# Can You Cook a Turkey with AA Batteries?

#### Today we try to answer the question of whether or not you can cook a turkey with AA batteries. How many do you think it will take?

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The answer is more complicated than you might think. There are a number of variables that will affect the number of alkaline AA batteries you need. Let’s list them off.

## Variables

Let’s go over some of the variables that we need to address for our estimate.

### Mass of Turkey 🦃

Obviously, the more turkey you have, the more energy it takes to cook. The more energy, the more batteries you’ll need. The average Thanksgiving turkey weighs in at 15lbs, or a mass of 6.8kg. For the purpose of this calculation, let’s pick a small bird of 10lbs, or 4.5kg.

### Specific Heat of Turkey

Specific heat is the amount of energy it takes to raise the temperature of a substance. We use the unit J/C°/g, meaning how many Joules (J) per degree Centigrade per gram. The specific heat of water is 4.186 J/C°/g. What’s the specific heat of turkey? That’s a good question, and it will vary with the fat, bone, and muscle composition of the turkey. We can estimate that the specific heat of turkey is slightly less than that of water. Several people have tried to calculate the specific heat of turkey, with varying results. I’ll cut the difference and use the value of 3 J/°C/g for my calculations.

With the specific heat of turkey, and the mass of a turkey, we can now use these values to start estimating how much total energy it would take to raise the temperature of turkey to a safe eating temperature.

### Cooking Temperature

The next question is what temperature do we need to cook our turkey to? Oh, and what temperature are we starting at. If we thaw our turkey in the fridge, we can assume a temperature of about 33°F, or 0°C. The recommended cooking temperature for turkey is 165°F, or 74°C. That is a difference of 74°C. We can use the following formula

$E = mcΔT$

Where E is energy in J, m is the mass in kg, ΔT is the change in temperature, and c is the specific heat.

$4500g * 74°C * 3 J/°C/g = 999kJ$

We need about a million Joules to cook a 10lb turkey. But…

### Cooking Efficiency

We know we need about 1MJ to cook our turkey, but there will be energy lost in the process of cooking. Just how much? Efficiency will change based on the cooking method, but regardless It would take some significant thermal modeling to truly accurately figure out the efficiency.

We can, however, make a rough estimate with a simple experiment. If we take a known mass of water, measure the initial temperature of the water, use our preferred cooking method to heat up the water, measure the final temperature, and measure the energy we used in the process. Once we settle on a cooking method, we can try to figure out the efficiency.

The efficiency in turning our battery power to heat will also affect how many batteries we need. The less in between our batteries and our cooking method, the better.

The other variable that will affect our total energy needs, will be the ambient temperature. Obviously, cooking the turkey with an ambient temperature of 110°F will take less energy than cooking it in freezing windy conditions. Unfortunately, snow was in the forecast when I planned on cooking the turkey, so my best option was to cook the turkey in my garage. Ambient temperature will also matter for something else I’ll mention later.

### The Batteries

Alkaline batteries often provide a number for the energy capacity in amp hours (Ah), which is how much current you can draw from a battery. However, there’s a catch. The number you get on the package is from ideal conditions. All batteries are affected by temperature and most batteries produce less energy at cold temperatures. Alkaline batteries have great energy density, but have relatively low power output when compared to lithium batteries.

Additionally, alkaline batteries have a fairly high internal resistance, which means the more power you draw from them, the more energy turns to heat inside the battery. The numbers given for capacity are calculated at 10mA, which at 1.5V is .015W. That’s about enough to blink an LED. The internal resistance varies significantly based on a number of other variables.

Since power will be the more limiting factor, In order to figure out how many batteries to use, we’ll need to figure out how much power we will draw from each battery. From the datasheets, we can find a diagram of the service time for a batter at a constant power output.

If we say the longest that we want to cook a turkey for is 8 hours, then we can use the diagrams in the battery datasheets to estimate how much power, and therefor energy, to cook our turkey. Based on the above diagram, we can draw just below 300mW and have the batteries last for 8 hours. Using that number, we should be able to run 300W slow cooker with 1000 AA batteries to fully cook a 10lb turkey in 8 hours. Depending on the cooking efficiency, 1000 batteries should provide us about 300W for those 8 hours. If we want to cook the turkey faster, we would need more batteries.

