Terry Spragg wrote:

JAD wrote:

I am adding a second battery to my small cabin sailboat, as a "house"
battery. Basically, it will run one thermoelectric cooler while away from
shore power. I am NOT planning on using a battery switch. I would sure
like to add a simple schematic to my post....but since binaries are frowned
upon (and I don't have the means to draw one on the computer anyway,)
here's the plan:

Engine (40hp Suzuki) NEGATIVE to battery #1 (starting) NEGATIVE. Engine
POSITIVE to center (feed) terminal of isolator. The isolator OUTPUTS will
go to the POSITIVES of batteries #1 and #2, respectfully.

CABLE from NEGATIVE of battery #1, to NEGATIVE of battery #2.

Accessories such as stereo, VHF, the feed for cabin and navigation light
panel will be attached to battery #1 + and -.

The accessory plug for the cooler will bridge the battery #2 + and -. (the
cooler draws 6 amps....my 100 a/h gel cell runs it all night after a good
charge now.)

I am NOT too worried about using battery #2 for starting.....I have a "jump
start" battery that is always well maintained....and have never had to use
it even after an overnight with stereo playing, judicious cabin lighting
use, and burning anchor light after dark.

Any potential for unwanted power feedback with this plan?

Any suggestions would be greatly appreciated!

My thanks in advance!


A third jumper battery?

with only two batteries, if you should need to use a jumper
(Why?) you will make a spark as you connect it. Aside from the
explosion and fire hazard if there are gas fumes about, you will
also charge the dead battery with the charged house one,
weakening it, so I would consider not using a jumper, but instead
a switch, disconnecting one before connecting the other, to
maximise emergency starting energy available. You might not want
to do this if the engine is running, if your suzuki has an
alternator instead of a [magneto -tk] in the flywheel, more so if you
have electronic ignition.

'Course, you can always pull the start string, eh?

Your isolator diode block will prevent your batteries from
attaining full charge as they 'contribute' a .6v or so voltage
drop in the charging circuit, unless the alternator regulator is
compensated to overcharge the battery / isolator diode
combination.

In principle, I like the solenoid charging combiner idea best, as
it is more efficient in one sense, but if a solenoid must conduct
staring current, it will be so big that it's coil might use more
power to energise it for charging than you might otherwise lose
from the diode block. This would affect fuel usage a very little,
and charging time. If the solenoid were to be connected only so
as to enable charging current in the house battery, it would use
a much smaller solenoid and coil, wasting less energy overall,
while still permitting a jumper cable to be used to start the
engine if you get tired of pulling the string. I know it's a
sequential non sequiter, but who cares?

So, my favorite plan is to hook up the one battery start circuit
as if there were no second battery used, without a blocking
diode, then add a smaller, 'smart' charge voltage sensing
solenoid that would enable charging the house battery, which
would drop out when the engine is stopped. As soon as the engine
started, the solenoid would engage, replenishing the charge taken
out of the starting battery, recharging it quickly from the house
battery, and discharging the house battery somewhat. A .1 ohm
(eg) resistor in the house battery charge / boost / recharge
circuit would prevent heavy equalisation charging if you did suck
down the start battery very far. That resistor would be in series
with the solenoid charge circuit and the direction of current
flow though it could be used to shunt it during normal charge
with an active sense circuit arrangement and another small
solenoid. It would take a little logic. That would be the most
elegant solution. There are several possible variations on this
theme, all the way up to 3000 dollar charge controllers.

You could still use a jumper cable or switching arrangement to
boost or replace the starting battery, if the other battery was
ok. If you have a third battery, I would connect it to the house
battery to increase the house system reserve, provided it was a
suitable type.

Heck, if the pull start works good, chuck all of the extra
batteries to save weight, and keep the fridge door (top opening,
right, with a block of ice in a camp drinking water bag on top of
the food?) closed, and the stereo down. If the o/b is working
right, it should start easily anyway.

Ain't nothin' like messin' with a boat?
--repost, my mistake -tk
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