[Nut-upsuser] Questions about FSD (Fast Discharge)

Roger Price roger at rogerprice.org
Fri Feb 26 21:29:33 GMT 2021

On Thu, 25 Feb 2021, Jey via Nut-upsuser wrote:

> Hi :)


> I use NUT on Home Assistant, the version of the add-on is 0.6.2. Home 
> Assistant is installed on a Raspberry Pi 4b.
> A have a APC 1400, the UPS is connected with USB to the Rasp.
> I'm working to built an automation, to receive notifications when there 
> is a power cut.
> So I used OB DISCHRG (On battery discharge) to detect the power cut and 
> trigger the automation.

It is sufficient to use OB.  Not all UPS's show DISCHARG

> Then I use OB CHRG to detect that the power is coming back and the 
> battery is charging.

It is better to use OL which says that you are receiving utility power, and 
therefore charging.

> Then OL the normal operation, with a full battery.

OL should appear before battery.charge = 100%

> In the same time I send notification to follow the % of battery (97 > 93 
> > 90 etc). It's working.

I assume you do this by monitoring variable battery.charge.

> During my real test, the load was to light (3%, only a rasp pi), so it 
> was slow to empty the battery. So I connected and started my main PC, 
> than the load was 18%.

If you want faster testing, you could also connect a 60W light bulb.

> Then the battery fell to 48%, and instead to have OB DISCHRG, I had FSD 
> OB DISCHRG (Fast Shutdown On Battery Discharge).

What is the value of battery.charge.low in your setup?  It is surprising to see 
FSD appear with no LB status.  Do you have upsmon running?  What is your system 
shutdown criterion in upsmon?  Do you rely on the built-in emergency shutdown on 

> Then I re-plugged the UPS to charge it.
> And I was surprised to see FSD OL CHRG until the 100%.

FSD is held fixed in upsd.  upsd does not turn it off, since it is assumed that 
there will be a system shutdown followed by a system restart which clears 

> Then, I had FSD OL, even everything was back and normal, and the battery 
> fully charged.
> I saw today, few days later, than now it's only OL (Online). I imagine 
> that FSD disappeared after a reboot or when I disconnected some USB cables.
> So, I have some questions. I didn't find this kind of information on 
> internet.
>  - please, could you explain why FSD is coming ? Is it depending of the 
> load, or the % of battery?
>  - Why FSD didn't disappear when the current was back and the UPS was 
> charging the battery or full?
> - Is there a list of all "modes" that NUT can show? (because my 
> automation didn't work any more when FSD appeared, so I would like to 
> know all modes).

What do you mean by "mode" ?  The possible status symbols, OL, OB, LB, ...?


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