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Peter Bennett wrote:
William P.N. Smith wrote:
Second, battery strings must be matched, so replace all the batteries in a string at the same time.
Sounds expensive. How well-matched must they be?
Quite well...
Got numbers?
Nick
nicksanspam@ece.villanova.edu wrote:
Given a max charging current, we could use these imbalances to predict the max temp rise.
But the meltdown doesn't come from charging current, it comes from internal shorting of a cell quickly converting all it's stored energy to heat.
Out of curiosity, would there be a detectable temperature increase prior to the shorting event within the battery? If you saw increasing internal temperatures at a point in time (like night) when outside temperatures are falling, might that be a clue that the battery's failure is imminent?
-- Will
"William P.N. Smith" wrote in message
nicksanspam@ece.villanova.edu wrote: Given a max charging current, we could use these imbalances to predict the max temp rise.
But the meltdown doesn't come from charging current, it comes from internal shorting of a cell quickly converting all it's stored energy to heat.
William P.N. Smith wrote:
nicksanspam@ece.villanova.edu wrote:
Given a max charging current, we could use these imbalances to predict the max temp rise.
But the meltdown doesn't come from charging current, it comes from internal shorting of a cell quickly converting all it's stored energy to heat.
And the heat shorts the neighboring cells. It works both ways.
Jerry -- Engineering is the art of making what you want from things you can get.
Will wrote:
Out of curiosity, would there be a detectable temperature increase prior to the shorting event within the battery? If you saw increasing internal temperatures at a point in time (like night) when outside temperatures are falling, might that be a clue that the battery's failure is imminent?
The shorting process is accelerated by heat. If you notice the external temperature of a battery rising spontaneously, you may have less than five minutes to cut it loose, probably less than half an hour.
Jerry -- Engineering is the art of making what you want from things you can get.
Jerry Avins wrote:
And the heat shorts the neighboring cells. It works both ways.
Right, it's a thermal runaway. Hard to predict, hard to control, easy to avoid.
On Fri, 02 Jun 2006 21:04:37 -0400, William P.N. Smith wrote:
nicksanspam@ece.villanova.edu wrote: Given a max charging current, we could use these imbalances to predict the max temp rise.
But the meltdown doesn't come from charging current, it comes from internal shorting of a cell quickly converting all it's stored energy to heat.
and the subsequent damage by a none-too-bright charging system.
On Fri, 2 Jun 2006 15:26:09 -0700, "Will" wrote:
"William P.N. Smith" wrote in message I wonder if you don't have a bit of confusion between cause and effect here. Old batteries can short a cell when the plates age (and swell) sufficiently, generating quite a bit of heat when the stored energy in that cell is released. This _can_ cause an avalanche effect in nearby cells and batteries, but the primary cause is ignoring the PM schedule on battery replacement...
When testing batteries, is there some key sign that might disclose imminent failure of one of the 12V bricks? Would we for example see the voltage decline in a non-linear way as it declines from 12V to 10V?
Sometimes - but certainly not always - you can detect slight bulging of the case which pre-empts failure in SLA's. Apart from that, as others have pointed out, failure can be rather sudden and without warning, especially if it results from internal shorting.
You MAY be able to pick other types of deterioration by ESR measurements, but I have never bothered. Also sometimes timing a regular test run to cutoff will give early warning. But neither reliably warns of an impending shorted cell.
On Fri, 02 Jun 2006 18:59:31 -0400, Jerry Avins wrote:
Will wrote:
"William P.N. Smith" wrote in message
I wonder if you don't have a bit of confusion between cause and effect here. Old batteries can short a cell when the plates age (and swell) sufficiently, generating quite a bit of heat when the stored energy in that cell is released. This _can_ cause an avalanche effect in nearby cells and batteries, but the primary cause is ignoring the PM schedule on battery replacement...
When testing batteries, is there some key sign that might disclose imminent failure of one of the 12V bricks? Would we for example see the voltage decline in a non-linear way as it declines from 12V to 10V?
Back in the days of six-volt car batteries, they tested individual cell's short-circuit current. Anything less than 250 amps indicated immanent failure.
and then the manufacturers decided to bury the inter-cell links where you couldn't access them without drilling into the wet chambers.
On Wed, 31 May 2006 21:45:02 -0700, "Will" Gave us:
Any information on why this overheating takes place, how to avoid it, and any referrals to third party UPS products that employ a more robust design are appreciated.
Lead acid gel cells exhibit heat when charged. It is a simple fact. Period.
On Wed, 31 May 2006 21:45:02 -0700, "Will" wrote:
You also have probelms keeping your posts just to appropriate news groups. SPAM is what is is.
Peter Dettmann
Our company has had a long-standing problem where UPS batteries will at various points in their lifetime suddenly overheat, sometimes catastrophically to the point where the battery casing starts to melt and you can actually smell the gases from the battery leaking. So far we have been lucky to catch such thermal events with temperature sensors but it has always been a goal of mine to better understand why this happens, and to find some UPS system where it can be avoided entirely. To date, we have seen these problems with APC Symmetra tower, Symmetra rackmount, and SmartUPS.
After working with an electrician, I have a theory about why this is happening, and if correct, the theory suggests a different design for UPS systems that would avoid the problem. I am hoping some manufacturer has already implemented this idea and someone can refer me to their products.
