Idea for cell, maybe...

[fisher]

You have to know a bit about electronics in order to understand what Im about to say, but...

What if you made 10 or 12 separate cells, each consisting of only two plates, each wired +-, each in separate housings so that they worked independently of each other. BUT, wire them in series. So that voltage goes into one plate of the first cell, then the wire from the other plate goes to the next cell, and so on.

This would be equavilent to wiring a cell with several neutral plates, each (if there were 10) would get about 1.4 volts each, (with 14 volts applied.)

Now, why have separate cells? As a cell heats up, it's resistance decreases. If you understand series circuits, you know that less resistance, gets less voltage. So assuming that all cells are equal, each cell of ten with 14 volts applied would get 1.4 volts. But as one particular cell began to heat up, it's resistance would decline, consequently it's voltage would decline, causing more voltage to be applied to the colder cells, warming them faster. The hotter cell would get less voltage, enabling it to cool somewhat.

That is the theory, what do you think? Actually, my idea is to have all cells tied together at the bottom so that I don't have to fill each separately, and the air lines exiting the top would connect them together there, but each should pretty much retain it's own water during operation, allowing temperature fuluctuations to cause voltage fluctuations, regulating temperature somewhat. (I hope.)

[Riddler250]

Now, why have separate cells? As a cell heats up, it's resistance decreases.

Isnt it the other way around? resistance=heat and heat = resistance? A 16ga wire with a load of 20a would get hotter than 12ga wire with the same load

[resago]

yeah, the series is the optimum way to go, but takes up about twice the space.

[fisher]

Riddler, we are talking about two different things here. For a wire, bigger wire can handle more current without heating.

But for cells, as a cell heats, it's current increases, which tells me that it's resistance decreased. Think of water flowing through a pipe with a valve. Open the valve wide open, maximum current (flow) due to low restriction (resistance) in the pipe. Close the valve halfway, reduced current due to higher resistance to water flow through the valve.

A bigger wire is a bigger pipe. More current can flow. That is why bigger wire can handle more current.

Resistance does not equal heat. Current equals heat in that more current (in a cell or in a wire) creates more heat. More resistance decreases current, reducing heat.

Clear as mud.

BTW, Im a college electronics instructor. I have splained this before (or at least tried to) on numerous occasions.

[Riddler250]

Riddler, we are talking about two different things here. For a wire, bigger wire can handle more current without heating.

But for cells, as a cell heats, it's current increases, which tells me that it's resistance decreased. Think of water flowing through a pipe with a valve. Open the valve wide open, maximum current (flow) due to low restriction (resistance) in the pipe. Close the valve halfway, reduced current due to higher resistance to water flow through the valve.

A bigger wire is a bigger pipe. More current can flow. That is why bigger wire can handle more current.

Resistance does not equal heat. Current equals heat in that more current (in a cell or in a wire) creates more heat. More resistance decreases current, reducing heat.

Clear as mud.

BTW, Im a college electronics instructor. I have splained this before (or at least tried to) on numerous occasions.

I mean no disrespect but what about joule's law, that states H (Heat) = I2 (Current squared) x R (Resistance) x T (Time the current is allowed to flow).

[fisher]

My best answer is to show you an example.
Heat = I^2*R*T

So lets say we run a generator at 12 volts, for one hour, and it has 4 ohms of resistance. The current will be 3 amps for this circuit. (If you know electronics I (current) = V (voltage) divided by R (resistance).
So a 12 volt power supply with 4 ohms of resistance would pull 3 amps because 12 volts divided by 4 ohms = 3 amps.

Heat = 3 amps ^2 * 4 ohms * 1 hour.
Heat = 9 * 4 * 1
Heat = 36 Joules.

Now, lets decrease the resistance to only 2 ohms, half of the previous value.
Using Ohm's law, the current in the circuit must be 6 amps.
12 volts divided by 2 ohms = 6 amps.

Now lets calculate the heat for the same voltage and time, but at half resistance of the original value, (which gives double current of the original value).

Heat = I^2 * R * T
Heat = 6 amps ^2 * 2 ohms * 1 hour.
Heat = 36 * 2 * 1
Heat = 72 Joules

So doubling the current (halving the resistance) doubled the heat generated.

I am not sure if I made the point but I am enjoying the discussion so let me know.

[Riddler250]

Riddler, we are talking about two different things here. For a wire, bigger wire can handle more current without heating.

But for cells, as a cell heats, it's current increases, which tells me that it's resistance decreased. Think of water flowing through a pipe with a valve. Open the valve wide open, maximum current (flow) due to low restriction (resistance) in the pipe. Close the valve halfway, reduced current due to higher resistance to water flow through the valve. .

I havnt built a cell yet, so I havnt witnessed it reaction to heat or resistance.

[BoyntonStu]

I havnt built a cell yet, so I havnt witnessed it reaction to heat or resistance.

As the cell temperature rises, the electrolyte resistance lowers.

That is the reason for cell runaway and the necessity for PWM.

BoyntonStu

[fisher]

My opinion that the resistance of a cell decreases as temperature increases is based on the fact that as the cell is powered on cold, there will be a smaller amount of current than there is after the cell has been on a while and gotten warm. I am sure that you have encountered the vicious cycle of a cell heating, drawing more current, more current makes it get hotter, which makes it draw more current, which makes it hotter...

I conclude from that fact alone, that resistance is decreasing as the cell warms. How else would current be able to increase?

I ran a cell today that began at 12 amps and as it ran, the current grew to 20 amps, and I shut it down because it was getting too hot. It is certain that current increases with heat. I don't know what resistance would be doing other than what Ohm's law would reveal and that is that as current is increasing, with a constant applied voltage, resistance must be decreasing. I don't think it is the temperature coefficient of the stainless, but of the electrolyte.