connecting batteries in parallel or series, myth and theory

[m II]

Uh no. the best two volt cells have "L" shaped posts that bolt
together and are then soldered.

You know what I was getting at. Let's not put too fine of point on it.
It is electrically more sound to make the proper voltage battery first,
THEN parallel them, if absolutely necessary, to increase capacity.





mike

 

[m II]

HeH. 1950"s John Deere diesel tractors used that "technology". Minus
the hydraulic starting motor. A small gasoline engine using the
communal cooling system was started electrically to warm the coolant
in the system then when the coolant reached a certain temperature the
starting pinion was hand levered to the main engine's ring gear to
crank it over. One benefit was that the diesel cylinder and head was
warm so that the intake air didn't need to be compressed quite so
tightly to make the diesel fuel combust. Resulting in cleaner exhaust
at startup without the unburned raw diesel being spewed up the pipe
and washing the lubrication oil off the cylinder walls.

Someone once said that there's nothing new under the sun. My only
experience with a Deere tractor was hand starting a two cylinder
gasoline version. You had to open the compression releases by hand, spin
the flywheel and when the thing was chuffing, close the valves. It was
really brutal on cold mornings.




mike

Densa International©
'Think tanks cleaned cheap'

Due to the insane amount of spam and garbage,
I block all postings with a Gmail, Google Mail,
Google Groups or HOTMAIL address.
I also filter everything from a .cn server.

http://improve-usenet.org/

 

[Archimedes' Lever]

Larger oil patch diesels often have hydraulic starters. Huge electric
ones are too hard on the ring gear and assorted fittings. A hydraulic
motor can be eased on slowly to full power.

There is a smaller auxiliary engine that first starts up to provide the
hydraulic pressure. That engine usually drives the alternator also. In
nicer set ups, the coolant for the large engine is also warmed up a bit
before start up. They have to, as in cold weather, the lubricating oil
is like molasses and 12 or 16 pistons don't like moving in it.


mike

--

Densa International©
'Think tanks cleaned cheap'

Due to the insane amount of spam and garbage,
I block all postings with a Gmail, Google Mail,
Google Groups or HOTMAIL address.
I also filter everything from a .cn server.

http://improve-usenet.org/

HeH. 1950"s John Deere diesel tractors used that "technology". Minus the
hydraulic starting motor. A small gasoline engine using the communal cooling
system was started electrically to warm the coolant in the system then when
the coolant reached a certain temperature the starting pinion was hand
levered to the main engine's ring gear to crank it over. One benefit was
that the diesel cylinder and head was warm so that the intake air didn't
need to be compressed quite so tightly to make the diesel fuel combust.
Resulting in cleaner exhaust at startup without the unburned raw diesel
being spewed up the pipe and washing the lubrication oil off the cylinder
walls.

They were not hydraulic, idiots, they were pneumatic. They used the
pressure that is always still in place in one's surge tank in the
braking/air system.

 

[[email protected]]

In alt.engineering.electrical [email protected] wrote:
| On Aug 9, 6:09 am, [email protected] wrote:
|> | On 8 Aug 2008 18:32:00 GMT, [email protected] wrote:
|> |
|> |>If the effect is that the _older_ strings gets _older_ faster,
|> |
|> | And if it is the newer string that gets older faster? ... Well, maybe you
|> | won't get the longevity you thought you'd paid for.
|>
|> Right. So we need to know which it is ... which string gets older faster.
|>
|> | In any event, the "best" setup does depend to some extent on the
|> | application and goals.
|>
|> How about a power backup system (charged by the grid) that gradually shifts
|> to a renewable power system (charged by solar, wind, etc) and then eventually
|> to a completely off-grid system (or at least a sell-only grid system).
|>
|> --
|> |WARNING: Due to extreme spam, googlegroups.com is blocked. Due to ignorance |
|> | by the abuse department, bellsouth.net is blocked. If you post to |
|> | Usenet from these places, find another Usenet provider ASAP. |
|> | Phil Howard KA9WGN (email for humans: first name in lower case at ipal.net) |
|
| Phil
|
| People have explained several reasons for "NOT" paralleling batteries.
| It should be avoided wherever possible.

