[mythtv-users] Power outages and UPSs

Michael T. Dean mtdean at thirdcontact.com
Sat Jul 28 15:37:49 UTC 2007

On 07/28/2007 05:06 AM, f-myth-users at media.mit.edu wrote:
>     > Date: Sat, 28 Jul 2007 02:48:02 -0400
>     > From: "Michael T. Dean" <mtdean at thirdcontact.com>
>     > I could make an UPS from a good battery (I've been looking at the
>     > Concorde Lifeline GPL-31T :) and a good-quality, high-efficiency
>     > benchtop power supply.
> I'd use a purpose-built battery charger, not a benchtop supply.
> Getting this wrong will kill the lifetime of your battery fast.
> Getting it -really- wrong can lead to messy venting (or bangs).  Even
> (good) lead-acid chargers are surprisingly complex; other chemistries
> (NiCd, NiMH, Li in various incarnations) are increasingly hairy.
> A purpose-built charger will give you much faster recharging and
> better charge termination, as well as being able to compensate for
> cell temperature.  Ignoring float will sulfate your battery

So, almost always a float charge with the occasional (manual) topping
charge, as described in the link I posted.

>  and
> undercharge it as well.  Attempting to recharge only via float can
> take days,

Not usually a problem with big enough batteries.

> and you need to set the float voltage typically to within
> 10mV/cell---and you've also got to make sure your charger (or your
> benchtop supply) is matched to the cell chemistry, of which there are
> half a dozen even in ordinary lead-acid varieties.  (Gel cells, for
> example, typically want somewhat unusual float voltages; AGMs have
> more typical voltages but will accept huge currents during fast
> charge; etc.)  A benchtop supply is likely to produce insufficient
> current for fast charge, be unable to differentiate between the
> various charging regimes and thus will either charge very slowly,
> undercharge, or incorrectly float, and otherwise do a poor job of
> keeping your battery charged and conditioned---and likely cost more
> than a purpose-built charger as well.  Especially after you factor in
> replacing the battery every few months because you're mischarging it.
> Note that the net is chock-full of various homebrew designs for all
> kinds of lead-acid chargers, of varying complexity and efficiency,
> and also chock-full of vendors willing to sell you commercial gear.

Interesting.  I'll keep this in mind.  Looks like the
appropriately-sized commercial chargers are around the same cost as
plain bench power supplies.

> [More details would be far off-topic and inappropriate here.  You
> could try searching for ``charging lead-acid'' and poke through the
> 1+ million hits therein.]
> P.S.  It's completely unclear to me why people are so hung-up on the
> "inefficiency" of the inverter;

Because the link I posted was to a dual-conversion (online) power supply
design that would always use the inverter.  Coupled with the fact that
it also uses a regular computer PSU, there's a completely-inefficient
AC-DC (for battery and inverter) followed by a DC-AC (for computer PSU)
followed by an AC-DC (from computer PSU) design.  By replacing the
inverter with a DC-DC PSU, it's AC-DC followed by a DC-DC conversion. 
Basically, as Lionel implied, why not cut out the middle man.

>  all consumer UPSes only run it when
> you're on-battery, which by definition is rarely.

Yeah, but considering my consumer UPS's have caused more downtime* on my
systems than running without them (since they'd just come back--a little
worse for the wear--when power is restored), I've unplugged them all
(and am running directly off the mains).  Basically, cheap "smarts" are
worse than "dumb."  At the least, I'll be going for a very large APC
SmartUps (SUA1500 or more) if I go with an OTS UPS.

* The UPS doesn't restore power after an outage, so my system is down
the 4 days while I'm traveling rather than the 10 minutes the power was out.
* The UPS decides that the battery isn't performing as expected, so it
cuts power--while there's still power to the mains--causing an outage
that wouldn't have existed without.
* The circuit breaker in the UPS triggers and the /only/ way to reset
the UPS is manually by holding the power button for 4 seconds or something.

Basically, I travel enough that these things cause days of outages
because no one is around to reset the UPS/hit the power button on the
computer.  When using the UPS's, I would have days or weeks of downtime
(generally about every 4th trip--and, it seems, usually losing the
system on day 1 or day 2 of travel).  Since removing all 7 UPS's from my
7 systems, my downtimes have only been in the minutes.

>   And you can only
> get rid of the always-on "inefficiency" of a typical switching power
> supply (already about as efficient as any supply you'll see) if you
> can supply exactly the required voltage from your batteries,

I'm not trying to improve efficiency, I'm trying to find a workable
solution that uses the batteries I choose.  However, the dual-conversion
design introduced a completely unnecessary inefficiency, so...


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