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Adusting battery float voltage on APC Smart-UPS SUA1000/XL

Posted: Fri Mar 09, 2012 11:02 am
by asavage
Calibrating charge float voltage on Smart-UPSs

List of Smart-UPS models that have been confirmed to be able to have their float voltage adjusted using these instructions, and the number of reports:
  • SUA750XLI (1) (2003 vintage) (Reported by Wim, 07Dec2017)
  • SUA750I (1) (2009 vintage) (Reported by Wim, 07Dec2017)
  • SUA1000 (1)
  • SUA1000XL (1)
  • SUA1500 (1)
  • SU2200 (1)
Units that respond to these instructions, but do not actually adjust the float voltage:
  • BP650S (1)
  • SU700 (1)
  • SU700NET (1)
  • SU1400 with firmware 70.9.I (reported by Marc van Dongen, 29Jun2012)
Contact me at to add models to this list.

Because I'm certain to forget this info, I write myself notes like this.

Problem: APC UPSs sometimes have a float charge voltage that is too high and tends to cook batteries. Here's a pair of gel/AGM batteries from a SUA1000 (not an XL) that have swollen so badly that I had to disassemble the case and pry the batteries out of the metal cage:

(click on any image for larger)
Image Image Image

In the past, I've used multiple Costco FLA (Flooded Lead Acid) batteries to power my UPSs, and over the years I've learned to carefully monitor the water levels, because various UPS' charging circuits overcharge and keep the electrolyte gassing.

I stumbled across one post by Jacob Joseph that gave some clues to programatically changing the float charge voltage on some APC Smart-UPS models. Jacob indicated that he could change software parameters to change the float charge voltage on his SUA1500RM2, but not his SUA700, and he gives specifics. He goes on to relate how to modify the circuit physically on a Smart-UPS 1000, when a programmatic solution doesn't work.

At first, I was not able to make Jacob's software recipe work: I could not achieve SM (Smart Mode) mode, due to a missing step in his instructions. Later, in the APC Support Forums, I came across a reference to using <Shift+Y> as a trigger to put the UPS into Smart Mode, and then found this post that agreed. My missing first step: start with Capital Y.

But, while I could get my SUA1000XL into Smart Mode, I couldn't get it into PROG mode.

Then I found a reference in a third place that any & all SmartSlot cards must be removed before beginning, and Voila!

Note that APC has many different firmware revisions for their Smart-UPSs (as opposed to the firmware in the SmartSlot devices!), and the features available in Smart Mode vary. My SUA1000XL has firmware Rev. 631.3.D
  • Remove any SmartSlot card from the UPS. This might be a Network Management Card such as a AP9606, AP9617, AP9618, AP9619, or an Environmental Monitoring (EM) card. Just removing the two retaining screws and pulling it out of the chassis 1" is sufficient to disconnect it. While the official APC recommendation is to "Brain Dead" the unit before removing or installing SmartSlot cards, many people routinely change them hot (with the UPS ON), and that's what I do.
  • Use the Serial port. Because the SmartSlot cards must be removed, no networking is available, so you have to use the serial connection (or, perhaps, the USB connection if your Smart-UPS has one, but I haven't tried that).

    Connect the Smart-UPS serial cable to the UPS & PC (or terminal emulator, or, I suppose, a real terminal if you have one ;) ). In my case, the Smart Signalling 940-0024C (black) DB9xDB9 serial cable. No, you can't use a straight-through or null-modem cable.
  • Disable COM-port apps. If using PowerChute, or another UPS Service, disable it. The terminal program must have exclusive control of the COM port. Figure out which port you're using, if there's more than one. I've read at least one report than a USB<->Serial converter does not work for this procedure.
  • Use a terminal program, such as HyperTerminal that's bundled with versions of Windows. Setup parameters are 8-N-1 (Google what that means, if you haven't used a terminal program before. HyperTerminal is not a very good terminal program, but it will suffice.). Flow control is variously recommended to be "None" or "Xon/Xoff". I was successful with "None", so I didn't try "Xon/Xoff".
  • Test the setup. If everything is correct, you should be able to type a capital 'Y' (that's <Shift+Y>) and the terminal should display "SM": the UPS is now in Smart Mode. Occasionally, I've had to type 'Y' twice to get a response.

    As a double-check, type a capital 'A'. Otherwise known as <Shift+A>. The UPS should beep and light up all front-panel LEDs for a couple of seconds. The terminal may display "OK".
  • Enter PROG mode by typing "1" followed by "1" again after two seconds. IF you pulled the SmartSlot card(s) out, and IF you got the timing of the "1"s right, the terminal will display "PROG". If not, try "Y" + "1" + "1" again, using different pauses. The "1" (pause) "1" is the sensitive part.

