Car battery charging below float voltage?

[Commander Kinsey]

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?

[Fredxx]

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.

[Brian Gaff]

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

[Brian Gaff]

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

[Fredxx]

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.

[mm0fmf]

You will charge a LeadAcid battery when then applied voltage exceeds the terminal voltage and the battery is not fully charged. If the battery voltage is below 12.6V when 12.6V is applied then a current will flow to charge the battery. You wont fully charge it with 12.6V applied but it will charge until its voltage matches the applied voltage. The difference between bulk charge and float charge is that you can "effectively" leave such a cell on a float charge voltage indefinitely whereas continuing to apply a higher voltage will cause gassing and all other kinds of nasties.

What should be being questioned is why either of us is reply to a Kinsey trolling thread.

[Jim GM4DHJ ...]

indeed

[Rod Speed]

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

[chop]

It isnt a trolling thread, its a real question, for once.

[Animal]

well, a lead acid at a fixed voltage isn't going to charge or discharge, so if he has a clue about them then it's deliberate trolling. Everything else is so I'd bet on that. And a battery is obviously not the right answer anyway.

[chop]

He clearly doesnt understand how that will 'work'

But clearly he needs to be convinced that it won't work although he is so bone headed that he will presumably try it and see that it doesnt work very well. No big deal.

[Commander Kinsey]

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

[Commander Kinsey]

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?

[Commander Kinsey]

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.

[Animal]

obviously he doesn't need to be convinced of anything

[chop]

That depends on whether it does what he wants.

[Hul Tytus]

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

[Sylvia Else]

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.

[Commander Kinsey]

[Commander Kinsey]

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.