UPS / Battery Query

[Feline]

Chez Feline (aka The Cathouse) is undergoing a UPS review (there being six active UPS's in use on a clear day). This was initiated by one UPS only giving 46 seconds output after losing mains power (despite replacing the battery).
As a replacement, I pulled a duff UPS out of the must-get-around-and-look-at-it--someday (!) pile.
On opening its case, I discovered there were two CS3 12 volt 7.2AH lead acid sealed batteries - both completely dead. Replacing both batteries is somewhat cheaper than replacing the whole unit (though not by very much).
What rather worries me is that the batteries are hard-wired in parallel rather than in series which seems rather dumb to me. Surely it would be better to wire them in series and design the converter/charger circuitry to run at 24 volts? I would add that this is a Brand "X" unit and was acquired on the basis of price rather than anything else.
I would have thought that wiring them in parallel is not a good design philosophy since if the internal resistance of one battery increases relative to the other battery, there will be an imbalance in both the discharge and charge rates, which will lead to premature failure of both batteries.

Question: Is this a valid concern? And should I be worried or very worried?

[jimtherev]

On the other hand, if one of the batteries in series goes open circuit, then phut goes your UPS. At least with them in parallel you have a chance to spot something wrong (maybe) and still have power while you take steps to rectify. Make sense?

[Avtrician]

The problem with Paralleled batteries is with one going flat, it will take the good battery down with it. Most UPS unit I have seen have been 24 Volt powered. When the batteries went flat, it probably strained the charge circuit meaning that when fresh batteries were fitted they werent charged properly.

Time to bite the bullet and replace the whole thing...

[FullOppositeRudder]

UPS Batteries

I would have thought that wiring them in parallel is not a good design philosophy since if the internal resistance of one battery increases relative to the other battery, there will be an imbalance in both the discharge and charge rates, which will lead to premature failure of both batteries.

A good point, but in practice it seems not to be a problem.

I can't comment with specific information on the wisdom of connecting SLA's in parallel, but I seem to recall that the concept was approved in theory on some data sheets for SLA's which I have come across. Certainly there are quite a few references to the practice on Google, all of those I saw were favourable, but with some precautions. The fact that your unit was so equipped in a commercial product suggests that the manufacturers were happy with the concept.

I might mention that connecting batteries in parallel seems to be fairly common practice in agricultural machinery and the concept is well accepted and proven there. Most of my equipment used in a former lifetime used 2 x N70ZZs in this mode. We were all cynical when it was first introduced, but no one questions it now.

In your case 12v 7ah SLAs are one of the most popular sizes and are very well priced - probably for this reason. If you were to use two SLAs in parallel, they should be commissioned as two identical new units from the same manufacturer, and I would charge them each as individual units to a full capacity before 'introducing' them in parallel. I would also give them separate fuses to minimize unhappiness in the event of catastrophic failure of one or the other, unlikely though it seems to be.

Having said all of that, there may well be advantages in replacing the entire UPS anyway. You best know the value of your data, and the cost of any interruption to the commercial AC supply, and these need to be given appropriate emphasis in the final calculation.

FORudder

[Bushfiva]

I test my UPSes monthly.　In general, I replace the batteries somewhere between 18 and 36 months. The smallest UPS eats batteries. There's also a huge difference in quality between the "best" brand I have and the "worst". They're certainly not low-maintenance items, whatever the manuals claim. For another location, I'm considering paying for the service warranty (which more than doubles the cost of the units) so someone else can visit once a month and do the testing.

[green granite]

Ideally batteries should be cycled occasionally down to say 10% and then recharged. Sealed lead acid batteries on permanent trickle charge shortens their life considerably.

[Feline]

Thanks for all the comments!

FWIW - Replaced both batteries with same make from same source. Terminal voltage of batteries initially not identical (one had been installed in another UPS for 24 hours). Ran unit on charge for 18 hours with no load - both batteries then reading same voltage (13.95V). Then disconnected mains and connected 100 watt globe as load - unit ran for 35 mins 41 secs before shutting down. At this stage, one battery at 11.61V and the other 11.64V - close enough for me.

Somewhat reassured by most of the comments (although I would still prefer two X 12V batteries in series rather than in parallel - but I'm not in a position to completely redesign and rebuild this UPS!).

Will not be deploying this UPS in a mission critical environment (where I have installed a very much more expensive unit with AVR and monitoring software), but there are a couple of less critical locations where it can be used.

Lesson I have learnt here is that I had better schedule a regular test and maintenance schedule for all the UPS's and keep track of all the SLA batteries.

Thanks for all the comments - most helpful!

[OFSO]

Thread drift but still about UPS - I've replaced the single 12v battery in mine several times - they only seem to last a couple of years. The problem is that these days they don't keep the PC etc going for long even when new and fully charged.

Would there be any problem running a couple of large-cross section cables out of the UPS to a 12v automotive battery under the desk ("under the desk" isn't part of my query !), IOW replacing the small internal UPS battery with a much larger capacity external one ?

[green granite]

No reason why not BUT Make sure you put FUSES in the lines at the battery posts.

[Bushfiva]

As long as the charging circuit can handle the battery, you should be OK. Many commercial units come with external battery terminals anyway.

[Feline]

OFSO - Seems to me that you are re-tasking your UPS.

In its existing form, your UPS is designed to provide you with enough time to close open documents and shut down your system should you have a mains outage. Depending on the design, it may also offer some power line conditioning as well (ie. it may protect your equipment from spikes and surges).

Hooking it up to a bigger battery will, on the face of it, give you the time to complete whatever you are working on - I have a UPS rated at about 500 watts output which runs off two 105 ampere/hour batteries. This gives me about 8 hours continuous operation of my PC and screen. Problem is that it's very noisy because of the fans installed.

The possible problem with using your existing system is the possibility of it overheating - it's designed to provide a certain amount of power for a few minutes - and using it for some hours might take it out of limits.

Secondly, a bigger battery may never get to recharge completely - the smaller units use a 7.2A/h battery which is designed to be recharged over 8 hours - so the recharging current will be limited to about 1 amp which is simply not sufficient to recharge a larger battery. UPS designed for long periods of operation typically have a more sophisticated charger which provides a high charge level (say 10 - 15 amps) when the battery is very low; they then step down to a lower charge rate (say about 5 amps) as the battery comes up (this lower charge rate is so that the battery doesn't overheat). Finally, when the battery reaches its fully charged state, the charger switches to trickle charge (less than 1 amp) to make sure they stay fully charged over a period of time.

Also note that you shouldn't use a car battery with a UPS: car batteries are designed to deliver heavy currents (300 - 500 amps) to crank your engine on a very low duty cycle (>1%). The rest of the time they are recharged by your alternator. UPS batteries (designated as "Deep Discharge" batteries) are designed to provide a much lower output current (typically about 10 amps) over a much longer period of time. A car battery will have a much shorter life when used with a UPS.

[green granite]

the smaller units use a 7.2A/h battery which is designed to be recharged over 8 hours - so the recharging current will be limited to about 0.1 amps

1Amp, your slide rule's faulty.