21V Mini DC UPS - too long switching delay

[lkondas]

Hello everyone!

I'd like to have a cost effective DC UPS for my home server, and need some help

I purchased a cheap chinese mini DC UPS - power bank , what is using 18650 Cells (https://www.aliexpress.com/item/1005002483138986.html?spm=a2g0s.9042311.0.0.5d7f4c4dB0307k)
The server is a micro form-factor PC, which has a 19.5V 65W power brick (Dell 7050 Micro)

The UPS - power bank is in passthrough mode when it has power from the DC barrel. When there is power loss, it generates the selected voltage level (19V, or 21V is available). However, there is a small (~1s) delay when switching, which is too much, and basically power-cycles the PC.

Can somebody help me, to create some kind of buffer, to circumvent the 1 second outage? I have very little knowledge, but maybe some capacitors could do the job?

Also, i'm interested in any expert advice on how this setup is feasible even, and if there are anycaveats that i don't know of.
Thanks in advance!

[GlennSprigg]

Firstly, hello, and welcome to the Forum!!
Hmmm...  Hope I'm not leading you astray here!!   
I looked at the specs for the Device you mentioned, and it only states that in 'UPS' mode...
"Upon power outage, the item will respond in LESS than one second".  Well I have 2 observations.
1- That for about $17/$18 is unfortunately not going to offer you much, in the way of real functionality.
2- A true 'UPS', is just that... 'Uninterruptible', as it is *always* creating/supplying the necessary power at the output.
It really shouldn't have to do any 'switching' as such? Especially if it's just DC in/out. A.C. ups's always make their own
sine-wave output, whether the supply is on or not. The technical issue is when the power comes back on, that the
sine-waves are in Sync when re-connecting.  For your case, you might need a slightly better unit !   

[Manul]

I guess "cost effective" backfired on you, sorry about that. That is not how such UPS should work, such delay is absurd. In fact, there should be no delay at all if done properly. Theoretically it seems you could use a big capacitor on the output, but in practice it might fail, because:

1. Huge capacitor might not be tolerated by original Dell power brick or the cheap UPS.
2. While in pass through mode, huge capacitor will slow voltage drop after mains cut-off, but we don't know at what voltage threshold cheap UPS will decide that its time to take over. It might be too low anyway, so the computer will still shutdown before take over. Hard to say without testing.

[lkondas]

Thanks for the replies.
Yes, this is a cheap powerbank mainly, falsely advertised as UPS. But i was hoping that maybe with a few extra components added it could be good for my needs.

I will test when does the unit "switches", regarding the input voltage. Also i will try to disassemble the unit and post picture.