UPS battery discharging question

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UPS battery discharging question

Postby Sou » Sat Oct 30, 2010 4:23 pm

Can anyone tell me how much lead acid batteries can be discharged before it becomes detrimental to the life of the batteries?

To explain the context - I have a couple of UPS's. One of them uses about 165 watts with a NAS, ADSL router and a couple of other small appliances plugged in. (The other one is a smaller simulated sine wave UPS and doesn't use much power at all.)

I've worked out that if I turn off the main UPS half the time (one hour on, one hour off) it will more than cover the running cost of the NAS.

The UPS has three 7AH 12volt DC lead batteries. The cycle of one hour on and one hour off means the batteries get to a minimum charge of about 70%, then recharge. I'm thinking this might put too much stress on the battery so am thinking about lessening the time off and increasing the time on a bit, so to discharge to a minimum of around 75% to 80%. Is that still too much discharge?

I'd be grateful for any advice on this. Thanks.
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Re: UPS battery discharging question

Postby Sou » Sun Oct 31, 2010 1:41 pm

To follow up this - and for anyone who's interested. I tried out a few different time combinations for turning on/off the UPS during the day. When it goes back onto power (after the power has been turned off), the UPS uses quite a bit more power than its normal base load, presumably to recharge the batteries. The manual says it recharges in four hours and this is about right.

In the end I've decided to get another smaller step wave UPS that uses about one fifth the power to run as the larger sine wave UPS. I'll keep the larger unit as an emergency backup and keep it recharged according to the storage recommendations (12 hours every two months). I doubt I'd be able to sell it easily for any decent price - and it's a good unit, so probably worthwhile to keep as insurance against a long-ish power outage.

The new UPS will pay for itself completely in just over three months. And our average (annual) daily energy usage will finally drop below what is being generated by the solar panels. :D
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Re: UPS battery discharging question

Postby Gordon-Loomberah » Sun Oct 31, 2010 5:33 pm

Sou, I'm afraid I dont understand your motivation for using a UPS... is it to "save power" or protect computers etc from blackouts?

From what you have written about turning them on and off, I suspect the former, and if you are charging Pb-acid batteries, you are working at something under 90% efficiency, so you will *always* use more power than you would if running the appliances directly from power with no UPS.
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Re: UPS battery discharging question

Postby Sou » Sun Oct 31, 2010 5:43 pm

Gordon-Loomberah wrote:Sou, I'm afraid I dont understand your motivation for using a UPS... is it to "save power" or protect computers etc from blackouts?

Hi Gordon. Thanks for your post. The reason I have UPSs is to protect equipment from blackouts and brownouts. I recently measured what power they were using and found the larger UPS is using nearly 3kWh a day with no load (edit: that's an annual cost of around $550/year, given that each sunlight hour is now worth/costing 66c since I've recently installed solar panels). (The smaller UPS only chews up about 0.55kWh/day with no load, for an annual cost of $77.) That's when I tried to figure out the best way of reducing the power usage of the bigger UPS while still protecting equipment.

I've decided that my equipment doesn't need the power hungry UPS and a smaller UPS that uses much less energy will suffice. Therefore today I've bought a second step wave UPS and will use that instead of the larger sine wave UPS.

Hope that clarifies things.

(EDIT: BTW, in the process I've learnt more about UPS and sealed lead acid batteries. Obviously it's not my area of expertise - lol.)
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Re: UPS battery discharging question

Postby Gordon-Loomberah » Sun Oct 31, 2010 8:22 pm

That big UPS sounds like a terribly inefficient machine!

OK, here are my thoughts- are most or all of the devices you are wanting to protect run from small plug pack 12VDC transformers? If so, how about al solar panel to charge a battery, which would power the device directly, independent of the unreliable grid? Use a 5V regulator if you need 5V instead of 12V. Charge the battery with a battery charger from the mains if its too cloudy for solar power.
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Re: UPS battery discharging question

Postby zzsstt » Sun Oct 31, 2010 8:43 pm

Sou,

As Gordon said, and you found out, it won't save power. Your equipment takes a given amount of power to run, the UPS supplies that power but also consumes a bit extra in inefficiencies. Cycling the battery cannot "create" more power so whether you run the UPS on mains power constantly or cycle between batteries and recharging from the mains you will never use less power, but may use more as the charging/discharge cycle of the batteries carries its own penalty.

Having said all that, depending on the sensitivity of your equipment and thus its power requirements, there may be other options to check out. For example, an online UPS may have a different efficiency to the normal (cheaper) offline units. Also, the inverter section of the UPS defines how much power it can provide in terms of kw (or KVA) but the size of the battery defines the run time. If your loading is not too high, you may find that a smaller UPS (from a KVA viewpoint) with additional batteries is a more efficient system that still gives sufficient loading and potentially a longer run time. When sizing your UPS, consider what needs to be protected. At home I can live without being able to use the computer, so the CPU unit is on a UPS but the screen is not, so the UPS can be smaller and the power use of the screen is not increased by the inefficiency of the UPS. In an office, the screen would likely need to be on the UPS too, to enable continued usage. Actually I cut my "computer" power requirements quite signifiantly by purchasing a modern LED screen, it uses far less power than the LCD I used previously. I also found that I used less power overall by having more computers - instead of one big one that needed to be running 24/7, I moved all the constant "monitoring" tasks to a nettop (runs at about 15W) and leave the "big" machine asleep until I actually need it.

