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1999\06\08@030142 by

What is the resistance of a lead-acid battery (say, 7 A.h) when it is
charged and when it is flat ? (And, to shed some light on this)
Since it changes during charge, isn't it enough to just supply the current
it draws ?

> What is the resistance of a lead-acid battery (say, 7 A.h) when it is
> charged and when it is flat ? (And, to shed some light on this)
> Since it changes during charge, isn't it enough to just supply the current
> it draws ?

The internal resistance of a lead acid battery (or any power source)
is a representation of that batteries ability to provide power to a
load. This can be thought of as a battery capable of delivering
infinite current at no voltage drop in series with a resistor, that
resistor being the internal resistance. So a small 12volt battery
will have a higher internal resistance than a big 12volt battery and
a damaged battery will have a higher internal resistance than an
undamaged battery.
watching the voltage drop across its terminals. If a 12 volt battery
drops  1 volt under a load of 1 amp then its internal resistance
is 1 ohm. Internal resistance will vary with the state of charge so
compare batteries at a common condition, such as full charge.
Manufacturers data is usually comprehensive on a batteries ability to
deliver power at different  rates, temperatures etc.
A lead acid battery is fully charged at a terminal voltage of 12.6
volts, 25% discharged at 12 volts and flat at 10.8 volts. DO NOT
discharge a 12 volt Lead Acid Battery below 10.8 volts or it will be
damaged over time.
So (from manufacturers data) a Hitachi 12 volt 7 Amp Hr Lead Acid
battery has a fully charged internal resistance of 25 milliohms and
can supply a maximum current of 40 Amps for 5 seconds. The maximum
charging current is 2.1 Amps.

Lance Allen

Lance Allen wrote:
>
> > What is the resistance of a lead-acid battery (say, 7 A.h) when it is
> > charged and when it is flat ? (And, to shed some light on this)
> > Since it changes during charge, isn't it enough to just supply the current
> > it draws ?
>

> A lead acid battery is fully charged at a terminal voltage of 12.6
> volts, 25% discharged at 12 volts and flat at 10.8 volts. DO NOT
> discharge a 12 volt Lead Acid Battery below 10.8 volts or it will be
> damaged over time.

The one Pb battery that you can discharge fully, is the Hawker
range. http://www.hepi.com/ IIRC, the short-circuit spec is
1.6kA for 5s (a D-size single cell), so internal resistance
is not related to size here...

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