Small UPS

I'm looking into getting a small UPS, that will handle a PI and an external disc drive. We seem to be a bit prone to very short power outages (~1 minute), but this is causing problems with remounting drive when the PI reboots. Longer term power outages I'll live with.

Any recommendations welcome.

Adrian

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How about one of those cellphone battery banks? They can simultaneously supply power while being charged from an AC converter, and will continue to supply power (for a reasonable time) after the AC has cut out.

Outside of the smarts, and the DC-AC converter, they seem to be capable of acting as a UPS for a Raspberry PI. Just plug the Pi into the battery pack's DC output port, and the wall wart into the battery pack's charger input port.

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Lew Pitcher 
"In Skills, We Trust"

Hello Mike!

Wednesday June 19 2019 11:33, you wrote to Mick Manning:

Thats one possible isse but the other is is the o/p from battery pack more than enough for your Pi system including any extra cards (secondary boards) or peripherals.

Vince

On the down side, I've heard it said that some won't supply power while they're charging themselves. I have no idea if that's true or not, but it has rather put me off the idea :-{ Can anyone confirm/refute this?

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Mike Scott 
Harlow, England

Some & some as far as I can tell.

Same with the small modules from that bay of ease ... some can some can't and do not rely on the seller to know the difference - don't ask how I know - I just do OK :-)

Avpx

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Others "dip" the voltage every few seconds/minutes to sense if anything is still consuming power so they can shutdown if nothing is ... of course an rPi doesn't like that.

I've got an Anker 79AN7925 13,000 mAhr battery. I'd occasionally wondered about using it as a UPS for my Pi, but I've just tested it and it does what you feared: the output power is turned off whenever the battery is receiving power from a mains source. Grrr.

I wonder if there are any batteries which maintain output power when input power is connected and turned on?

It would be a useful backup in case of power cuts. I use my Pi for logging data from a weather station and for recording TV programmes (using TVheadend and DVB-T or DVB-S adaptors) so some resilience would be useful. Mind you, when recording from satellite (DVB-S) I'd still need to find a way to provide 12V for the dish LNB, and from terrestrial aerial (DVB-T) I'd need to provide mains to the distribution amplifier in the loft. Maybe I just need a UPS for the whole house ;-)

I have used my 12V-to-240V inverter that I keep in the car, to supply emergency power at work to essentials like routers and telephone VOIP switchboards during a lengthy power cut (JCB through high-voltage cable - nasty), so they could continue to make/receive phone calls, even if they couldn't use desktop PCs. I earned a few brownie points for coming up with that solution ;-) They left the engine of the company van ticking over (to avoid flattening its battery) and had an extension mains cable snaking from the van in through the door to the office - probably contravening loads of health and safety rules, but what the hell.

Yes there are, but the problem is finding them.

I have an old "New Trent iCruiser IMP120D" which is perfect for this job, though since I also have an actual UPS in the machine cellar it mostly now gets used for charging phones.

Ooh nasty

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If you don't mind the physical size, use a 7Ah 2v SLA (SLA = Sealed Lead Accumulator, Aka A motorcycle BATTERY) with a continuously attached float- type charger and a solid state 12v->3v DC converter. This will definitely work and looks like it can be assembled for around 40 quid.

OTOH the GP Batteries GPACCMP10002 M-Series Portable PowerBank, 10,000mAh Teal for 30 quid and you'll need a 5v wall-wart to charge it plus input cable (another 10 quid, so $40 in total). The Rapid Electronics write-up says that this unit has "Pass-through charging - recharge your devices and the power bank at the same time" which seems like what you need.

The price estimates given above are all from the Rapid Electronics catalogue, so it looks as if there's not much price difference between using a Lithium-based power or an SLA-based solution.

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Martin    | martin at 
Gregorie  | gregorie dot org

Sounds good

No, te problem is not finding them, any LIPO will do that. The problem is charging them while they are in effect connected to a load.

Th trouble si that a LIPO likes a constant current charge abiut te same as its ampere hour rating. (I.e. a 10Ah battery likes to be charged at not more than 10A) that cuts off at 4.2V per cell. It doen not like for example a constant current that suddenly trebles because someone has disconnected a Pi....

