On Monday, January 9, 2023 at 8:57:38 PM UTC-5, Paul wrote:
> On 1/8/2023 11:37 PM, Commander Kinsey wrote:
> > I have 8 computers with 15 graphics cards running science projects. The graphics cards run from 12 volts, adding up to a lot of current between them. I have three 1kW power supplies connected to a big bus bar and have set those power supplies to 12.6 volts, as the graphics cards expect 11.4 to 12.6 volts, so I'm playing safe and allowing the biggest voltage drop not to make it fall outside that range. Currently I'm close to the limit of the power supplies, and since the current draw of the graphics cards is uneven, I thought it would be a good idea to add a car battery (actually a 130 Ah leisure battery) to the bus bars, to help out if there was too much current draw momentarily. The power supplies are current limited so don't mind if I try to draw too much, they will just limit the current.
> >
> > My question is, is it ok to have the battery sat at 12.6 volts? This is the voltage the battery sits at with nothing connected to it when it's 95% full. If the battery were to supply a fair amount of current for a while and become a little discharged, would it manage to charge back up with only 12.6 volts supplied to it? Or does it require a float voltage of 13.2 volts or more?
> Power outages, have statistics.
>
> Here, a power outage is 1 second. Or a power outage is 2+ hours.
> In fact, the long power outages have been lengthening in time,
> in the last decade. One lasted a day. The last one was two days plus.
> The power company is on a safety kick, where power repair trucks
> sit idle on the street, with staff sitting on their hands.

Why? Because it's safer to sit in an idling truck than to drive it somewhere??? I guess they screw up their courage to drive it back to the shop at quitting time.

> As such, a single leisure battery and three 80 ampere loads, that's
> a huge load. And the 130 Ah leisure battery, you're not really supposed
> to be running those flat. This means you have well-less than an
> hour of capacity. How many BOINC units can you do in half an hour ?
> Is it worth XXX pounds currency, for the privilege of doing
> so few units ?
>
> With a UPS, the objective is to allow clean shutdown of all
> computers. You could buy a consumer UPS for each 1kW supply.
> Maybe this would give you 8 minutes holdup time, or 4 minutes
> holdup time. You would need to send the shutdown signal,
> to all the computers, so they would begin shutting down.
>
> You cannot buy the lowest tier of UPS either, if you
> really plan on handling a full kW load. There are some
> really awful UPS that will smoke if you do that.
>
> A commercial UPS, a double conversion rack mount, might have the
> power rating to run your entire computer room. But, you will
> be charged a commercial rate for such a beast. In your IT days,
> you might have had such rackmount UPS in the server room. They
> seem to be quite common. As double conversion, they have a
> cooling fan that runs constantly (unlike a consumer SPS which
> runs cool until it flips to battery).
>
> Buying three UPS, would be an intermediate solution, compared to
> buying a Tesla Powerwall (price has gone up 2x since introduction),
> or some of the less well thought out consumer "battery bank" thingies.
> There is one product, which does not even work as well as a
> double conversion UPS, which would be cheaper than a powerwall,
> and they're about 1kWh each.
>
> *******
>
> The video card uses 3.3V and 12V
>
> The motherboard uses 3.3,5,12,-12,+5VSB.
>
> The leisure battery only has one voltage, not
> six or seven voltages.
>
> You need to pick the logically correct point for
> backup powering this mess.
>
> Your plan right now, is just plain wrong.

I don't think I've seen anything about wanting to cover power failures. His stated objective is to handle loads that temporarily exceed the capacity of the power supplies. Even running them at 12.6V, he's looking to ruin a perfectly good battery that will very likely drop below his desired 11.4V if the power does fail. Being shut down and not able to calculate is very different from continuing to run, but at a voltage that produces erroneous results... without being flagged.

--

Rick C.

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