I have a 12V battery, not used on a car but is connected to a 12V
recovery winch in a boat shed.
This is kept between use connected to a CETEK MXS 5 - an 8 step charger
with correct Float charge once up to full charge.

Over winter I remove and put indoors, and leave it on the charger.
The same charger wired in parallel across a pair of batteries (worked
fine on previous years)
Fitted it on w/end and it was dead ..... So to be sure put it on the
charger again for 24 Hrs
Now it appears to be working fine.

Checking voltage ..... it measures 13.5V after charge - which I can only
think must be a surface charge. (i.e greater than ~12.6)
I used the winch to pull in a 2.5T load, and seemed OK, Voltage after
the recovery was 12.75V

Would this suggest battery is probably OK ?

Perhaps having the charger across 2 batteries, when one of them was new
and higher capacity, may have provided imbalance and it shut down
charging due to back emf of the larger battery and then failed to charge
the winch battery ?

Next year I'll use separate chargers.

On 06/04/2021 20:00, rick wrote:
I have a 12V battery, not used on a car but is connected to a 12V
recovery winch in a boat shed.
This is kept between use connected to a CETEK MXS 5 - an 8 step charger
with correct Float charge once up to full charge.

Over winter I remove and put indoors, and leave it on the charger.
The same charger wired in parallel across a pair of batteries (worked
fine on previous years)
Fitted it on w/end and it was dead ..... So to be sure put it on the
charger again for 24 Hrs
Now it appears to be working fine.

Checking voltage ..... it measures 13.5V after charge - which I can only
think must be a surface charge. (i.e greater than ~12.6)
I used the winch to pull in a 2.5T load, and seemed OK, Voltage after
the recovery was 12.75V

Would this suggest battery is probably OK ?

Perhaps having the charger across 2 batteries, when one of them was new
and higher capacity, may have provided imbalance and it shut down
charging due to back emf of the larger battery and then failed to charge
the winch battery ?

Next year I'll use separate chargers.

Or just one charger, swapped between batteries once a week.

On 06/04/2021 20:00, rick wrote:
I have a 12V battery, not used on a car but is connected to a 12V
recovery winch in a boat shed.
This is kept between use connected to a CETEK MXS 5 - an 8 step charger
with correct Float charge once up to full charge.

Over winter I remove and put indoors, and leave it on the charger.
The same charger wired in parallel across a pair of batteries (worked
fine on previous years)
Fitted it on w/end and it was dead ..... So to be sure put it on the
charger again for 24 Hrs
Now it appears to be working fine.

Sorry I don't follow. What was dead, the charger, battery or winch.

Checking voltage ..... it measures 13.5V after charge - which I can only
think must be a surface charge. (i.e greater than ~12.6)
I used the winch to pull in a 2.5T load, and seemed OK, Voltage after
the recovery was 12.75V

Would this suggest battery is probably OK ?

Flattening a battery will always damage it to some extent.

Perhaps having the charger across 2 batteries, when one of them was new
and higher capacity, may have provided imbalance and it shut down
charging due to back emf of the larger battery and then failed to charge
the winch battery ?

Next year I'll use separate chargers.

I see no issue with using one charger for 2 batteries. A charged battery
on a float charger should take very little current.

On Tue, 6 Apr 2021 20:00:59 +0100, rick
wrote:

snip
Over winter I remove and put indoors, and leave it on the charger.
The same charger wired in parallel across a pair of batteries (worked
fine on previous years)

snip

That can be ok and especially if the batteries are of the same voltage
/ charge status (when you hook them together), because ...

You should also connect them via fuses (or at least one fuse in the
case of just two batteries, between the batteries) in case one battery
goes short and tries to discharge the other one ... very quickly. ;-(

Cheers, T i m

On 06/04/2021 21:54, T i m wrote:
On Tue, 6 Apr 2021 20:00:59 +0100, rick
wrote:

snip
Over winter I remove and put indoors, and leave it on the charger.
The same charger wired in parallel across a pair of batteries (worked
fine on previous years)

snip

That can be ok and especially if the batteries are of the same voltage
/ charge status (when you hook them together), because ...

