Switching 12V with 60A

Hello forum

My current project is a solar powered inverter with UPS, so to say a symbiosis of an offgrid solar system and a standard UPS. To choose between power sources (main line/solar), I have to install a 12V wifi switch between the inverter and the battery. I am using a UPS capable of 700W peak.
So if the maximum output is drawn from the battery, if I am not mistaken, the 12V switch would see 700W/12V=58A. If I see my little Sonoff SV, my gut feeling tells me “smoke”. I could not find the maximum current in the docs of that switch.
Till now I was only tinkering around with 230V or low current 12V. Never thought of currents frying a switch. Has anybody an idea how much amps the Sonoff SV can handle? Or an idea how to set up a switch which can handle a big relais which is made for these currents?

Again… as always… thank you for your input.

Use the SV to switch a big automotive relay.

Can you give us a rough diagram showing where you intend on switching and what’s attached to where (solar/solar battery/UPS battery…)
There is considerable risk in switching battery current - you will need a fuse to protect the wire at least as well as a switch/relay which is reliable, and you need oversize the circuit components so that you don’t induce voltage drop or overload if the current exceeds 700W/58A.

Hi,
Going through your requirements suggests a few areas where clarification would help.

  • What is the purpose and use of the power source switch?
    You’ve mentioned line/solar, however line switching is usually performed automatically by a UPS or similar “island mode” features of an inverter - be it on the “source” or “load” sides.
    If this is a critical function (and possibly safety related), using a switch controlled only via Wi-Fi increases the risk of failure as you need a working WLAN.
    Unlike AC where the inverter needs to line-sync, DC is a little simpler but sequencing the switch over may be more than just “throwing a switch” due to high inrush-currents, or the risk of connecting two sources together (e.g. break-before-make).

  • When you say the UPS is capable of 700W peak, can I check that’s not 700Wh - e.g. total energy stored, rather than the instantaneous power that can be provided?
    I’d expect the UPS specs to be more explicit about the continuous and maximum output current (and have BIG fuse and/or RCD protection with obvious ratings).
    Simply, how big are the UPS terminals? Are there fuses nearby?

  • A contactor is basically a relay capable of switching high current loads. These are regularly used in mains control systems (e.g. 3-phase motors) where one relay controls another - the contactor.

  • Please don’t underestimate the risk working with DC. Under 50V (SELV) the risk is not “shock” heart or muscle damage, but flash and burns. High DC currents can have the same energy as mains.
    High DC currents may “make” fine, but heat up and when you try to “break”, DC creates a continuous arc meaning DC kit may be more complex than AC rated devices. 60A AC != 60A DC
    My suggestion is to go to the manufacturer and get data sheets, application notes, and ask lots of questions. If your design isn’t similar to their expected use, you’re probably using the wrong product.

For an example of the issues connecting high-current DC, see Quindor’s video series where he is designing a solar/mains battery storage system. The manual crimping tool needed for 60A Anderson connectors is almost half a meter long, never mind the cable cross sectional area! (hydraulic crimpers look like medieval torture devices for a reason…)

As others have said, sketch what you are trying to achieve on paper - “engineers go back to the drawing board” very regularly as trying to communicate an idea is a great way to think!

If this helps, :heart: this post!

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Hello everyone

Thank you all for the awesome replies. I feel like playing with fire after reading them. I threw together a rough layout:

I have the UPS and an offgrid-solar system (100W panel, 60Ah battery and an 1200W inverter) laying around. Everything was working “standalone” till now. The UPS is Netstar XT1200 with 1200VA/600W (sorry, wrong number in the previous post) and 5.2A Max, 1Ø. The total power consumption of the devices connected should be around 100W, although I still have to measure that.
Wifi switch #1 will switch the line power and the charger for the battery on/off. Since it is a Sonoff 4 channel, I could use that one to control the relays. #2 is just to measure the consumed Watts and is always on, to estimate the SOC of the battery. #3 will switch off the inverter and cut off the 230V supply for the UPS, #4 cuts off the 12V from the solar panel.
The automation must make sure that there are never two power sources supplying to the battery. I did not draw the fuses here, but I thought of putting one each between battery - UPS, UPS - devices and 1. wifi switch - charger.