Other things to check out are whether the UPS has an adjustable output voltage, which shouldn't make any odds (if the power requirement is the same, a lower voltage simply increases the current) but you never know. Also check the input voltage settings, in our case we have a high mains voltage and the standard settings cause the UPS to switch to battery every time there is a voltage fluctuation, which is detrimental all round and a slightly wider tolerance does not cause any problems.

Check the PSU's in your computers, and any external transformers. The older ones tend to be quite inefficient compared to the "80+" and upwards units that are now available. A grossly oversized PSU running far below it's best efficiency is not the best option!

Lastly some motherboards now have on the fly power saving in addition to any power saving built in to the operating system. Mine allows me to slow everything down to reduce power consumption when I'm just typing emails etc., but then speed everything up if I need to do some photo editing or other intensive task.
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Re: UPS battery discharging question

Postby Tracker » Sun Oct 31, 2010 9:16 pm

.
My only thoughts are that I have some cheapie UPS's to cover any power outages on computers that I do not want to power cycle - ever. ( Computer phone answerer /Fax for my business )
Those UPS's are ALWAYS Significantly HOT, and that tells me that they are most power inefficient.

I would like to investigate a highly efficient computer system and UPS, that I could run 24/7 from the UPS, and using a time clock so as to charge to float, during the off-peak rate time, and then run during the day.

Alternatively, Gordon's preferred use of PV-Solar and a battery would be fine if one can get some OLD PV-Panels at the right cost... Just so much more efficient - IF the panels are disposal units..

In the interest of saving power, I am going to look for a "Fair" lap-top, that might work on 12V rather than the usual 19V, and actively cut back on computer/technology power consumption.. for phone/communications..
.
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Re: UPS battery discharging question

Postby Sou » Sun Oct 31, 2010 11:02 pm

One of these days when solar prices and battery prices fall sufficiently I'll think about getting some off grid solar. But my immediate concern was to ensure protection for my main work computer and NAS/server (plus phone, Spa 3k ATA, switch and modem/router), while minimising power draw. I bought the sine wave UPS many years ago when the power supply here was much less reliable than it is today, and it did the job well. I was very surprised at how much electricity it's consuming. Maybe the current sine wave ones are more efficient or maybe mine got less efficient with age. Of course it cost much less to run before I got solar. Household electricity economics changes radically once solar is installed :)

In any case, as I said, I'm going for the simplest and cheapest option, which is to retire the larger UPS and spend $130 incl freight on an 800va / 480w UPS that uses 23 watts with no load, which seems okay to me. I don't need an expensive sine wave UPS. Nor do I need it for more than graceful shutdown, as I have a laptop and wireless modem that I can switch to if the power is down.

A couple of months ago I bought the same 800va model that I purchased another of today, for an HTPC/computer and other stuff in the lounge room and was pleasantly surprised. It's more than ample for the job (silent, small, runs cool, decent software, and only 12% load with the computer, fish tank and various bits and pieces hooked up).

I probably wasn't very clear in my posts above. Hopefully I've made it more clear now.
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Re: UPS battery discharging question

Postby Sou » Sun Oct 31, 2010 11:20 pm

zzsstt, those are good tips. I've recently built a new computer for work and it's got the latest and greatest efficient PSU :)

I have multiple monitors that chew up a bit of power, but these days turn them off at the power point when not in use. They aren't on the UPS of course. It's worth checking what power things use up even when switched off. A lot of things use quite a bit and it all adds up. Even the computer draws about 15 watts when it's been shut down unless I turn off the switch on the PSU - possibly from USB ports or just the PSU itself, not sure.
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Re: UPS battery discharging question

Postby zzsstt » Mon Nov 01, 2010 5:27 am

Sou wrote:It's worth checking what power things use up even when switched off. A lot of things use quite a bit and it all adds up.


They certainly do. My desk has about 12 plug in transformers powering speakers, switches, sat. modem, various powered external drives and widgets. I spent some time figuring out which could be switched off independently (most USB devices will auto connect when they are powered up, but eSATA drives are not always so accomodating!), and then bought a large foot operated switch. I can now stamp on it when I arrive at my desk and switch on all the widgets (and monitor), and switch them all off when I leave. Not quite as efficient as switching each one on and off as needed, but far easier!

By the way, depending on budget, even though the new "green" hard disks are very efficient, solid state drives use even less power and are faster. I put one in the little nettop I built for 24/7 monitoring, because the hard drive cannot be "slept" by the OS as it is frequently written to, and it is quite impressive. They do have a finite life, but the modern algorithms mean that the finite life is still longer than I'll ever need!
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