Looking for a UPS Design That Doesn't Overheat Batteries

[Will]

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.

 

[Mike]

Seen this before, most UPS dont have individual temp sensors
on each battery and/or dont have them in the right place and/or
maybe more than one per battery is needed. I have a solution I
have been working on with a battery manufacturer which I cant
discuss for commercial reasons but I can say we havent pursued
it much because the issue hasnt been so major - its just been
'occasional' and didnt warrant a whole new topology, this might
have changed, so thanks for your report. There are intermediate
fixes you can do for any UPS if you can open it up, so I'd
suggest that route first, esp for UPS that are mostly unattended.
I heard of one situation where an office was left overnight with
the UPS functioning online and the owner entered the premises
in the morning to find the room was quite warm and a bad acrid
smell throughout. Turned out the batteries had been overcharged
because one or more cells had shorted and the UPS wasnt the
smart one they said it was <sigh>

Cheers


--
Regards
Mike
* GMC/VL Commodore, Calais VL Turbo FuseRail that wont warp or melt !
* High grade milspec ignition driver electronics now in development
* Twin Tyres to suit most sedans, trikes and motorcycle sidecars
http://niche.iinet.net.au

 

[Peter A Forbes]

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.

Thermal runaway has been an aspect of sealed batteries for some years, we saw
this in the early 1980's when we were distributors for Gates Energy.

Temperature compensation on the charge circuit will help get around this, but
cell failure in a block, batteries too close together with no airflow around
them and unbalanced batteries in a string all contribute.

It is more prevalent in UPS's with enclosed cases and where they are trying to
screw the maximum output from the minimum battery.... )

I have only replied to this newsgroup, not the long list of others it was posted
to.

Peter

 

[[email protected]]

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.

Or multiple emails, as in:

while true
do
mail [email protected] < replacemybatteryfile
sleep 3600
done

An overtemp shutdown might be simpler

Nick

 

[dnoyeB]

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.

If yoru UPS is not detecting this then get a different UPS. Just because
you supposedly have a good brand does not mean you actually have a good
brand. Especially when you single handedly identify a flaw in their
system.

In fail open situation the low battery will not absorb the energy and the
voltage will raise right away. You will find our your batt is bad when you
try to use it and it runs out quickly.

In fail short situation the low battery will absorb the energy and the
voltage will not raise and the batts will slowly heat up. It should be
detected.

Looks like they just are not checking for fail short.

How can there be enough heat to melt a battery but the temp sensor system
not complain?

 

[budgie]

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.

IMOE the main reason for the failure of SLA/VRLA batteries in SOHO UPS units is
the charging regime - no more, no less.

Recovery after a discharge is a tradeoff. Do you want minimum recovery to
(substantially) full charge to best position you for a possible follow-up
outage, or a leisurely recovery which treats the battery properly?

The bad news is that you don't get to choose - the UPS manufacturers have made
that decision for you and have opted for the over-zealous rate. This is usually
compounded by an excessively high float voltage. To further shorten the life of
the batteries, the regime is often one of current-limited (too high) constant
voltage (also too high) charging. An example of a far better regime is set out
in the data sheet for the Unitrode/TI UC2906/3906 SLA charge controller chips.
Unfortunately - and yes, there seems to be only bad news - it is usually
impossible to splice in a better charging system without causing the UPS'
monitoring circuits to go apeshit.

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.

Indeed it would be good sense to monitor the individual batteries, but these are
price-sensitive products in the SOHO sizes. Certainly there is scope to add a
monitor which reports on voltage differential.

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.

You are mixing up discharge and charge in the above, UIAVMM.

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.

By the time one battery in a series string needs replacing, they all should be
replaced. You create the exact situation you referred to above by replacing one
of a series string.

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.

The other consideration that compounds battery heating is the way they are
tightly packed into the case. Again, short of mounting them outside - as some
users do - there is little that can be done in mitigation as there simply isn't
room to provide ventilation or additional conduction paths.

Both of the UPS units I have here - 1000VA (5x12V) and 300VA (2x12V) - have the
batteries almost shoe-horned into their place. In fact, there is no way a
shoehorn could fit in there between them. What I have done, though, is
re-engineer the charging circuitry to "better" recovery and float charging
conditions.

 

[William P.N. Smith]

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.

Are you replacing the batteries every few years as per the
manufacturer's reccomendations? New batteries are much cheaper than
the elaborate battery monitoring, charging, and warning system you
want to engineer.