If you'd like, feel free to summarize the thread. I plan to look it all over
again once it seems all the responses are done.


| You response tells us that you have/want parallel batteries and you
| are looking for someone to tell you it's ok.

No. If there is an alternative, I'm willing to consider that. So what would
you suggest as an alternative? More in series and increasing the voltage?


| So, Yes it is ok. As long as you also accept that it is the second
| best option and are prepared to take all responsibility for your
| choice of battery bank.

It's 2nd best compared to what is 1st best?

 

[Archimedes' Lever]

Rather than calling you an idiot I prefer to just say that any motherfucker
that calls everyone he encounters an "idiot" obviously has such a small dick
that he MUST use stupidities to soothe his damaged ego.

Wanna bet, dipshit? How about a year's salary that mine is bigger than
yours?

 

[[email protected]]

| On 8 Aug 2008 20:09:04 GMT, [email protected] wrote:
|
|>| On 8 Aug 2008 18:32:00 GMT, [email protected] wrote:
|>|
|>|>If the effect is that the _older_ strings gets _older_ faster,
|>|
|>| And if it is the newer string that gets older faster? ... Well, maybe you
|>| won't get the longevity you thought you'd paid for.
|>
|>Right. So we need to know which it is ... which string gets older faster.
|
| Read that paper again and work out the physics. Assume two strings of the
| same make/model/size but one is older. Note that the older string will
| have a lower capacity. Think about the relative resistances. Which string
| will supply most of the initial load? That will be the string that cycles
| most for shallow discharges, and will most likely age the fastest.

As long as there are no other "hidden" issues, that makes sense. I just
don't want to jump to such a conclusion before ensuring that all the issues
are known. I am not trusting any one single paper/article to cover all the
issues.


|>| In any event, the "best" setup does depend to some extent on the
|>| application and goals.
|>
|>How about a power backup system (charged by the grid) that gradually shifts
|>to a renewable power system (charged by solar, wind, etc) and then eventually
|>to a completely off-grid system (or at least a sell-only grid system).
|
| I would only use parallel strings if I had no other choice. In my case, I
| have two parallel strings, but that's because I could not obtain batteries
| of the size and capacity I wanted to set up a single string. When these
| need to be replaced, it will be with a single string of 2V cells (which are
| now available in a suitable capacity for me). Maintenance is much easier
| with fewer cells to care for.

Are you going for the 33 plate Surrette 2KS33PS cells?

 

[[email protected]]

| [email protected] wrote:
|
|> | I have never seen single cells paralleled. They are always connected in
|> | series to get the desired voltage. Then, another group of identical
|> | series cells may be paralleled with it.
|>
|> So the "reason" is "everyone else does it".
|
| Not at all. If you want 48 volts and have two volt cells to play with,
| that is 48 jumpers for the paralleling the 48 cells. Add another 24 for
| the series portion. That is a total of 72 jumpers. It's easier to make
| two batteries first, then parallel them with two jumpers.

I was planning on looking at this, and other, costs later on. I wanted to
know the technical, not economic, issues. Then I would balance between them.
If method X costs more, but works better, I might still consider it over
method Y that costs less, but doesn't work as well.

I'm focusing on the technical issues right now.


|> Aren't all the plates within one cell already parallel? By extension of that,
|> paralleling individual cells would make sense because it retains the very same
|> concept.
|
| No. They are each in series with the one next to it. The power comes in
| on one terminal, through the electrolyte and out the other plate. There
| is no chance of any circulating currents within one cell. They all share
| the same electrolyte bath. If there were only two large plates in a
| battery, we wouldn't call them paralleled. Take those two plates and
| roll them up in a cylinder shape. It may look a lot different, but
| they're still not paralleled.