    If you made it this far, the hard part is over.
  • Enter "Battery Gain Adjust" mode. Now that the UPS is in PROG mode, capital "B" (<Shift+B>) will display a voltage (for the SUA1000XL; apparently, it shows you the Battery Gain Constant for Jacob's UPS). I consider it a reference only and near-useless. I ignore it and use the voltmeter exclusively.
    Jacob Joseph wrote:B: Battery voltage gain.
    0=maximum gain, FF=minimum gain.
    Adjust with the batteries fully charged, and an accurate voltmeter connected.
    Factory setting was "E9" on my SUA1500, and "E6" gives me the desired 27.30V battery voltage. . . .
    I agree that the battery pack must be fully-charged and a good voltmeter attached to perform any changes. Otherwise, you're going to spend a lot of time wondering why the voltage isn't changing the way you think it should.

    Keep these Adjust Battery Gain points in mind as you change this:
    • Pressing 'B' (<Shift+B>) will display a voltage. It also puts the unit in "Battery Gain Adjust" mode (my terminology). It's very easy to have it drop out of this mode!
    • In this mode, only the upper row '+' and '-' keys apply. If you press any other key, it will drop out of Battery Gain Adjust mode.
    • Pressing '+' will raise the Battery Gain constant one notch, and lower the float voltage.
    • Pressing '-' will lower the Battery Gain constant one notch, and raise the float voltage.
    • Each keypress changes the float voltage ~0.10v.
  • Adjust Battery Gain.
    With the terminal displaying your supposed battery voltage (Adjust Battery Gain mode), take note of your voltmeter's reading, then press a '+' (to lower voltage) or '-' (to raise it). All changes are immediate. The terminal should now display a two-character Gain Constant, in the range of 00 to FF, your new Battery Gain Constant. You won't know your old constant, it never showed it to you (for this model, anyway; apparently, it does show you the constant for Jacob's UPS). To determine (and record, if you wish) the old constant that you just changed, press the complementary adjust key ('+' or '-') and your old (previous) constant value will be displayed. You just moved one step away, then one step back.

    Watch the voltmeter. Wait for the new voltage to stabilize. This can be anywhere from a few seconds (for my FLAs, <10) to a couple of minutes. The battery voltage will change about one-tenth of a volt. As long as you only press '+' or '-', you can adjust up & down forever. The UPS will remain in Adjust Battery Gain mode.

    Chances are, though, that you'll eventually forget to hold down <Shift> to get the '+' key, and it will display "NO" and drop out of Adjust Battery Gain mode. To continue adjusting, re-enter the mode by pressing 'B' again. You'll be picking up where you left off.
  • End the session using 'R' (<Shift+R>). The terminal will show "BYE". You can close the terminal, disconnect the cable, etc.: you're finished. All changes are persistent.
Information on float voltage
According to Wikipedia:
Continuous-preservation (float) charging:
  • 13.4 V for gelled electrolyte
  • 13.5 V for AGM (absorbed glass mat)
  • 13.9 V for flooded cells
  • All voltages are at 20 °C (68 °F), and must be adjusted −0.0235V/°C for temperature changes.
  • Precise float voltage (±0.05 V) is critical to longevity; insufficient voltage (causes sulfation) which is almost as detrimental as excess
  • Float voltage recommendations vary, according to the manufacturer's recommendation.
My SUA1000XL has been charging to 27.66v for a 24v nominal system. This is consistent with 2.30v per cell (2.3v * 12 cells = 27.6v) for my Costco Flooded Lead Acid type batteries, and the batteries would probably be just fine if I left this charge float voltage alone. As long as the float voltage remains under the gassing voltage, the FLA batteries are fine (setting aside periodic equalization). For a 25°C/77°F temperature, that works out to

2.25 to 2.30 per cell =
13.5 to 13.8 per battery (27-27.6v for the pack)

2.39 is the gassing voltage at that temperature: 28.7v for the pack, the voltage to stay under.

However, this unit originally ships with AGM VRLA batteries, mounted inside the unit and running about 85°F+. AGM & esp. gel batteries prefer cooler temperatures, and while AGM can handle slightly higher charge voltage, gel batteries absolutely can't.

Which helps to explain why so many UPS batteries don't even make it to 3 years.

I was able to change my SUA1000XL (with external Costco FLA batteries) float voltage from a Battery Gain Constant of (unknown) and 27.67v, to constant E7 and 27.45v .

I was able to change my SUA1000 (with internal AGM VRLA batteries) float voltage from a Battery Gain Constant of (E2) and 27.77v, to constant EA and 26.92v .

