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Active Member
Aug 1, 2014
Hi Everyone,

There really isn't a subforum for UPS:es and other power solutions, so I decided to post here.

I bought a couple of APC SMT1500RM2U units for my rack used on ebay last year. The seller put fresh batteries in them before selling them, and they have gone from 50 minutes of runtimes when new in August last year, to absolute trash, sub 5 minute runtimes now.

There are three potential contributors to this short battery life from what I can tell:

1.) Were they just cheap garbage batteries?

2.) Was the high heat (30C-35C) environment they were running in killing them early?

3.) Is this a symptom of APC's notorious high float voltage problem, which kills batteries early (presumably on purpose, because their Schneider overlords want to sell more batteries)?

I have since addressed the high heat. Older server room (lol, closet) had no options for cooling. New one in the new house does.

I also plan on trying to buy decent batteries this time around (but is it just m, or have they become really pricy? Batteries Plus used to be affordable, but now 12V 8AH batteries are $40 each, and I need 8 of them!)

From various sources I have read that on older models of APC UPS:es you used to be able to configure the desired float voltage via hidden configuration settings, over a serial interface. Does anyone know if this is still possible with the newer SMT1500RM2U's I have?

I appreciate any suggestions!


Active Member
Aug 1, 2014
So, upon a 8th googling of the same terms, I randomly came across this page which never popped up before.

Some reading of this page, and the linked reference page has resulted in the following conclusion:

There is a proprietary RJ45 Serial interface port on the back of these things (requiring a specialty RJ45 to RS232 cable), but apparently it is no longer connected for "SMART" functionality like the old ones, so it cannot be used to - among other things - reprogram the float voltage.

That said, the funtionality is still in there, and there is still a header on th econtrol board, so you just need to pop it open and connect your own.

Looks like it is relatively simple. (Just don't touch anything high voltage!) All I need is a cheap TTL USB to serial adapter ($5 on amazon) and some female to female breadboard jumper wires.

I'm going to give it a shot.


Active Member
Nov 18, 2014
EDIT: IGNORE MY ORGINAL POST - update: i was able to adjust the Float Voltage of my SMT-2200 (ID:18) w the latest FW (as of dec2022) -- the key sequence is a bit different than the others, once connected via TTL/Serial, you press 1, then 1, (should get PROG back) then Shift+B , then Shift-+ or Shift- - (+ to go up , - to go down) - Shift-B will output the current float, i set mine to 54.00 (will have to wait to see what multi-meter shows once batts have time to adjust) (once done you are finished, you press R )

see this guide (but key sequence seems a tad different on the smt-2200)

I hoping someone can confirm this , i have done the float voltage mod on 2x of my APC SMT-1500 UPSes (they very much needed it btw). This has helped the batteries last longer! (all of my upses have a NMC added (network management card)

However, i now have a larger SMT-2200 , I updated its firmware to the latest (i think its v15.0 , but i know this FW has a released date of Jul 2022 , which is the latest as of now, dec2022).

The issue however is that once im connected to the Serial headers, it seems that the FW/UPS is not letting me change this value! (im wondering if APC has blocked this in the newer firmware (s)). Or is the smt-2200 just not capable of changeling the float voltage (this is the first 2200 ive tried it on)

as you can see in this image (when i press + or - , i should be getting a hex value back, AND see the float voltage change on my multi-meter, just as was the case when i did my other smt-1500s) -- instead its replying back NO , and my multi-meter is NOT changing:


(the good news however is that my SMT-2200, stock, w latest FW is reading a float voltage of 54.6v dc , which actually seems good (id prefer ~52v), but its not way out of spec like my other SMT-1500s were showing before i did the mod/change. My batteries label shows: "Standby use: 13.5v to 13.8v (13.5 * 4x batts = 54.0v)

Can someone confirm this??
(ie that we are no longer able to adjust the float voltage on either the SMT-2200 , OR on the SMT-2200 with the latest firmware)
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