How is it that the common electrolyte bath prevent circulating currents?
Does it have circulating currents in the bath itself when plate surfaces
are uneven?

Yes, I would still call a 2 plate (one "+", one "-") as parallel ... it is
paralleling multiple spots of each plate with other spots.

More plate surface means more current and/or capacity. The distinction is
whether the plates are in the same common bath, or separate. The problems
that can happen because the bath is separate, I want to know about. But
it needs to make sense in terms of the bath, so I'm looking also for theory,
not experience alone (even if the theory is just theoretical).


| Now, there *are* multiple plates on each polarity because of space
| constraints. There's no room for two huge plates with the same surface
| are. Changing the shape of each polarity plate won't make it parallel.

Well, you could have a long battery

I understand how they are physically constructed. There are lots of plates
in an interleaved configuration so that instead of it being a big long thin
cell, it's a convenient semi-cubic shape (and you can have most plates do
double duty front and back facing).

If any part of a plate is in a weaker charge state, the other parts of the
plate that are in a stonger charge state effective charge it, and it all
gets balanced out. If two cells are paralleled, that should happen between
the cells, as well. Now I could imagine that if the electrolyte condition
is what causes the cells to be slightly different, that can impact how well
the cells balance out their charge. Presumably within a single cell the
electrolyte can circulate around (not considering AGM or gel-cell here) so
each point on the plates gets an equivalent electrolyte.

 

[[email protected]]

| [email protected] wrote:
|
|> Uh no. the best two volt cells have "L" shaped posts that bolt
|> together and are then soldered.
|
| You know what I was getting at. Let's not put too fine of point on it.
| It is electrically more sound to make the proper voltage battery first,
| THEN parallel them, if absolutely necessary, to increase capacity.

If you are building a system from scratch, and selecting the cells/batteries,
would it be your intent to find invidual cells/batteries of the intended AH
capacity so you can have a single string (nothing paralleled)? If so, then
what percentage of economic savings would half capacity cells/batteries need
to be priced at to make you choose to split it into 2 parallel strings? Or
the same for quarter capacity for 4 parallel strings? Would a 15% savings
(in dollars per AH) be enough to make it worth splitting?

 

[Vaughn Simon]

If you are building a system from scratch, and selecting the cells/batteries,
would it be your intent to find invidual cells/batteries of the intended AH
capacity so you can have a single string (nothing paralleled)? If so, then
what percentage of economic savings would half capacity cells/batteries need
to be priced at to make you choose to split it into 2 parallel strings? Or
the same for quarter capacity for 4 parallel strings? Would a 15% savings
(in dollars per AH) be enough to make it worth splitting?

But there is more to battery selection than just capacity vs. money,
sometimes design issues get in the way.

As I stated earlier, my battery bank consists of two strings of two 6-volt
batteries, but the two strings are not even in the same building! Electrically,
they are all the same battery bank, but I keep one string near my generator to
assure plenty of starting current, and the other string inside my house to
potentially run inverter loads following a power failure. (I have never done
it) Heavy paralled conductors connect the two strings, which make them
effectively one battery bank, except for that tiny bit of time when there is a
heavy load on one side or the other. (Ohm's law, if there is hardly any
current, there is hardly any voltage drop) Also, that configuration gives me
the option of splitting the strings to assure that I always have sufficient
capacity to start my generator.

So you see, one string of big cells would not do.
[/QUOTE][/QUOTE][/QUOTE]
Nothing personal, but if you are posting through Google Groups I may not receive
your message. Google refuses to control the flood of spam messages originating
in their system, so on any given day I may or may not have Google blocked. Try
a real NNTP server & news reader program and you will never go back. All you
need is access to an NNTP server (AKA "news server") and a news reader program.
You probably already have a news reader program in your computer (Hint: Outlook
Express). Assuming that your Usenet needs are modest, use
http://news.aioe.org/ for free and/or http://www.teranews.com/ for a one-time
$3.95 setup fee.
Will poofread for food.

