Car battery charging below float voltage?

I would think a directly connected battery would be a poor choice for this arrangement. As you have indicated, the output of the battery will vary significantly with the current and the direction of the current. You seem to be worried about the battery charging, but the voltage would sag when current is being drawn.

3 kW at 12V is 250A. How much do you expect the battery to provide? I can't see this working very well without electronics to deal with the mismatch.

BTW, when the power supplies go into current limit mode, they are no longer regulating the voltage. You get to pick one, limit the current by letting the voltage sag, or maintain the voltage by supplying more current. You can't both limit the current and maintain the voltage, unless you can somehow talk the graphic cards into slowing down their calculations.

Why not add another supply? Power is not something you should skimp on if you want a reliable system.

If you do your homework, you will discover that 13.6-13.8V is considered a float charging voltage.

A voltage of 13V is considered a charge maintenance voltage.

Anything less than 13V won't supply any charge.

You might like to rethink this.

In my experience it would be fine, of course if the extra current charging the battery exceeded the load for everything on the psu, you would be back to square one if you are not careful. These must be very power hungry cards. I would not want you electricity bill. Brian

In practice though, it does charge at the lower current, but it can take a long time. I used to do this sort of thing in a caravan and monitored the voltages, so unless the batteries have fundamentally changed you should find it works. However as I said before, if you do get a very substantial charge going and adding that to the drain of the cards, yoyou will still run out of power. In a way the battery as you describe it is kind of working like a very big capacitor most of the time.

Brian

Most articles I have read state at 13.0V there will be no charging current. Even at 13.7V there will be a tail such it can take days for the last 2% or so be put into the battery.

I would be grateful if you could cite an article that says you will charge a Lead-Acid battery between 12.6 and 13.0V. I've never seen one.

You would in fact be paying it if your entire income wasn't govt benefits just like with him.

Typical GPUs, 200W each, depends what they're doing. Double precision stuff they get very warm indeed. Much cooling fans.

I don't need it to be full or anywhere near full. A battery at 50% charge will sit at 12V. Surely applying 12.6V to that, current will flow?

Well at the moment I'm loading the PSUs at 75% (running programs requiring less power), and the battery has less than 0.1A flowing into or out of it. However there's 0.5A of AC, which suggests it's acting as a nice smoother. Strange since the ripple voltage on those supplies I measured as tiny, I think it was 0.02V under heavy load when the battery wasn't there. If 0.5A AC exists, it must be charging as well as it's discharging.

I've found 12.6 volts applied to car batteries effective, though slow, for charging. I've been monitoring a battery that's been charging from a current source limited to 12.6 volts. Current yesterday morning measured .029 amps & 12.55v.

Hul

A battery is not a capacitor. In theory, it would be better than a capacitor since the voltage doesn't droop as much. But it has a much higher internal resistance, which kills it for this project.

A large amount of capacitance would help here, but it would need to be truly massive and charging would need to be done carefully.

Bottom line is, neither the battery or a capacitor is a good fit by simply tying it onto the power rail. What is needed for the battery, is a charging controller, as well as a regulator to allow the variable voltage of the battery to power the graphic cards. Probably the best arrangement would be to use a serial approach, where the PSUs put charge on the battery, and the battery powers the graphic cards. But this would require very different PSUs. So the easier path is to just buy another PSU and parallel connect it to the others.

Capacitors are not batteries, and batteries are not PSUs.

So if you connect a battery with low charge to a 12.6V supply, it will not charge at all? I'm pretty sure it will.

I would question the economics. Lead-acid batteries don't last very long at the best of times, and they're quite pricey. Unless this is a short-term endeavour, I expect it would be cheaper to increase your power supply capacity.

Sylvia.

It was £75 for 130Ah. They last for 5 years in a car, and this one won't be experiencing hundreds of amps to start an engine, so should last a lot longer.

I am increasing the capacity, but it takes almost a month to get a new one (they come from China). Now I've put it there I might aswell leave it there. It means I have more flexibility, I can remove a supply for repair, or move it around etc.

There's a ripple current of 0.5A, so it's being a nice smoothing capacitor too. I think I might add a fuse though, I looked it up and a battery that big will give out 8000 amps if shorted with a big enough spanner. The cable is rated at only 400 amps so I think some flames would result. I'll have to check what the resistance of a fuse is though, don't want to lose a couple of tenths of a volt.

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.

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.

Paul

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.

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.

Or you could get real radical and get a generator for the longer times without power and use the car battery to give you time to start it and switch over to it.

A UPS and generator would do that fine with the car battery supplying the very high 12V demand with his unusual config.

Not if he never get long power failures.

Do you live in India? I can't remember my last power outage. This battery is not for power outages.

The computers are not powered by that battery. They run directly off the mains. They do nothing in a power outage.

I have one for the computer in here, plus the screen, stereo, internet router, and lights for the whole house. That runs off two of those batteries. The pathetic little batteries inside UPSs are useless, I bought a 2nd hand one without batteries and connected the leisure batteries to it. Lasts 20 times longer.

My parents bought a 720W UPS which will power a 15W LED standard lamp for 8 hours, or that plus two fridges for 2 hours. Pathetic. I'm trying to persuade my dad to let me connect a couple of leisure batteries to it. The original intention was to power the pumps for the oil boiler but they changed their mind and said they can just use the fireplace. For some reason he doesn't trust me connecting stuff to his fusebox.

My APC 1kW UPS can power a 1kW vacuum cleaner, even though motors at the full load rating are not supposed to be connected to a UPS (they draw 5 times the current at startup).

Nah, I'd get a 5kW solar invertor, and add lots of leisure batteries which are dirt cheap. A 240V powered relay will easily switch the load between that and the mains instantly.

Actually most of the load could run straight off the battery as it's 12V.

If the doom sayers are correct and we end up with mass power shortages, I may end up with the whole house on UPS.

We did, it held the (on wheels) server cabinet very steady!

No, ours did not make any noise. If there was a fan it was totally silent when not under load.

Those are a stupid idea. Lithium Ion batteries?!? Way too expansive. Li Ion is for when you need light weight, otherwise lead acid is far cheaper.

No it does not, my video cards run entirely off 12V.

A very small amount of current. The CPU is 12V for example.

You might care to read what I'm doing first.