when cranking my battery voltage drops to around 8V or so
When my alternator died my battery read 5 volts and the car was still=20
(just) running, with the headlights off (at night time of course)

On 19/05/12 11:56, Dave Tweed wrote:
> Sean Breheny wrote:
>> On Thu, May 17, 2012 at 2:21 PM, Spehro Pefhany<speff@interlog.com>  wro=
te:
>>> The current taken by the charging battery isn't important, because
>>> the net flow is OUT of the 'dead' battery when the engine is being
>>> cranked. Unless the 'dead' battery is internally knackered (shorted
>>> cell etc), it will have an OPEN CIRCUIT voltage that is more than 10V.
>>> It's more the internal resistance that goes up as the battery discharge=
s.
>> I don't think this is always the case. You are assuming that the
>> cranking current will be much greater than the battery charge current.
> No, you've missed Spehro's point: The drop in the cables due to the crank=
ing
> current will cause the voltage at the dead battery to drop well below its
> open-circuit voltage, so it will not be charging at all. In other words,
> the combination of the rescue vehicle plus the jumper cables is anything
> BUT "a stiff 13V voltage source", as you put it.
>
> -- Dave Tweed

--=20
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