It will take at least a year to find out, the hard way, whether a battery you bought on eBay is a good deal or bad.  Yet eBay only allows 90 days for feedback. Feedback is not a good indicator, obviously.  A little research will go a long way and this guide is intended to give you a start.

This guide primarily addresses small UPSs from 200 up to 2200VA, although there is information here that is relevant for all sizes. 

I refer to APC a lot in this guide but we have no connection or relationship with APC.  We replace batteries in lots of UPSs each month and we consistently have good luck with their SmartUPS and BackUPS products.  These UPSs rarely go bad and they are designed to make it easy to change batteries.

Contents:

  Quality
       Brands
       Price
       Age, shelf-life and storage
       Avoiding low quality
   Why replace a battery before the UPS tells you to?
   When to replace the batteries
       APC's battery alarm
       Run-time calibration
  Compatibility
       Battery technology
       Sizes
       Terminal style

Quality

APC rates the brands they use for 3 to 5 years in normal service and this an accurate rating, based on our experience.  A low quality battery may fail after a year of service where a high quality battery can last for over three years.  There are lots of ways for a battery manufacturer to affect the cost and quality of the battery being produced, starting with, for example, the formula, purity, and consistency of the lead alloy. 

Brands - There are many, many manufacturers of  Sealed Lead-Acid (SLA) batteries and there is significant variation in the quality (reliability and longevity) among all these manufacturers.  Many battery manufacturers don't manufacture batteries.  Admittedly, this sounds more like a tongue-twister than a statement of fact.  In fact, some "manufacturers" contract with real battery manufacturers to do their production for them.  Other battery manufacturers do only final assembly of batteries using lead plates purchased from larger battery manufacturers.  There are relatively few manufacturers that actually make their own batteries, entirely, from raw materials.

Price - In 2007 the commodity price of lead more than doubled in less than a year and is only recently starting to drop.  Since SLA batteries are mostly lead, by weight, it is reasonable to expect to see corresponding increases in the price of batteries.  The wholesale prices from reputable battery manufacturers did indeed rise with the price of lead.  The prices on eBay didn't change for almost a year, and today many eBay prices are below the wholesale cost of the corresponding good-quality battery.  This should be cause for concern. 

And then there is fraud.  I have personally seen an off-brand, low-priced of "17Ah" battery, disassembled to reveal that inside were the guts of a 12Ah battery surrounded by a lot of empty space.  Clearly, it is dangerous to buy a battery based on price alone.

Age, shelf-life and storage - New, high-quality batteries can be stored under properly controlled conditions for over a year with little or no deterioration. Temperature should always be below 75degF and the battery's charge should be checked and topped off every 6 months or less.  Reputable, factory-authorized distributors do this.  I'm sure very few Internet sellers do this.  Improper storage will shorten the life of the battery.  Lower quality batteries don't tolerate storage as gracefully as high-quality batteries.

You can store your own, new batteries for many months by keeping them cool all the time.  Don't refrigerate them, but most basement floors (away from furnaces and other sources of heat) are very good so long as moisture isn't excessive.  Even though they are "sealed" there may still be traces of acid remaining from the manufacturing process, so don't store them around paper, fabric or anything that could be affected by acid.

Avoiding low quality - There is no reasonable way for the normal UPS user to research the SLA battery industry and determine which is the best brand to buy.  The major UPS manufacturers, like APC, employ engineers to test and qualify batteries and brands.  I think the best path for users is to follow the lead of the largest UPS manufacturers and buy a brand that they buy.  In APC's SmartUPS UPSs we find "CSB," "Panasonic" and "BB" brands consistently. We know, from from experience and from dealing directly with these manufacturers, that these brands are high quality, so these are the brands that we always use. 

Another helpful tactic is to look for certifications and agency approvals, like UL, VDE, CE, etc., printed on the batteries.  These approvals are time-consuming, difficult and expensive for the battery manufacturer to obtain.  The approvals are primarily required only by OEM customers (like APC.)  The smaller battery manufacturers know they will never be able to sell to an OEM customer like APC, so they don't go to the trouble and expense of  meeting the requirements for approval.  Regrettably, some small manufacturers fraudulently print approval logos on their products and UL has told me that they can't police this type of fraud.

Why replace a battery before the UPS tells you to?   

The picture above shows what happens to a battery that is left in service too long.  At the end of its life it overheats, softens the plastic and swells up.  The battery in this picture is actually a high quality battery.  It lasted almost four years.  Despite the abuse, it did not leak!!  A cheaper, lower quality battery would have overheated much sooner and probably would have leaked acid into the UPS

When to Replace Your Batteries

Just because APC rates their batteries for 3 to 5 years doesn't mean you should leave them in service that long.  Most models of APC UPSs will beep and light an alarm indicator to tell you that the battery has gone bad.  There's a big problem with this indication: it is too late!  If the "replace-battery" alarm does come on, you must remove the batteries immediately or they may very soon start overheating, swelling up and/or leaking. 