If this is a Giant UPS for (say) a machine room, there are are other
options, but for small individual UPSen, you aren't going to find what
you want.

 

[JoeSP]

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.

Best I can offer, is to avoid deep discharges that can sulfate and short the
plates. Then avoid putting batteries in parallel, that can short through
those sulfated plates. Maybe a fusible link between the banks would burn up
instead of the batteries themselves. Another idea is to put 8-10 cells in
series, to give you 120V (number varies with type of cells used) and then
use the inverter alone, without the transformer to convert it to AC.

 

[Jerry Avins]

Our company has had a long-standing problem where UPS batteries will at
various points in their lifetime suddenly overheat ...

There's not much to add to the good comments already given. The
important concept is *battery*, i.e., a group of cells. Each 12-volt
battery is 6 2-volt cells in series. It makes no difference if the 20
cells of a 120-volt battery are in one case or several provided they are
well matched. Mixing cells from different production lots is not good
form unless production tolerances are very tight. The only safe way to
use a mix of cells of varying capacity is by charging the cells
individually and discharging them in parallel. Then you need to make
sure that the discharge is stopped when any one cell is depleted.

That's just not practical. You wouldn't mix old and new cells in a
flashlight. Don't do it in a UPS.

Jerry

 

[Hawk]

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.

Here is an example of one thing to consider. In a string of cells (or batteries)
you could have one that over time has fallen in usable capacity faster than the
others. In the situation where the entire string is being discharged, the moment
that the weakest cell runs out of capacity it will no longer be contributing to
the load and will essentially be CHARGED BACKWARDS by the discharge current of the
remaining cells.

You will get plenty of gassing and other bad things happening under this scenario.
Depending on the design of the system, it may not even be able to detect that a
cell has effectively dropped out. Obviously this is a bigger problem in higher
voltage battery configurations. For example, you will notice a dead cell in a 4
cell pack much easier than in a 12 cell pack.

(*>

 

[Will]

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

Thank you to everyone for all the good comments made so far in this thread.
The bottom line seems to be that most smaller UPS systems use a
too-aggressive charging scheme, and pack the batteries too close together,
so that thermal events in one battery quickly spread to other batteries
touching that one.

Does any UPS manufacturer make a unit that will do about 3000VA - rackmount
or tower configuration - that will treat the batteries better and use less
aggressive charging in favor of long battery life? And of those units,
does anyone make a unit where the batteries are given enough space between
them that overheating in one battery stays localized to that battery?

 

[Eric Sears]

Does any UPS manufacturer make a unit that will do about 3000VA - rackmount
or tower configuration - that will treat the batteries better and use less
aggressive charging in favor of long battery life? And of those units,
does anyone make a unit where the batteries are given enough space between
them that overheating in one battery stays localized to that battery?

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) with a quality seperate charger - something
that can provide the power you normally need for you computers, but
also has a proper float charging regime. Run everything through the
inverter all the time.
(Personally I prefer "wet cells" like T105's - but that's probably not
suitable in an office situation - it just that I think they are harder
to kill).
It will be more expensive, but it much easier to monitor. You might
even have a staff member learn to measure the the battery voltage on a
regular basis.

My tuppence worth

Eric Sears

 

[Cameron Dorrough]

One idea that has been missed so far: If you are really serious about
reliability (most UPS users aren't) and don't like dealing with batteries,
consider installing a Rotary UPS. You'll find plenty of good info via a
Google search.

I know it's "old school" and more expensive initial outlay, but Rotary UPS's
are used by the majority of the world's Stock Exchanges and major data
centres for all of the reasons you mentioned in your post - and on power
failure, they just work.

HTH,
Cameron

 

[William P.N. Smith]

One idea that has been missed so far: If you are really serious about
reliability (most UPS users aren't) and don't like dealing with batteries,
consider installing a Rotary UPS. You'll find plenty of good info via a
Google search.

Don't Rotary UPSes just provide a few seconds of power while your
diesel generators are firing up?

 

[Cameron Dorrough]

Don't Rotary UPSes just provide a few seconds of power while your
diesel generators are firing up?

I'm sure some are configured that way - that would be a TPS (Temporary Power
Supply). Although it takes a lot longer than a few seconds to run the
flywheel down.

It depends what you buy. The Pillar UPS powering the Melbourne Stock
Exchange (that's Melbourne, Australia - not Melbourne, USA) is rated for 30
minutes at full load.

Cameron

 

[budgie]

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.