You should also connect them via fuses (or at least one fuse in the
case of just two batteries, between the batteries) in case one battery
goes short and tries to discharge the other one ... very quickly. ;-(

Cheers, T i m




I think that one issue could be that the two batteries were NOT exactly
the same voltage to begin with. They have low internal resistance, so
even a small voltage difference between them could drive rather large
currents through them.

I think it would be better to charge them separately in future.

In article ,
rick wrote:
I have a 12V battery, not used on a car but is connected to a 12V
recovery winch in a boat shed.
This is kept between use connected to a CETEK MXS 5 - an 8 step charger
with correct Float charge once up to full charge.

Over winter I remove and put indoors, and leave it on the charger.
The same charger wired in parallel across a pair of batteries (worked
fine on previous years)
Fitted it on w/end and it was dead ..... So to be sure put it on the
charger again for 24 Hrs
Now it appears to be working fine.

Checking voltage ..... it measures 13.5V after charge - which I can only
think must be a surface charge. (i.e greater than ~12.6)
I used the winch to pull in a 2.5T load, and seemed OK, Voltage after
the recovery was 12.75V

Would this suggest battery is probably OK ?

Perhaps having the charger across 2 batteries, when one of them was new
and higher capacity, may have provided imbalance and it shut down
charging due to back emf of the larger battery and then failed to charge
the winch battery ?

Next year I'll use separate chargers.

If the battery is good and not connected to anything, no need to leave it
on float, as they keep their charge for a long time. Float is useful when
installed in a car etc where there is a quiescent load.

But if using the float feature, I'd make sure both batteries were fully
charged first.

--
*America is so advanced that even the chairs are electric.

Dave Plowman London SW
To e-mail, change noise into sound.

On 07/04/2021 10:32, GB wrote:
On 06/04/2021 21:54, T i m wrote:
On Tue, 6 Apr 2021 20:00:59 +0100, rick
wrote:

snip
Over winter I remove and put indoors, and leave it on the charger.
The same charger wired in parallel across a pair of batteries (worked
fine on previous years)

snip

That can be ok and especially if the batteries are of the same voltage
/ charge status (when you hook them together), because ...

You should also connect them via fuses (or at least one fuse in the
case of just two batteries, between the batteries) in case one battery
goes short and tries to discharge the other one ... very quickly. ;-(

Cheers, T i m




I think that one issue could be that the two batteries were NOT exactly
the same voltage to begin with. They have low internal resistance, so
even a small voltage difference between them could drive rather large
currents through them.

I think it would be better to charge them separately in future.

Definitely.

On 06/04/2021 21:19, Fredxx wrote:


Sorry I don't follow. What was dead, the charger, battery or winch.


The battery ....



I see no issue with using one charger for 2 batteries. A charged battery
on a float charger should take very little current.

On previous years charged each fully individually then put them in
parallel and hooked up charger and left it on over winter and all was
fine ... this year the one battery was flat.

Maybe I had a bad connection but all were in place OK

On 07/04/2021 10:32, GB wrote:


I think that one issue could be that the two batteries were NOT exactly
the same voltage to begin with. They have low internal resistance, so
even a small voltage difference between them could drive rather large
currents through them.

I think it would be better to charge them separately in future.

That is my thought - Voltages were not going to match
One was a new 170 Ah
The other a 9 yr old 74 Ah

On 08/04/2021 23:00, rick wrote:
On 06/04/2021 21:19, Fredxx wrote:


Sorry I don't follow. What was dead, the charger, battery or winch.


The battery ....



I see no issue with using one charger for 2 batteries. A charged
battery on a float charger should take very little current.

On previous years charged each fully individually then put them in
parallel and hooked up charger and left it on over winter and all was
fine ... this year the one battery was flat.

Maybe I had a bad connection but all were in place OK

I would say you had a bad connection or the battery failed o/c. I guess
a bad connection is more likely when you string two batteries in parallel.

On 08/04/2021 23:05, rick wrote:
On 07/04/2021 10:32, GB wrote:


I think that one issue could be that the two batteries were NOT
exactly the same voltage to begin with. They have low internal
resistance, so even a small voltage difference between them could
drive rather large currents through them.

I think it would be better to charge them separately in future.

That is my thought - Voltages were not going to match
One was a new 170 Ah
The other a 9 yr old 74 Ah

A float charge voltage from such a charger should be independent of
battery capacity or age, unless one of the batteries is taking current,
usually evident by gassing.

