I've been looking around for a sealed lead charger. A piece of
equipment I'm building has to be run off a battery, 6V or 12V,
and I prefer to use an SLA. The PIC which runs the equipment
is available as a charge monitor, as the equipment will not be
run when the battery is under charge

Not much is turning up for PIC chargers. One that looked promising
was a dead link unfortunately. There are plenty for other chemistries
like NiCd, Li, NiMH etc

On this page is a charging diagram

http://www.technick.net/public/code/cp_dpage.php?aiocp_dp=guide_bpw2_c04_06

I figure I could make a constant current supply from an LM317
(or is that just current limited ?) and a constant voltage supply from
a 7805 or 7812 baseline

Following the diagram, for a 12V SLA, a current is applied to the
battery until the terminals measure 14.4V (6 x 2.4V), at which point
the constant current is replaced with a 14.4V constant voltage

Now, do I then start monitoring the current into the battery (the dashed
line) until it reaches some lower plateau ? How would this be done ?
Measuring the voltage across a series resistor from 14.4V CV to
B+ ? It's easy to measure the voltage across a sense resistor from B-
to 0V, because that falls within the Vcc range of the PIC. Would
a resistor there also provide top-up charge current information or is
the charge going into B+ "not reaching" B- ?

Can you consider the battery a resistor that reduces in value as it
approaches full charge, and therefore the voltage at the junction of
B- and a low-side sense resistor lowers as charge progresses ?

TIA, Joe
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