 

[budgie]

I'm sure some are configured that way - that would be a TPS (Temporary Power
Supply). Although it takes a lot longer than a few seconds to run the
flywheel down.

It depends what you buy. The Pillar UPS powering the Melbourne Stock
Exchange (that's Melbourne, Australia - not Melbourne, USA) is rated for 30
minutes at full load.

When I had to re-engineer the power systems at a large govt computing centre
here in Perth (Oz) we looked at several rotary systems. They all relied on the
flywheel energy to cover the break until the backup diesel genset(s) start. One
rather large bank's head office had this config with ONE diesel alternator. The
outlook if it fails to fire is rather bleak.

I opted for three parallelled 300kVA UPS units, with inherent redundancy as we
had a base load below 600kVA. I also added a third diesel set to the existing
pair, bringing total diesel-generated output to ~1500kVA. Again, redundancy -
and necessary capacity.

I don't "like" large battery banks, but I would never seriously consider a
rotary storage system in any critical application. And I doubt there is a
rotary offering in the O/P's 3kVA size.

 

[Jerry Avins]

When I had to re-engineer the power systems at a large govt computing centre
here in Perth (Oz) we looked at several rotary systems. They all relied on the
flywheel energy to cover the break until the backup diesel genset(s) start. One
rather large bank's head office had this config with ONE diesel alternator. The
outlook if it fails to fire is rather bleak.

I opted for three parallelled 300kVA UPS units, with inherent redundancy as we
had a base load below 600kVA. I also added a third diesel set to the existing
pair, bringing total diesel-generated output to ~1500kVA. Again, redundancy -
and necessary capacity.

I don't "like" large battery banks, but I would never seriously consider a
rotary storage system in any critical application. And I doubt there is a
rotary offering in the O/P's 3kVA size.

Years ago, when wet cells were the only game, we ran a synchronous
generator as a motor driving a flywheel. When power dropped, the
contactor to the line did too, and a clutch engaged an internal
combustion engine that ran on illuminating gas. The system never failed,
but short dropouts -- they're more frequent than we had imagined -- were
painful. Switching back to line power required that the generator be
phase locked by hand before the contactor was re-energized. We couldn't
let the flywheel carry the load while the contactor remained closed for
the obvious reason.

Jerry

 

[Will]

Recovery after a discharge is a tradeoff. Do you want minimum recovery to
(substantially) full charge to best position you for a possible follow-up
outage, or a leisurely recovery which treats the battery properly?

The bad news is that you don't get to choose - the UPS manufacturers have made
that decision for you and have opted for the over-zealous rate. This is usually
compounded by an excessively high float voltage. To further shorten the life of
the batteries, the regime is often one of current-limited (too high) constant
voltage (also too high) charging. An example of a far better regime is set out
in the data sheet for the Unitrode/TI UC2906/3906 SLA charge controller chips.
Unfortunately - and yes, there seems to be only bad news - it is usually
impossible to splice in a better charging system without causing the UPS'
monitoring circuits to go apeshit.

Could you reformulate the above statements as a set of requirements I can
put into a requirements document for a new UPS? I'm guessing that the
requirements would be something similar to the following, but I am hoping
you will make these statements more precise:

- UPS should use a recharge / recovery scheme that maximizes battery
lifetime rather than minimizing recharge times, or should as a worst case
allow this to be a user configurable option.

- UPS should use an appropriate float voltage for the batteries it uses.

- UPS should allow the use of deep cycle batteries.

- UPS should provide a way to use an appropriate topping charge every six
months with the battery.

By the time one battery in a series string needs replacing, they all should be
replaced. You create the exact situation you referred to above by replacing one
of a series string.

If I take an older UPS gel-cell battery which is in series as 2 x 12V, and I
test each 12V and find that one is giving a good 12V reading when charged
and the other never goes above 10V, why can't I just replace the dead 10V
battery with a fresher 12V? How is replacing the dying battery promoting
overheating?

 

[Jerry Avins]

Will wrote:

...

If I take an older UPS gel-cell battery which is in series as 2 x 12V, and I
test each 12V and find that one is giving a good 12V reading when charged
and the other never goes above 10V, why can't I just replace the dead 10V
battery with a fresher 12V? How is replacing the dying battery promoting
overheating?

You can replace only the bad one*, but expect short life from the one
not replaced.

Jerry
__________________________________
* You might even revive the bad one by floating it separately. Don't
rely heavily on it, though.