It is a good idea to check battery voltages periodically. Float charge
should be ~13.7V Anything over 13V will maintain charge.

In article ,
rick wrote:
On 07/04/2021 10:32, GB wrote:


I think that one issue could be that the two batteries were NOT exactly
the same voltage to begin with. They have low internal resistance, so
even a small voltage difference between them could drive rather large
currents through them.

I think it would be better to charge them separately in future.

That is my thought - Voltages were not going to match
One was a new 170 Ah
The other a 9 yr old 74 Ah

The voltage on a lead acid isn't dependant on the capacity. But rather
obviously different sizes are going to take different times to charge from
low.

--
*I pretend to work. - they pretend to pay me.

Dave Plowman London SW
To e-mail, change noise into sound.

On 09/04/2021 11:05, Dave Plowman (News) wrote:
In article ,
rick wrote:
On 07/04/2021 10:32, GB wrote:




The voltage on a lead acid isn't dependant on the capacity. But rather
obviously different sizes are going to take different times to charge from
low.


I realise its Volts per cell that makes up the 12V and number of cells
is the same.... but with massively bigger cells thought there may be
other factor, differnet back emf, resitance, etc.

On 12/04/2021 19:48, rick wrote:
I realise its Volts per cell that makes up the 12V and number of cells
is the same.... but with massively bigger cells thought there may be
other factor, differnet back emf, resitance, etc.

The bigger cells will give you more current for a longer time.

Andy

In article ,
rick wrote:
On 09/04/2021 11:05, Dave Plowman (News) wrote:
In article ,
rick wrote:
On 07/04/2021 10:32, GB wrote:




The voltage on a lead acid isn't dependant on the capacity. But rather
obviously different sizes are going to take different times to charge from
low.


I realise its Volts per cell that makes up the 12V and number of cells
is the same.... but with massively bigger cells thought there may be
other factor, differnet back emf, resitance, etc.

Yes, but in terms of charging none of that makes much difference. It's the
charger which determines the charge rate.

--
*Organized Crime Is Alive And Well; It's Called Auto Insurance.

Dave Plowman London SW
To e-mail, change noise into sound.

rick wrote:
On 09/04/2021 11:05, Dave Plowman (News) wrote:
In article ,
rick wrote:
On 07/04/2021 10:32, GB wrote:




The voltage on a lead acid isn't dependant on the capacity. But rather
obviously different sizes are going to take different times to charge
from
low.


I realise its Volts per cell that makes up the 12V and number of cells
is the same.... but with massively bigger cells thought there may be
other factor, differnet back emf, resitance, etc.

The voltage is "electrochemistry". There's a professor in the
chemistry building at your local university, that's all he does :-)

It's an electrochemical potential. If you sit still long enough,
the electrochemistry prof will teach you the Nernst equation.

See the third page - the battery acid is roughly 12 molar when
the battery is fully charged. That is giving slightly over 2 volts
per cell. There's also a temperature term in the equation, and battery
chargers should really know how warm the battery is.

http://ecee.colorado.edu/~ecen2060/m...tery_paper.pdf

It is modified by the boundary layer (the concentration of materials
there can be different than the bulk material). There is a layer of material
next to the plate, and after the battery is charged, that's a bit
"disturbed". After a few hours, the cell settles to its final (real) voltage.

A cell has resistance, but that only affects the voltage under load.
If the cell has a resistance of one ohm, one ampere of current is
drawn, then the cell will appear to be delivering one volt less
voltage than before the load was applied. A car battery will have
resistance values, well under an ohm. For example, by measuring my
own car, one operating point is "9V @ 150A". When the starter
starts cranking, the battery voltage drops due to its internal
resistance, and those two values are what I measured with two meters
set to "peak hold" mode.

The behaviors of the battery are also measured by various pieces of
equipment. At the Battery University website, they talk of
"impedance spectrum", which is related to the transient behavior
during charging. It is possible to collect health information about
a battery, by studying the impedance. And doing it with pulses of current.

There is no back-EMF as such. Back-EMF is a property of motors
(and by analogy, likely generators or alternators as well).

If you change the chemical composition of the lead paste used
in the plates, that's more likely to alter the potential. Just as
lithium iron and lithium cobalt have radically different voltages.

Paul