I have a Smart-UPS SC1500. Admittedly quite old, but works fines, except...
Even after a full charge (battery voltage almost 27V), when disconnecting mains power it immediately reports the battery charge dropped to 0% and initiates a server shutdown. It has 4 new batteries. I performed a manual calibration. This means disconnected it from the server, attached a 500W load, and left it to shut down, which it did after about 30mins. No luck. I then checked the battery constant, which was 38, so I set to 8F (according to http://www.apcupsd.org/manual/#resetting-the-ups-battery-constant), firmware is 738.3.I so that matches. Still no luck, same behaviour. After it does this, I need to wait several hours for the battery charge parameter to slowly climb back to above 50% (despite a battery voltage well above 25V, as reported by the UPS, not measured manually, though I believe that value).
All the parameters from "apcaccess" (Linux apcupsd) look fine, and nothing on the front LEDs indicate anything unusual.
BTW, if I disconnect the serial cable, it'll run for over an hour on batteries no problem. Disconnecting the cable obvious prevents a server shut down, but on the flip-side I lose the ability to monitor the UPS from the SW and for the UPS to initiate a shut-down. Not great.
One more bit of info: it's as if that as soon as it switches to batteries, it senses no voltage on the batteries, and sets the battery charge parameter to 0%. When on mains power again, the battery charge parameter climbs back to 100% over about 10 hours. The battery voltage is 26.9V the whole time (i.e. fully charged), both measured with a meter and as reported by the UPS over the serial link. However, when the server is not connected to the UPS, it lasts 30min easily on battery power before the UPS shuts down.
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