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'[OT] Why do car batteries go dead?'
2008\07\30@215459 by Tomás Ó hÉilidhe

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It's common knowledge that if a car's left sitting for a few months, the
battery will be dead when you go out and try start it.

Assuming no current's being drawn from the battery (because the
circuit's broken), does anyone know why it goes dead?

2008\07\30@223736 by Herbert Graf

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On Thu, 2008-07-31 at 02:54 +0100, Tomás Ó hÉilidhe wrote:
> It's common knowledge that if a car's left sitting for a few months, the
> battery will be dead when you go out and try start it.
>
> Assuming no current's being drawn from the battery (because the
> circuit's broken), does anyone know why it goes dead?

Batteries have internal leakage currents, they will self discharge no
matter what.

Leave any battery long enough and it'll be empty.

Car batteries are optimized for large startup currents during cold
weather. Internal leakage is not a very important issue when they are
designed.

TTYL

2008\07\30@225903 by Carlos Marcano

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2008/7/30 Tomás Ó hÉilidhe <spam_OUTtoe_listTakeThisOuTspamlavabit.com>:
>
> It's common knowledge that if a car's left sitting for a few months, the
> battery will be dead when you go out and try start it.

Indeed.

> Assuming no current's being drawn from the battery (because the
> circuit's broken),

Not completely true. As much as the circuit seems to be "broken" there
is still a big chance that little current will be drawn from the
battery (internal leakage).

>does anyone know why it goes dead?

Besides what was written before, there are also chemical processes
involved (gargoyle for "cell reverse"). Heat will also affect them.

Regards,

-Carlos Marcano-
*Guri, Venezuela*
>
>

2008\07\30@230119 by Apptech

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{Quote hidden}

However, SLA batteries have very good shelf lives compared
to eg NiCd and conventional NimH. Recent NimH developments
have improved on this. A good SLA will have useable charge
at 1 year (?20%+?)

Above a certain voltage (Gargoyle can remember where I
forget) sulphation does not occur. Below this value cells
may be damaged by sulphation. Storage temperature is
extremely significant. A LA battery at 10 degree C  has
several times the shelf life of a LA at eg 30C. Also, total
battery life will be much reduced at elevated temperatures.



       Russell


2008\07\30@230743 by Herbert Graf

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On Thu, 2008-07-31 at 15:00 +1200, Apptech wrote:
> > Car batteries are optimized for large startup currents
> > during cold
> > weather. Internal leakage is not a very important issue
> > when they are
> > designed.
>
>
> However, SLA batteries have very good shelf lives compared
> to eg NiCd and conventional NimH. Recent NimH developments
> have improved on this. A good SLA will have useable charge
> at 1 year (?20%+?)

You're right, a GOOD SLA will do pretty well.

Car batteries don't. I know that in colder weather after say only 2
months most car batteries I've tried can no longer start the car.

In warmer weather I'm sure the interval is much longer.

Note this is all based on personal experience, "the numbers" may say
otherwise.

TTYL

2008\07\31@002729 by microsoftwarecontrol

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there are talking about "car battery reparing machine".
I am going to make it and to see see.



{Original Message removed}

2008\07\31@012831 by Sean Breheny

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I was under the impression, too, that some of the circuits in a car
are always powered and draw a few mA all the time. This is on the same
order of magnitude as the self-discharge rate, but together they
determine the life.

Sean

2008\07\31@072304 by Michael Rigby-Jones

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> -----Original Message-----
> From: .....piclist-bouncesKILLspamspam@spam@mit.edu [piclist-bouncesspamKILLspammit.edu] On
Behalf
> Of Sean Breheny
> Sent: 31 July 2008 06:28
> To: Microcontroller discussion list - Public.
> Subject: Re: [OT] Why do car batteries go dead?
>
> I was under the impression, too, that some of the circuits in a car
> are always powered and draw a few mA all the time. This is on the same
> order of magnitude as the self-discharge rate, but together they
> determine the life.

Any adaptive ECU draws a standby current to maintain the correction
tables in RAM.  Car alarms can also draw a surprisingly high quiescent
current, especially if they have sirens with battery back-up.  Stereo
head units can also draw a standby current to maintain programmed radio
stations, last CD track etc.  Altogether the drain on the battery can be
fairly high, and coupled with the trend for smaller, lighter batteries I
suspect few modern cars will start after more than a few weeks without
use.

Regards

Mike

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2008\07\31@082739 by Apptech

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>> I was under the impression, too, that some of the
>> circuits in a car
>> are always powered and draw a few mA all the time. This
>> is on the same
>> order of magnitude as the self-discharge rate, but
>> together they
>> determine the life.

A mA of current adds up to 730 mAh/month - ay 1 Ah to round
it off.

10 mA => 10 Ah = 25% of a substantial battery fully charged.
Many modern batteries are in the 20 - 30 Ah range.
Start with a 20 Ah battery half charged and 10 mA will
deplete it in a month.

The original question was

> It's common knowledge that if a car's left sitting for a
> few months, the
> battery will be dead when you go out and try start it.
> Assuming no current's being drawn from the battery
> (because the
> circuit's broken), does anyone know why it goes dead?

I think that "because the circuit's broken" MAY mean
"because the cable has been removed from the battery".
ie the battery is genuinely open circuit. If that's the case
then.

- Initial voltage low enough to allow sulphation to set in.

- Leakage across battery due to acid conduction (10 mA =~ 1k
ohm)

- Self discharge a natural feature of all chemistries. SLA
are 'not too bad'. Automotive LA may be somewhat worse.
Elevated temperature doesn't help.

- That's the way it is with car batteries ... .

The last answer is probably the most authoritative.



           Russell


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