Looking for a UPS Design That Doesn't Overheat Batteries

[JoeSP]

IN both charge and discharge, this is true
THe same current flows through both.



What you want here is not that (Separate chargers for each battery
gets expensive) but a cell/ battery monitor system. Such as Cell
watch, or Alber.

"Real" ups systems use them all the time.
jk

Back in the days when batteries were mostly considered a power source, and
ran 36 VDC lights and appliances, charged by a donkey engine or a
windcharger, it was part of regular maintenance to test each 2V glass-jar
cell with a hydrometer and an ammeter. When one was faulty, you simply
unbolted the lead bars, inspected the lead plates for sulfation and erosion,
replaced the electrolyte, cleaned the plates, and you were back in business.

 

[no one that you know]

Have ya tried Alpha UPS?
I maintain about 590 of them all with three 12volt batts each.

 

[daestrom]

Quite well - since two batteries permanently connected in series are
acting as a single battery, all cells will have the same "life
experience". Chances are that when one cell fails, most of others are
nearing end-of-life.

There are exceptions to this. If one cell shorts because of some bridging
across the seperator, it may be caused by sediment buildup in the bottom of
the cell jar. This sediment *could* also be building up in other cells at
the same rate, but there's no guarantee. One cell could also be damaged
from some contaminant getting in that wouldn't affect others (using high
mineral water for watering the cell is one way they can become
contaminated). If the battery was left deeply discharged for a while, a
'tree' of sulphate may have formed in one or more cells bridging the
separator, but not formed in others before recharging (the importance of not
leaving the battery discharged for too long).

Certainly is preferable to have all cells in series, no series-parallel
connections. In series they all have the same charge/discharge current.

Now, if all the cells are nearing the number of cycles the manufacturer
rates them for, and one shorts, then yes, the others are probably not long
for this world either. That's one reason to use an ah meter and replace all
the cells at the same time. Otherwise its pretty hard to keep track of
which ones are getting old and which ones are still 'young'.

Of the many shorted cells I've seen, I have yet to see one 'run away' if the
cells are all wired in series. The result is usually the cell takes a
charge but quickly discharges again in a few hours. If only a single string
of series cells, the two volts available internally to the shorted cell is
the only voltage available to generate short-circuit current and the
resistance of a 'short' is actually fairly high. When loading the battery,
the shorted cell actually reverses polarity and terminal voltage of the
battery drops to the minimum much sooner than expected.

Now, if you wire two or more strings in parallel, then when one cell shorts
it not only discharges itself, but the string its in now becomes a lower
voltage battery in parallel with one or more higher voltage batteries.
*This* can become dangerous pretty fast since the energy of all the other
parallel strings tries to 'charge' the lower voltage string. And all that
energy goes into heat & hydrogen.

daestrom

 

[repatch]

I haven't described any symptoms.

Sorry, thought you were the original op, fine, change my line to "symptoms
described by the original op".

According to "solar consultant" George Ghio?

I don't know who that is.

Spoken like a hide-bound bureaucrat How about matching voltages
within some range or equivalent series resistances? Given a max charging
current, we could use these imbalances to predict the max temp rise.

Sure, go for it, how much is your time worth really?

You'd have to discharge each battery and insure the discharge curves are
the same, at several different loads. Then you'd have to confirm that
their charge performance is the same. How much time do you expect that to
take?

It's up to you, if you feel that spending DAYS matching up different
batteries is worth it then more power to you.

Personally, why ANYBODY would consider doing anything less then the UPS
and battery manufacturers recommend with regards to battery
replacement when dealing with something as critical as a UPS system just
doesn't make sense to me.

Nickel and diming sometimes does make sense, in the case, I don't
personally believe it makes any sense.

TTYL

 

[[email protected]]

... if you wire two or more strings in parallel, then when one cell shorts
it not only discharges itself, but the string its in now becomes a lower
voltage battery in parallel with one or more higher voltage batteries.
*This* can become dangerous pretty fast

I doubt that.

... since the energy of all the other parallel strings tries to 'charge'
the lower voltage string. And all that energy goes into heat & hydrogen.

We might somehow come up with a "dangerous" example of this kind, but
it seems to me that the electrical energy in a battery is insufficient
to raise its temp more than a few degrees. For example, a Diehard might
store 1 kWh (3400 Btu) and weigh 30 pounds. If it contained half water,
the temp rise might be 3400/15 = 226 F. The water might boil, but it
seems unlikely to explode, even if every cell in the battery shorts at
the same instant.

Nick

 

[[email protected]]

You'd have to discharge each battery and insure the discharge curves are
the same, at several different loads. Then you'd have to confirm that
their charge performance is the same.

I disagree.

Nick

 

[repatch]

I disagree.

Well that's fine, but care to explain WHY you disagree?

Any time you have several batteries in series ensuring they are a match is
very important. Otherwise you'll have cases where some batteries are over
charged and others are undercharged. The series will perform below par due
to this, it will overly stress both the undercharged (due to deeper
cycling) and overcharged (due to being, well, overcharged!) batteries
resulting at best in a shorted life for all batteries concerned. At worst
you may have a battery fail in a very bad way (think acid, heck maybe
kaboom if enough hydrogen is emitted).

