cross-battery voltage
 

I am taking the "bold" (for me) step of leaving my two batteries in the boat
over the long, cold Canadian winter, a common practice, based on the theory
that a good, well-charged battery won't freeze, and if it does, it wasn't
good and should have been replaced anyway. Furthermore, keeping it cold
should prevent it from self-discharging as quickly as it would in a warm
place.

So, I disconnected the negative leads (which sounds backwards, but that's
what many pros do), turned the switch OFF, and tested the voltages with a
digital multimeter. This was after an hour or so off the trickle charger,
which has been on for weeks. The water levels are fine.

One battery, my new-this-season deep cycle, read 13.45V. The older one, the
starter battery, said 13.33V. These, of course are resting voltages, and I
realize not as valuable as voltage under load, and I didn't test the sg's -
I may still do that. (Although my deep cycle batterry which failed
mid-season had had good sg's until it died, then they went way south in a
hurry.)

However, the thing which really puzzles me is that when (for some reason) I
tested the voltage between the positive and negative posts of the opposite
batteries, i.e. neg on battery one and pos on battery two, and vice versa,
there was a momentary reading in the 5V range, which within a split second
dropped to 1.5V, and from there plummeted. I watched it down to about 0.6V,
but it was still dropping rapidly.

If I let them rest for a minute, the same sequence would repeat. So, I
disconnected the positive leads too, and wiped off any dust and grime from
the battery surfaces, but the same thing kept happening.

Any explanation? I doubt very much that it indicates any kind of a problem.
But I'm not sure. Mainly, however, I'm curious and puzzled. Thanks in
advance.

Charles

====

Charles T. Low
- remove "UN"
www.boatdocking.com
www.ctlow.ca/Trojan26 - my boat

====

cross-battery voltage
 

On Sun, 9 Nov 2003 19:33:18 -0500, "Charles T. Low"
wrote:


However, the thing which really puzzles me is that when (for some reason) I
tested the voltage between the positive and negative posts of the opposite
batteries, i.e. neg on battery one and pos on battery two, and vice versa,
there was a momentary reading in the 5V range, which within a split second
dropped to 1.5V, and from there plummeted. I watched it down to about 0.6V,
but it was still dropping rapidly.

If I let them rest for a minute, the same sequence would repeat. So, I
disconnected the positive leads too, and wiped off any dust and grime from
the battery surfaces, but the same thing kept happening.

Any explanation? I doubt very much that it indicates any kind of a problem.
But I'm not sure. Mainly, however, I'm curious and puzzled. Thanks in
advance.
================================

What you describe sounds like a capacitor bleeding off its charge
through your voltmeter. Other than that it's difficult to say what is
going on. If you have a voltmeter with very high input impeadance,
there's a possibility that you are just picking up stray electrical
noise originating somewhere else. If you disconnect both the positive
and negative leads there should be no possibility of stray leakage. I
usually don't bother however on my own boat, just turn off the
battery switches. The only exception is my generator battery which
has three leads to the positive terminal. Those I disconnect.

cross-battery voltage
 

I'm no battery expert but I'm an electrical Engineer.
What you saw I think is capacitance.
Did you have the positives both hooked up to any sort of battery switch or
some other device?
Were the negatives still disconnected?
That could explain that.

Typically, when I'm working with electronics boards, a voltage measurement
that starts high then drops is due to a discharge of a capacitor. A volt
meter has a resistance to it (usually pretty high for a digital meter).
Likewise, if you are measuring resistance, the meter is putting out a
voltage, so capacitance will affect any resistance measurement.

Thats my 2cents worth.

David

"Charles T. Low" wrote in message
...
I am taking the "bold" (for me) step of leaving my two batteries in the
boat
over the long, cold Canadian winter, a common practice, based on the
theory
that a good, well-charged battery won't freeze, and if it does, it wasn't
good and should have been replaced anyway. Furthermore, keeping it cold
should prevent it from self-discharging as quickly as it would in a warm
place.

So, I disconnected the negative leads (which sounds backwards, but that's
what many pros do), turned the switch OFF, and tested the voltages with a
digital multimeter. This was after an hour or so off the trickle charger,
which has been on for weeks. The water levels are fine.

One battery, my new-this-season deep cycle, read 13.45V. The older one,
the
starter battery, said 13.33V. These, of course are resting voltages, and I
realize not as valuable as voltage under load, and I didn't test the
sg's -
I may still do that. (Although my deep cycle batterry which failed
mid-season had had good sg's until it died, then they went way south in a
hurry.)

