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

[Simon Hobson]

Michael Watson 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.
>Not exactly an energy efficient way of doing it, unless your battery 
>charger is smart enough to stop charging when battery is fully 
>charged, or not charged at all when battery is charged and directly 
>power the PC.  Could be super efficient if you used a solar charger 
>to keep battery charged, and charge generated was enough to power 
>the machine during non daylight hours.

You would only *EVER* do this with the right sort of charger/PSU. At 
it's most simple, this can be a 13.8V (assuming 12V battery and 
average temperatures) regulated supply with current limiting (not 
overcurrent shutdown). On a fairly flat battery the current limiting 
will control the power supplied until the battery reaches a certain 
state of charge. After that, the PSU just maintains the fixed voltage 
while supplying whatever current the battery requires plus the load.

It's not about efficiency, but about not destroying your battery by 
boiling it dry. For a VRLA (aka Sealed Lead Acid, aka Dry-fit) you 
can dry them out in a matter of hours with the wrong sort of abusive 
overcharging. Wet batteries will take somewhat longer but will still 
dry out eventually.

It's not so much a case of "stop charging and power the PC directly" 
as simply "float the battery". Connect the load directly to the 
charger with the battery in parallel for backup, and use a regulated 
charger that will maintain a correct float voltage. The battery will 
charge very very slowly at this voltage, but the charger will be 
supplying the load directly all the time.

BTW - most intelligent chargers will not work in this situation. They 
will see the current being drawn by the load as charging current and 
hence the intelligent features just won't work properly - they will 
think they are in the bulk charge regime all the time.


Incidentally, this is the basis of a double-conversion UPS which is 
used where you want clean regulated power as well as backup. Power 
always goes through the AC-DC and DC-AC conversions and so the output 
is completely separated from disturbances on the input. The batteries 
may be connected directly across the DC link, or there may be a 
separate DC-DC converter involved (as is the case with the unit we 
have at work).
As you point out, the downside is the extra losses. They are 
typically rated at 95%+ efficiency - so a few % of the load gets 
added in conversion losses.

-- 
Simon Hobson

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