Replaced two 12V, 9AH batteries in my APC Back-UPS XS 1500, Model BX1500LCD, with two 33AH, AGM Deep Cycle batteries for more than triple the backup time.
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Step 1: Purchase Needed Items
I purchased two Werker, 33 AH, deep cycle, AGM batteries from a local Batteries Plus store for about $80 each. At the hardware store, I purchased 5 feet each of red and black, stranded, 10 gauge wire; a 1 foot piece of stranded 6 gauge wire; 2 copper terminal lug connectors; crimp (butt splice) connectors; and spade terminal connectors.
Step 2: Cut Connectors Off Battery Leads
After removing the old batteries, I cut the existing battery connectors off the battery leads in the UPS battery compartment.
Step 3: Connect 5-Foot Wire Extensions
I stripped a little less than a cm of insulation from the ends of the leads and connected the 5-foot wire extensions with crimp connectors. CAUTION: I DISCOVERED THAT THE LEADS HAVE VOLTAGE EVEN WHEN THE UNIT WAS UNPLUGGED! SO KEEP THEM APART! Must be an internal battery.
Step 4: Cut Holes in Case for Battery Leads
I filed two small slots in the battery compartment cover to allow the extension wires to pass through the case. I made sure I placed the slots a few inches apart.
Step 5: Wire Batteries Together in Series
I wired the batteries in series (positive to negative) to achieve the 24 volt input my UPS requires, using lug connectors and a 1-foot piece of 6-gauge wire.
Step 6: Connect the Batteries to the UPS
I positioned the UPS and batteries on apposite sides of a small chest located next to the computer, ran the wires behind the chest and made the final connections using spade connectors.
Step 7: Test
Tested using the built in breaker on my GFCI outlet. This allows me to simulated a power outage without unplugging the unit. This way the unit and my computer equipment remain grounded at all times. I ran my computer for 1.5 hours on battery before ending the test.\
Note: At first, the UPS did not detect the added battery capacity and would shut down my PC after only 10 minutes if I didn't unplug the USB signaling cable. This was OK if I wasn't working on the computer at the time of an outage. However, if I was on the computer and wanted to keep working for longer than this, I would have to unplug the data cable to prevent the UPS from shutting the computer down. I was able to recalibrate the UPS by unplugging the data cable and discharging the batteries with with over a 30% load until they were almost completely discharged (14%) and then hooking everything back up and letting the batteries recharge. Now the UPS indicates 30 minutes backup time, which would be accurate for the 9AH batteries specified for the unit.
Word to the Wise: Do not do this with a low end (under 1500 VA) UPS, as the inverter is likely to overheat during extended battery run time. Also, such units might not be able to keep the larger batteries charged, or take too long to recharge them after a power outage. Be sure to use heavy gage wire and connectors and check to make sure that the wiring, batteries and UPS do not get hot while running on backup power. Use on non-sealed batteries NOT recommended due to hydrogen gas emission and slightly different charging profile compared to the AGM batteries the UPS is designed to work with.