However, the thing which really puzzles me is that when (for some reason)
I
tested the voltage between the positive and negative posts of the opposite
batteries, i.e. neg on battery one and pos on battery two, and vice versa,
there was a momentary reading in the 5V range, which within a split second
dropped to 1.5V, and from there plummeted. I watched it down to about
0.6V,
but it was still dropping rapidly.

If I let them rest for a minute, the same sequence would repeat. So, I
disconnected the positive leads too, and wiped off any dust and grime from
the battery surfaces, but the same thing kept happening.

Any explanation? I doubt very much that it indicates any kind of a
problem.
But I'm not sure. Mainly, however, I'm curious and puzzled. Thanks in
advance.

Charles

====

Charles T. Low
- remove "UN"
www.boatdocking.com
www.ctlow.ca/Trojan26 - my boat

====

cross-battery voltage
 

Downside to leaving even fully charged batteries aboard over winter storage
period is freezing. If temps get low enough(?) and a battery charge becomes
low enough the electrolyte will freeze. This often leads to a fractured
battery case. When the thaw comes the electrolyte gets everywhere in the bilge
you don't want it to be. This leads to a major clean up in the Spring plus the
time and trouble to replace batteries.

Saw this happen a few times at our marina in Northern Virginia so I always
removed my batteries after hauling the boat. Now that I live on the Gulf Coast
I no longer remove the batteries but I boat pretty much year round here.

IMO, you're talking about really cheap insurance.

Butch

cross-battery voltage
 

Thanks, David (and Wayne),

This phenomonen occurred even after I disconnected all the leads, neg and
pos, both batteries.

So: capacitance from what?

Charles

====

Charles T. Low
- remove "UN"
www.boatdocking.com
www.ctlow.ca/Trojan26 - my boat

====

"David Ditch" wrote in message
...
I'm no battery expert but I'm an electrical Engineer.
What you saw I think is capacitance.
Did you have the positives both hooked up to any sort of battery switch or
some other device?
Were the negatives still disconnected?
That could explain that.

Typically, when I'm working with electronics boards, a voltage measurement
that starts high then drops is due to a discharge of a capacitor. A volt
meter has a resistance to it (usually pretty high for a digital meter).
Likewise, if you are measuring resistance, the meter is putting out a
voltage, so capacitance will affect any resistance measurement.

Thats my 2cents worth.

David

cross-battery voltage
 

A modern DVM has very high input impedance, which is another way of saying
it will measure a voltage even when there is almost no power behind it -- a
voltage that, in a sense, isn't really there. So what you were measuring is
essentially meaningless. If you want to demonstrate this, try putting a
large value resistor (say, 100,000 ohms) across the red and black posts of
your meter and doing the same tests -- you'll find that the batteries
measure exactly the same (there's a theoretical, unmeasurable difference),
but the strange readings won't happen.

BTW, at least down here in the warm climates (Boston), it is assumed that a
even a good lead acid battery will self discharge enough over the winter so
that it will freeze -- a trickle once in a while is good.


--
Jim Woodward
www.mvFintry.com


...
"Charles T. Low" wrote in message
...
I am taking the "bold" (for me) step of leaving my two batteries in the
boat
over the long, cold Canadian winter, a common practice, based on the
theory
that a good, well-charged battery won't freeze, and if it does, it wasn't
good and should have been replaced anyway. Furthermore, keeping it cold
should prevent it from self-discharging as quickly as it would in a warm
place.

So, I disconnected the negative leads (which sounds backwards, but that's
what many pros do), turned the switch OFF, and tested the voltages with a
digital multimeter. This was after an hour or so off the trickle charger,
which has been on for weeks. The water levels are fine.

One battery, my new-this-season deep cycle, read 13.45V. The older one,
the
starter battery, said 13.33V. These, of course are resting voltages, and I
realize not as valuable as voltage under load, and I didn't test the
sg's -
I may still do that. (Although my deep cycle batterry which failed
mid-season had had good sg's until it died, then they went way south in a
hurry.)

However, the thing which really puzzles me is that when (for some reason)
I
tested the voltage between the positive and negative posts of the opposite
batteries, i.e. neg on battery one and pos on battery two, and vice versa,
there was a momentary reading in the 5V range, which within a split second
dropped to 1.5V, and from there plummeted. I watched it down to about
0.6V,
but it was still dropping rapidly.

If I let them rest for a minute, the same sequence would repeat. So, I
disconnected the positive leads too, and wiped off any dust and grime from
the battery surfaces, but the same thing kept happening.