 

[m II]

How is it that the common electrolyte bath prevent circulating currents?

Ever wonder why the cells in a car battery are separated? If it didn't
matter, they wouldn't do it. The electrolyte is conductive, but it's
rectification qualities are nil.

Does it have circulating currents in the bath itself when plate surfaces
are uneven?

No. The protruding pieces of plate probably get eroded faster as the
cell is used.

Yes, I would still call a 2 plate (one "+", one "-") as parallel ... itis
paralleling multiple spots of each plate with other spots.

Don't confuse the plates, which are in series, to the current flow in
the electrolyte, which may have millions or billions (biwwions, for you
Carl Sagan fans) of parallel paths between the plates.

The electrolyte provides parallel paths. How the area of one plate is
arranged is immaterial, outside of structural constraints.

More plate surface means more current and/or capacity. The distinctionis
whether the plates are in the same common bath, or separate. The problems
that can happen because the bath is separate, I want to know about. But
it needs to make sense in terms of the bath, so I'm looking also for theory,
not experience alone (even if the theory is just theoretical).

I understand how they are physically constructed. There are lots of plates
in an interleaved configuration so that instead of it being a big long thin
cell, it's a convenient semi-cubic shape (and you can have most plates do
double duty front and back facing).

I don't know if that is important or not. Off hand, I would think that
there is very little difference between sandwiched plates and two large
ones of the same total area. I can't say for sure. I don't know.

If any part of a plate is in a weaker charge state, the other parts of the
plate that are in a stonger charge state effective charge it, and it all
gets balanced out.

I think not. At that point it's a chemical state. I can't see that
'migrating' across the surface. The most chemically active portions
would be the ones to see most of the current flow to the other plate,
however, until physical erosion teaches them a lesson.

If two cells are paralleled, that should happen between
the cells, as well. Now I could imagine that if the electrolyte condition
is what causes the cells to be slightly different, that can impact how well
the cells balance out their charge.

Sure. Electrolytes can get gassed off at different rates. The distance
between the plates, surface condition of plates and the sediment at the
bottom of each cell also count. Not all cells are created equal. The
weak ones put more of a load on the good ones. The gassing rate changes
because of unequal current flow in a parallel setup. The specific
gravity varies with charge and how much water has been forced out or
mutated into hydrogen. Unequal current flow is where all the problems
start.

These guys can explain it in a better fashion than I've been able to do:

http://www.ece.osu.edu/~fasiha/thes...of Automotive Lead-Acid Batteries (final).pdf

or:
http://tinyurl.com/57pp26




mike

--

Densa International©
'Think tanks cleaned cheap'

Due to the insane amount of spam and garbage,
I block all postings with a Gmail, Google Mail,
Google Groups or HOTMAIL address.
I also filter everything from a .cn server.

http://improve-usenet.org/

 

[Vaughn Simon]

He moved a lever which
shifted the lifters of one cylinder to an aux cam that was timed to make that
cylinder act like a steam engine. Then compressed air was introduced to that
cylinder. The rest of the valve were lifted off their seats by another lever
to relieve compression.

That is basically how the diesel aux generator in my nuclear submarine
started, compressed air straight into the cylinder. After a 60 day submerged
cruise we would place bets on how many tries it would take to get the thing to
start. It was seldom pretty.

Vaughn

 

[m II]

The biggest problem we had was that some of the carburetors had been stolen
from the pony motors and TVA was too cheap to replace them. So during
starting, a second operator would stand by the pony motor dribbling ether from
a spray can into the open intake to keep the pony motor running. Can you see
someone doing that today? The safety nazis would go into cardiac arrest!

