Backup power supply

It's a APC Back-UPS ES 8 Outlet 700VA 230V, paid 70 eur for it (clearance sale). Replacement battery was 54 eur. Like mentioned it's NOT just for the Pi but also for the fiber router/VOIP as cell phone is unreliable here.

Since then for other purposes I've also got an Eaton Ellipse ECO 1600 @ 184 eur (Amazon daily sale).

SO I guess looking around for reasonable prices may make sense....

This one

...

Speaking of Eaton, I've found they have Eaton 3S Mini UPS which seems to be good a match for my original purpose. And price of around 50 eur makes it a cheapest brand-name option so far :)

If you want 150 hours, you need to be looking at a backup generator, so your UPS only needs to last as long as it takes for your generator to fire up.

That's usually a couple of minutes for professional diesel ones, but as its home setup it might take you a few minutes to connect it up and pull the starter cord on a cheap petrol one.

It might then take a few more minutes to drain the tank of the sludge that used to be fuel last time you used it, and to run to the garage to get some fresh petrol.

It might take a few more minutes if your spark plugs have been fouled as you didn't clean it after use, and your air filter has a hundred different types of bugs living it.

So make sure the UPS lasts an hour or 2.

Oh and once you've managed to get it going, and it's making a tremendous racket, just remember everyone else in the neighbourhood who is without power will be turning to look in your direction...

---druck

Of course - but I don't think anyone wanted 150 hours they were just confused about Watts and Watt-hours and seemed to think that's what they would get.

Not as a ramp rate for a power station :-)

Seems the rechargeable bit is the issue. For my part, and purely to use as a short-term standby for the Pi until mains resumes or the system can shut down cleanly - I'd be happy with something with a bundle of AA cells or similar. I doubt a clean switchover circuit would be difficult to design, but my electronics knowledge is decades out of date :-{

Since I started mucking about with inverters the complexity of line detection has gradually dawned on me. I started out thinking of a simple SPDT relay with coil across the line. Line drops, relay goes to normal contacts, job done. With enough filter caps on the Pi to sustain it through a few line cycles maybe that would work. For a larger load the caps get big, if you want (as I do) to back up several different devices (DSL modem, router, switches) that amounts to a custom DC supply interconnecting all devices.

If you want significant endurance (minutes or hours) capacitors become impractical. Primary cells in series with a blocking diode (6 volt battery with one silicon diode in series) would get you close to 5 volts with the diode drop. Maybe that's your ticket. The key is finding the right battery chemistry to give the voltage you need. Still, it's a handmade gadget. And, whatever voltage matching device you select must not drain the primary cells. I think a three-terminal regulator would cause at least some trouble on that count. On the good side, you could put it all downstream of the Pi power supply and skip line detection entirely. That's a big advantage. When line voltage drops (and returns) it's often noisy, dropping, returning and dropping again. Much better if the UPS takes over with a hair trigger and waits for a period of quiet before retiring. That's where the logic gets intricate.

I puzzled over the custom DC supply versus off-the-shelf AC UPS approach and settled on the latter. For a single Pi, that no longer looks so attractive.

Thanks for reading,

bob prohaska

On second thought, this is probably wrong. If one connects to a Pi a 5 volt supply in parallel with a blocking diode and a 6 volt battery, the diode is still forward biased and will drain the battery eventually. Some kind of switch is needed, controlled by line detection.

Maybe a three terminal regulator set for 5 volts out powered by a 9 (or higher) volt battery pack could be substituted. Still, one has to consider the idle draw of the regulator. TI's LM340 takes 6 mA roughly. Still, that's a smallish supply for a Pi. You'll likely want something larger.

Apologies for the red herring!

bob prohaska

It is possible to switch between mains and backup using MOSFETs, so you can get the voltage drop of the switching device arbitrarily low, and a switchover time of a microsecond or so - it doesn't need to be quite that quick, but it probably wants to be quicker than a mechanical relay. /If/ you want the negative line to be common, and the switch to be in the positive line, you need to use P-channel MOSFETS. And remember the presence and direction of the body diode, so you'll want to connect source and drain the opposite way round from what you might imagine.

You'll need a reliable detector of the presence of mains, and probably also that the output is high enough before you swap back to mains.

Most people are accustomed to working with N-channel devices and not thinking about the body diode, so you might find the mental gymnastics very hard, but it can be done!

The prize is that normal running will definitely be from mains, even if the output voltage of the mains supply is slightly lower than that of the backup regulator.

David

I think the chances of finding an 'ideal' battery are relatively slender. You are likely forced into some sort of 5v switched mode regulator - and its probably wisests at least for a single Pi - to have that permanently delivering power and just have the battery on permanent mild charge

Re: Re: Backup power supply By: Andriy D to Richard Falken on Thu Mar 14 2024 07:57 am

Yes, I was thinking a full UPS with an USB port for control. APC has some models that would do. Cyberpower also does have some very nice ones for domestic purposes.

Re: Re: Backup power supply By: Mike Scott to Joe on Thu Mar 14 2024 04:32 pm

Well, any battery (without controllers or anything, just the battery) is going to be expensive as heck. I think the trick is using the same battery for a bunch of mini-appliances so you don't feel stupid spending many hundred bucks for backing a single machine worth 30 bucks...

Please note that the Pi still draws power after shutdown, because it has no power-off circuit of any kind. There is a small but significant stand-by current (not just any LEDs) that will eventually drain the UPS/batteries/whatever. A dumb power source should at least be over-discharge protected (think of protected Lithium batteries) or not be damaged by total discharge.

Another issue: If grid power is restored before the UPS is empty, the pi will not boot automatically. It only knows how to handle unplug - replug the USB power.

So a good solution should remove power from the Pi after shutdown, and switch it on again once the grid power is back.

Like David Higton suggests, MOSFETS are great for that.

It's certainly the simplest solution from off-the-shelf parts. Standard dc-dc converter, standard battery, standard charger. No need for a handmade line power loss detector.

Efficiency won't be great. For one, or even several, Pis that's not a problem. But it will likely be somewhere close to $100 for a halfway decent battery and charger.

bob prohaska

On 15/03/2024 10:51, Ahem A Rivet's Shot wrote: >> Make that 5 minutes * 500W/5W or 500 minutes which is indeed a >> long time but a lot shorter than the 150 hours you were expecting.

On 3/15/2024 6:43 AM, druck wrote: > If you want 150 hours, you need to be looking at a backup generator, > so your UPS only needs to last as long as it takes for your generator > to fire up. >

I read this and feel first-world superiority for having last year installed a 14kW Generac system, powered by our municipal natural gas supply. In case of a power outage, it kicks in within about 10 seconds. When the grid comes back up, it syncs the AC to the grid before pulling the relay to reconnect. And every other Wednesday, it tests itself for

-- Lars

That is what I used for my CPAP which has a 12V input for use when camping. A motorcycle battery and a trickle charger.

But since then, I installed a backup generator for the whole house, so the wife wanted the jerry-rigged UPS out of the house.

-- Lars

I think I will print a small nuclear reactor on my 3D printer and go off grid entirely :-) :-)

It also amuses me how people think that the electricity that powers their homes is all the energy footprint they have.

I wonder how many lumps of coal a typical Raspberry Pi burns in its manufacture and shipment.

Or how much diesel goes into the organic vegetable basket they buy...