Any explanation? I doubt very much that it indicates any kind of a
problem.
But I'm not sure. Mainly, however, I'm curious and puzzled. Thanks in
advance.

Charles

====

Charles T. Low
- remove "UN"
www.boatdocking.com
www.ctlow.ca/Trojan26 - my boat

====

cross-battery voltage
 

Charles,
Strangest electrical problem I came across was when a car battery would
go dead all the time. I hooked up a test light in series with the positive
cable and sure enough had a current drain, commenced to pull out the fuses
to isolate the circuit but with all the fuses out and the clock disconnected
I still had the drain. I eventually started to remove the battery and
noticed that as soon as I lifted the battery off of its metal holder the
test light went out. Turned out to be a cracked case on the bottom of the
battery. Also realize that a good quality digital volt meter puts virtually
no load on a battery, it can at times show the battery to have sufficient
voltage but in reality it's only measuring a surface charge that quickly
dissipates under a load. If you put a small load across the battery such as
a small light, you'll get a more realistic reading.
Paul

"Charles T. Low" wrote in message
...
I am taking the "bold" (for me) step of leaving my two batteries in the
boat
over the long, cold Canadian winter, a common practice, based on the
theory
that a good, well-charged battery won't freeze, and if it does, it wasn't
good and should have been replaced anyway. Furthermore, keeping it cold
should prevent it from self-discharging as quickly as it would in a warm
place.

So, I disconnected the negative leads (which sounds backwards, but that's
what many pros do), turned the switch OFF, and tested the voltages with a
digital multimeter. This was after an hour or so off the trickle charger,
which has been on for weeks. The water levels are fine.

One battery, my new-this-season deep cycle, read 13.45V. The older one,
the
starter battery, said 13.33V. These, of course are resting voltages, and I
realize not as valuable as voltage under load, and I didn't test the
sg's -
I may still do that. (Although my deep cycle batterry which failed
mid-season had had good sg's until it died, then they went way south in a
hurry.)

However, the thing which really puzzles me is that when (for some reason)
I
tested the voltage between the positive and negative posts of the opposite
batteries, i.e. neg on battery one and pos on battery two, and vice versa,
there was a momentary reading in the 5V range, which within a split second
dropped to 1.5V, and from there plummeted. I watched it down to about
0.6V,
but it was still dropping rapidly.

If I let them rest for a minute, the same sequence would repeat. So, I
disconnected the positive leads too, and wiped off any dust and grime from
the battery surfaces, but the same thing kept happening.

Any explanation? I doubt very much that it indicates any kind of a
problem.
But I'm not sure. Mainly, however, I'm curious and puzzled. Thanks in
advance.

Charles

====

Charles T. Low
- remove "UN"
www.boatdocking.com
www.ctlow.ca/Trojan26 - my boat

====

cross-battery voltage
 

Are your fingers touching the probes?
Is it a good $200 meter or a cheaper $40 meter.

Unless you can see leakage I'd not worry about it.
If your battery tray is metal like a car and it measures differently when
you remove the battery from the tray, then you may have a leak somewhere.

David
"Charles T. Low" wrote in message
...
Thanks, David (and Wayne),

This phenomonen occurred even after I disconnected all the leads, neg and
pos, both batteries.

So: capacitance from what?

Charles

====

Charles T. Low
- remove "UN"
www.boatdocking.com
www.ctlow.ca/Trojan26 - my boat

====

"David Ditch" wrote in message
...
I'm no battery expert but I'm an electrical Engineer.
What you saw I think is capacitance.
Did you have the positives both hooked up to any sort of battery switch
or
some other device?
Were the negatives still disconnected?
That could explain that.

Typically, when I'm working with electronics boards, a voltage
measurement
that starts high then drops is due to a discharge of a capacitor. A volt
meter has a resistance to it (usually pretty high for a digital meter).
Likewise, if you are measuring resistance, the meter is putting out a
voltage, so capacitance will affect any resistance measurement.

Thats my 2cents worth.

David

cross-battery voltage
 

Thanks again, David,

These batteries are enlosed in wood. Fingers not touching probes. It's the
cheaper meter. I won't worry.

Now, to answer other messages all in this one: Butch (JDavis1277) says that
freezing is bad. Yes it is. But a charged battery will not freeze. We leave
our cars outside, and it gets down to minus 30 C here some nights in
Jan/Feb. (Burr.) People who store "summer" cars in their unheated garages
often leave the batteries in, and never give them a second thought until
Spring. Lots of boaters leave their batteries in. If there have been
problems at your marina, are they due to poorly maintained and incompletely
charged batteries being left in the cold? Is this reasonably avoidable by
good maintenance and monitoring?