I like that. Amazing how people adapt to the situation. You could
probably tell how much coffee he had to drink by how steady the small
engine ran...





mike

--
__ __ __ __ __ __ __ __
/ /\ / /\ / /\ / /\ / /\ / /\ / /\ / /
/ /\ \/ /\ \/This space for rent/\ \/ /\ \/ /
/_/ \/_/ \/_/ \/_/ \/_/ \/_/ \/_/ \/_/

Densa International©
'Think tanks cleaned cheap'

Due to the insane amount of spam and garbage,
I block all postings with a Gmail, Google Mail,
Google Groups or HOTMAIL address.
I also filter everything from a .cn server.

http://improve-usenet.org/

 

[m II]

These guys can explain it in a better fashion than I've been able to do:

http://www.ece.osu.edu/~fasiha/thes...of Automotive Lead-Acid Batteries (final).pdf

or:
http://tinyurl.com/57pp26

Here is another way to look at it. Series strings have enough problems
of their own. No two series strings can be 100 percent identical. When
we parallel two or more strings, the chance for problems is multiplied.

He's a bit repetitious, but it's readable.

http://www.battcon.com/PapersFinal2004/SymonsPaper2004.pdf



mike

--

Densa International©
'Think tanks cleaned cheap'

Due to the insane amount of spam and garbage,
I block all postings with a Gmail, Google Mail,
Google Groups or HOTMAIL address.
I also filter everything from a .cn server.

http://improve-usenet.org/

 

[[email protected]]

| Here is another way to look at it. Series strings have enough problems
| of their own. No two series strings can be 100 percent identical. When
| we parallel two or more strings, the chance for problems is multiplied.

So I can build a string of 24 2V cells, or 4 12V batteries, with a given
capacity, for about X dollars. I'd be inclined to go with the 24 2V cells
in one big string. But, suppose I could build a string of 4 12V batteries
that are smaller with only 1/4 capacity, for much less than X/4 dollars.
In that case, 4 such strings in parallel gets me the same capacity, but it
costs less than X. How much less than X does the 4 parallel strings option
begin to look attractive, balancing the economic gains against the technical
issues?

FYI, I'm looking at having a 20A loading capacity at 100 hour rate and 48V.
The Surrette 2KS33PS (x24) certainly looks like it would do the job well.
But I'd rather find something less pricey. It seems multiple strings of
6V or 12V batteries would do the same in capacity at half the price. But
is it worth it?

One factor in favor of multiple strings is the ability to take a string out
of service when there is a cell/battery issue. If something goes bad in a
single string, then it's all down at once. I'd rather be able to replace a
bad cell/battery in a derated system than in a shutdown system.

Other options: separate strings to separate syncronized inverters that are
then paralleled on the AC side.

 

[m II]

FYI, I'm looking at having a 20A loading capacity at 100 hour rate and 48V.
The Surrette 2KS33PS (x24) certainly looks like it would do the job well.
But I'd rather find something less pricey. It seems multiple strings of
6V or 12V batteries would do the same in capacity at half the price. But
is it worth it?

I struggled with the same problem. My conscience finally let me connect
three groups of two batteries. I didn't like doing it and I'm not proud
of it. It's never mentioned around the dinner table.

I'm using 225 amp/hr 6 volt golf cart batteries. Interstate model 2200.
63 lbs. They come in different company names. It was the most cost
effective solution as the huge two volt cells were going for almost 500
dollars each. These batteries were a bit over a hundred each. (cheaper
in the States) It was a compromise, but if I can get 5 to 8 years use
out of them I'll be happy. The Surrettes and Trojans were, I thought,
overpriced. We will see if these last.

These are what I'm using:
http://tinyurl.com/6l6w7u

Looks like even Costco sells them
http://www.rv.net/forum/index.cfm/fuseaction/thread/tid/20618277.cfm

I use a 12 volt inverter as anything higher was either not available or
more money than it was worth. The banks are connected to 2 inch copper
water pipe buss bars, which are fed/tapped at opposite ends in an
attempt to balance out the charge/discharge. So far each battery has
taken the same amount of water when refill time comes around, which is
every three months. Roughly two litres/half a gallon of water are used
in total.

