I'm about to wire up a battery switch so that I can switch between two
batteries. (one or the other or both)
Just wondering if I should put the switch on the positive or negative?? Or
doesn't it matter?
Also, do both the batteries have to be the same age/rating??? I was hoping
to use my old battery for powering accessories when I'm stopped, but charge
it when the engine is running with the other battery.

Thanks in advance!

"bowgus" blathered....

in Message-id: ...

"GC" wrote in message
...
I'm about to wire up a battery switch so that I can switch between two
batteries. (one or the other or both)

Don't use the "both" setting ... read on

Just wondering if I should put the switch on the positive or negative??
Or
doesn't it matter?

Positive is the norm around here (geez ... hurts the head to even think
about switching the negative since that's normally the electrical reference)

Also, do both the batteries have to be the same age/rating??? I was
hoping
to use my old battery for powering accessories when I'm stopped, but
charge
it when the engine is running with the other battery.

In your situation, I would ... (get myself a deep cycle for the accessories
... e.g. my accesory is my electric trolling motor)

a) make sure the switch is a make before break type which they normally are
so that the alternator is never without a load if switched while running;
b) when stopped select the old battery (if it's a regular starting type
battery it won't tolerate many cycles of being discharged), before starting,
switch to the new battery;
c) while running charge batteries by switching between the two (hmmm ...
when are they charged ... well, maybe your batteries have indicators of the
green it's charged variety?);
d) never connect the batteries ... it's my understanding that the charged
one could pump a lot of current into the discharged (my theory ... a
battery (let's just say for the sake of argument rated 800 cold cranking
amps) has no current limiting feature like a battery charger does) which
could result in 800 amps going through wiring/switch resulting in fire,
exploding battery, ...


Thanks in advance!


My 2 cents worth ...


And worth slightly less than 2 cents.

Where do you people get your information? Disneyworld?

A typical engine start battery would be 600 to 650 CCA

As you have quoted 800 CCA we shall use that as the example.

A brand new 800 CCA engine start battery will have an internal resistance of
around 0.004 ohms and an _open circuit_ voltage of around 12.7 volts (you can
calculate the fully charged internal resistance of an engine start battery
using the given CCA and the _definition_ of what CCA means - but of course you
didn't know that).

A flat deep cycle battery will have an _open circuit_ voltage of around 11.9
volts and an internal resistance of around 0.06 ohms.

The voltage _difference_ between the 2 batteries is therefore 0.8 volts dropped
across the two internal resistances of 0.004 + 0.06 = 0.064 ohms = 12.5 amps.

And that is making the simple assumption that the voltage of the fully charged
engine battery does not fall under load (in addition to the voltage drop caused
by the internal resistance) which in practice it will, by about 0.2 volts at
this sort of loading.

So the _real_ current draw will be approximately 9.4 amps.

Further, there is a dead band between the open circuit voltage and the voltage
at which charging will commence. This dead band varies quite dramatically
depending upon the state of charge of the battery in question. A typical figure
for a flat deep cycle battery would be about 0.2 volts.

So an even closer figure for the current draw (that you assert is 800 amps)
would be (12.5V - 12.1V) 0.4V/(total internal resistance) 0.064R = 6.25 amps.

And of course you won't believe me. So instead of using half a theory that you
don't understand anyway, either go learn the rest of the theory or try it and
take some measurements.

Connectng the batteries in parallel is perfectly safe, quite normal, will
achieve what is desired and is used in millions of installations the world
over.

Idiot

Folklore Killer.

"GC" wrote in message
...
I'm about to wire up a battery switch so that I can switch between two
batteries. (one or the other or both)

Don't use the "both" setting ... read on

Just wondering if I should put the switch on the positive or negative??
Or
doesn't it matter?

Positive is the norm around here (geez ... hurts the head to even think
about switching the negative since that's normally the electrical reference)

Also, do both the batteries have to be the same age/rating??? I was
hoping
to use my old battery for powering accessories when I'm stopped, but
charge
it when the engine is running with the other battery.

In your situation, I would ... (get myself a deep cycle for the accessories
.... e.g. my accesory is my electric trolling motor)

a) make sure the switch is a make before break type which they normally are
so that the alternator is never without a load if switched while running;
b) when stopped select the old battery (if it's a regular starting type
battery it won't tolerate many cycles of being discharged), before starting,
switch to the new battery;
c) while running charge batteries by switching between the two (hmmm ...
when are they charged ... well, maybe your batteries have indicators of the
green it's charged variety?);
d) never connect the batteries ... it's my understanding that the charged
one could pump a lot of current into the discharged (my theory ... a
battery (let's just say for the sake of argument rated 800 cold cranking
amps) has no current limiting feature like a battery charger does) which
could result in 800 amps going through wiring/switch resulting in fire,
exploding battery, ...


