Generally adding extra batteries is not a good idea as most of these UPS's are only designed to charge up what they were originally designed for. Batteries can be improperly charged and/or damaged (and not work when you need them to) or the UPS can be damaged or shutdown due to a sensed fault (ie- battery impedance wrong, etc). Interestingly enough UPS prices can be so low that the replacement battery cost can almost be the same as the UPS itself (cheapest one I have heard of approached around $35 US), of course the quality is a little dubious .

Thanks for the info. One site that I was reading mentioned that you should recalibrate your UPS' battery sensor every 6 months or so. Is this "sensor" only in higher end units, and if not, wouldn't it be sufficient to simply recalibrate the sensor after installing a new battery? Granted, I assume from your statement that I'd be limited to the size of the battery, but going from a 5 amp/hour to a 7.5 or maybe even 10 wouldn't be pushing the unit/circuitry that far, would it?

Actually, I agree with everything jack of all trades has said. However, almost ten years ago I did exactly what you describe: I bought a new battery with about 1.5 times the capacity of the original (same voltage of course) and put it just outside the original UPS box (into which it wouldn't fit). Worked fine, and for longer (more years) than the original battery. Then the economics made sense -- I don't know that they would now.

From what I know about batteries and chargers (which is what your UPS does, at least part of it), a charger will charge at about 10% of the amp-hour rating. If the original battery is, say, 10 amp-hours, the charging current will be 1 ampere and the charging time will be about 14 hours. When the battery voltage reaches a certain level, the charger switches to trickle charge (5% current or 0.5 amperes). Then your battery's charge can be continuously maintained. Some units will automatically switch off the charging circuit and reconnect if the battery's voltage drops below a certain level. The switching levels are what require the calibration that was mentioned earlier.

If you put in a battery with a larger amp-hour rating, the charger will not automatically sense the difference. It will charge at the same rate (in this case, 1 amp). What this means is, it will take longer to charge.

Ergo, putting a larger capacity battery will work except for the charging time. The converse is not true, however. A smaller capacity battery will be charged faster, which may cause it to get hot and/or blow up.

Of course, as ken mentioned, you need a battery with the same voltage.

One wouldn't be doing much harm by replacing the dud battery for one with a similar voltage and amp-hour rating, checking the battery polarity was correct when connecting.

It is likely (check the manufacturer's information) that the circuitry within it has sufficient protection for all modes of abuse.

If the circuitry is dud and the battery is good then keep the battery in case another application comes along and salvage/recycle the rest.

Charging batteries in parrallel-->GERNERALLY BAD! Discharging is a whole 'nuther story. In theory it might work but one will always reach capacity before the other and then you start either over charging the full cell or the detected voltage will trigger the charger to stop too soon resulting in the two batteries coming to equilibrium at less than their total capacity. I thought I could do this with RC batteries and about fried one. Perhaps Lead-Acid batteries (i.e. in a big truck) can handle it but the deep-cycle batteries in a UPS probably wouldn't handle it.

Howdy, FYI lots of batteries are charged in parallel. Motorhomes, boats, aircraft just to name a few. Batterries of all types incliding ni cads, lithium, lead acid, alkaline and so on are charged in parallel in many devices. If you pry open your cell phone battery odds are it will consist of many cells connected in series/parallel. Addititional capacity is always easiest to obtain by paralleling battery packs. I don't know what happened to your battery set up to cause it to be "fried" but parallel charging is done all the time including the batteries in my old EV-1. My twin Beech has dual 24 volt batteries which via selection can be charged in parallel from either engine's generator. So check out you charging setup. I fly RC and those batteries can cost a bundle especially if you fly electric R/C. Hate to see another R/C buddy lose flying time because of battery probs. John

The general rule for charging/discharging batteries in parallel is that they be a "set": identical in every way, and ideally manufactured at about the same time and/or in the same lot number.

Did you perhaps parallel a new battery with an old or different one?

I'm not familiar with your NCR UPS, but depending on the UPS model and elctronics inside, sometimes adding an extra battery is possible. In fact, some companies make their UPS' specifically for adding an extra battery - sometimes one, sometimes a chain of them. Chaining batteries will get you longer run-time (but usually not more current), and they take a lot longer to charge if you run them down - same rate of charge, just more to charge. They really are an economical way to go in some applications - I once needed longer runtime on a UPS for a certain piece of equipment that drew a fairly small load. Rather than buying up to a bigger UPS (and throwing away my previous investment) I was able to buy piggy-back batteries to increase run-time. The gear I was using in that specific instance was Tripp-Lite, but I think APC makes some similar gear. Go to http://www.tripplite.com and look for the SmartPro line to see more (rack mount as well as tower models), or you might visit http://www.apc.com - both have quality gear. I don't sell or endorse any UPS gear, just sharing my experience!

Hi

Times before I used the same what you are saying. I connected one more baterries of the same model and rating (A_H) packed by the manufacturer of the UPS. It worked for longer time and for more power (more equipment). Finally, also I recharged the batteries but the baterries capacity was going down gradually. Hence, I hope you can parallel them when ever you apply the twince approach.

On another time, I tried to connect big A-H but single battery. It failed to be reconized by the UPS. The battery specification was both 12 Amp but AH was diffrent

As result, I recommend you to test your UPS with similar batteries and if it work fine try to purchase similar. I hope you will get some where for testing purpose e.g. some repair shop.

Alex

Thanks for all the great info, everyone, I knew this would be the best place to ask. What I think I'm going to do is this: Once I get the money all saved up (So I can buy them all at once, same mfg'r, mfg date, lot, etc), hook them in parallel, and hook up one of my pieces of surplus equipment to it, as well as a voltmeter and such, and make sure that when I do get my compy hooked up to it, I don't have to worry about killing anything. I'll repost here with a link to my site when I get the project done/moving.