[mythtv-users] Power outages and UPSs

[f-myth-users at media.mit.edu]

    > Date: Sat, 28 Jul 2007 11:37:49 -0400
    > From: "Michael T. Dean" <mtdean at thirdcontact.com>

    > So, almost always a float charge with the occasional (manual) topping
    > charge,

And what about the constant-current phase first, aka bulk charge?
Who's doing that?  Or is it your intent to take many times as long as
necessary to recover from an extended power outage which discharges
your battery?  [And see PS for a totally different approach.]

[There's also the issue that your load probably consumes many times
the current of the float for the battery, so getting the Vreg correct
on both legs of that will be interesting, and voltage drops down wires
become noticeable as well.]

	   >  as described in the link I posted.

Which?  Where?  You posted several, but I didn't see that.  In any
event, if all you're going to do is trickle, then you'll have very
poor recovery from an extended outage, and you don't want to bulk
charge at the topping voltage (excessive current -> gassing).

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

    > Not usually a problem with big enough batteries.

Are you saying "I don't expect to -ever- discharge my battery"?
If that's not what you're saying, then the problem will only get worse
as cell size increases, because your trickle-charge current will
increase with it.  [I'm not sure if the intent is "shut down at
the first sign of trouble and don't come back up until the mains
are back" or "run as long as possible and -then- shut down if the
mains haven't returned."]

Basically, what I'm saying in this increasingly off-topic thread is,
"Some of us are electrical engineers who have designed high-rate
lead-acid chargers, and some of us aren't."  A good charger that
minimizes your time discharged (and vulnerable to the next outage)
and which also maximizes your battery lifetime is a lot more
complicated than a benchtop supply that's just set to a single
voltage and forgotten---but it's still not rocket science.*  It
takes some good power engineering for the high-A paths and (these
days) a -lot- of what used to be an annoying analog computer is
now available in IC form to do all kinds of great charge regulation,
e.g., the UC2906, as well as many others from Maxim et al.

[* When charging a single 12V lead-acid at a few A, that is.  But easy
commercial solutions don't really exist for 72V AGM strings at many
hundreds of A, though, especially given other numerous constraints,
which is where I came in.]

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

And they'll perform better.  Consider also what's going to happen with
your load/charge regulation if you have a power supply that's not
designed with sufficient output current to recharge a discharged
battery.  The battery will drag your charge rail down sufficiently
that your -load- (e.g., the computer) may not work until the battery
has been able to recharge---which, at trickle rate, will take days.
Stiffening Vreg on the charge rail is going to take a fair current
output capacity; going into current-limit on your benchtop supply
means your -load- may be current-limited as well as your battery,
and that's unlikely to do what you want... :)

Thus, since you're using a nonbypassable online UPS (e.g., your load
-must- run from the battery rail, not from the AC mains), your load
is hosed until your battery resistance rises.  And if you oversize
a bench supply so it can simultaneously bulk-charge the battery and
supply your load, then (a) it has a risk of recharging the battery too
fast unless you've got everything tweaked just right [don't add or
subtract hardware!] and (b) bulk-charging at the trickle-charge or
topping voltages may gas your battery.

    > 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

I don't understand this sentence.  Is the problem here that you get an
extended outage, the UPS runs flat, the power comes back, the CPU comes
up, and then you get an immediate outage again when there isn't enough
UPS runtime to do a controlled shutdown?  (If so, and you don't want to
spend the extra money on a SmartUPS or equivalent with its "hold down"
logic, there are very simple circuits (amounting to a pair of one-shots)
that can say "if AC fails for more than x minutes, then don't turn the
power relay back on until y minutes after it's back".  Not as good as
something that's actually monitoring the state of charge of the battery, 
but not horrendous either, and very cheap.)

Oh wait.  "they'd" is your CPUs, right, not the UPSes?

    > (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.

WTF?  All of these sound like broken or misconfigured UPSes.  I've
used a couple dozen midrange consumer APC units (Back-UPS Pro &
ordinary Back-UPS; SmartUPS, and also much older ones like the 1200VX
and others) for 15+ years and I've never seen any of these misbehaviors.
I assume other vendors' UPSes are similar or we'd hear lots of screaming.
(a) & (b) are just dead wrong; (c) sounds like you're overloading the UPS;
the only time I've seen (c) is when some numbnut plugged a laser printer 
into a UPS, which managed to both pop its breaker -and- permanently blow
up its inverter (!).

Also, how old are (were) your batteries again?

    > 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.

What brands/models of UPSes are you using?  

    > >   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...

My inefficiency comment wasn't aimed at you, but at someone else in
the thread who bemoaned those "inefficient" CPU switchmode PSUs, which
are likely more efficient than anything else in the entire DC powerchain.

P.S.  If your currents are low enough (doubtful), and you only want to
ride out a few seconds of interruption (not enough for a controlled
shutdown, but to get around flickering) then another solution which
bypasses all the complexities of battery charging regimes would be
to use some supercapacitors on your DC rail.  You'll get more usable
capacity out of 'em (due to the exponential in the discharge curve)
if you can either make a high-V string (20-30V) and use a buck
converter to generate your 12V (or whatever), or make a low-V string
(e.g., 5, 7.5, or some other multiple of the basic 2.5V rating of most
supercaps) and a boost converter that can supply your 12V (or whatever)
until the whole set falls below your boost's min Vin.  So compute the
load in W, remember that caps store 0.5 * C V^2 in joules, and do the
math to compute the F required for a given t; who knows, if you need
few enough wattseconds aka J, it could work---and it will last "forever",
rather than decaying in a few years the way batteries do no matter how
well they're treated.  If you're willing to spend a couple hundred bucks
for that AGM battery you were talking about, you could get a few thousand
F of supercap and that might be good enough.  Send me the VA of what you'd
like to power and I'll spend a couple minutes computing whether it's even
-possible- to survive more than 5s of flicker without going broke...:)