Here is where ambient temperature comes in again. AA batteries, like most other batteries, perform differently at different batteries. Unfortunately for us, the batteries lose capacity at colder temperatures. This Thanksgiving brought us early snow, so even if we do this in my garage, we’ll be using the batteries below their nominal temperature. If you look at the diagram below, you can see that even with a small temperature drop, you can lose a significant portion of the battery capacity.

Temperature brings us to one more important factor. While batteries discharge, they produce heat. This might help us overcome ambient temperature in the Rockies in November, but also poses a potential danger. Heat can cause alkaline AA’s to fail and leak. And possibly explode. At 300mW, or around 250mA, and an internal resistance between .1-.3ohm, each battery will produce between 6-19mW. With 1000 AAs, you are looking at 6-19W of heating. If we were to even double the current draw and halve the number of batteries, each battery would produce 25-75mW of heat. Multiply that by 500, and you are now at 12.5-37.5W of heat for the entire pack. That’s less batteries, but more heat. A little heat will help our experiment, and help the batteries perform better. A lot of heat will pose a problem, and could be a severe safety concern.

## Cooking Method

Originally, I considered using a plug-in roasting pan. Unfortunately, I found that they consume 1450W, which would drain 1000 AA’s in less than an hour. Additionally, with that power draw the batteries would produce hundreds of watts of heat themselves. Now, the roasting pan would only draw 1450W for short stretches, but we would probably still need around 3000 batteries to power the roasting pan long enough to cook a turkey. After figuring out that we should have about 300W using 1000 AA batteries, we can find a cooking method that might work.

My next thought was an electronic pressure cooker, because it could cook a turkey in an hour. Although this might work, the pressure cooker I have is rated at 1000W. At this point, I assumed the next most efficient cooking method would be to build my own oven powering nichrome wire directly from the batteries. Due to the cost of fire brick, needing to coil my own heating elements, and the fire danger, I decided to look elsewhere. After going through my entire kitchen, I found a slow cooker rated at 275W. Perfect.

The slow cooker turned out to be exactly what I wanted, and consisted of about a 50 Ohm heating coil that would draw 275W at 120V. Even better, I had one with a simple switch that would work with DC and had no AC transformers.

## The Math, and the Most Useful Equation Ever

Now that most of the variables have been defined, and we have the most feasible cooking method, we can create a spreadsheet to help us figure out the right number of batteries to use.

Pulling data from the datasheets, we can also generate an equation using a line fit algorithm to generate a function to estimate how long a battery will last given any power input. Then, using this function, we can compare the service time for a battery to how long it would take to cook a turkey at that power, given our cooking method efficiency and specific heat of turkey. Using some constants I generated for a AA battery, the equation to estimate the number of AA batteries we need to cook a turkey is

$N = P(\frac{4011.0 Pη}{mcΔT})^{-0.670}\\$

Where P is your cooking power, η is the efficiency of your cooking method, m is the mass of the turkey, c is the specific heat of turkey, and ΔT is the change in temperature.

If we plug in some values, you can see that if we cook a 15lb turkey at 1450W, assuming 35% efficiency, we’d need about 2600 AA batteries. Take with a grain of salt, because my line fit is optimistic at the extremes. If we use our 275W slow cooker with a 7lb turkey, we’ll need about 900 batteries. Now, if we use 80 batteries in series, we’ll go from 275W down to 115W when the batteries are depleted at 0.9V, we’ll cook at an average power of 195W, which would require about 815 batteries.

960 AA’s should cook our 7lb turkey with the slow cooker. Now just to put it all together.

## Battery Holder Design

Building a holder for 1000 AA batteries ended up being one of the most difficult tasks. I opted for a design that would accommodate an extra layer of batteries in case things are looking questionable. You can purchase batter holders for up to 8 AA batteries, but it would have cost over \$200 in just holders. The more economical solution was to print and build a custom holder. Initially I designed a holder that would support each battery and would space the layers evenly.

However, this design turned out to use too much PLA and would have taken days to print a single block. The full assembly would have taken about 3-4 weeks to print. I only had a week to finish the build. With that, I had to start over and try something different. The next design consisted of a number of parts that ultimately could be combined using a plywood backer to hold the batteries.

In order to get 120V DC, I figured 80 batteries in series should give me the starting voltage I needed. 80 factors into 5 and 16, which means I would make a layer of 5 batteries long, and 16 batteries across. By using spring terminals, I could snake the batteries back and forth and then connect all the positive and negative terminals to a wire that I could run to an outlet.