The advantage of twelve volts is in the ease of charging and really
common automotive accessories. I have a 200 watt 12 volt stereo wired
into the shop. I use 12V wherever I can, like in the lighting. No sense
turning on the inverter if it isn't needed. The disadvantage is the need
for monster wiring and paralleling. I have 2/0 cable feeding the
inverter. Voltage drop becomes an issue if your use is any distance from
the source.

One factor in favor of multiple strings is the ability to take a stringout
of service when there is a cell/battery issue. If something goes bad in a
single string, then it's all down at once. I'd rather be able to replace a
bad cell/battery in a derated system than in a shutdown system.

That makes sense. Some large knife switches could isolate each bank.

Other options: separate strings to separate syncronized inverters thatare
then paralleled on the AC side.

These look very nice. I'm sure there are others out there. I don't know
if 86 percent efficiency is good or not. As far as I know, the waste may
be less in normal stand alone inverters.

http://tdi.wonatech.com/pdf/lpe_inverter_series.pdf


For the rest of us:
http://www.fieldlines.com/story/2008/4/3/22554/96299

--
__ __ __ __ __ __ __ __
/ /\ / /\ / /\ / /\ / /\ / /\ / /\ / /
/ /\ \/ /\ \/This space for rent/\ \/ /\ \/ /
/_/ \/_/ \/_/ \/_/ \/_/ \/_/ \/_/ \/_/

Densa International©
'Think tanks cleaned cheap'

Due to the insane amount of spam and garbage,
I block all postings with a Gmail, Google Mail,
Google Groups or HOTMAIL address.
I also filter everything from a .cn server.

http://improve-usenet.org/

 

[[email protected]]

In alt.engineering.electrical [email protected] wrote:
| On Aug 10, 5:09 am, [email protected] wrote:
|> In alt.engineering.electrical [email protected] wrote:
|>
|> | On Aug 9, 6:09 am, [email protected] wrote:
|> |> | On 8 Aug 2008 18:32:00 GMT, [email protected] wrote:
|> |> |
|> |> |>If the effect is that the _older_ strings gets _older_ faster,
|> |> |
|> |> | And if it is the newer string that gets older faster? ... Well, maybe you
|> |> | won't get the longevity you thought you'd paid for.
|> |>
|> |> Right. So we need to know which it is ... which string gets older faster.
|> |>
|> |> | In any event, the "best" setup does depend to some extent on the
|> |> | application and goals.
|> |>
|> |> How about a power backup system (charged by the grid) that gradually shifts
|> |> to a renewable power system (charged by solar, wind, etc) and then eventually
|> |> to a completely off-grid system (or at least a sell-only grid system).
|> |>
|> |> --
|> |> |WARNING: Due to extreme spam, googlegroups.com is blocked. Due to ignorance |
|> |> | by the abuse department, bellsouth.net is blocked. If you post to |
|> |> | Usenet from these places, find another Usenet provider ASAP. |
|> |> | Phil Howard KA9WGN (email for humans: first name in lower case at ipal.net) |
|> |
|> | Phil
|> |
|> | People have explained several reasons for "NOT" paralleling batteries.
|> | It should be avoided wherever possible.
|>
|> If you'd like, feel free to summarize the thread. I plan to look it all over
|> again once it seems all the responses are done.
|>
|> | You response tells us that you have/want parallel batteries and you
|> | are looking for someone to tell you it's ok.
|>
|> No. If there is an alternative, I'm willing to consider that. So what would
|> you suggest as an alternative? More in series and increasing the voltage?
|
| A single string of 2V cells of the correct Ah rating.
| If there is no other option then limit the parallel strings to 2.

Let me summarize/rephrase, since my intentions didn't pass through the layers of
conversation. What is the alternative for paralleling cells/batteries/strings
when the desired capacity is NOT available in single cells or small batteries?

