On Tue, 3 Jul 2018 00:06:33 -0700 (PDT), wrote:

I have a SU3000 that works from AC power and inverter will start if you remove power, but it wont start up from battery power.

Originally I didn't have batteries to test it so I used a couple of small power supplies in series to produce the 48V it requires to operate. They were only a 1 amp supply or so but it worked.

Now with nothing plugged in, it draws a lot more amperage on startup from battery and instantly goes to overload..
When using the power supplies I originally used to start it up, it draws too much power from them and they shut down, and it doesn't get a chance to do anything but beep once and the lights blink once.


So obviously the unit is using more power than before and not just a software issue.


I am thinking it may have a bad mosfet. Cant remove them until I get my hot air station. Does this seem like a reasonable place to start troubleshooting? Maybe the mosfets were damaged from using the power supply to power the unit?

Thanks in advance for any advice

In the APC SmartUPS, some of these run the switching power supply
section full time, whether on AC or battery. On AC, the voltage from
the switching section is slightly below the input AC voltage, so the
load draws most of its power from the AC line. Of course, they are
phase locked. When the AC line voltage disappears, the switching
power supply section takes over running the load with zero switching
time. I'm not sure if the SU3000 SmartUPS works this way but it's
worth checking.

A schematic should be useful:
https://elektrotanya.com/apc_smart-ups_su2200_3000.pdf/download.html
(175K)

Offhand, I would guess(tm) that you blew something up with your 48V
power supply. The SU2200/3000RM uses a pile of 4ea 12V 18AH AGM
batteries. I believe that they are wired 2 in series and 2 in
parallel for a total of 24VDC, not 48VDC. You probably blew up the
electronics with the overvoltage.

If it draws lots of current from the battery section, the fets are the
likely culprit. However, as I vaguely recall, the SU2200/3000 series
has 4 strings of 8 FET's in parallel. It's not going to be much fun
finding the one FET out of 32 that is blown. I built a crude current
probe using a Hall effect device at the end that is useful for finding
which device is sucking the most current.

Most of the value and cost of a UPS are in the batteries. In this
case, a set of batteries is about $130 plus shipping. At the local
recycler, such large UPS's without batteries are plentiful because the
price of a new or refurbished UPS is only a little more than the cost
of decent batteries. Incidentally, beware of junk batteries and look
at the weight of the battery for a clue as to how much lead is inside.
I have my own collection of UPS's with dead batteries.

https://excessups.ca/refurbished-ups-systems/apc-ups-refurbished/apc-smart-ups-rackmount-refurbished

Good luck.

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558