When the power goes out (like it did during Hurricane Isaac), what do you do? Well, I have a small generator – so that's good. It's big enough to run some lights, fans, and my refrigerator (to keep my beer cool). But what if you didn't have a generator? You could make a penny battery. Here is the one I made (following these instructions - PDF warning).

It actually works. The key to these types of batteries is to have two different types of metals with some material in between them. Usually, an acid will work. For this battery, it uses salt water and vinegar as the acid. The two different metals come from the same penny. Today's U.S. penny is 97 percent zinc with a copper coating. So, if you shave off the copper on one side you have a two-sided metal penny. One side is copper and one side is zinc. The thing that makes this battery so cool is that you probably don't even need to go to the store to make it.

So, I could make this if the power went out, right? But could I use it to run stuff in my house? Of course - the question is how many of these penny cells would I have to make? Bring on the data.

How Much Energy? —————-

This is the first thing to look at, the amount of energy per cell. I have actually looked at the energy stored in a battery before. For that case, I compared the stored energy in name-brand versus generic AA batteries. To do this, I measured the current coming out of the battery and the voltage across the battery over a few hours as it was connected to a load. The power for an electrical device is calculated as:

To determine the total energy, I just need to break the data up into small time-steps and add up all the power-times-time intervals. Like this:

I can get the potential-current data using Vernier's Logger Pro. Here is a plot of the power from five cells in series for a penny battery. Also included is a numerical integration to get the total energy. Notice that there were some weird things happening at the very end of this data. If you look at the electric potential data, the current actually drops to a negative value. I suspect this is just a measurement error since the current values are so low (around 0.1 mA).

That's 2.204 x 10-3 Watt*hrs of stored energy. Since a Watt is a Joule per second, I need to convert this value to Watts*sec to get the energy in Joules. That is 7.93 Joules for five cells or 1.39 Joules per cell.

How Much Energy Do I need? ————————–

After the hurricane, we without power for two days. How much energy do we need? It would probably be best to look at our average power use to determine the total power. I previously looked at the power my refrigerator uses and it averages around 100 Watts. I guess I could get by with about 300 Watts average (for lights, fans and internet modem and stuff).

If I use 300 Watts for 48 hours, this would be (300 Watts)*(2 days)*(24 hours/day)*(3600 seconds/hour) = 5.18 x 107 Joules. If each penny battery cell has 1.39 Joules then I would need:

If I just stack these penny cells in series, I would only need like one penny per cell (plus one for the end). This would cost $373,000 in pennies alone. I doubt you would have that many pennies with you. Oh, but you do? OK then, how much space would that take up? Here are the dimensions of a U.S. penny.

I have also included a diagram showing how they could be stacked. In this case, the top-down area would just be d2. But what about the height? If the height of the penny is h, I am going to estimate that the height of the cell (with the stuff in the middle) is about 2h. This puts the volume per cell (not including the top penny) at 2hd2. So, if I need 3.73 x 107 cells, this would take a total volume of:

So, about 40 cubic meters. If arranged in a cube, this would have a side length of about 3.5 meters. This would take up a significant portion of a room. A room that would smell like copper and vinegar. Oh, I didn't even mention all of the wires you would need. You could connect vertical cells by just stacking, but you would need something to connect adjacent cells. Also, if you want to run stuff like your refrigerator, you would need a DC-to-AC converter. Just so you know.

There is something else. What about the maximum current you could get from these penny cells? I will look at that in a future post. Just letting you know I didn't forget.