(click on any image for larger)
CSB HR 1251W F2FR wrote:Constant voltage charge, voltage regulation (20°C)
Standby use: 13.5 - 13.8v
Cycle use: 14.4 - 15.0v
Initial current: 5.1a MAX.
Image Image Image

Posted: Thu Apr 18, 2013 11:05 pm
by HKPolice
Thank you so much for this post. I found out that one of my Smart-UPS 1500 (IBM rebranded) was charging at 27.9v! and the other was at 27.4v. I've adjusted one of them to 27.1v with genuine APC batteries and the other will be adjusted soon.

Is ~27.1v ideal for genuine APC batteries made by CSB?


Posted: Fri Apr 19, 2013 4:37 pm
by asavage
I would think so.

CSB says: Standby use: 13.5 - 13.8v

That's 27.0 to 27.6v @ 20°C

I'm running mine at just under 27v (26.9) which is possibly too low.

Posted: Sat Apr 20, 2013 1:22 pm
by HKPolice
Do you know if it's possible to adjust the fan settings in PROG mode? Both of my IBM branded 1500 units are old so the fans don't turn on until the power goes out or running with over 75% load. I recently purchased a Dell branded SUA1500 (DLA1500) and the fan is constantly on, even when unplugged from the wall.

I did some googling and apparently all units manufactured after ~2007 are like this. The fan is supposed to spin constantly at a "low" speed, then speed up when running on batteries. Problem is that even on "low" speed, it's way too loud for home use.

Posted: Sat Apr 20, 2013 1:34 pm
by asavage
I've heard of the fan problem before; fortunately, both of mine are old enough that the fan stays off until the power is out, and I like it like that!

If you hear of a way to turn off the fan on the later ones, I want to know! It's not a current problem, but if I acquire a newer one . . .

Posted: Wed May 01, 2013 11:10 pm
by HKPolice
I took apart my UPS today to clean out the dust inside (spring cleaning) and realized that the float voltage is actually temperature sensitive. When it started out cold around ~27C it was charging at 27.3V and after a while it got up to ~31C, dropping the voltage to 27.1-27.2V. All measured with DMM and the temperature reading was from PowerChute.

I run my battery externally to keep the UPS and battery cooler so this doesn't really benefit me but it's an interesting hidden feature.

Posted: Thu May 02, 2013 6:31 am
by asavage
Temperature compensation for charging voltage has been around for a long time. I didn't think to mention what the temp was when I was adjusting mine. I think the CSB spec voltages were at room temp.

I run one of mine with a stock-style CSB inside (the SUA1000), and for another that has the external battery connector (SUA1000XL) I bought the mating connector from Grainger* and a bit of 10ga wire from Home Depot, and have it running on a pair of Costco (Johnson Controls) deep-cycle marine batteries in a plastic tub.

In the past, before I lowered the float voltage, I'd have to top off the batteries' water every few months, but now it does not seem to use any water.

Image Image Image

* I think the connector I purchased was an Anderson Power Products 6319 SB50, a 50A (nominal) rated unit that can accept 6 ga wire and is less than $5 from Mouser or Allied, though it's $9 at Grainger, but Grainger is my 7-Eleven and they had it in stock, so I didn't order it, I just paid the premium. I needed only the one.

Car show vendors often stock these type of connectors, too.

Posted: Thu Sep 05, 2013 12:21 am
by HKPolice
I picked up a SUA2200 today and as expected, the float voltage was too high: 55.4v so I followed this procedure but the actual DMM voltage was not responding even though everything looked normal in Hyperterminal.

I fixed it by running APC Fix: with Auto batt calibration checked.

Once this prog connected to the UPS, the voltage I set in HyperTerminal took effect. Running the prog again would revert the voltage back to 55.4v, requiring another connect to lower the voltages again. It's as if the UPS has 2 "modes", one where the voltage can't be adjusted and another where it can.

I have a feeling some of the UPSes that didn't respond to adjustment might just need to run the APC fix prog.

Posted: Wed Sep 18, 2013 5:43 pm
by asavage
Good info, Thanks for posting it.

Re: Adusting battery float voltage on APC Smart-UPS SUA1000/

Posted: Tue Jul 22, 2014 6:24 am
by drescherjm
Hopefully this thread revival is okay..

Anyways I have a Smart SUA1400 model that allows me to change the float voltage in the PROG mode everything appears correctly but when measuring the voltage with a DVM it is still 27.8V instead of the 26.9V I set it to even though when I query the UPS it does say the battery voltage is 26.9V I guess I will have to try APCFIX on a windows box.