Caring, Feeding and Enhancing UPS Battery Systems?

cdn-programmer asks: "I've got a couple UPS's that are now about 5 years old. They are MGE EL4 units - rated at about 450VA /280W. These originally came with a battery rated for 2x6V - 7.2Ah I've found an inexpensive replacement battery: SBS60 rated at over 50Ah and I've done a little mod so the unit 'fit' together. I've also done as much reading as I can on float currents and ripple currents and so forth basically covering the care and feeding of your UPS battery system. People can find some good information in the SBS web site and the specific data sheets are here. Look under the SBS section for the manuals. If people find alternative batteries there seems to be some excellent tech specs here. Having read these it seems to me that there should be no reason that I cannot 'redesign' the UPS to give it a significantly increased power reserve. The load is the same... the only difference is that a larger battery is used so this means that the recharge times will potentially be much much longer. So what? We are not interested in cycling the battery. Indeed the power here is quite stable and we get very few outages per year."

"Now - the SBS people have been very professional and have provided good technical engineering data. I haven't been able to find much data on the MGE EL4 and I wonder if this cheap little UPS has the proper smarts to take good care of my new battery? If it does not - then why not and what models/manufacturers should we be considering?

For instance, what is the ripple characteristics of the EL4 and how can I measure it? Since the SBS60 is HUGE in comparison to the original batteries (Panasonics - 7.2Ah - 2x6V) is ripple even something to worry about?

Does the EL4 charging system 'cycle' in a harmful way? I tested the float voltage levels and found that they varied from 13.89 to 13.42 over the course of a day. But this battery has only been hooked up for a day so maybe its stabilizing. The nominal float should be 2.27 per cell according to SBS so that works out to 13.62 for the battery.

Does the EL4 have a temperature sensor? This is something else that the charging system should do according to the SBS people because optimal float voltage varies with temperature.

Finally, I'm interested in doing a load test to determine how healthy by batteries are. I'm thinking that a very simple test can be a couple lamps - say 100 watt - that can be plugged into the UPS. Since I've never done anything like this before the thoughts in my mind are that all I would need to do is take a voltage reading say every 5 minutes over the next few hours and if I can find the proper curves this should yield enough data to determine the life expectancy of my battery.

If anyone has actually done tests like this it would be wonderful if they could tell us how to do this."

Comments

Larger batteries = more charging current/voltage

[RobKow]

Larger batteries draw more charging current at a given voltage than smaller ones. Depending upon the charger configuration in the UPS you could either end up undercharging the battery or shortening the life of the charger by increasing power dissipation in it.

I've seen car batteries work connected to small UPSes for years until the power went out for an extended time, the battery was significantly drained, and the charger failed when the power came back on. Just something to be aware of.

A couple of problems...

[stienman]

The only issues you'll have to deal with are

Smaller chargers are not meant to charge larger batteries - you may well be overstressing your UPS charger by expecting it to charge your new, larger battery for so long.

Lead Acid batteries and their variants (gell-cell, deep cycle, etc) do NOT like to be discharged more than 50%, yes, that includes so-called deep cycle batteries. Deep cycle means that deep discharges won't hurt the battery as much as it would hurt a regular gell cell, but it'll still be damaged.

Most consumer and low end UPS systems do NOT monitor battery temperature. They simply charge the battery so slowly that there is little risk of overheating, boiling, or overcharging.

Cycling the battery with light bulbs may not be a good idea, because many UPS systems allow more than 50% battery discharge. You'd have to monitor the voltage, then shut it off when it drops below 11 or 12v.

Light bulbs will not pull power the same way your computer will, so the best load test is the real load you intend to use. a 400W powersupply doesn't draw 400W. Depending on how you measure it, it may pull more or less from the AC line (read about Power Factor and power factor correction). This is one of the reasons these supplies are rated in VA and not Watts. Of course, the real question is, Why? When you have few power outages, what is the reason to use such a large capacity battery, but more important, why do you even need to characterize it?

Lastly, make certian you aren't pulling more current than the supply is regulated for. As you suggest a larger battery does not make it more able to handle larger loads. You'll be tempted in the future to add more stuff because 'it should handle it', but it'll only make it fail faster.

-Adam

Overheating on long use is the issue.

[Futurepower(R)]

UPS units are often cheaply and poorly designed, especially the older ones. The ONLY issue is whether the unit would overheat with the longer time of use that is possible with a more powerful battery.

Earlier posters mentioned that a bigger battery would draw more current at a particular charging voltage. This is true, but irrelevant. The chargers are designed to be constant-current, or close to it. The current drain does not depend on voltage.

I've powered a telephone answering computer from an 18 volt UPS using a 6 volt and 12 volt car battery in series, with no problems. However, the unit was arranged that there would be much more air flow for cooling than it would normally get.

Re:Overheating on long use is the issue.

[GigsVT]

We have a winner. :)

I agree, the main thing is that the designers of the UPS were expecting a certain length of operation, given the normal battery of the unit, and therefore lie about specs.

An example:
A 500VA UPS is rarely able to handle 500VA continuous and sustained for long periods of time. The battery it comes with might give only 5 minutes of usage at full load, so the designers usually cut corners and just made sure it wouldn't overheat in 5-10 minutes at 500VA, but that's about it. Run the unit for 50 minutes at 500VA, and it's entirely likely that solder will start melting and components will burn up.

This becomes less of an issue once you get to the higher end of UPSs, the kind that have the external plugs that you can string extra batteries on. Those are designed with specs that usually don't lie, targetted for indefinite operation at full rated output.

Two things you can do to combat overheating with a cheap UPS, as you pointed out, you can increase cooling, which may be difficult if the components are spaced tightly, or you can run it at some smaller percentage of "rated" load, i.e. 50%.

Re:Stay close to the same battery

[ivan256]

DigiKey sells the individual cells that make up the battery packs in all APC UPSes. You should check it out if you ever need cheap exact replacements of APC UPS batteries.

My two cents

[fruity_pebbles]

UPS batteries usually don't last more than two or three years. Test on a regular basis, or just buy new batteries every two years. Heat kills UPS batteries. A lot of UPS's connected to desktop PC's are sitting on a carpeted floor under the desk. That carpet makes wonderful insulation that helps keep the batteries nice and warm. Put the UPS somewhere that has good airflow all around the UPS's case. I simply laid a couple large pens under either end of the UPS to get it an inch or so off the carpet, and that works fine.