Again, it doesn't matter what ANY of us say, you should simply follow what
the UPS and battery manufacturers recommend, and in all cases I've seen
where you've got batteries in series, both manufacturers recommend
replacement of the whole string at once, NEVER battery per battery.

TTYL

 

[repatch]

I doubt that.

The problem is overcharging. Any time you over charge a lead acid battery
you risk emission of hydrogen gas. This is a VERY series issue, people
have been killed because of this issue.

We might somehow come up with a "dangerous" example of this kind, but it
seems to me that the electrical energy in a battery is insufficient to
raise its temp more than a few degrees. For example, a Diehard might store
1 kWh (3400 Btu) and weigh 30 pounds. If it contained half water, the temp
rise might be 3400/15 = 226 F. The water might boil, but it seems unlikely
to explode, even if every cell in the battery shorts at the same instant.

You don't seem to understand. The battery exploding due to internal
pressure is NOT what's really dangerous (although really, I wouldn't want
to be in a room where an acid filled battery exploded) since they are
designed to relieve the pressure before that happens.

The issue is the pressure is due to hydrogen production. Hydrogen is very
explosive, it's NOT something you want your string of batteries producing
and emitting.

TTYL

 

[[email protected]]

Well that's fine, but care to explain WHY you disagree?

I'm an electrical engineer.

Nick

 

[repatch]

I'm an electrical engineer.

OK, umm, so am I, is that supposed to be enough on it's own?

Is there something specifically wrong with what I'm saying?

I'm here to learn about things, if what I'm saying is false, I WANT people
to correct me. Saying your an EE doesn't tell me anything.

 

[William P.N. Smith]

I doubt that.

Thank you for your input.

 

[Will]

We might somehow come up with a "dangerous" example of this kind, but
it seems to me that the electrical energy in a battery is insufficient
to raise its temp more than a few degrees. For example, a Diehard might
store 1 kWh (3400 Btu) and weigh 30 pounds. If it contained half water,
the temp rise might be 3400/15 = 226 F. The water might boil, but it
seems unlikely to explode, even if every cell in the battery shorts at
the same instant.

I think you are talking past each other. daestrom and others were trying
to explain the original result, which is that in our facility we have stacks
of batteries that have overheated to the point the battery swells. When
one of these overheating events take place, it usually takes out not just
the defective battery, but all of the other batteries around it. Your own
facts suggest a large temperature increase, which seems sufficient to
explain overheating and deformation of the battery. No one was talking
about explosions.

 

[Will]

You also have probelms keeping your posts just to appropriate news
groups. SPAM is what is is.

I rarely cross post this widely, but for this thread there was not a single
newsgroup that best covered UPS. Which specific newsgroups do you think
this subject didn't belong in, and why?

 

[Jerry Avins]

I doubt that.

Do you actually believe that the charging current is the culprit? Why
doesn't is affect new batteries? Put your thinking cap back on.

Jerry

 

[[email protected]]

... Your own facts suggest a large temperature increase, which seems
sufficient to explain overheating and deformation of the battery.

That doesn't seem likely, given the internal energy and the battery heat
capacity, even with no hydrogen generation during the "sudden discharge."
Then again, we have boiling.

Charging a string with a shorted cell seems more of a problem. This can
be solved with an overtemp sensor.

Nick

 

[William P.N. Smith]

it seems to me that the electrical energy in a battery is insufficient
to raise its temp more than a few degrees. For example, a Diehard might
store 1 kWh (3400 Btu) and weigh 30 pounds.

Or a T-105 might store 5.2KWHR (17KBTU) and weigh 62#.

If it contained half water,
the temp rise might be 3400/15 = 226 F.

17K/31=570F

What's it take to convince you?

 

[Jerry Avins]

I'm an electrical engineer.

So am I, but that's not a /reason/. As an EE, what would you rate a
high-voltage capacitor made by connecting low-voltage units in series?
How practical would that be with 'lytics? Rechargeable cells are only a
little better than 'lytics in that regard. Think about it.

Jerry

 

[Jerry Avins]

I doubt that.




We might somehow come up with a "dangerous" example of this kind, but
it seems to me that the electrical energy in a battery is insufficient
to raise its temp more than a few degrees. For example, a Diehard might
store 1 kWh (3400 Btu) and weigh 30 pounds. If it contained half water,
the temp rise might be 3400/15 = 226 F. The water might boil, but it
seems unlikely to explode, even if every cell in the battery shorts at
the same instant.

If your car battery won't crank when it's -20F out, do what I do: short
the battery briefly by bridging the terminals with the handles of a pair
of pliers. The initial Short-circuit current won't be large at that
temperature, but the battery quickly warms up, so don't tarry. About two
seconds will raise the battery temperature to where it will crank the
cold engine just fine. Only a small fraction of the energy raised the
battery temperature 20F or so. Imagine the rise if all the energy went
to heat!

You came for advice, and you got it from people with experience. If you
value your "logic" more than their experience, have it your way.

Jerry

 

[Jerry Avins]

and then the manufacturers decided to bury the inter-cell links where you
couldn't access them without drilling into the wet chambers.

At first, there were small access holes to the links. It was also
possible to remove the fill caps and contact the plates directly.

Jerry

 

[budgie]

I'm an electrical engineer.

Oh, so that makes your view beyond question and everyone else's a load of
nonsense?

I happen also to be an electrical engineer, and I diagree with YOU.