"silverback" wrote in message
news:tLBgg.8127$771.3210@edtnps89...

You can not overcharge a battery with a properly functioning alternator.

Deck Gorilla

"In typical automotive use we have a thin plate cranking battery that is
rarely discharged by more than a small percentage of it's capacity. The act
of cranking draws the charge from the surface of the plates, which are then
recharged before the battery has time to equalize internally-the inner
plates retain their charge. In general, the engine and alternator run far
longer than is needed to replace this charge. Little charge is needed; it
has to be applied only to the surface areas of the plates; and we have
excess charging time. In this undemanding environment voltage regulator
settings are kept deliberately low in order to avoid damaging the battery
through overcharging during extended periods of engine running time.

In contrast, in many marine applications a thick-plate deep-cycle battery is
deeply discharged over a long period of time, allowing the battery to
equalize internally and so draining the charge from the inaccessible
inner-plate areas. The engine and alternator are then run for far less time
than is needed to restore a full charge. If a typical automotive regulator
voltage setting of around 14.0 volts(for a 12 volt system) is used during
charging, the rising surface voltage of the battery will cause the regulator
to curtail alternator output soon after the battery is 50% charged, which is
well before battery safety demands that it be cut back. This unnecessarily
prolongs charging times.

Since a well designed deep cycle battery bank will be cycling primarily in
the region of 50% to 80% of full charge, a typical regulator cripples
charging performance in the region of interest to boat owners! As a result,
many batteries are perennially undercharged, and die prematurely from
sulphation. In addition, if battery charging is accomplished by running the
boat's main engine at anchor, the engine will run long hours at low loads to
little effect, except to increase wear, tear, and maintenance.

In these circumstances the charge rate needs to be accelerated to the limit
the batteries can accept, driving the voltage(pressure) as high as can be
tolerated, so that the inner plate areas will be charged as rapidly as
possible. But if a regulator's voltage setting is raised to produce the
maximum safe charge rate in the 50% to 80% of full charge region, during
extended periods of engine running the batteries will be overcharged. This
will result in an excessive loss of electrolyte, destruction of the positive
plate grids, and perhaps overheating and buckling of the battery plates.

The dilemma is clear. In many pleasure boat situations, batteries are
periodically discharged deeply (to at least the 50% level), but charging
times are restricted. Thus, fast charges are required, which calls for
higher voltage regulator settings than in automotive applications; otherwise
the batteries will suffer from undercharging, sulphation, and a permanent
loss of capacity. But if voltage regulator settings are raised, extended
engine running will likely overcharge the batteries, causing excessive
gassing and plate damage".

Nigel Calder

"Boatowner's Mechanical and Electrical Manual"

The key, IMHO, is how batteries discharge and accept charges.

John Cairns