On Wednesday, January 11, 2023 at 6:16:56 AM UTC-5, Paul wrote:
> On 1/10/2023 11:52 PM, Commander Kinsey wrote:
> > On Wed, 11 Jan 2023 04:10:27 -0000, Jasen Betts <use...@revmaps.no-ip.org> wrote:
> >
> >> On 2023-01-09, Commander Kinsey <C...@nospam.com> 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?
> >>
> >> the voltage is too low for a 6 cell lead-acid battery, and too high
> >> for a 5 cell battery. maybe try supercapacitors instead.
> >
> > Why is it too low? 12.6V means it's almost full.
> You have to read this very carefully. One reading, is not enough.
>
> https://batteryuniversity.com/article/bu-403-charging-lead-acid
>
> Lead acid batteries can sulphate or they can suffer plate corrosion.
>
> They are quite sensitive to how they are treated, over long
> periods of time.
>
> My APC 650 battery lasted for 11 years, the first one.
> At changeover, I verified the float was 13.5V .
>
> The replacement RBC4, with an APC sticker on it, wore out
> in only three years. Three of six cells were shorted, so
> only half of the normal voltage was evident. When this
> happens, the chassis of the UPS gets warm, which is your
> warning that something bad is happening.
>
> Obviously, the characteristics of that "new" battery are not
> a match for the control settings on the APC 650 circuit board.
> But if the battery does not admit to not being an RBC4,
> it is pretty hard to guess at what setting would make a
> new battery last 11 years.
>
> When the 11 year battery failed, it had full terminal voltage,
> but little amp-hour capacity left.
>
> 14.4V End of charge cycle on a fully charged battery.
>
> 12.6V The terminal voltage one day after a fully charged lead acid battery
> is allowed to settle. In such a case, no bias is being applied to the
> battery. You can't leave it sitting forever like this.
>
> 13.5V The float voltage used for battery maintenance (as measured on my UPS)
> The UPS seems to be applying this all the time.
>
> 12.0V You have to decide when to stop discharging a lead-acid battery.
> A leisure battery might be OK down to 11.0V, but then maybe
> only one hundred charge/discharge cycles is the result. Automotive
> batteries, you cannot be too greedy, during discharge, and stopping
> at a higher value is advised.
>
> When you start a car, the 12.6V battery drops to around 9.5V (at terminals)
> while 150 amperes flows to the starter motor. Some battery chemistries
> are stiffer than others. This is not a violation of anything (it does
> not mean a cell is reverse biased necessarily), since the condition is
> transient for the battery. If the battery was no longer stiff and had a
> high impedance, the terminal voltage would be a lot lower and you would
> not get the 150 amperes. It could not maintain such a load (150 amps)
> for long anyway. The conditions in this example, were measured on my
> car, using a clamp-on DC ammeter with peak hold, and a multimeter
> on DC volts with peak hold. A healthy starter motor might have been
> 100 amps.

I think you will find the OP already has planted firmly in his mind, that a battery is much like a capacitor. He isn't going to understand that the battery simply is not capable of supplying the extra current he seems to be talking about, at a constant voltage. His discussion of his arrangement seems to be saying he doesn't understand that his power supplies will also dip voltage when current limited. It's hard to tell what he actually is thinking, as he often leaves large gaps in what he describes.

Oh, well. You can lead a pig to the trough, but you can't make him swallow..

I think Peeler is posting in a different group. I've mentioned him at least once and did not see his reply. I guess he's not reading s.e.d. I wish he wasn't posting here either. Good reason to not crosspost.

--

Rick C.

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