that you've got to the point where the battery capacity is down to around 5Ah - still enough to start a car first time (petrol engine, at least), but not enough capacity to run the hazards for an hour.

You will find if you keep it that quite soon, it won't be able to start the car, possibly on a cold damp morning when the car needed a second attempt. It also deteriorates much faster each time you run it flat, which you surely will do in this state.

In my student days, I actually managed to drive a mini in this state for a few months - I got used to always parking it where I could run it down a slope to start it, and it did at least always fire first time, or I would have been stuffed!

[tony sayer]

In article , Another John scribeth thus

If we're sure the battery is really OK then,

Are you sure its being charged OK in the car?, i.e. is the alternator drive belt as tight as it should be?..

Its not usual these days with modern cars to have much bother. Generally batteries work less well when the temperature drops and modern alternators usually dump plenty of amps in the battery to charge it so shouldn't be a problem.

Unless its not being charged as it ought be..

[Bob Henson]

The effect you describe is due to the plates becoming "sulphated up", as it used to be called in my Grandad's Ford Garage and elsewhere. There's a description of the problem, and how to remedy it, here.

[Another John]

OP here - thanks for all the very useful replies, so far.

I should say that the car is a 2003 Toyota Corolla, and the battery (I think) is about eight years old. See [1] below for boring enlargement on this point.

I have now charged the battery, which took about 90 minutes.

I'm intrigued by the idea first raised by OG:

I've just re-read the relevant bits of my owner's manual, but there's no mention of this feature -- there again, it might not be something suitable for the owner's manual (since the general indication is that a new battery's required, saver circuit or not).

Anyway thanks *so* much for the enlightenment from all concerned: brilliant!

John

[1] Er-um ... ah... Yes! so maybe I ought to just replace it. But I'm thinking about changing the car this year and wanted to avoid replacing the battery just now. If I decide (as I have done for the last 3 years) to keep the car (best car we have ever owned!), then I'll definitely replace the battery. As it was, I could do with a battery charger anyway, so I bought one of those instead. Spent ?30 ... not exactly top-drawer stuff I suppose.

[Michael Chare]

How long is a few years?

With a 2 litre car I have usually found that batteries last 5-6 years. Modern cars start so easily I never really notice the deterioration in the battery until one morning the car just won't start. On one occasion I charged the battery, drove 6 miles to get a replacement, then found my car would not start to drive home. I had to fit the new battery to get home.

[Nick]

+1

[meow2222]

4x21w indicators, 50% duty cycle = 42w = 3A x45min = 2.25Ah.

Seems it was close to flat that day. I'd take that as a caution to use less leccy or charge it.

NT

[Scott M]

Friend of mine has a 1 year old Transit with 50,000 on the clock. Had the hazards going for a couple of hours while parked up and had to get the AA out for a jump. So even a new and totally topped off battery won't cut the mustard.

[The Natural Philosopher]

Had a flat battery in a car towed with lights on, Next day it started no jumps. Depolarised?