Jim Woodward says "down here in the warm climates (Boston), it is assumed
that a even a good lead acid battery will self discharge enough over the
winter so that it will freeze -- a trickle once in a while is good." It's
good to err on the side of caution, but if the battery is clean (to avoid
trace currents between the posts), the electrolyte good, the charge high,
and the plates in good condition (but how would you know?), then a battery
will last longer in the cold that in the warm. I know knowledgeabe boaters
with batteries over ten years old that they leave in the boat over the
winter, with never any trouble. Maybe they're just lucky. I used to keep
mine in the basement, then moved them to my unheated garage, and put them
back on the charger for a few days mid-winter - but they never needed it.

Paul Schilter says "If you put a small load across the battery such as a
small light, you'll get a more realistic reading." I'm going to do more
research into this. Some dockside "authorities" recommend a big load, like
monitoring the voltage while starting the engine. Is a small load really
enough?

Thanks to all.

Charles

====

Charles T. Low
- remove "UN"
www.boatdocking.com
www.ctlow.ca/Trojan26 - my boat

====

"David Ditch" wrote in message
...
Are your fingers touching the probes?
Is it a good $200 meter or a cheaper $40 meter.

Unless you can see leakage I'd not worry about it.
If your battery tray is metal like a car and it measures differently when
you remove the battery from the tray, then you may have a leak somewhere.

David

cross-battery voltage
 

A little research is a wonderful thing. Nigel Calder has several relevant
tables -- Boatowner's Mechanical and Electrical Manual, second edition,
International Marine, 1996, page 39 (Calder was by far the single most often
seen book on voyaging boats on our circumnav.)

At 10C, a battery will self discharge about 5% per month (I'm interpolating,
but that's probably a good number). So, if you start fully charged, after
five months, you'll be around 75% charged and have a freezing point
around -40 (C or F). That's probably safe, as Charles says, particularly in
an.area where 10C may be a hot day. This may even be one of those strange
situations where something is less likely to freeze in a really cold climate
than in a more temperate one.

I suspect the standard practice here may relate to several things:
a) the temperature during storage is often above 10C
b) the storage season may be as long as 8 or 9 months, particularly in Maine
c) the battery may not be fully charged to begin with -- many marine
batteries are never fully charges
d) general conservatism


--
Jim Woodward
www.mvFintry.com


..
"Charles T. Low" wrote in message
...
Thanks again, David,

These batteries are enlosed in wood. Fingers not touching probes. It's the
cheaper meter. I won't worry.

Now, to answer other messages all in this one: Butch (JDavis1277) says
that
freezing is bad. Yes it is. But a charged battery will not freeze. We
leave
our cars outside, and it gets down to minus 30 C here some nights in
Jan/Feb. (Burr.) People who store "summer" cars in their unheated garages
often leave the batteries in, and never give them a second thought until
Spring. Lots of boaters leave their batteries in. If there have been
problems at your marina, are they due to poorly maintained and
incompletely
charged batteries being left in the cold? Is this reasonably avoidable by
good maintenance and monitoring?

Jim Woodward says "down here in the warm climates (Boston), it is assumed
that a even a good lead acid battery will self discharge enough over the
winter so that it will freeze -- a trickle once in a while is good." It's
good to err on the side of caution, but if the battery is clean (to avoid
trace currents between the posts), the electrolyte good, the charge high,
and the plates in good condition (but how would you know?), then a battery
will last longer in the cold that in the warm. I know knowledgeabe boaters
with batteries over ten years old that they leave in the boat over the
winter, with never any trouble. Maybe they're just lucky. I used to keep
mine in the basement, then moved them to my unheated garage, and put them
back on the charger for a few days mid-winter - but they never needed it.

Paul Schilter says "If you put a small load across the battery such as a
small light, you'll get a more realistic reading." I'm going to do more
research into this. Some dockside "authorities" recommend a big load, like
monitoring the voltage while starting the engine. Is a small load really
enough?

Thanks to all.

Charles

====

Charles T. Low
- remove "UN"
www.boatdocking.com
www.ctlow.ca/Trojan26 - my boat

====

"David Ditch" wrote in message
...
Are your fingers touching the probes?
Is it a good $200 meter or a cheaper $40 meter.

Unless you can see leakage I'd not worry about it.
If your battery tray is metal like a car and it measures differently
when
you remove the battery from the tray, then you may have a leak
somewhere.

David