On all of the UPS systems we use generic "brick" batteries are joined together in a series, then the leads from the ends of these battery chains are connected to the UPS. The problem is that batteries rarely fail all together. If a 12V battery should be considered discharged and not useful at around 10V, and you have two 12V batteries joined in series, what happens when one of the batteries maintains a full charge at 12V but the other battery in the series starts to lose its ability to hold charge and slips below some critical level? From the point of view of the UPS, it doesn't see anything about the state of individual batteries. The UPS only sees that the total voltage of the two 12V batteries in series has fallen from 24V to 22V.
Maybe an electrical engineer can step in here and explain what is happening, but my pure guess is that to maintain the same power output, an increased amount of current probably has to flow through the batteries. That creates problems with heating for the "good" battery, which is still measuring 12V. Now that 12V is receiving too much current, overcharges, overheats, and at some point the casing of the battery starts to melt. I haven't done enough experimentation to determine if it is the good battery or bad battery that is overheating. To be honest, in such situations I have often seen evidence that both batteries start to melt. Perhaps this is nothing more than one battery being in physical proximity to the overheating battery and therefore gaining heat from its physical contact. The only thing that is common to all cases is that one of the two batteries has discharged and should have been replaced before the overheating event took place.
Regardless of the actual mechanism for the overheating we are observing, it seems to me that the obvious solution is to design UPS systems to physically connect to each 12V battery individually. Forget connecting multiple batteries in series, at least don't do that at the battery itself. By connecting to and monitoring individual batteries, now the UPS can see when an individual battery falls below some critical voltage threshold and put it into a special recharge state (not put any load on it). If the battery fails to recharge, the UPS can declare the battery defective and can signal the condition by an LED on the battery's compartment. If there is a network attached monitoring system, the UPS can send an e-mail.
Aside from increasing safety and utility of the monitoring system, such a design would allow much easier re-use of off-the-shelf batteries, improving ease-of-use in making battery changes and lowering cost. While I realize that APC in particular has no desire to make anything regarding batteries non proprietary, maybe some other vendor has a UPS design that puts a direct monitoring circuit on each individual 12V brick battery, thus avoiding the overheating problem I have described?
Any information on why this overheating takes place, how to avoid it, and any referrals to third party UPS products that employ a more robust design are appreciated.
On Sat, 03 Jun 2006 16:51:18 +1000, Peter Dettmann Gave us:
On Wed, 31 May 2006 21:45:02 -0700, "Will" <westes-usc@noemail.nospam wrote:
You also have probelms keeping your posts just to appropriate news groups. SPAM is what is is.
Peter Dettmann
Yet your retarded ass had to quote and repost the entire chunk of crap again.
You TOFUd it too.
Looks like you need to learn a few things about Usenet as well.
http://en.wikipedia.org/wiki/Top-posting
William P.N. Smith wrote:
nicksanspam@ece.villanova.edu wrote: Given a max charging current, we could use these imbalances to predict the max temp rise.
But the meltdown doesn't come from charging current, it comes from internal shorting of a cell quickly converting all it's stored energy to heat.
I doubt that.
Nick
Peter Dettmann top-posted:
You also have probelms keeping your posts just to appropriate news groups. SPAM is what is is.
Oh, I dunno, I invited him over to alt.energy.homepower from the battery newsgroup, as there are a lot more people here with battery experience. He might have ranged a bit wider than I would have, but it's certainly marginally on-topic for these groups.
And SPAM is unsolicited commercial advertising, which I don't see any of here. If you don't like the thread, click that little "ignore thread" button underneath where it says "Tools".
On Fri, 02 Jun 2006 09:24:51 +0800, budgie wrote:
On Thu, 01 Jun 2006 19:58:26 GMT, phoneme@025379386.for.email.address (Eric Sears) wrote:
I don't know what is available, but if this is what you want (3000VA), it seems to me that you might be better to put together your own UPS from discrete units. It might be larger (do you have room for it?), but it ought to be more reliable. Find a reasonable inverter, perhaps one that runs on 12volts (to mitigate needing too many "cells"), and perhaps use about 20 to 30 amphr SLA (or even bigger)
(snip)
???? a 3000VA inverter is going to want 250A @12V input, assuming 100% efficiency. There IS a reason why higher input voltages are used as the output rating climbs. Even my 1000VA unit runs a 60V string of 5*12V.
I absolutely agree. However, since the originator of this thread wanted something that was easier to "fail-proof" I thought that less cells might be better (even if larger amperage capacity). My 350 watt UPS has a single 12v/6ah battery - which I guess means that at full power it might draw 30 amps (not for long!). So maybe I should have said about 50 ah for 3kw if scaled correctly.
But you are correct about input power. I suppose that most UPS's have the "inverter" part running in sync with the grid power all the time, and the battery only gets called on when the grid fails.
Perhaps a standard 48v inverter would be better, but then there is the problem of "all those cells".
I guess in the end, as others have said, the best system is a regular maintenance schedule of replacing batteries at a set time period. If the regular batteries in the UPS were re-sited OUTSIDE the box, it might be possible to a) prevent the heat from one battery affecting the adjacent ones and b) monitor the individual battery voltages more easily.
Thanks for your comments budgie.
Eric Sears.
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