I am redoing the wiring on a small powerboat (6.2 meters). As part of
this I intend to use a dual battery system, I have seen a number of
different circuits that would work. The circuit I prefer users a
latching relay, which is activated by the ignition switch, this places
the batteries in parallel when starting and allows the charging of
both batteries at the same time. When the ignition is off the
batteries are separated allowing one be the house battery without
flattening the other.
At long last comes the question; if the house battery is flat what
effect will this have on the other battery during starting, charging
and the circuit in general.

Thanks in advance
Bruce

Bruce,
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.

For the few times you need the extra power, just set the battery switch to
"both", then run off battery number "one" or battery number "two" as you see
fit.

Larry


"bruce" wrote in message
m...
I am redoing the wiring on a small powerboat (6.2 meters). As part of
this I intend to use a dual battery system, I have seen a number of
different circuits that would work. The circuit I prefer users a
latching relay, which is activated by the ignition switch, this places
the batteries in parallel when starting and allows the charging of
both batteries at the same time. When the ignition is off the
batteries are separated allowing one be the house battery without
flattening the other.
At long last comes the question; if the house battery is flat what
effect will this have on the other battery during starting, charging
and the circuit in general.

Thanks in advance
Bruce

"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.

Larry

That's ok as long as you don't forget to change the switch from "both"
back to "1" or "2" or you will end up with two flat batteries.

Bruce

"bruce" wrote in message
m...
I am redoing the wiring on a small powerboat (6.2 meters). As part of
this I intend to use a dual battery system, I have seen a number of
different circuits that would work. The circuit I prefer users a
latching relay, which is activated by the ignition switch, this places
the batteries in parallel when starting and allows the charging of
both batteries at the same time. When the ignition is off the
batteries are separated allowing one be the house battery without
flattening the other.
At long last comes the question; if the house battery is flat what
effect will this have on the other battery during starting, charging
and the circuit in general.

Well, it works a little different: the relay connects both batteries in
parallel *after* starting, when the generator starts charging. Thus, the
engine always starts from one and the same battery.

Meindert

bruce wrote:
I am redoing the wiring on a small powerboat (6.2 meters). As part of
this I intend to use a dual battery system, I have seen a number of
different circuits that would work. The circuit I prefer users a
latching relay, which is activated by the ignition switch, this places
the batteries in parallel when starting and allows the charging of
both batteries at the same time. When the ignition is off the
batteries are separated allowing one be the house battery without
flattening the other.
At long last comes the question; if the house battery is flat what
effect will this have on the other battery during starting, charging
and the circuit in general.

Thanks in advance
Bruce

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. The chargers vary the
voltage to keep the current in spec.

I don't think charging in parallel is the best idea, IMHO.

Donald

--
I'm building a Steel Robert's 434. You can sneak a peek if you wish by
clicking on me link below.
http://bellsouthpwp.net/d/o/donrayp/
'USA, Home of the best
politicians money can buy'

Donald Phillips wrote:
bruce wrote:

I am redoing the wiring on a small powerboat (6.2 meters). As part of
this I intend to use a dual battery system, I have seen a number of
different circuits that would work. The circuit I prefer users a
latching relay, which is activated by the ignition switch, this places
the batteries in parallel when starting and allows the charging of
both batteries at the same time. When the ignition is off the
batteries are separated allowing one be the house battery without
flattening the other.
At long last comes the question; if the house battery is flat what
effect will this have on the other battery during starting, charging
and the circuit in general.

Thanks in advance
Bruce


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. The chargers vary the
voltage to keep the current in spec.

I don't think charging in parallel is the best idea, IMHO.

Most (if not all) charging circuits rely on the voltage to indicate a
full charge, if I remember correctly roughly 14.7v means a fully charged
lead acid batter. Now if you connect them in parallel the voltage seen
at the regulator is going to likely be wrong for both batteries, leading
to slower charging or overcharging.

And don't connect them in series, cause they you've got 24v ;-)

I'd suggest a switch for 1 - both - 2 batteries, and put a seriously
bright red light on the dash to indicate when you're in 'both' mode...
or a buzzer, or something.. Two seperate meters for the batteries, and
you can switch over to one that needs charging the most...

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?

You don't want to convect both batteries together during starting. The house
battery will draw from the starting battery ( just like jump starting a car,
only the good battery is in the running car ). If you are separating the
batteries separate the load also. All engin related loads on the starting
battery all house loads on the house battery. Ether get a battery combiner or
a battery isolator ( each has its good and bad points ).

Have a dedicated starting battery with the ability to select the house battery
if needed.

Mike D.
************************

I am redoing the wiring on a small powerboat (6.2 meters). As part of
this I intend to use a dual battery system, I have seen a number of
different circuits that would work. The circuit I prefer users a
latching relay, which is activated by the ignition switch, this places
the batteries in parallel when starting and allows the charging of
both batteries at the same time. When the ignition is off the
batteries are separated allowing one be the house battery without
flattening the other.
At long last comes the question; if the house battery is flat what
effect will this have on the other battery during starting, charging
and the circuit in general.

Thanks in advance
Bruce

Bruce, Check out http://www.yandina.com/

He makes and sells battery combiners that might help you out (as well
as issolaters) Also, he has some good education material on the ++
and - - - of different approches.

-al-



On 11 Aug 2003 17:41:48 -0700, (bruce)
wrote:

I am redoing the wiring on a small powerboat (6.2 meters). As part of
this I intend to use a dual battery system, I have seen a number of
different circuits that would work. The circuit I prefer users a
latching relay, which is activated by the ignition switch, this places
the batteries in parallel when starting and allows the charging of
both batteries at the same time. When the ignition is off the
batteries are separated allowing one be the house battery without
flattening the other.
At long last comes the question; if the house battery is flat what
effect will this have on the other battery during starting, charging
and the circuit in general.

Thanks in advance
Bruce