[mythtv-users] Netbook as low-power combined back-end / front-end?

[Simon Hobson]

rsh1k wrote:

>  > On the other hand, if you pick a system that runs off 12V (as some of the
>>  ION boards do, then in principal you should be able to just hook this up to
>>  a 12V lead acid battery and use a 13.8V supply/battery charger instead of
>>  going with a full UPS. Also, for things like the mini ITX boards, there are
>>  PSUs designed for in-car use that run from a battery and will auto-shutdown
>>  on low battery etc.
>
>Now there's a whole new aspect!   A car battery would need to be
>outside (hydrogen venting, etc), but could provide a substantial
>amount of run time.

You'd want a deep cycle battery - normal car batteries really aren't 
suitable as they are designed to provide a lot of power for a short 
time (starting) but can't handle deep discharges. Deep cycle (often 
called leisure batteries due to their use for 'hotel' loads in 
caravans and campers etc) are designed to handle very deep discharge 
but are generally not capable of providing the same short power 
bursts.
Unless you abuse it, it won't produce a lot of hydrogen so should be 
OK as long as it's in a well ventilated area.

>Plus, it opens up new charging opportunities -
>solar, wind, etc - things that with an up-front investment could lower
>the annual operating costs even more.     Hmm......

Why do I have visions of the cash registers going wild !

>At best, when
>new, it had about 15 minutes of runtime and would continue on with
>only a brief warning squeal to indicate that the power had been
>interrupted.

Ah, that's probably part of the problem. IMO a lot (most ?) UPS have 
too small a battery capacity which means they work the batteries very 
hard - quite high discharge rates, plus to get 'good' runtimes I 
think manufacturers tend to set them to discharge a bit too much. The 
combination can kill batteries fairly quickly - if pulling hard on 
the battery till it's voltage is very low, a very slight difference 
in cell state can mean you overdo it on one cell. That cell then gets 
damaged, reducing it's capacity, so next time it's a bit more out 
from the rest, and so it goes.
If you've a 12V battery (6 cells), and discharge it down to 10.5V, 
that's nominally 1.75V/cell. If one cell is a bit weak, you could get 
that down to (say) 1.5V while the others are still at 1.8V. 
Discharging to 1.5V/cell is likely to damage even a good battery.
It's even worse on the big UPS we have at work. That's nominally 120V 
(10off 12V blocks in series). By the time you've got that down to 
105V, you could have several cells missing altogether and the UPS 
wouldn't know. That's why I've got 1/4 ton of scrap batteries at home 
waiting till I can have a sort through to see if any will do for 'odd 
projects' before they get weighed in at the scrapyard.
I've been looking at forklift (wet) batteries so I can at least top 
them up as required - though size, cost, and weight are a factor.

>I thought about getting new batteries, and I thought about upgrading
>to a bigger UPS.

Just bigger batteries would probably have dealt with that.

-- 
Simon Hobson

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