More UPS hacking

Posted on September 15, 2009 by
I ran a 60 odd watt load on the autozone powered UPS for 24 hours or so…no issues, and lots of run-time being reported via ‘nut’. So I moved it over to a bigger load. Here is what is being reported currently:
root# upsc autozone
battery.alarm.threshold: 0
battery.charge: 096.0
battery.charge.restart: 00
battery.date: 10/07/02
battery.runtime: 1320
battery.runtime.low: 120
battery.voltage: 13.85
battery.voltage.nominal: 012
driver.name: apcsmart
driver.parameter.pollinterval: 2
driver.parameter.port: /dev/tty.serial
driver.version: 2.4.1
driver.version.internal: 2.00
input.frequency: 60.00
input.quality: FF
input.sensitivity: H
input.transfer.high: 127
input.transfer.low: 106
input.transfer.reason: O
input.voltage: 107.2
input.voltage.maximum: 107.2
input.voltage.minimum: 106.5
output.voltage: 122.4
output.voltage.nominal: 115
ups.delay.shutdown: 060
ups.delay.start: 000
ups.firmware: 33.7.D
ups.id: UPS_IDEN
ups.load: 046.8
ups.mfr: APC
ups.mfr.date: 10/07/02
ups.model: Conext 900 ups.serial: QC0240244442
ups.status: BOOST OL
ups.test.interval: 1209600
ups.test.result: NO
The UPS is a Conext 900, which is a re-badged APC. I picked them up from COMPUSA probably 5 years ago, they were < $100 each and had a rebate. They have been durable, and at 900VA(AVR) and 500watts reasonably powerful. Though they came with a simple signal cable, putting an APC 024C cable on them lets NUT grab all the info described above. They are plain-old 9-pin RS232, so Macs and newer computers will need a USB to serial (the above dump is from an Xserve using its built-in serial port).
Probably these numbers are not accurate, as the firmware is calibrated for the OEM battery pack. It’s showing about 22 mins runtime on a 240 watt load. An OEM unit with a 11% load (55W) is reporting a 66Min runtime. Theoretically I should see more — the OEM battery is 12Ah, and the Autozone unit is 85Ah, so conservatively I should have 8x the runtime. At some point I’ll do some power-off experiments and symmetric load comparisons.
Still this is inspiring enough that I’m shopping for a used, battery-less large UPS, ideally a 110v 3Kva. I’ve located a few which look promising. I first found a Compaq/HP unit which was quite inexpensive and claimed to be ‘digital’, using a DSP to generate the 60hz cycle. It was also only 2U. Further investigation revealed that it used 10 12V batteries in series to produce 120V DC, which it then ‘switched’ to AC. While this may be efficient, 12 deep cycle batteries would be large and expensive.
I have my eye on an APC 3kv with no batteries and a severed cord. It’s < $100, but will close in on $200 when shipped, even w/o batteries these are heavy gadgets. This _should_ be a 24V unit, so 2 larger DC batts should handle it, if it turns out to be 48V as some of the newer APC 3Kvs are, 4 smaller batteries will only be about $50 more, though another $50 in cables and boxes.
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