Thanks in advance!


My 2 cents worth ... mainly because I'm up at 5:00 in the morning mulling
over a work related situation ... er ... challenge :-)

On 06 Nov 2004 11:10:38 GMT, Folklore killer wrote:

....snip

Connectng the batteries in parallel is perfectly safe, quite normal, will
achieve what is desired and is used in millions of installations the world
over.

If one connects a battery near the end of it's life in parallel with a
relatively new, fully-charged battery, are you saying that the old battery
will not discharge the new one? By your own calculations, it appears that
it will. If that's the case, then shouldn't one be careful paralleling two
batteries of different age and condition?

Of course one could prevent this by the use of high current diodes, but
that lowers voltage by about 0.7 volts per diode.

I think that's the reason the practice tends to be discouraged, but I'm not
an expert.
--

Larry
email is rapp at lmr dot com
11/06/04 9:38:03 AM

Larry asked...

in Message-id: ...

On 06 Nov 2004 11:10:38 GMT, Folklore killer wrote:

...snip

Connectng the batteries in parallel is perfectly safe, quite normal, will
achieve what is desired and is used in millions of installations the world
over.

If one connects a battery near the end of it's life in parallel with a
relatively new, fully-charged battery, are you saying that the old battery
will not discharge the new one?


No, that is not what I wrote.

By your own calculations, it appears that
it will.


It will, until the charges equalise.

If that's the case, then shouldn't one be careful paralleling two
batteries of different age and condition?


Yes, if they are to be paralleled while _not_ being charged.

But even then it is not so simple. The internal self drain of an old, tired is
only _slightly_ higher than a new one (unless the battery is U/S). So the
discharge is not as heavy as some people seem to believe.

Of course one could prevent this by the use of high current diodes, but
that lowers voltage by about 0.7 volts per diode.


And makes a mess of the charge voltage.

Remeber that the charge current into the battery is proportional to the square
of the voltage difference between the presented charge voltage and the open
circuit voltage at that particular charge state. So reducing the charge voltage
by 0.7 volts could actually completely prevent charging.

I think that's the reason the practice tends to be discouraged,


It is not discouraged whilst under charge. It _is_ discouraged when no charge
source is present.

Regards
Folklore Killer

"bowgus" miserably failed in attempting to insult his superior.....

in Message-id: ....

By blathering.....

So what you're saying is no need to turn in those old batteries, just hook
up a new one in parallel ...


Which is not what I wrote so you clearly need to go back to "baby school" to
learn how to read.

and oh yeah, no problem running a starting
battery (my understanding that's what GC has) for accessories until it's
dead ...


That is because either

A) You misunderstood what OP wrote or

more likely

B) You did not read it (or perhaps _couldn't_ read it).

or even more likely

C) were so busy trying to prove your superior knowledge (and failing quite
dramatically) that you forgot to read the entire thread before collapsing your
lower jawbone muscles.

they can be recharged over and over and over?


Now please quote where that was stated by anyone.

Geez ... even a deep
cycle's life is shortened by doing that.


You don't say!

I do agree that a healthy lead acid
fully discharged has a very high internal resistance ... as a matter of
fact, so high that it cannot be charged using the alternator ... my jeeping
buddy found that out.


Well you've improved your standing because now you have actually posted
something that is correct. Well done. Have a banana. As they say... "there's a
first time for everything".... Now the whole of usenet knows that you know
_one_ thing, whereas before they all thought you knew _nothing_. Well done.

But about that old starting battery (that GC has)
that's maybe dry to the plates or maybe bridged or ... my quick and dirty
risk assessment


So you forgot the read the sentence that said... "unless the battery is U/S"

U/S being "un-servicable". As in broken, faulty, kapputt. Can you actually
read?

Or perhaps that sentence did not fit in with your plans of trying to attack a
superior knowledge?

... adios old batteries ... I for one would do not connect
an old battery in parallel with my only way back to shore.


Again you show your inability to read. I did not tell OP to do that. Learn to
read. Apparently, even in your hick ridden town, there are some people who
read. Even better there are some people who can _teach_ you to read.

So post us a
picture of your batteries configuration ... I'm always up for a good laugh
... adios ... "killer" :-)


You can laugh at your own inability to read.

That is assuming you didn't actually post using your own name. You didn't did
you? Post using your own name I mean.

Not really?

Well now you must feel really stupid.