So what do you guys think? What do the number of the UPS A Max and 1Ø mean?

Definitely implement that rule in hardware, for example with a dual-throw relay that physically cannot connect two things at once.

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Do you have a nice little screenshot like above please? Is a 200A-version sufficiently oversized?

AC UPS expose the complicated issue of reactive power or power factor. 12000VA means 1.2kW at a power factor of 1, or unity. Connect a computer, and it’s more complicated…

Incandescent bulbs are resistors - resistive loads, at a unity power factor = 1. Simple.

Most computers are reactive loads - both inductors, and capacitors, so actually place more demands on a UPS, which electrical engineers cope with via a non-unity power factor.

Without two days in Wikipedia, or a course on imaginary numbers, plus AC phasors, it’s complicated.

De-reate the UPS by (say) x0.8 for inefficiency (heat losses), de-rate for power factor by (say) x0.8 (as the PSU is capacitive and stresses the UPS more), then measure that the computer manufacturer’s data is worst case, so the steady state current draw is really only a fraction of that on the back of the PSU so it might still work!

I’ve seen a computer draw 1/10 of the data plate when running happily, with more for startup - just not the full whack. Testing is needed as inrush current can blow fuses (e.g. lots of amps are needed to start spinning NAS mechanical disk platters; SSD less so).

Data centres turn on kit in stages to avoid inrush currents when racks are first turned as once on the load reduces a lot. If you have several larger computers, several relays can do the same to allow start up to be staggered.

Safety

Looking at the diagram, the risk is several independent switches that rely on Wi-Fi, and could connect two mains sources to the UPS, and two chargers to the 12V battery.

Some UPS have multiple inputs, but here, it would be much better to use a break-before-make arrangement guaranteed in hardware to make it impossible for a microwave oven to spam 2.4GHz and cause a serious loss of magic smoke. APC used to offer 1U boxes for this, but a change over relay might be enough as you have a UPS to smooth out the AC going to the IT kit.

Efficiency

As mentioned in passing above, conversion of AC to DC or the other way is not perfect. An inverter or AD to DC charger may only be 80% efficient, loosing a chunk to heat.

You’ve effectively drawn two UPS in series, as a UPS is just a charger + battery + inverter in a box.

Going from solar DC to AC (inverter), then DC (inside the UPS), then to AC (although some UPS bypass the AC->DC->AC automatically unless in test), then down to DC (in the computer) will loose energy at each stage.

The hard bit is solar battery designers aren’t used to IT kit, and UPS designers only expect one power source.

I’d run the router (assuming 12V consumer CPE, not a Cisco 6U box) directly from the 12V battery. I use a cheap CCTV 12V SLA battery-backed supply as a UPS for mine.

12V replacement for ATX power supplies exist, but realistically unless you are building a remote mast site, a cheap UPS or separate inverter will be cheaper.

Accepting the losses of a cheap UPS to smooth out breaks in power might be OK, but I’d suggest you’re being too ambitious in trying to connect everything to every power source.

(I did wonder about schotky diodes to attempt to charge the UPS batteries directly from solar, but my guess is the UPS will freak out with higher than expected charging voltages on the battery monitoring terminals. Not worth the risk.)

The screenshots where I simply typed your questions into Google and posted the results? I was hoping that’d prompt you to think “oh, I could have done that”.

If you need me to do searches for you, I’d humbly suggest that you may be out of your depth with this project.

I did… but I went to the official homepage and could not find that info there. Relais are new to me, and the images of the dual-throw are totally far away.
I humbly accept your advice. Thank you for pointing me into the right direction.

Just a NAS, a fanless small PC and a router running 24/7 here.

Safety

I should implement the mentioned dual-throw relay as a hardware solution.

Efficiency

Yes… bad point… at the moment, with using stuff that is laying around and waiting for a proper solar kit… I can live with it.