APC Smart-UPS 1000 on car batteries: battery calibration

What we had

• An APC Smart-UPS 1000 (SUA1000I). A 24 V battery system: two 12 V batteries in series.
• Instead of the stock batteries, two identical Varta Black Dynamic 6ST-45 (B20) car batteries, 45 Ah each. Each read 12.7 V before connecting, so they were charged.
• After a day on the UPS all five LEDs showed a full charge, yet the UPS behaved as if the batteries were flat: its runtime estimate was wrong.

The UPS estimates runtime with its own battery model. After a battery change, and all the more so after fitting much bigger ones, that model has to be updated by calibration.

⚠ Before you repeat this with car batteries:

• a starter battery is not made for deep discharge: every full discharge, like the one during calibration, shortens its life;
• an ordinary (not AGM or gel) battery gives off hydrogen while charging. Keep it in a ventilated place, not in a closed cabinet;
• use battery leads rated for 15–20 A, with a fuse next to the battery;
• the UPS charging current is small, because it is sized for the stock batteries. Big batteries take a long time to recharge after a discharge, a day or more.

Attempt 1. PowerChute over USB — no luck

First we ran the calibration from APC PowerChute Business Edition 9.1.1 (Windows), with the UPS on a USB cable. The calibration did not help: the UPS still considered the batteries flat.

The COM cable

The service tools for Smart-UPS work over a COM port, not USB. APC's own cable is the 940-0024C. We soldered an equivalent from the COM-port cable that came with a motherboard.

⚠ Do not connect an ordinary serial cable to a Smart-UPS. UPS pin 1 is also the “shut down” input, and a standard cable can switch the UPS off together with its load.

Attempt 2. Service tools over COM

Once the cable worked, the APC FIX service tool (apc-fix.com) found the UPS and showed its internal constants. Constant 0 read BC.

Then, following advice from a repairer’s blog, we wrote FF into register 0 with UpsDiag 2.0 (the calibration tab). According to that description, this register affects how the UPS rates the battery, and FF makes it rate the battery anew during calibration. We did not verify that separately, we just did it. The UPS clicked its relays, and register 0 read FF.

⚠ These tools change the UPS memory. Write down all constants before changing anything, so you can go back. Change only what you understand.

Then we unplugged COM, plugged USB back in and entered the new battery replacement date in PowerChute.

Calibration

Calibration is discharging the batteries under load all the way down. The UPS measures how long it lasted and updates its estimate. The tool warns that the load must be at least 35 %.

First try, at night: a 300 W incandescent lamp, the UPS showed 37 % load. We stopped the test once it was clear it would take hours.

Second, in the daytime: a load of about 250 W (30 %).

Summary

Two 45 Ah car batteries (in series, 24–27 V) carried a 250 W load through an APC Smart-UPS 1000 for more than three hours, with no extra cooling for the UPS. What it took: a 940-0024C-style COM cable, constant 0 = FF, and a calibration under load.

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