Don't worry. Usenet only has a memory of about, er.... forever. So you'll only
look stupid for about, er.... forever.

I love people like you.

You are so clever in your own little fairy world, that you forget, you are
actually very stupid.

You girlfriend told me so.

I love you all.
Cuddles.
Folklore Killer.

So what you're saying is no need to turn in those old batteries, just hook
up a new one in parallel ... and oh yeah, no problem running a starting
battery (my understanding that's what GC has) for accessories until it's
dead ... they can be recharged over and over and over? Geez ... even a deep
cycle's life is shortened by doing that. I do agree that a healthy lead acid
fully discharged has a very high internal resistance ... as a matter of
fact, so high that it cannot be charged using the alternator ... my jeeping
buddy found that out. But about that old starting battery (that GC has)
that's maybe dry to the plates or maybe bridged or ... my quick and dirty
risk assessment ... adios old batteries ... I for one would do not connect
an old battery in parallel with my only way back to shore. So post us a
picture of your batteries configuration ... I'm always up for a good laugh
.... adios ... "killer" :-)

"Folklore killer" wrote in message
...
"bowgus" blathered....

in Message-id: ...

"GC" wrote in message
...
I'm about to wire up a battery switch so that I can switch between two
batteries. (one or the other or both)

Don't use the "both" setting ... read on

Just wondering if I should put the switch on the positive or negative??
Or
doesn't it matter?

Positive is the norm around here (geez ... hurts the head to even think
about switching the negative since that's normally the electrical
reference)

Also, do both the batteries have to be the same age/rating??? I was
hoping
to use my old battery for powering accessories when I'm stopped, but
charge
it when the engine is running with the other battery.

In your situation, I would ... (get myself a deep cycle for the
accessories
... e.g. my accesory is my electric trolling motor)

a) make sure the switch is a make before break type which they normally
are
so that the alternator is never without a load if switched while running;
b) when stopped select the old battery (if it's a regular starting type
battery it won't tolerate many cycles of being discharged), before
starting,
switch to the new battery;
c) while running charge batteries by switching between the two (hmmm ...
when are they charged ... well, maybe your batteries have indicators of
the
green it's charged variety?);
d) never connect the batteries ... it's my understanding that the charged
one could pump a lot of current into the discharged (my theory ... a
battery (let's just say for the sake of argument rated 800 cold cranking
amps) has no current limiting feature like a battery charger does) which
could result in 800 amps going through wiring/switch resulting in fire,
exploding battery, ...


Thanks in advance!


My 2 cents worth ...


And worth slightly less than 2 cents.

Where do you people get your information? Disneyworld?

A typical engine start battery would be 600 to 650 CCA

As you have quoted 800 CCA we shall use that as the example.

A brand new 800 CCA engine start battery will have an internal resistance
of
around 0.004 ohms and an _open circuit_ voltage of around 12.7 volts (you
can
calculate the fully charged internal resistance of an engine start battery
using the given CCA and the _definition_ of what CCA means - but of course
you
didn't know that).

A flat deep cycle battery will have an _open circuit_ voltage of around
11.9
volts and an internal resistance of around 0.06 ohms.

The voltage _difference_ between the 2 batteries is therefore 0.8 volts
dropped
across the two internal resistances of 0.004 + 0.06 = 0.064 ohms = 12.5
amps.

And that is making the simple assumption that the voltage of the fully
charged
engine battery does not fall under load (in addition to the voltage drop
caused
by the internal resistance) which in practice it will, by about 0.2 volts
at
this sort of loading.

So the _real_ current draw will be approximately 9.4 amps.

Further, there is a dead band between the open circuit voltage and the
voltage
at which charging will commence. This dead band varies quite dramatically
depending upon the state of charge of the battery in question. A typical
figure
for a flat deep cycle battery would be about 0.2 volts.

So an even closer figure for the current draw (that you assert is 800
amps)
would be (12.5V - 12.1V) 0.4V/(total internal resistance) 0.064R = 6.25
amps.

And of course you won't believe me. So instead of using half a theory that
you
don't understand anyway, either go learn the rest of the theory or try it
and
take some measurements.

Connectng the batteries in parallel is perfectly safe, quite normal, will
achieve what is desired and is used in millions of installations the world
over.

Idiot

Folklore Killer.

