Not so. Re-read my post. The only time the extra battery is connected
is when the engine is running; all batteries are at 13+ volts at the
terminals, regardless of their charge state, b/c the alternator is
running. The small battery will never be charging the house bank while
the alternator is running.

If the house bank is depleted, the worst that happens is the alternator
is trying to keep up with the house bank and runs at 13 volts or so
until the house bank charges up some, at which point the alternator's
output will rise in voltage. This is still above 12.8 volts, the
full-charge resting voltage of a battery. Even if the small battery
lost no charge during the month and is at 12.8 volts, there's no way the
small battery can drain into the house bank.

Ditto the sparking issue. Connecting a battery at 12.5 volts and one at
14 volts won't likely cause any sparking. Shouldn't be an issue anyway,
b/c your box is properly vented, right?

Finally, the loss issue. True. But: High quality, well maintained
jumpers make a difference. Clean the connections first. Make sure the
"small" battery isn't sized to just barely start your engine, but has
some hefty reserve to it. Did this with an old 8D starting battery for
a 120 h.p. diesel. Tried it out, worked fine. How big is your engine?

Rich Mechaber

MIDEMETZ wrote:

The only possible problem with your idea is if you run down the main battery it
will over load the small extra battery. There is lot of loss with jumper
cables plus if you can't disconcert the dead batteries they pull a lot out of
the good battery.

Not to mention the probably sparking during hook up. Problem never happen in
good conditions. At least for me anyway.

Mike
***********

I would echo what others have contributed about not routinely drawing
from the house battery for starting if you set up as you describe. What
surprises me is that almost no one selects a setup I prefer: a
large-ish bank of house/starting batteries, without switches, relays, or
problems. "What if you drain the batteries down?" For this purpose,
and for this purpose _only_, I bought a cheap starting battery.
Unconnected to anything. Once a month, I would affix heavy gauge jumper
cables from it to the main bank during a 6-hour motor to re-charge the
self-dischaged capacity. Left the cover off the battery compartment
while under way so there was no way to forget to remove the cables
later. ("Gee, why are the batteries visible from the saloon?") Cables
served for the emergency start, if needed, connected directly to the
starter solenoid.

HTH,
Rich Mechaber