[4gkenzy]

For this upcoming lab assignment, we are to design a battery.

Last week, we used a banana to power a stopwatch. A copper cathode and magnesium anode were placed in the banana, salt was added (to stir up the ions) and it powered the stopwatch via two clips attached to the electrodes.

Now for this week, we need to design a battery with the following specifications:

- Powered a digital clock (it takes, I think it was, 1.5V to do)
- Battery did not drip (area must be dry after testing)
- Mass of battery is less than 15.0g
- The measured voltage of battery is >2.5V
- The measured current of battery is >300mA
- Powered a buzzer for more than 30 seconds
- Powered a mini-motor for more than 30 seconds
- Powered maxi-motor for over 30 seconds

Does anyone have any ideas what I could use? I was thinking maybe a chapstick tube but I am not sure if that would be enough voltage at >2.5V.

That is all the information I have. I don't have any info on the amount of voltage needed to power the mini-motor or maxi-motor, unfortunately.

edit: we use series or parallel circuiting

[4gkenzy]

*Ignore me, I am impatient*

[Enthalpy]

Must all requirements be fulfilled by one answer, or shall one answer only go as far as possible, or can you provide one answer per question? The requirements seem to be listed by increasing difficulty more or less.

2.5V is an awful lot for one cell. I suppose this demands metals that are not compatible with aqueous electrolyte, check that. Something like lithium in a carbonate electrolyte. Depending on whether you study chemistry or something else, it might be too difficult. Assembling two cells in series (which is still one battery, but may not be the answer expected for the assignment) would keep easier choices.

300mA isn't trivial neither. You may want to measure the "maxi motor" consumption and check which is worse (motors exist well over 1MW). It needs electrolyte concentration, electrodes area, possibly a decent depolarizer (but 30s help here).

[4gkenzy]

All requirements must be fulfilled by one design, yes.

Series/Parallel circuiting.

The things supplied are: Copper, zinc, magnesium, lead electrodes

Salt and salt bridges are also supplied.

[4gkenzy]

Also, I managed to get 1.5V with just a banana...

[Enthalpy]

Then, put two bananas in series if you believe it's a good electrolyte. I don't.

To decide how many cells for 2.5V, you could look around you what voltages exist at commercial single cells and how convenient their metals are. And if you are supposed to use Cu, Zn, Mg or Pb, compute or measure what voltage they give. Maybe Mg gives the voltage for a short time, but it's not used commercially, I believe due to self-discharge.

What kind of "salt"?

I don't see any depolarizer in your inventory. Your battery won't work for long.

[4gkenzy]

Dude I think you are overthinking this.

The banana, potato, lemon, whatever is only there to make this more fun. I did not power my stopwatch with a banana, the power was stored in the piece of magnesium. The banana was only there to provide an enclosure to hold the electrodes and to provide a rather poor electrolyte solution. As I said, electrodes were used.

edit: Also we used table salt

[Enthalpy]

If you try a magnesium battery, please tell us how well it works! Up to now I know them only as sacrificial electrodes supposed, meant, and claimed to protect steel from oceanic corrosion, but I've proposed to power underwater drones with a battery where seawater is the electrolyte, the other electrode is essentially durable and leached by flowing seawater, while the magnesium electrode is replaced before the drone dives to a new mission. Big autonomy, cheap, safe - if it works.

I suppose magnesium isn't used commonly in batteries because it corrodes even if no current is consumed. If the electrode is stored dry and inserted before the drone consumes permanent power, self-discharge is a lesser worry.

Maybe purity is an answer to the supposed self-discharge of magnesium batteries. Or some additive in magnesium, like Hg was added to Zn before Zn purity solved the self-discharge problem.

I admit a strong tendency to overthink. But in case your battery can't power the maxi-motor for the 30s, consider a depolarizer.

[4gkenzy]

Here's what I ended up doing.

It worked surprisingly well!

I'd cut little square pieces of cardboard and soak them in vinegar. Then I would sandwich electrodes inbetween them. I ended up making two of these sandwiches and series-circuited their asses. Then the voltage reading was 4.2V!!! Crazy! Didn't expect that at all.

Unfortunately, due to poor surface area, we weren't able to generate much of a large current at all. Oh well, that was just bonus points anyway. Current ended up being like 1mA lol. If we had the time/patience, we could've easily made much more current if we simply improved the surface area of the electrodes but we got lazy and ended up getting like 16/20 of the bonus points so we called it a day.

[Enthalpy]

Vinegar conducts electricity badly because it contains too few ions. The first step to a strong current is a concentrated electrolyte.