I have to do a project for my school science fair. Since my dad is the editor-in-chief of Pro Tool Reviews, I had no trouble integrating powerful tools into my projects. The purpose of my project is to see if a higher watt-hour battery can provide more work for a longer period of time. I'd also like to see if it's cheaper for consumers to buy a battery with more watt-hours over time, even if it's more expensive initially.

## Table of contents

- my hypothesis
- How to Test Higher Watt-Hour Batteries
- set up experiment
- Calculate future operation based on 14 Wh control
- Running tests is harder than math
- evaluation result
- Battery capacity and price per watt-hour
- in conclusion

### my hypothesis

My hypothesis is that a higher watt-hour battery would yield more overall runtime. If my assumption is correct, the battery with the most watt hours will run the longest and drive the most screws. This can be very useful information for consumers. I'm also assuming (big word!) that you can do the math to figure out how many screws a higher capacity Li-Ion battery can drive. This will "control" the battery based on the test, then use the math to calculate the rest.

Accurately testing batteries involves many aspects. You will need adequate supplies and fixtures to run all the tests. Milwaukee Tool actually provided me with an M12 brushless drill kit (2404-22) to run my tests on. They also provided two M12 batteries of four different capacities for my testing. They sent extended run models in various capacities. For simplicity (and to save screws and time) I just used the compact M12 pack.

## set up experiment

To experiment, I first fully charged all the batteries. My dad then cut a 4×8 sheet of 3/4 plywood into 4" by 48" strips. We then fastened the 3 strips together to make the test board. In all, we need 12 test boards – three for each test cell. Next, my brother and I marked a half-inch grid on each test board with a marker, a tape measure, and an Empire Level laser-etched square.

After marking, we mounted the plywood on a pair of sawhorses. My dad then inserted the depth setting #2 Phillips bit into a Milwaukee brushless 12V drill/driver. This gives me a consistent depth of fastening to help balance the results. I started with a freshly charged 14Wh battery to see how many screws it tightened before the battery died. Then, I charge the battery and run this test two more times to get an average. On higher watthour batteries, I use a LOT of screws!

## Calculate future operation based on 14 Wh control

If you recall, I like *to count* the number of screws I'm going to make *before* actually doing the physical testing. For this purpose, I defined X as the "average number of screws secured per Wh for a 14 Wh battery (control)".

I came up with the formula for finding X by dividing the average number of screws "controlled" by a 14 Wh battery (181 screws) by the watt-hours. This yields the following equations and solutions:

If 14 Wh produces 181 screws, then *X = 181/14 = 12.929 screws/Wh*

Then I calculated the number of screws *that should be* fixed with the other battery using the following formula and results:

- 16 Wh • X = 207 screw
- 22 Wh • X = 284 screws
- 36 Wh • X = 465 screws

I'm now ready to test my calculations with an actual battery and compare my results to the calculations.

## Running tests is harder than math

I had to test each battery three times for accuracy. I did this by driving screws into our 3 ply plywood until each battery was drained. Then, I charge the battery and retest. Next, I averaged the number of screws driven in for each of the three tests. Finally, I compare my results with the calculated quantities.

Overall, the actual and calculated results are very close – as shown in the table below. Only 36 Wh calculations and estimates are significantly off compared to the rest. Note the average number of screws per battery compared to the estimated number.

It turns out that the more watt-hours a battery has, the more screws it can screw in. It also yields longer runtimes. The battery with the fewest watt hours uses the fewest screws overall. The batteries with the highest watt-hours have the most screws in. This more or less proves my assumption correct. The results also showed that even though the 36 Wh battery had the same voltage as the 14 Wh battery, it was able to drive twice as many screws.

## evaluation result

I had a talk with my dad about why my results deviated from the calculated values as the battery capacity increased. He explained that batteries with higher watt-hours are also *newer.* He further explained that better battery technology could affect the results because there are more factors. Batteries are not just batteries. They involve electronics and controls to monitor heat buildup. This prevents overloading and damage to the tool and battery pack.

## Battery capacity and price per watt-hour

After calculating the above results, I decided to do a few more calculations to find the price per watt-hour (Wh). What I found surprised me. Higher watt-hour batteries actually cost less per watt-hour. This could be due to volume cost savings, or an update to a larger battery. I calculate the price based on the total cost of the battery divided by the watt-hours. Here's a chart showing what I found ( **note** : the price for the 14 Wh battery is based on its original list price. It's no longer available):

My chart shows a 16 Wh battery at $39, so the cost per Wh is $2.44. For a $49 22 Wh battery, the cost drops to $2.23 per Wh. Finally, since the 36 Wh battery costs only $59, the cost per Wh is only $1.64. Interestingly, the 14 Wh battery used to cost $39, making it the most expensive battery per watt-hour. My calculations revealed some very interesting things. It will be cheaper for consumers to buy more expensive, more watt-hour batteries over a longer period of time. Due to the higher Wh the battery will also do more work before needing to be recharged. This means they also save you time.

## in conclusion

In conclusion, I think that when shopping for batteries for your tools, you should be looking for higher watt-hour batteries. Watt hours are more important than higher voltages in terms of run time. Consider this the next time you shop for batteries for your cordless power tools.

I had so much fun doing this project for my school science fair. I even finished third and made it to county! Who says science can't be fun?