I was shopping for a small UPS for my home networking lab closet; total load under 300w at 120v. But then realized I have a spare AGM car battery, 12v solar panel maintainer and 12-120v sine wave inverter. So I'm DIY home brewing up a project.
I sourced a 15A automatic transfer switch designed for this application – essentially a DPDT relay from what I can tell online. This is for line voltage loads and utility to backup inverter seamless switching. And I'll mount and integrate it to home electrical standards in metal boxes and EMT conduit on its own circuit. The 5v and 12v modem and wifi loads are getting 7812 and 7805 voltage regulators to keep them pure DC off the battery.
While I'm comfortable with the plug and play aspect of this design I am a bit concerned with the reported 80ms cutover time of the relay and power transfer. Goal being to keep a small server and network gear with internal AC/DC supplies from rebooting or struggling during any power flickers and utility to inverter switching.
Is there a capacitor or induction coil design you can help spec that I can place on the load side of this to help smooth this out? A big motor start capacitor or something would be my uninformed thought but don't have the EE skills to know what to do with it. I realize the computer power supplies likely have similar in their circuits but would like to help if I can. I will have TrippLite ISOBARs in line which I believe have induction coils in them.
Am I over thinking this at 80ms delay? Or is there a drop in big coil or cap I should integrate? I realize that commercial UPS designs probably have the inverter in use at all times with a trickle charger, so it's a constant AC > DC > AC flow but I'd like this transfer design to save wear on my basic inverter. Power outages are rare enough here I'm not battling ongoing outages or anything; maybe a few times per year. Any other holistic design aspects I'm missing? TIA!
tldr: Is 80ms too long of a cutover to keep a server and big network switch from rebooting?
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Best Answer
That is a bad choice of battery. It's a starting battery, not a deep cycle battery. You'll destroy it lickety split. You may want to hang out with the off-grid homesteader / solar / wind crowd for awhile and learn how to set up systems like this.
Also since you're willing to spend this kind of scratch on a battery, maybe evaluate some of those Tesla modules that are out there. They respond much better to deep cycling than even a deep cycle battery, which this is not.
80ms is typical of a mechanical transfer switch. But further, you will have trouble because of it throwing automatically, and possibly having some false starts, as power cuts out and returns. To be clear, 80ms is five full AC cycles - high low high low high low high low high low. So there is just no chance of doing this with DC-style inductors or capacitors. There's no such thing as a DC battery. And even then, how would it be cut in? The transfer switch's contacts are in transit.
Really, you should be reinventing the whole kit-n-kaboodle to run on low voltage DC. The server's power supply should input LVDC. The router, switch, modem whatever should input low voltage DC (probably already are). Essentially it will be hung off the battery at all times with makeup power coming from a battery charger.
Now, you don't need a transfer switch.
The kind of money you're spending on that gear, you could get a LV power supply for a server, and also a nice lithium Tesla style battery with some range.