Steve Foley wrote:
"RST Engineering" wrote...

Trivial to design and build.

Jim

Not for me

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Yes, even for you :-)

Although he hasn't stated it very well, the original poster is probably
looking for a voltage comparator.

Some years ago I kitted and sold a simple on-off 'controller' based on
the LM-339 voltage comparator chip that would probably fill the bill.
Although sold as a solar controller, the circuit also served as a
low-voltage indicator, a means of turning on a motor-generator set and
a number of other functions, depending on what voltage was being sensed
and how you adjusted the set-points.

The circuit itself was little more than the practical application of
one of the several circuits provided by the chip manufacturer
(National, as I recall), making the 'design' a trivial task indeed.
The real value was in the circuit board and the accompanying article,
which appeared in 'CTM' magazine, in that you needed a precisely
adjustable power supply to adjust the set-points (ie, turn-on &
turn-off). In the article I provided a schematic for such a power
supply, cobbled up from a salvaged wall-wart, regulation provided by an
LM-317T.

Unfortunately, the original poster's use of the term 'charging' needs
to be defined in electrical terms, and the chances off finding exactly
what he wants in an off-the-shelf package is probably somewhere between
slim and none. But there are numerous examples of such circuits out
there, assembled as well as kits.

-R.S.Hoover