On SmartUPS you can adjust the float voltage via programming, and you have to reset the battery constant to prevent early shutdown from a badly calculated runtime. If you buy a used UPS, universally they will be screwed up either in float voltage, batt constant, or likely both. I'll copy and paste from my post of years ago, sorry for the novel even though I chopped a lot out from the original post but it goes over in depth everything about repurposing a Smart UPS:
Edit 11/21/14: I've recently discovered that the APCs line of SURTA UPSs and Eaton 9130 UPSs are full online double conversion machines, which means that it has two sections in it; a battery charger and a inverter.
For APC up to 2200VA has a 48V battery pack, however the 3000s and bigger use a 192V battery pack.
Eaton 9130s use a 72V or 96V battery pack depending if its a tower or rackmount model.
The rectifier operates full time producing DC, the inverter portion also operates full time, drawing power off the DC & producing a nice clean sinewave for the load. At the same time the battery charger is doing its job of keeping the batteries charged. The two portions work independently and the only connection is the DC bus which provides the load a rock-solid stable power source completely independent of the power line conditions. Just the thing that sensitive electronics need like mission-critical servers, or medical equipment.
If the power is lost the charger just drops offline leaving the inverter to just keep merrily making sinewave power until the batteries are discharged.
This means the inverter is designed and built to operate continually for years, decades even and don't have to be modified to keep from cooking itself.
So basically a SURTA UPS _IS_ a inverter/charger, just one with its own internal battery bank. The XL models are designed for charging/using really large external battery banks, the chargers are really slow though - only 3-6 amps. A SURTA is perfect, just the way it is, and a hell of a lot easier to find and cheaper than a "inverter/charger".
They also have to be reprogrammed so it knows there is an big external battery bank, the battery constant adjusted, and to make sure the float voltage is correct (they usually aren't).
If you dont reprogram it the UPS will just quit when it reaches what it thinks is the end of its runtime. You can also turn off the beep and a couple dozen other things, more on all this later.
Here is a pic of the completed cabinet, it started out as a garage cabinet I got at Sams Club. There is a APC SU2200 Smart Ups sitting on top that I was originally going to use before I found the SU3000 on craigslist. Note the top air intakes and lower door exhausts. Even when sitting idle just floating the batteries the UPS generates a good bit of heat so constant airflow is required.
I have two 120mm 55 CFM fans pushing air in and they're just barely adequate, if the UPS goes on battery power or doing heavy charging I have to open the cabinet. I did the in-top/out-bottom arrangement so the UPS and batteries are at the same temperature so the UPS will push the correct temperature-compensated float voltage.
The next thing I need to do is mount two more fans at the top of the door so they're blowing onto the UPS and tie them to the 24v fan that the UPS turns on when under load. I'll tape a bit of plastic on them so air doesnt escape back out through them but will flap up out of the way when they're on.
PROGRAMMING
To get the best battery life and runtime you can the UPS has to be reprogrammed. The UPS has a internal protocol you can communicate with and modify to an extent. A good description and a list of the various things it in it is here:
http://www.apcupsd.com/manual/#apc-smart-protocol" onclick="window.open(this.href);return false; .
You'll need a APC communication cable to talk to your UPS, it looks exactly like a standard serial cable but they've switched around the pinouts. You can also make your own if you google for the "how to". I just took one home from work
The first thing you need to do is make sure the battery constant is showing a good battery. The battery constant is what the UPS uses internally to calculate runtime, when a UPS (and battery) is new there is a set value, that value is changed as time goes by to show the degredation of the battery. The constant is particular to each model, most of them can be found here:
http://www.apcupsd.com/manual/#resettin ... y-constant" onclick="window.open(this.href);return false; . My SU3000NET wasn't on the list so I corresponded with APC and got the correct value of 96, it was sitting at 86.
EDIT 11/22/14: If you set your battery constant to "FF" it will set your battery to "magic-super-duper-uber" battery and it'll give you the maximum possible runtime the machine can calculate per battery pack. Your calculated runtime will be wildly over-inflated but that's okay since the intent here is to prevent premature shutdown due to a bad calculated runtime. It'll still shut down when the battery voltage gets too low (~10.5V per battery) so you're not risking a over-discharge that'll damage your battery bank.
Changing the battery constant
Use hyperterminal (or telnet, or whatever) @ 2400 baud with 8-N-1 Xon/Xoff settings, under Advanced make sure FIFO is unchecked. There can not be any network cards in the UPS for this to work.
- Put the UPS in "Smart" mode by typing Shift-Y (capital Y) it should return "SM"
- Now put it in "Program" mode by typing "1", wait 2-3 secs and type "1" again. It should return "Prog"
NOTE:
It is very easy to turn your UPS into a nonresponsive brick if you type in the wrong thing while in Program mode, be careful.
- Enter a "0" (numeral 0) and the UPS reports the present value of the battery constant.
- If this value is not correct, press "+" or "-" until the correct value is returned IAW the linked list.
- "R" save your new value and exit program mode, it should come back with "Bye"
External battery bank count
- Put the Smart Program mode (Y, 1-1)
- ">"
- "+" or "-" to change the battery count, every additional 18AH at the system voltage is considered another battery. For example my 48v 270AH battery bank is considered 15 external batteries. Depending on your model you might not be able to go past 10 external batteries.
- "R" to save and exit
Float voltage
Google for your batteries correct float voltage, make sure you use the correct temperature, most float voltages are given at 25 degrees C, most peoples houses/garages are usually less than that. Gel, flooded, and AGM batteries will all work. AGM and especially Gel batteries are sensitive to overcharging. Flooded lead acid batteries require more maintenance (watering, periodic equalization).
What you're actually changing is the UPSs battery gain, you're making the UPS think the battery voltage is higher or lower than it actually is and it adjusts its float voltage accordingly. The voltages the UPS will spit back are irrelevant, you need to have a decent digital voltmeter attached to the batteries. Just .1 volt can make the dif between getting the maximum life out of your batteries or an early battery death. When adjusting, adjust it and then let it stabilize before further adjustments. It can take a LONG time for it to stabilize depending on the size of your bank.
- Smart Program mode (Y, 1-1)
- "B"
- "+" or "-" to adjust, anything other than + or - will drop it out of battery gain adjustment mode
- "R" to save and exit
Other useful commands:
"s" (lowercase) for sensitivity to line conditions
"k" (lowercase) "+"/"-"for alarm (0=5 sec delay, T=30 sec delay, L=Low battery only, N=No alarm)
"r" for calculated runtime
"t" system temperature, this is what it uses to calculate the temperature compensation.
"m" manufacture date
Hope this helps somebody, I can't take credit for any of the above, a lot of people have done all the intellectual heavy lifting prior to me, I just found it via Google. The big push for me to build this is that our wintertime power out here is terribly unstable and outages are common.