Also, it may be obvious but bears pointing out, by the time the "replace-battery" alarm comes on you will have zero reserve capacity to carry you through a power-outage.  Since reserve capacity and load-carrying run-time are the reason you bought your UPS in the first place, it makes no sense to let the capacity go to zero and leave yourself vulnerable to power outages again.  So, you should replace the batteries before they get near zero and before the alarm indicator comes on.

How do you know when to replace the batteries?  Schedule!  The batteries that APC originally supplies in their SmartUPS and BackUPS products are rated for 3 to 5 years of normal service.  These good quality batteries are fairly predictable and after 30 months of service the batteries most likely will have declined to less than half of the original capacity they had when new.  They will still be capable of carrying your computer through a short power-outage but they won't be able to carry it as long as when new.  If you've previously replaced your batteries with a lesser quality brand than APC uses, your batteries may not even get you to 30 months.  Whenever you replace your batteries, write the replacement date with a marker on the the battery cover.  Then replace them after 30 months or whatever period of time makes you feel comfortable.

APC's SmartUPS line has a "run-time calibration" function that will test and predict how many minutes your current batteries will carry your computer during a power outage.  Their PowerChute software can perform this test whenever you want but only if you have installed it on your computer and connected the UPS with the USB or serial cable that it came with.  This "run-time calibration" will use your computer as a test load to discharge the batteries to 50% of their stored energy and report to you how many minutes your computer should be able to run during an outage.  You can run this test once a month if you want to, without hurting your batteries. Then you can decide when to replace the batteries based on how much run-time you think you should have for the type of power-outages you expect in your area.

Compatibility

APC supplies APC-labeled replacement batteries, identified as RBC2, RBC4, etc., complete with wires, fuses and connectors as required depending on the UPS configuration and requirements.  They also supply a box with a pre-paid shipping label for returning the old battery to APC for recycling.  These are some of the reasons why APC's prices for replacement batteries may seem high. 

APC does not manufacture batteries.  It buys batteries from large battery manufacturers and adds wiring, connectors, fuses, labels and packaging as needed to make it into an APC product.  This explains why you are able to save significantly by buying naked batteries from a battery supplier and re-using your old, original fuses and wiring.  The fuses and wires don't wear out although they can, rarely, be rendered unusable by acid from a battery that was left in service too long.

Battery technology - All commonly available UPSs use "valve-regulated lead-acid" (VRLA) batteries, often called SLA ("sealed, lead-acid"),  sometimes AGM ("absorbent glass mat.") .  Wikipedia has a very good tutorial on VRLA battery technology.  Despite the name, SLA batteries are not truly "sealed" and acid leaks are frequently seen when they are left in service beyond their useful lifespan.

Sizes - Most older APC UPSs and many of the current SmartUPS and BackUPS lines typically use one of three common sizes of 12-Volt SLA batteries: 7, 12 and 17Ah.  The most common and most economical UPS battery is the 12Volt 7Amp-hour (12V 7Ah) size because it is produced in large quantities by the manufacturers, is widely distributed, is readily available and is relatively inexpensive. (Some manufacturers rate them for 7.2Ah but this is just specsmanship on the part of the manufacturer; 7 and 7.2 ratings are equivalent.)  All 12Volt 7Ah batteries are the same physical size.  I routinely recommend buying UPSs that use 12V-7Ah batteries because they are most economical in the long run.  In recent years some new, small, cute APC models have begun using newer, smaller, lower-power batteries that are less commonly available and much more expensive.

Terminal style - The older UPSs and many current models use either quick-disconnect blade terminals or bolt terminals.   The smaller, under-1400VA UPSs use batteries with "quick-disconnect" terminals, which are sold with either 3/16in. and 1/4in terminals.  Watch out.  Many vendors will sell you the wrong size.  All the APC SmartUPS and BackUPS models that I've ever seen have used 1/4in. terminals except for one, very old BK200.  If your UPS is not APC, measure the terminals carefully because 3/16in. terminals are more common outside of APC.

Don't try to use a battery with 3/16in. terminals in a UPS that's designed for 1/4in. terminals.  The connectors will seem to fit on the smaller battery terminals but the connectors do not grip the undersized terminals properly and will not be as reliable in handling the UPS' current demands.  They could overheat and cause reliability problems. 

It's not physically possible to use a 1/4in. terminal battery in a UPS that requires 3/16in. terminals because the connectors won't fit at all.

The larger UPSs, 1400VA and up, often use batteries with large terminals with holes for bolts.  The position of these terminals on the top of the battery is not always the same.  Manufacturer CSB, for instance, produces the same battery in two models with different  bolt-terminal locations.   Depending on  the type of fuse  you are attempting to re-use, your new battery might not work because the terminals are in different locations than on the original batteries.  You will need to find out which battery model your fuse is designed for.