To monitor the voltage and current draw of the battery pack, I purchased a 0-200V Voltmeter/ 10A Ammeter display from Amazon. The display worked quite well and was invaluable for tracking the progress while cooking the turkey.

After some initial tests, the 3d printer’s tolerances and tolerances in the AA manufacturing made it so that I had to add some slop to the fit of the batteries. The additional clearance eventually resulted in some issues with the assembly. The rows of batteries began to sag in the middle, and caused the terminals on the endcaps to not line up by the 11th layer. The pack was built to accommodate 1040 batteries, but only ended up holding 880.

The other design issue was the spacers between the rows of batteries. I opted for a simple design that would support the batteries on all sides. There was one crucial mistake. I added some tabs to go between the gaps between the button top of one battery and the next batter, but those gaps did not line up between the directions of the rows. During assembly, we snipped off thousands of these tabs and it cost us a lot of time.

One last point, I opted to add some rectifying diodes to each row of batteries to prevent any row of batteries receiving a reverse current. I’m quite glad I did this, because I accidentally placed two batteries in reverse on the top row (I blame fatigue). Without the diode protecting that row, the rest of the pack would have put a reverse current through that entire row and probably caused the entire row to fail. You almost never regret adding a little bit of safety.

## Cooking the Turkey

You can criticize me for wasting a ton of batteries, but simulations only get you so far. Sometimes, you have to actually do the experiment to see if you accounted for everything and to see if your estimates were close. I can’t tell you how many times I’ve seen an actual experiment reveal some major blind spots in designing an experiment.

One thing we learned, when we opened the turkey and removed a package of gravy and giblets, the bird we were cooking was closer to 5.5lbs, or about 2.5kg. Fortunately, the smaller turkey pushed us further into the green and away from the margins.

The batteries started at 127V and dropped to about 97V under load. I forgot to track the temperature rise at the beginning of the cook, but I do have some data points that validate our estimates. Below is the final capture of the battery packs. There is no load, so the voltage jumped back up.

Overall, about 21% of the energy put into cooking actually went into the turkey, depending on the specific heat of a whole turkey.

At about 2 1/2 hours, I could measure a rise in temperature of the batteries on the top row. With 11 layers of batteries, the rise became concerning because depending on how the heat moved through the battery pack, the batteries in the center of the pack could have become significantly warmer than the top. Once the top reached about 80F, I actively cooled the battery pack with a fan.

## Conclusion

So, how accurate were our estimates? We erred enough on the optimistic side that we had a large safety margin. We succeeded, but some of our estimates could use some refinement.

#### Cooking Efficiency

You can see that our cooking efficiency drops significantly as the temperature rises. Near the end of cooking, efficiency drops sharply. This is caused probably because the temperature of the turkey gets close to the temperature of the slow cooker and far less energy transfers to the turkey. We begin to lose energy to the environment.

I actually suspect that my estimates for efficiency are a little low based on cooking the turkey in 5.5 hours on a lower power than expected. A greater cooking efficiency or lower specific heat of turkey might explain the difference from my estimate.

#### Specific Heat Capacity of Turkey

The specific heat of turkey was always a rough estimate. Without an accurate knowledge of how much energy we put into the turkey, we can’t get a good estimate of the specific heat capacity. Additionally, the specific heat will change between any two whole birds based on weight and muscle and fat composition. Based on our results, I suspect our estimate was high, partially because the size of the bird. A smaller bird will probably have a higher bone to muscle ratio.

#### AA Battery Specifications

The temperature rise of the battery pack seemed to greatly impact performance. Additionally, we cooked the turkey on LOW, which greatly reduced the load on the batteries. After an initial sharp drop in voltage, the batteries held at around 100V for quite some time, about when I noticed the temperature rise in the batteries. The batteries began at 55F, and I measured the top layer at 80F near the end. We probably drained the batteries around 50%, and could have cooked for another 5+ hours on low.

If we had cooked on high, the batteries would have heated up significantly more and may have drained significantly more. Ultimately, on HIGH, I’m not 100% confident we would have completed cooking the turkey. My estimates conclude it probably would have succeeded and there would have been enough energy, but it would have much more stressful.

If I were to do this again, I would change the design of the battery pack to better monitor and manage the heat dissipation of the AA batteries.

### How did it taste?

The turkey ended up being cooked perfectly. The white meat was juicy and delicious. The skin, however, was not crispy or golden brown. The dark meat could have been cooked to a higher temperature in order to render it more tender.

Overall, I call this a success.