[RE-wrenches] Still walking it back

Exeltech exeltech at yahoo.com
Fri Nov 2 10:23:24 PDT 2012


Hi Larry and Wrenches,

Battery "resistance" is a matter of semantics depending on
the direction of the current flow.

For charging, the applicable term is "acceptance".  In this
context, a fully discharged battery, when connected to a
source of charging current, allows for a rate of current flow
generally limited only by the charging source.  Hence, the
opposition to current flow when charging a very discharged
battery is quite low.

If one were to connect a discharged battery to a source of
extremely high current capability (let's say in the many
thousands of amps), a fully discharged battery would allow a
current flow limited only by its own internal resistance.
This can (and does) lead to spectacularly high currents,
and unpleasant consequences.

Conversely, it's the battery's internal resistance that in turn
limits the current flow when connected to a load and delivering
current.  When fully charged, internal resistance is quite low,
and increases as the charge level decreases.

So, to be as correct as possible, we need view "acceptance" and
"internal resistance" essentially as reciprocals of each other.
Their applicability in context will be determined by the direction
of current flow into or out of the battery.

Keep up the good work all....



Dan Lepinski, Senior Engineer
Exeltech Solar



--- On Fri, 11/2/12, Larry Crutcher, Starlight Solar Power Systems <larry at starlightsolar.com> wrote:

> From: Larry Crutcher, Starlight Solar Power Systems <larry at starlightsolar.com>
> Subject: Re: [RE-wrenches] Still walking it back
> To: "RE-wrenches" <re-wrenches at lists.re-wrenches.org>
> Date: Friday, November 2, 2012, 11:35 AM
> Dan is correct if the discussion is
> about the open circuit test of a charged battery: resistance
> is lower than when discharged.
> 
> However, the resistance I was describing to Mick was the
> effect of a battery under charge. The "resistance" to
> current flow increases because the electrochemical process
> slows down as active material is converted back to it's
> original state. I believe this is what Dan meant by
> "equilibrium", PbSO4 is converted to PbO2, Pb and H2SO4
> increasing specific gravity, reducing current flow. 
> 
> A common question I am asked is "why can't I force my
> charger (PV solar or otherwise) to put out full power?" The
> reason is the battery simply can't absorb any more current
> as described above.
> 
> Larry Crutcher
> Starlight Solar Power Systems
> 
> 
> 
> On Nov 2, 2012, at 8:41 AM, Mick Abraham wrote:
> 
> Hi, Mechanix~ I'm sorry to add more "noise" to the List but
> I sort of
> mis-spoke again.
> 
> Dan Lepinski @ Exeltech sent a mail to me Off List; below I
> am posting
> the part of his
> mail which addresses my latest faux pas:
> 
> +++++++++++++++++++++++++++++++++
> 
> "As battery state of charge increases (toward 100%), the
> internal resistance
> actually goes *down* -- which is why you can get a very high
> short-circuit
> current from a fully charged battery .. and a much less
> current from a
> nearly dead one.
> 
> "What is happening instead is a voltage equilibrium -- that
> of the battery
> becoming more equal with the voltage of the charging
> source.  This is
> what reduces the charge current flowing from a
> constant-voltage source
> into the battery.
> 
> "Think of it this way: If you're adding air to a car tire,
> and have a compressor
> that produces a maximum of 35 psi, as the tire pressure
> approaches 35 psi,
> the air flow from the compressor to the tire will slow, and
> then eventually stop
> when the two pressures become equal."
> 
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