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user9253
Joined: 28 Mar 2008
Posts: 1921
Location: Riley TWP Michigan
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 Posted: Thu Sep 15, 2016 3:46 pm Post subject: battery misconceptions |
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Below is quote from another forum. Are the statements true or false? Or part true and part false?
| Quote: | | . . . .batteries (in parallel) need to be the same; same type, same brand, same model, same age. . . . I think parallel batteries should have diodes between them on the charging leg to prevent one draining the other. |
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Joe Gores |
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lyleapgmc
Joined: 19 Feb 2014
Posts: 57
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| Posted: Thu Sep 15, 2016 5:20 pm Post subject: battery misconceptions |
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A couple on interesting concepts are revealed there. Theoretically the
batteries should be identical in all the factors mentioned. In reality,
I doubt you will find that situation in the wild. There may be two that
are close enough for government work but not identical
The diodes mentioned would be to permit two slightly dissimilar
batteries to be connected in parallel and function well.
Why the need or desire for batteries in parallel? The output voltage is
not increased. The capacity is increased but a better plan would be one
battery of sufficient capacity for the task at hand.
If higher voltage is needed, but by using two batteries in series, the
same concept the writer mentions should be applied or use one higher
voltage battery. Even then you have a series of batteries, or cells, in
series each operating on its own.
Regular cell checks would help to reduce differences in the cells thus
the performance of the whole battery package.
On 9/15/2016 6:46 PM, user9253 wrote:
| Quote: |
Below is quote from another forum. Are the statements true or false? Or part true and part false?
> . . . .batteries (in parallel) need to be the same; same type, same brand, same model, same age. . . . I think parallel batteries should have diodes between them on the charging leg to prevent one draining the other.
--------
Joe Gores
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=460501#460501
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nuckolls.bob(at)aeroelect
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| Posted: Thu Sep 15, 2016 7:05 pm Post subject: battery misconceptions |
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At 06:46 PM 9/15/2016, you wrote:
| Quote: | --> AeroElectric-List message posted by: "user9253" <fransew(at)gmail.com>
Below is quote from another forum. Are the statements true or false? Or part true and part false?
> . . . .batteries (in parallel) need to be the same; same type, same brand, same model, same age. . . . I think parallel batteries should have diodes between them on the charging leg to prevent one draining the other. |
There is no foundation in physics for having
paralleled batteries 'match' . . .
When you charge a battery from a constant
voltage bus, there is no way that one battery
feels the effects of any other batteries
irrespective of their numbers, size or
condition.
Further, when discharging paralleled batteries,
each battery will deliver what ever energy
it contains . . . again, there is no exchange
of performance between batteries beyond each
battery's ability to deliver energy.
I have never understood the diodes for preventing
one battery from 'discharging' another. Unless
the discharged battery suffers badly trashed
chemistry or shorted cells, the exchange of
energy between a fully discharged battery and
a fully charged battery is minuscule.
I think it was about 2005 when Paul M. mounted a
ambitious effort to illustrate design flaws in
the crowbar ov protection system. I think it was
that same series of threads where he cited "sparks"
that occur when you jumper-cable connect a 'dead'
battery with a charged battery . . . perhaps
sustained at 14+ volts by the rescue vehicle.
Yes, connect a battery with less that 5% capacity
to a fully charged battery, you do indeed see
some sparks. But plot a curve of the energy exchanged
between the two and you'll find that it is
insignificant. A battery delivers significant
energy at 12.5 volts and down. But it takes
a sustained application of 13.5 volts or more
to push significant energy into a battery. That's
why we call them 14-volt systems.
Which explains the sparking observed when
the dead battery is being connected to a running
rescue vehicle. NOW . . . we're hooking our
dead patient to a !4-Volt source . . . that's
a different situation.
But the idea that diodes are useful for mitigation
of energy exchange between two good batteries
in different states of charge is simply unsupported
by the physics.
Bob . . .
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nuckolls.bob(at)aeroelect
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| Posted: Fri Sep 16, 2016 6:36 am Post subject: battery misconceptions |
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At 08:18 PM 9/15/2016, you wrote:
| Quote: | AeroElectric-List message posted by: Lyle Peterson <lyleap(at)centurylink.net>
A couple on interesting concepts are revealed there. Theoretically the batteries should be identical in all the factors mentioned. In reality, I doubt you will find that situation in the wild. There may be two that are close enough for government work but not identical
Why the need or desire for batteries in parallel? The output voltage is not increased. The capacity is increased but a better plan would be one battery of sufficient capacity for the task at hand. |
Paralleled batteries have been proposed in numerous
AEC architectures over the years . . . but paralleled
only for the purpose of (1) improved cranking performance
(Z-14) or (2) offering a means for separating two batteries
into separate tasks during battery-only endurance
operations. In these cased, the batteries were only
CHARGED in parallel . . . a situation were differences
in size and condition of the battery is transparent
to functionality of all batteries.
During battery-only ops, individual batteries are
partitioned into separate tasks. Again, the capacity
and condition of any one battery has no influence on
performance of other battery(ies).
| Quote: | | If higher voltage is needed, but by using two batteries in series, the same concept the writer mentions should be applied or use one higher voltage battery. Even then you have a series of batteries, or cells, in series each operating on its own. |
This is what happens when a battery is crafted to
meet system design goals. Earliest cars were 6v
(3 lead-acid cells in battery). Later cars were
12v (6 cells), some vehicles are 24v (12 cells),
many electric vehicles are 36v (18 cells). Now,
wether that array of cells is crafted in one device
of necessary cells or multiple devices with smaller
numbers of cells in series is driven by design goals.
| Quote: | | Regular cell checks would help to reduce differences in the cells thus the performance of the whole battery package. |
Exactly. Cells connected series are no more capable
than the weakest cell in the string.
Back in the days when I shot a lot of film professionally,
I used Vivitar 283 flashguns with 4xAA battery paks. I
tried using NiCd . . . for a time . . . but they proved
labor intensive to mitigate risk of a poor shoot. If
only one cell of the 4 was weak, it would fall out of
bed before the other three cells . . . but the whole
array became useless. For a time, I would check the cells
for capacity and build 'teams' of cells with similar
capacity but it took too much time. Clients were paying
all expenses of the job and a hand-full of batteries
for radios, cameras, telemetry and data acquisition
systems was perhaps 0.1% of the mission costs. I quit
fiddling with NiCd and loaded fresh alkaline cells
into all electro-whizzies at the start of the day.
Cell matching becomes important for the case of
building a large array battery from a series
connected array of smaller batteries, . . . not just
within any one battery but the whole string.
This is the condition that prompts cap checks with
benchmarks that call for replacement of the whole
array at some established value . . . usually 80%
of new. This benchmark is picked to meet battery
only endurance goals while insuring that no single
cell gets so far ahead in the down-hill run to
failure that it puts all other cells at risk.
Bob . . .
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