From: Dave Baker on
Dave Plowman (News) wrote:
> In article <giquol$vus$1(a)news.datemas.de>,
> Dave Baker <Null(a)null.com> wrote:
>>> What's the explanation?
>
>> Assuming there wasn't an unusual parasitic drain or bad connection
>> after he'd moved the car to wash it then the battery is probably
>> near as dammit dead with very little reserve capacity left. It may
>> be partially shorted inside already. However the charger would have
>> put enough surface charge into the cells after an hour to start the
>> car easily enough. If the engine and ignition is in good nick you
>> only need a fraction of an ampere hour to turn the starter over at
>> say 200 amps for 1 second. The true test will be to check the
>> voltage after the surface charge has been removed by putting the
>> headlights on for a few minutes and then letting it sit for a few
>> more - or just wait until next morning before he starts it next
>> time. If it shows 12.65V then well and good but if it's really on
>> the way out the voltage will be much lower than that and it probably
>> won't hold a charge for very long.
>
> I'd say it did come up to the correct voltage as the charger switched
> to float mode. They're off on holiday now - so I'll keep my fingers
> crossed for them. ;-)

I don't think that really tells you anything other than the battery wouldn't
take any more charge. The post charging "surface charge", even on bad
plates, will bring the battery voltage up to a level that makes the charger
switch to float mode but when you remove the charger and discharge the
surface charge the true voltage and battery condition will be revealed.

If you put a voltmeter on a battery, even a bad one, immediately after
removing the charger you'll see very high open circuit voltages for a short
while. Possibly well over 13V depending on what the final float voltage of
the charger was. Those high voltages are created by an uneven mixture of
sulphuric acid and water on the plates which aren't representative of the
average specific gravity of the electrolyte or the true output voltage of
the battery. That's why you have to remove this surface charge before you
can test properly.

I've tried to soldier on with knackered batteries in old cars many times
which would show a decent surface charge voltage making you think the
battery was fully charged, start the car fine soon afterwards but still be
dead again in a day or two. I've even trying reviving batteries by replacing
the electrolyte by saving the acid from other old batteries, concentrating
it by letting some of the water evaporate with it in a glass bowl on the
central heating boiler until it reached an SG of 1.26, charging the battery
as high as it will go and then removing and washing out all the old acid and
sludge and putting the new brew in but it never achieves anything.

Some of these new fangled pulse chargers are meant to be able to remove hard
sulphate to some extent but I haven't tried one. They cost more than a new
battery.

I think your neighbour's experience isn't really that unusual. He started
the car just to move it a few feet which took some load out of the knackered
battery but didn't replace it which a longer trip would have done and it's
showed its true colours. Your charger gave it enough for one more start and
the overnight charge might have given it enough for a few more starts but
next time he leaves it for a day or two it'll be dead again.
--
Dave Baker


From: Dave Baker on
Harry Bloomfield wrote:
> My own guess would be that the 9.5v was not a true reading of the
> actual voltage across the battery terminals, that you too the reading
> from the cable lugs and that there might have been some high
> resistance between lug and battery. Your disturbing the connections
> testing and/or putting the charger across them, improved the
> connection enough to make it start.

I think Dave knows more than enough about electrickery to measure a battery
voltage properly so the above really isn't a possibility.
--
Dave Baker


From: Dave Plowman (News) on
In article <gir2fv$6pl$1(a)news.datemas.de>,
Dave Baker <Null(a)null.com> wrote:
> I think your neighbour's experience isn't really that unusual. He
> started the car just to move it a few feet which took some load out of
> the knackered battery but didn't replace it which a longer trip would
> have done and it's showed its true colours. Your charger gave it enough
> for one more start and the overnight charge might have given it enough
> for a few more starts but next time he leaves it for a day or two it'll
> be dead again.

Yup. He didn't buy the car new - but the battery is a BMW one so in all
probability original. So at 7 years old has done well enough. Eurocarparts
charge 60 quid inc. for one so not the end of the world.

--
*i souport publik edekashun.

Dave Plowman dave(a)davenoise.co.uk London SW
To e-mail, change noise into sound.
From: Duncan Wood on
On Tue, 23 Dec 2008 16:19:13 -0000, Dave Plowman (News)
<dave(a)davenoise.co.uk> wrote:

> In article <gir2fv$6pl$1(a)news.datemas.de>,
> Dave Baker <Null(a)null.com> wrote:
>> I think your neighbour's experience isn't really that unusual. He
>> started the car just to move it a few feet which took some load out of
>> the knackered battery but didn't replace it which a longer trip would
>> have done and it's showed its true colours. Your charger gave it enough
>> for one more start and the overnight charge might have given it enough
>> for a few more starts but next time he leaves it for a day or two it'll
>> be dead again.
>
> Yup. He didn't buy the car new - but the battery is a BMW one so in all
> probability original. So at 7 years old has done well enough.
> Eurocarparts
> charge 60 quid inc. for one so not the end of the world.
>


It's always worth ringing bmw for a price at that age, they where cheaper
last time I bought one.
From: Mark on

"Conor" <conor_turton(a)hotmail.com> wrote in message
news:6rccqqFj6bsU1(a)mid.individual.net...
> In article <5012386bcddave(a)davenoise.co.uk>, Dave Plowman (News)
> says...
>
>> This morning he returned the charger and said on a whim - or more likely
>> to stop the earhole gbh - he'd tried to start it after an hour or so and
>> it did. With a charger who's peak output is less than 5 amps?
>>
>> He kept the engine running while she shopped then put it back on charge
>> overnight. By the morning the charger had gone to float mode so had
>> decided the battery was fully charged.
>>
>> What's the explanation?
>>
> Starter motor is possibly fucked - maybe brushes. Just changed the one
> on the Capri last week - similar symptoms. Turned over fine if the
> voltage was there but once the voltage dropped a bit, i,e when parked
> overnight, it didn't want to know. What was really wierd is it'd turn
> real quick for a few seconds then if it missed firing and you instantly
> tried again, it'd turn over like the battery was flat. All connections
> absolutely fine, battery in good nick.
>
> I put a new starter motor on and it's been fine since.

My Sierra is exactly like that too. Even with a freshly charged battery, it
can spin engine really fast, but next go its sluggish. Been the same way for
a few years, even though its had a new battery and all connection have been
removed and cleaned, including the engine block neg/earth and the starter
solenoid. I suspect the starter/solenoid too.

Mark