On Mon, 17 Mar 2008 23:46:20 -0500, James Waldby wrote:

On Tue, 18 Mar 2008 10:07:45 +0700, Bruce in Bangkok wrote:
...
While there is no question that high voltage will damage a battery, in
real life most battery chargers will be perfectly suitable since a
battery's internal resistance increases as the battery is charged making
the system is pretty much self limiting.

Internal resistance vs. charge depends a bit on battery type and
history. [1][2] For lead-acid batteries, internal resistance
decreases (not increases) as charge goes up. For typical car
battery chargers, current drops as charge approaches 100% because
the difference between charger voltage and battery voltage drops
toward zero. Open-circuit charger voltage and charger resistance
will be the main limiting factors.

Try connecting a volt meter and an amp meter in parallel with the
battery charger. Turn the charger up to about a 10 amps charging current
and watch the volt meter. As voltage climbs the amperage decreases. With
about a 10 amp initial charging current by the time the battery voltage
reaches about 13.5 volts the current will have decreased to about 1 or 2
amps. Hardly a battery killing combination.
...

[1] http://www.arttec.net/Solar_Mower/4_...20Charging.pdf
in section "Lead-acid Internal Resistance and SOC" at bottom of 4th page
says "A fully discharged lead-acid cell has virtually no sulfuric
acid in its almost pure water electrolyte" [so would have high resistance]

[2] Near end of http://www.batteryuniversity.com/partone-22.htm ,
the paragraph before Figure 6 (resistance vs. voltage) says
"The resistance of lead acid goes up with discharge", etc.
-jiw


Of course you are correct and electricity flows from negative to
positive and transistors work because of "electronic holes".

But the average user has a bit of trouble visualizing that so the
explanation that the internal resistance increases and thus the
charging current decreases (which it does) is easily understood. It
may not be correct but it does explain the action of the usual
transformer battery charger in terms that the average user can relate
to.








Bruce-in-Bangkok
(correct email address for reply)