So you need to current sense only the charge current. Non trivial. Then you also need to detect both when the cell voltage reaches 'fully charged', and reduce charge to near zero, and also detect when battery volatage is really low and disconnect the pi because preusamably the charger has lost mains input.

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Thanks for the suggestion, but I'm not sure that it would work in this case. Whilst the sort of solution (noting the comments below about charging) might keep the Pi going, there is the issue with the disc drive, which is also mains powered (which looking at my original posting wasn't clear), so I'm looking for something which would handle both.

Adrian

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When you say the disk is mains powered, does that mean its in an external enclosure with either a mains socket on the enclosure or a wall-wart feeding low voltage DC into its enclosure?

And, of course so we can see what its power requirements are:

- What type/model/size is the disk drive and how old is it? Age can have a bearing on its power requirements and size (2.5"or 3.5") definitely does. [1]

- How is it connected to the RPi? IOW is there any interface device, such as a USB adapter that also needs to be powered.

[1] A quick check shows that WD Blue 3.5" drives have a peak power requirement of around 2A at 12v (20 - 24 watts) while WD Blue 2.5" drives have a peak requirement of 1A at 5v (5 watts), which, of course has to be added to the start-up power needed by the RPi when considering what peak load the UPS must be able to handle. It probably also affects the capacity vs. length of power outage that can be supported by your chosen battery capacity, because a disk that can spin down when idle probably uses less power than one that has to remain spinning while the system is running - unless, of course the applications you're running has a pathological activity pattern that lets the disk time out and spin down and then almost immediately spins it up again for the next active period.

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Martin    | martin at 
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It is in an external enclosure powered by a wall wart.

3.5" disc WD red 3 years old. Connection is via USB, the disk's enclosure has an adapter built into it.

Adrian

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I bought one similar to this:

I confess I haven't got around to testing it yet.

There are some more options described here:

Theo

Ah, I was assuming that it was powered by the USB from the Pi, via a mains PSU that could handle the normal current for a Pi plus the extra for the HDD.

If it's a mains powered HDD, then you need a mains UPS - or a battery that can supply the voltage that the HDD needs (which is often 12V rather than

5V) in addition to the 5V for the Pi.

Anything based on lithium batteries is going to be complex. SLAs are far more easy going, especially when on constant trickle charge.

There is a relatively simple way this can be achieved, and get constant genuinely uninterrupted power to the Pi.

You use a 12V SLA charged from about 15V via a series diode and small 12V light bulb. The light bulb not only gives you current control, it also gives you visible indication.

Both the battery, and the original supply have further diodes, both cathodes going to a small DC-DC converter to give you the stable 5V out.

Under normal conditions the original supply is higher than the battery voltage so the battery's output diode is backed off. If the mains fails, the voltage sags to the battery voltage minus the diode drop, which will still be plenty. At the same time the light bulb will go out, and the series diode prevents power being push in reverse to the power supply.

I've used variations of this on many, many occasions going back into the 1970s and never had a problem.

If you want to be really fussy, add a relay to the output that drops out if the battery is below a certain voltage.

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W J G

That tells me that you need a mains voltage UPS with the capacity of maintaining 30 watts output (22-25w for the disk and 5 watts for the RPi

- and, yes I know that's an overestimate but not by much) for as long as the longest power outage you want to bridge.

Also, unless you're planning to convert the disk's enclosure to run off a

12v supply (3.5" disks run on 12v, 2.5" disks on 5v) and you're planning to use a 12v supply (i.e. an SLA rather than a Li battery with 5V USB output, then you are stuck with using a mains voltage UPS. Worse, a basic UPS won't have the ability to tell the computer that the mains is off and they're now on battery power - this is necessary info if you want your RPi to shut down cleanly if the power outage outlasts the UPS battery capacity. That alert capability still seems to be missing from

UPS I got from them a few years ago and regularly buy 7AH SLA batteries from them because their prices are as good as any.)

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