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From: Mike Scott <usenet.16@scottsonline.org.uk.invalid>
Newsgroups: comp.sys.raspberry-pi
Subject: Re: Backup power supply
Date: Sat, 16 Mar 2024 15:27:05 +0000
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On 14/03/2024 18:11, bp@www.zefox.net wrote:
> Mike Scott <usenet.16@scottsonline.org.uk.invalid> wrote:
>> On 14/03/2024 15:51, Joe wrote:
>>> I'm using a cheap APC, I believe 650 or 750, with USB monitoring hookup.  Also have my router connected (with VOIP) to keep
>>> communication possible for a little while (runtime abt 30 minutes).  Battery replaced once, I believe it was some 7 years old.
>>
>> Somehow that makes no sense to me..... current 650 is 227 pounds (maybe plus
>> VAT) on the APC website. 5 times the cost of the Pi. I'm sure it's a
>> very nice UPS, but.... ???
>>
> 
> Sense or not, it seems to be a fact of life. The inverter/charger/transfer units
> I used have vanished from Amazon, replace by units with four times the power and
> double the price. Battery prices seem to have dropped very slightly, but are
> still a whisker over $200/kWH.
> 
> A 12 volt to 5 volt stepdown converter appears to be less than $10, small 12 volt
> batteries range from $20-100 in the few AH range, a 12 volt 5 amp charger ranges
> from $11 to $50. The charger would be the iffy part, since a smart charger on a
> loaded battery is apt to get confused by what looks like high self-discharge..
> 
> Something reliable for less than $100 will take both luck and ingenuity. One
> possibility is finding a used UPS unit in e-waste and replacing the battery.
> 
> bob prohaska

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to design, but my electronics knowledge is decades out of date :-{


-- 
Mike Scott
Harlow, England