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UPS Selection for an inverter
#1
Thought this might be useful for those using or selecting a UPS instead of a specific grid/off grid inverter. Lots of posts in the past covering UP units and thought it might be useful to have a single post with the main list of bullet points and benefits / drawbacks and some UPS models that other people are using.


This is an APC UPS unit model number : 2200RMI2U - the 2U signifying 2 rack slot units in height. 2200VA rated and 1980 Watts, so good active (1980W) power capability compared to the reactive/apparent (2200VA) rating.



The APC units seem to have 3 or 4 generations with this 2nd or 3rd as the newer 3000VA units have a battery pack with 72V (compared to the older 3000VA with 48V) and the older 3000/2200VA units have a different front display unit and no USB interface.

Main benefit of these units is the heavy duty transformers, which allow for higher loading and the two cooling fans that will pull airflow through and over the switching FET's and the transfomers. Some of the smaller desktop / tower case units either lack fans or the cooling flow is not as directed over the "hot" components.

The units tend to have charge current of around 1 to 2A (typically around 100W) so do not rely on them to charge your battery unless the battery is less than 2kWh and you have a lot of time to charge.

When using them, leave them plugged into the mains and switch the mains off at the wall, the reason is that this then the easiest way to earth the unit (older units can leak current as I found out with a 120V live case). Switch the unit on and you then pull power from the grid and put a small charge into the battery. Switch off again to back to battery (or put on a time switch...).
Korishan likes this post
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#2
Very good points. Another reason for having it connected to the mains is that it is synced up with at frequency. This is very beneficial if you have multiple units feeding the same device (this is actually not recommended unless these two devices are designed to be serialized and/or parallelized). Also, would allow for a clean signal to the devices it is feeding.
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#3
The holy-grail of UPS re-use would be to parallel them via a small signal 50Hz direct into the UPS boards, which would prevent them from trying to re-connect to the grid randomly... That would be well cool and very, very useful...

My APC SUA2200 battery light (silently still flashing 1 LED...) appears to increase charge LED's as the battery voltage declines to 49V, so have little faith the APC units would not randomly re-connect to the grid after a brownout..

Are there any small UPS units that can be used in parallel by design ?
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#4
I think you can trick most UPS units into maintaining a solid frequency with each other by introducing a common AC input, but at a low voltage. Most UPS will switch to backup mode when line voltage drops below a threshold, usually about 108V here in the US. Some are able to be custom set. So, if you take several UPS units, and feed in a 100V signal @ X Hz, then they will stay in backup mode, but still get their signal from the AC line.
Again, I think this would work. I have not tried this in practice.

There's also a way to "force" the UPS into bypass mode. knarlnar on Youtube showed how to do this by applying a steady 5V to a specific leg on an IC on the board. I don't know enough to do this myself or to find that particular IC (with different labeling) that does that job. He triggered the "slave" UPS into bypass mode. Then connected the slave to the master UPS. Turn on the master first, then turn the slave on. The slave syncs up to the master and you can actually connect the units in Parallel or Series. The waveform stayed steady during his tests.
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#5
I had seen that youtube before and watched it again to double check his method... interesting points were :
1. Low AC input voltage will not maintain sync as the UPS will not follow a low input signal - I did not realise this.
2. Series connection is less of an issue as worst case you end up no output voltage if both outputs move 180 degrees out of sync. Limits to 2 units and you need 110/120V rated output units.
3. You have to hack the analog input registering the line voltage and pull that to 5V to make the UPS think there is over voltage and the unit will remain in sync.
4. Still need one unit with a grid/line voltage input to use as the sync source signal, so could use a single smaller sine wave inverter.

If the circuit boards do not have significant changes to them between models the pinout for the ADC is likely the same.

Still, at least it provides a method to get a 6kVA inverter... with the APC units rated at 2700W that means a real output around 5kW. Hmmmmmm.....
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#6
Ah yeah, that's right. I forgot about the low voltage/high voltage issue. Low voltage is just flat out ignores the input. Overvoltage it at least syncs with it, but bypasses it. Been awhile since I've seen the video. I wish there more videos showing the principle either of him or others going more into depth with it. But seems like his was the most advanced and no one took it any further.
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