Simplish maths question

[blobrana]

@kenny
ok you said...
T=time, in years
D=discharge (per year)
C=charge level (1 being full, 0 being empty)

I worked out that C=(1-D)^T, but got stuck trying to get it for T. But with your help:

T = ln(C)/ln(1-D)

Cheap alkaline or zinc chloride batteries, for instance, have a self-discharge rate of 4% per year, therefore:

T = ln(0.85)/ln(1-0.04)

= 3.98

So where did you get the value D from....

It`S a bit like saying D= 1 x (D x 1)

 :-)

[KennyR]

That's the formula for cumulative percentages, blob. It's (1+D) for additive, such as when working our interest on an account, (1-D) for subtractive. D is the percentage you lose or gain every iteration.

[blobrana]

Ah,
you mean:

first year = 96% charge ( d=4%)
second year = 92.16 (96 - 3.84 , d=4%)
third year =  88.4736    (92.16 - 3.6864, d=4%)
forth year = 88.4736 - (88.4736  x 0.04 = 3.538944) = 84.934656%

 :-)

(rather than the rough  method of 15 / 4 = 3.75)

[Karlos]

It's basically an exponential decay curve, just like any other proportional (as in the current absolute rate is always dependent on the current amount) decay.

The manufacturers could have chosen the half life of the cell to make it more obvious, but I doubt that a cell at 50% charge is not much use so they picked the 85% threshold instead.

[KennyR]

Karlos wrote:
The manufacturers could have chosen the half life of the cell to make it more obvious, but I doubt that a cell at 50% charge is not much use so they picked the 85% threshold instead.

Right. It's a matter of internal resistance. Most cell chemistries gain internal resistance as they lose capacity, decreasing the output voltage of the cell. At 50% capacity, your average alkaline battery would have an output of only 1.1-1.2 volts rather than the advertised 1.5, which is actually just not enough for some stuff. Their performance gets worse as they drain.

Incidentally, NiCd and NiMH don't have this effect (low internal resistance), which is why I like them. Pity they only have 1.2V even when fully charged, and they lose charge like fiends. And even worse, shops only sell padded crap like 2300 mAh NiMH D batteries (the theoretical capacity of an alkaline D battery is around 18,000 mAh, which should mean NimH D should be around 9000 mAh. These do exist, but they can only be bought specially and cost about £12 each!!).

Someone should do the world a favour and invent rechargeable batteries with 1.5V, low self-discharge and low internal resistance, but which aren't full of explosive ether like Li-Ion are. They'd be a rich man/woman!

Until consumer fuel cells came out that is... :-P

[Karlos]

You sure are into cells dude :-)

-edit-

That said, I can think of worse cells to be in of a Saturday night ;-)

[KennyR]

Electrochemistry is my forte. I didn't want it to be, that's just how it ended up. I'd rather have specialised in nerve agents or explosives, but unbelievably they wouldn't let me at uni, and gave an oscilloscope and some electrodes instead. Now I'm cursed with being specialised in perhaps the most horribly boring of all the sectors of chemistry.

Still, given today's choice between researching mind-numbingly mundane battery facts and anthropod/sock interaction, I made my choice and I'm sticking to it. It's better than getting drawn into the bickering on the front page, for sure.

[Karlos]

You'd find it easier to stick to the sock, er...

As for explosives, I do recall some very interesting reactions I had to set up when I embarked on my PhD.

I needed to prepare isopropyl 1-napthyl sulfoxide (and also the sulfone) in order to ascertain the barrier to rotation (if any).

The first stage was to prepare the thioether and then oxidise it up. I ended up having to perform a reflux of neat propene at -6 C directly on napthan-1-thiol with an acid resin catalyst.

I had a dry ice cold finger and gently passing dry propene direct from a gas bottle, diluted with nitrogen to be on the safe side.

I went away for lunch, came back and found someone had been in my fume hood to get something and had knocked the assembly, pulling out the N2 line. They'd reattached the wrong one and were now diluting the propene with compressed air.

Good job nothing sparked in there!

[adz]

Still, can only be as bad as aloe vera

Your comparing a crusty old sock to Aloe Vera??? Thats a bit of an understatment I would say, crusty old socks have far more appeal than a hearty glass of Aloe Vera juice anyday :-P

[blobrana]

@ Karlos
 @ kenny
You may be interested in this article I found, er, in Platinum Today...

“Scientists in the US have succeeded in breaking through the "oxo-wall" to forge multiple stable chemical bonds between oxygen and platinum.
The breakthrough is being widely hailed as a significant achievement because the process was previously thought to be impossible because oxygen is extremely unstable when combined with certain metals”

I wonder what the properties that would have,

 Ect…

[Karlos]

Are they talking about multiple bond order between oxygen and platinum?

If they are talking about oxoplatinate species, my first guess would be that they are good oxidising agents ;-)

Mind you, nothing beats perxennate as a source of reactive oxygen...

/gets coat :lol:

[KennyR]

Conincidentally, I was just thinking about using one of the platinum group in an oxygen related reaction.

I was actually wondering if we could trap platinum ions in a heme and make biomechanical blood cells. Synthetic pink blood that gives human beings superhuman capabilities! Yay!

[Cymric]

Is the power of a human limited to the transport of oxygen to his cells then? You'd think that there are other limitations too in ATP, ion and CO2 transport.

[Karlos]

Yes, there's more to it than oxygen transport :-)

However, if someone did improve upon haemoglobin, it would effectively improve the aerobic fitness of the individual, giving them better endurance before they get lactic acid build up and so on...

Of course, platinum, like most heavy metal elements is toxic as hell - a slight oversight perhaps should any of these biomechanical corpuscles break down ;-)

[Karlos]

adz wrote:

Still, can only be as bad as aloe vera

Your comparing a crusty old sock to Aloe Vera??? Thats a bit of an understatment I would say, crusty old socks have far more appeal than a hearty glass of Aloe Vera juice anyday :-P

What, even with the septic cat juice chaser?

[adz]

Karlos wrote:

What, even with the septic cat juice chaser?

Yes, even with a septic cat juice/ear wax cocktail...hmmm, what could we call that and would you frost the glass with sugar or salt??? Thinking caps peoples :-D