In message , Martin Gregorie
 writes
>On Wed, 19 Jun 2019 21:37:13 +0100, Adrian wrote:
>
>> In message , Martin Gregorie
>>  writes
>>>On Wed, 19 Jun 2019 18:55:43 +0100, Adrian wrote:
>>>
>>>> In message , Lew Pitcher
>>>>  writes
>>>>>Adrian wrote:
>>>>>
>>>>>> 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.
>>>>>>
>>>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.
>>>
>>>
>>>
>> 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.
>>
>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
>any UPS under around £70 to £100 (the Eaton 5EiUSB, from VPS
>www.vps-ups.co.uk is £67.42 including VAT. They're OK - I have a Riello
>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.)
>
>

Thanks very much, some useful information there that I can follow up on.

Adrian
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