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

"Larry" wrote

There is not a reliable way to charge the batteries together
without using
a battery isolator. Batteries ALWAYS charge at different rates and an
isolator will allow for this.

Bunk. Multibank battery systems were on boats for decades prior to the
marketing of "combiners", "isolators" and other solutions to the
"forgot to switch from Both to House after engine shutdown problem."

Paralleled banks of differing charge states will charge just fine when
hooked up to an alternator. The banks will automatically charge at
different rates until all banks are fully charged, without additional
devices. Consider the 2 bank worst case scenario, a fully charged
bank at 12.7v resting voltage and a dead flat bank @ 10.5v. When the
alternator kicks in the system voltage jumps to 12.5v or so depending
on bank capacity and alternator output. The charged bank is
essentially at system voltage already and accepts little to no
current, while the discharged bank sees a 2v differential and accepts
all the current the alternator can deliver. The charged bank will not
accept charge until the voltage of the discharged bank rises to the
resting voltage of the charged bank, then, as the system voltage
further rises, both batteries charge until the system voltage is
around 14.4v. The charged bank accepts about 2% of its capacity
because it's already charged (its highest charge rate during this
engine run, so it's not even warm), and the discharged bank slowly
accepts less charge until its charge rate is 2% of capacity. Both
batteries are (nearly) fully charged.

Some caveats here. The banks must be electrically similar, mixing
GelGell batteries with regular flooded lead acid batteries is a no-no,
for example. If the banks differ in capacity, the alternator must be
no larger than 30% (GelCell, etc. 50% or even higher) of the smaller
bank's capacity, to prevent damage from too high of a charge rate as
the smaller bank sucks up the current. And, the cabling system must
be properly sized, so all banks see the same voltage; a windlass
battery in the bow with too small wiring will show system voltage when
no charging devices are on line, but a half volt or more drop when the
alternator's pumping 70 amps through it to the discharged battery.

Donald Phillips wrote

The way batteries are charged in the factory where they are made
is in
series string. This way all batteries receive the same amount of
current and thus the same amount of amp hours.

Wouldn't another advantage be the high voltages involved, so much
smaller wires serve for the hookups and charging equipment?