If a 24V system needs to be doubled in capacity, and larger cells is not an
option, maybe you'd suggest a 48V system, especially if no investment has yet
been made in the 24V ssytem. But what if the system is already at 48V? Then
what? Even higher in voltage?


|> | So, Yes it is ok. As long as you also accept that it is the second
|> | best option and are prepared to take all responsibility for your
|> | choice of battery bank.
|>
|> It's 2nd best compared to what is 1st best?
|
| A single string of 2V cells of the correct Ah rating.

As you go up in single cell capacity, other issues come up. These can be things
like cost (double capacity single cell might be 3x the cost) and handling (you
can't lift the double capacity cell).

What I want to know is not so what _the_ best system design is, but information
about the advantages and disadvantages so that I could weigh one design over
another. ONE way to explain this might be in terms of the cost of everything.
For example, just how much of a savings on smaller cells/batteries makes it
worth going that route, in your opinion/experience. If I can build a system of
one string of single cells for $30000, and would like to reduce the price, at
what price level would _you_ decide it's worth going with parallel strings?
$10000?

For me, once I can get a good handle on the _actual_ issues of parallel strings
vs. parallel cells vs. parallel batteries, then I could answer the above for
myself. Right now I'm getting answers like "batteries obey the laws of physics"
without any explanation of what laws apply (there might be more laws that apply
beyond the obvious ... and that is crucial to know).

Right now I'm not getting much better answers here than I got from Googling.

But I did get one useful answer that the measurement of a single cell can be
masked when another is in parallel to it ... but that just tells me it is
better to parallel the strings rather than the cells (which also happens to
be a lower cost option).

And maybe some big rectifiers to isolate the strings from cross-charging might
be called for. It would then seem to me the only way to keep the strings
charged with the rectifiers in place is to separately charge each string.
But that might be an economic benefit from smaller chargers.

 

[[email protected]]

In alt.engineering.electrical [email protected] wrote:
| On Aug 10, 11:40 am, [email protected] wrote:
|>
|> | Here is another way to look at it. Series strings have enough problems
|> | of their own. No two series strings can be 100 percent identical. When
|> | we parallel two or more strings, the chance for problems is multiplied.
|>
|> So I can build a string of 24 2V cells, or 4 12V batteries, with a given
|> capacity, for about X dollars. I'd be inclined to go with the 24 2V cells
|> in one big string. But, suppose I could build a string of 4 12V batteries
|> that are smaller with only 1/4 capacity, for much less than X/4 dollars.
|> In that case, 4 such strings in parallel gets me the same capacity, but it
|> costs less than X. How much less than X does the 4 parallel strings option
|> begin to look attractive, balancing the economic gains against the technical
|> issues?
|
| No gains, just more problems.The economics will cost more in the long
| run.
|>
|> FYI, I'm looking at having a 20A loading capacity at 100 hour rate and 48V.
|> The Surrette 2KS33PS (x24) certainly looks like it would do the job well.
|> But I'd rather find something less pricey. It seems multiple strings of
|> 6V or 12V batteries would do the same in capacity at half the price. But
|> is it worth it?
|>
|> One factor in favor of multiple strings is the ability to take a string out
|> of service when there is a cell/battery issue. If something goes bad in a
|> single string, then it's all down at once. I'd rather be able to replace a
|> bad cell/battery in a derated system than in a shutdown system.
|
| Okay, Say you have 4 parallel strings. If you lose a cell in one
| string you are left with 3 parallel strings. It's no good replacing
| one cell in the affected string and just as bad replacing that string
| with a new one. One bad cell in a string and you are up for a new set
| of 4 parallel strings, or, forever chasing problems related to the
| different ages of batteries/cells.
|>
|> Other options: separate strings to separate syncronized inverters that are
|> then paralleled on the AC side.
|
| So you save money by using 4 parallel strings so you can afford 4
| inverters that can be synchronized. Shock, horror, 4 of this type of
| inverter will probably cost more than a single series string of cells
| of the correct Ah capacity.

