Comparing Apples, Oranges, and Batteries

I happen to like food, science, and electronics. Naturally, that means that from time to time, I play with my food.

I think it's important to understand and conceptualize by doing - unfortunately, most off the shelf batteries are not friendly or safe to being taken apart and experimented with. Likely, you've heard of lemon batteries, potato batteries, and any number of variations - so let me bring you one more. The apple battery.

It's been a helpful analogy for me to understand several properties of batteries, and I hope it'll be a fun evening diversion.

*Apologies - no oranges here.

Like most experiments, you'll need some materials. Luckily, most are fairly easy to come by... with the exception of a multimeter perhaps.

You'll need:

-several zinc nails or screws*

-several steel nails or screws*

-an apple you don't mind not eating

-some alligator clips or wires and multimeter probes

-a multimeter (used to measure current or voltage)**

-an LED (Light Emitting Diode - Red is recommended, as will be explained later)

*Any pair of two dissimilar metals will work in principle. For instance, copper wire and steel nails would also work. However, the chemistry is such that the zinc and steel combination yields the greatest voltage per cell.

**Multimeters aren't necessarily costly. You can pick a very cheap one for $10-$15, or a pretty decent one for $40-$50

First we'll start hooking up a basic cell, and it's pretty simple and easy.

Stick a zinc screw in one end, and a steel nail in the other.

Attach a negative (shown in black) wire to the zinc, and a positive (shown in red) wire to the steel.

Next, we're going to hook up the multimeter to our single cell apple battery.

A multimeter can measure several electrical quantities (hence the "multi").

Here, I measure the open-circuit voltage (the battery isn't powering anything) with the multimeter set to measure voltage. We get nearly 0.45 Volts from a single cell! Not bad at all.

I also measure the short-circuit current (allowing the maximum current to flow) through the multimeter. We measure only about 66 uA*. This.... is not as impressive. Let's see if we can improve on that.

*microAmperes, or 0.000001 Amperes. For reference, your typical handheld flashlight might take 0.5 A.

Now, we could get a bunch more apples and make more battery cells that way, but it turns out the voltage and current of a cell is dependent only on the chemistry of that cell, and not the size. This is why AAA, AA, and D cell batteries can all have the voltage, but differ widely in the current delivering capabilities and lifetime.

To save more apples for eating, cut your apple into several sections. (Again, don't eat an apple once you've been experimenting on it). I've chosen to cut them in quarters.

First position the steel nail and zinc screw at opposite ends, and measure the current and voltage.

I measure 0.443 Volts and 39.1 uA. Notice that the voltage has pretty much stayed the same despite changing the size significantly, while the current has dropped. The current has dropped because while the nail and screw are the same distance apart, there is less of a cross sectional area for the current to pass through. This increases the internal resistance of the cell.

Okay, back up, you may be saying - an analogy might be useful. Voltage is analagous to water pressure, while current is analagous to water flow. Similarly, the internal resistance of the battery can be thought of as the diameter of the pipe that the water (current) flows through. So we've found that by reducing the cross section of the apple, we've increased the resistance and decreased the current.

Next position the steel nail and zinc screw closer together at about half the distance.

I measure 0.447 Volts and 71.4 uA. This is better! We've gotten more current out of our cell by reducing the internal resistance by shortening the path. This is analagous to decreasing the length of pipe water has to flow through.

So, if we want a useful battery cell, how should we shape the apple, and where should the metals be placed?

We've seen that no matter what we do, the cell voltage stays the same. So be it (for now)

Decreasing the cross sectional area of the apple decreases the current, and increasing the distance between the metal electrodes also decreases the current.

So for a better current output, we want a large cross sectional area and closely spaced electrodes.

I measure the apple slice and get 0.429 Volts and 82.2 uA. Great! some improvements, but we'll do even better.

As a side note, I had to leave this set up for a little while to take a call. When I returned, both the voltage and current of the battery had decreased, much like a real battery would throughout its lifetime. In this case, there could be several mechanisms for this - the apple could be drying out, the electrodes could be developing an oxide layer, or even slipping / making poorer contact with the apple.

Okay, so we can do even better. We really just need small cubes of apple to provide a decent cross sectional area while spacing out electrodes closely. (Be careful not to accidentally short the electrodes, or you'll get no current or voltage!)

We can increase our limited voltage by putting several of these cells in series. This means connecting the positive of one cell to the negative of the next, and so on, adding successive cell voltages. So, putting 4 cells in series should give up about 4*0.43 V = 1.72 Volts

I measure 1.677 Volts for 4 cells in series. Note that we still measure only 87.5 uA for the short circuit current - we would need to put cells in parallel to increase our current capability.

With this voltage, we have a shot at turning on a Red LED which has a forward voltage of 1.8 Volts.* We get a nice, dim glowing red light, if you look at the correct angle.

*An LED will begin to turn on, pass current, and emit light around its forward voltage. This is why a Red LED was recommended. A Blue LED will have a forward voltage of around 3.3 V (not a problem if you have more patience than I at constructing more battery cells).

So we've just scratched the surface, but I hope I've introduced you to exploring with a (safe) battery that you can actively change parameters and dimensions of.

Placing batteries in series allows you to increase the voltage.

Placing batteries in parallel allows you to increase the current (though be careful when doing this with real batteries, as their voltages will not match perfectly).

What would you expect the internal construction of a 9V battery to look like?

Feel free to reward yourself with any of the apples you didn't experiment on.

I've really just grazed the top of a lot of subjects here. For more information, you could check out some of the following links.

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