Hi GC,
We'll just let those have their fun.
I have had the type of setup you're talking about in both a boat & a motor
home.
Go with the positive to the switch even though it's not quite so important
in a boat but in a vehicle (metal frame) it's essential as unless you have
an old 1955 bedford or something, most are negetive earth.
ALWAYS put your best battery on your motor start side. (say position 1 on
the switch)
NEVER be tempted to use that battery for accessories while on anchor. (it's
hard to push start a boat!)
Never forget to turn it over to No 2 when stopped.
Never forget to return to battery No 1 before starting motor.
Get a switch that will rotate either way through full 360degrees.
Have an amp gauge to monitor charge.
Once under way for several minutes when amp meter drops to zero switch to No
2 but AWAYS go in direction of No1 to BOTH to No2 so (like he said) you
always have current to alternator.
Doing it that way you also don't lose your GPS briefly.
I've said enough.
What ever you do make sure that bilge pump is ACTIVE!!!!! (get
that?........ )
Regards..... BruceM


"Folklore killer" wrote in message
...
"bowgus" miserably failed in attempting to insult his superior.....

in Message-id: ....

By blathering.....

So what you're saying is no need to turn in those old batteries, just
hook
up a new one in parallel ...


Which is not what I wrote so you clearly need to go back to "baby school"
to
learn how to read.

and oh yeah, no problem running a starting
battery (my understanding that's what GC has) for accessories until it's
dead ...


That is because either

A) You misunderstood what OP wrote or

more likely

B) You did not read it (or perhaps _couldn't_ read it).

or even more likely

C) were so busy trying to prove your superior knowledge (and failing quite
dramatically) that you forgot to read the entire thread before collapsing
your
lower jawbone muscles.

they can be recharged over and over and over?


Now please quote where that was stated by anyone.

Geez ... even a deep
cycle's life is shortened by doing that.


You don't say!

I do agree that a healthy lead acid
fully discharged has a very high internal resistance ... as a matter of
fact, so high that it cannot be charged using the alternator ... my
jeeping
buddy found that out.


Well you've improved your standing because now you have actually posted
something that is correct. Well done. Have a banana. As they say...
"there's a
first time for everything".... Now the whole of usenet knows that you know
_one_ thing, whereas before they all thought you knew _nothing_. Well
done.

But about that old starting battery (that GC has)
that's maybe dry to the plates or maybe bridged or ... my quick and dirty
risk assessment


So you forgot the read the sentence that said... "unless the battery is
U/S"

U/S being "un-servicable". As in broken, faulty, kapputt. Can you actually
read?

Or perhaps that sentence did not fit in with your plans of trying to
attack a
superior knowledge?

... adios old batteries ... I for one would do not connect
an old battery in parallel with my only way back to shore.


Again you show your inability to read. I did not tell OP to do that. Learn
to
read. Apparently, even in your hick ridden town, there are some people who
read. Even better there are some people who can _teach_ you to read.

So post us a
picture of your batteries configuration ... I'm always up for a good
laugh
... adios ... "killer" :-)


You can laugh at your own inability to read.

That is assuming you didn't actually post using your own name. You didn't
did
you? Post using your own name I mean.

Not really?

Well now you must feel really stupid.

Don't worry. Usenet only has a memory of about, er.... forever. So you'll
only
look stupid for about, er.... forever.

I love people like you.

You are so clever in your own little fairy world, that you forget, you are
actually very stupid.

You girlfriend told me so.

I love you all.
Cuddles.
Folklore Killer.

What ever you do make sure that bilge pump is ACTIVE!!!!! (get
that?........ )

Thanks for your tips
When you say the bilge pump is ACTIVE, what do you mean by that? and it
directly across one of the batteries, not through the switch??
Do you think I'm better off just getting a cheapy battery for my
accessories?? So at least its new
Thanks again!

On Sun, 7 Nov 2004 18:10:25 +1030, "BruceM"
wrote:
Once under way for several minutes when amp meter drops to zero switch to No
2 but AWAYS go in direction of No1 to BOTH to No2 so (like he said) you
always have current to alternator.

===================================

I consider switching batteries with the engine running to be a high
risk regardless of whether or not the switch is claimed to be
"make-before-break". Contacts and switch wipers can become worn or
corroded creating switching transients that can toast the alternator
in a heart beat. It happens all the time. If you have to switch with
the engine running for some reason, I'd recommend doing it only at
dead slow idle speed, and never switching it to the off position.

For what it's worth, here's what I do if the #2 has become run down.
Start the engine on #1 and let it warm up so that it will restart
easily. Shut down and restart on "both". In the "both" position the
charge will flow to the battery that needs it.

Periodiaclly test #2 by trying to start the engine with it. If it
will not crank and start a warm engine it is shot and should be
replaced. Ideally it should be capable of starting a cold engine also
but that may be asking a lot if it has been frequently deep cycled.