Many many years ago I worked at computer services company (as a programmer
and system administrator) that had a battery based "UPS" system sized for
5 mainframes. I never found out how the batteries/cells were actually wired.
I didn't spend a lot of time in the UPS room and when I did need to go in
there to get some readings, the place was HOT! But what I do know is there
were 216 "batteries" (which may have been cells). They were in a black case
and each was significantly larger than a typical car battery by about 8 to 12
times in volume. The system would drain down in about 25 minutes if we left
all 5 mainframes on. If we lost power, the usual procedure was to shutdown
3 of them if the power was not back in 10 minutes, giving us about 45 minutes
on the remaining 2. Management was unhappy with the time frames but did not
have the room to install a larger system.

Would it be your guess that this system, installed by the vendor that sold it,
was a 432VDC system? Sorry, I didn't get any specs on the true capacity and
I can't tell you what the mainframes used (but I can say there were 600+ disk
drives of mostly IBM 3330 size). I wish now that I had enough interest in it
back then (1970s) to have checked up on the power capacities and requirements.

 

[[email protected]]

In alt.engineering.electrical clare wrote:

| Use 2 SEPARATE strings - one backing up the other, if redundancy is
| required.

[...]

| Or just split your loads. Unless a single load excedes the capacity of
| your "half system" it is much simpler to just run 2 separate systems.

That might make more sense.

Assuming all the DC systems are the same voltage, is it safe to do a closed
transition switch when changing over, where the transition is several seconds
(as in manually switching one on then switching another off)?

Based on what I see about inverters (Xantrex specifically), they are 120V only
and 120/240V is achieved by syncronizing two of them and running them in series.
So what about _feeding_ these inverters with separate DC systems? Unless they
are self-syncronized by some signal on the DC feeds, I would think there should
be no issue with this (but I've been surprised by other things before).

 

[daestrom]

I've heard from way back that connecting batteries in parallel is a
bad idea.

I've heard a couple reasons. One is that it can be hard to make sure
the batteries are loaded equally. Another reason I heard is more
complex. It says that parallel connection should be done only on a
per-cell basis, and that a series/parallel combination would thus
have parallel connections at every cell ... and therefore multi-cell
batteries are bad in this regard
while single cell (2V) "batteries" are the way to go.

Two 12V in parallel for occasional surge like starting is one thing, but a
large number of banks in parallel and having frequent charge / discharge is
another.

You've heard from folks that parallel two batteries for starting
(with/without diodes) and not had any problems.

But for large storage applications, parallel batteries are the second best
option, always. If you want to keep all the parallel strings 'up to snuff',
you will need a way to separate them and charge/test them individually.
When in parallel, it becomes more and more difficult to have them 'share the
load' as they age or as you add more parallel strings.

Series connections do result in much higher voltages and you *can* have a
problem with reversing a cell. This can be minimized by performing good
equalizer at manufacturer's recommendations.

Testing individual sp. gr. and ICV (individual cell voltage) during charging
is also a good way to find 'weak cells', regardless of connection.

FWIW, when commercial applications such as large UPS and even larger
(submarine batteries are about the largest in the world), the engineers have
always opted for special 2V cells that have the right A-H rating and as many
such cells in series as needed to get the desired voltage. I've never seen
a commercial UPS system (or submarine battery) that was connected in
parallel. (maybe they know more about this than alt.energy.homepower
posters

Depending on the voltage needed, I've seen UPS systems with 36 12V batteries
connected in series (about 430V). Submarines used about 128 2V cells
(about 270V).

daestrom

 

[Vaughn Simon]

In alt.engineering.electrical clare wrote:
So what about _feeding_ these inverters with separate DC systems?

I see no particular advantages, and I see one disadvantage...unequal
discharge on your strings. (unless your 120/240 system is perfectly balanced.)

Vaughn