18 August 2012

Experiment: Battery Profiling Update

I want to get the final results of the Battery Profiling experiment put up for everyone to see, as it came up with an interesting result.

150 Ω Circuit

Average of two runs using matched resistors.
The immediate conclusion is that Duracell does in fact last longer than Energizer. But stopping there doesn't really tell the whole story; yes, the Duracells did maintain a voltage for about 10 hours more time, but look more carefully at the initial period-- the Energizers actually perform better for the first half.  In fact, the Energizers stayed consistently at a higher voltage until around 120 hours, where both batteries were found to be at 1.05 V.  At that point the Energizer drops off quickly, while the Duracell holds on for a little longer. Big deal you say? Well, that depends on how you're using the batteries.

Consider a flashlight as a case example. The higher the voltage, the brighter the light. So the Energizer powered flashlight will stay brighter than the Duracell powered flashlight for a significantly longer time. The Duracell powered light may last longer overall, but how useful is that last 10 hours, really?

This behavior is consistent even at higher power draws:

33 Ω Circuit

Average of two runs using matched resistors.
The same result is found for the higher current case, with the cross again occurring at 1.05 V and the Duracell lasting about 10 hours longer.  This result suggests that the Energizer design optimizes maintaining the higher voltage at the expense of overall lifetime.  Integrating the actual current of each circuit over time gives an average energy capacity of 1380 mAh for Duracell and 1340 mAh for Energizer-- roughly the same!  If the Energizer maintains a higher current for a longer period of time, it would make sense for it to finish draining first as it uses more of its capacity during that initial time.

What we conclude from this is that the brand you choose may depend just on your needs-- specifically on the minimum voltage required in your circuit. If you need the voltage to stay above 1 V to power your application, Energizer will give you that power for a longer time.  If you only need the voltage to be above a threshold below 1 V, go with the Duracell.

What Next?

One reason I'm including these little experiments is as a catalyst for others to start investigating. This simple idea could lead, for example, to an excellent science fair project for a young student. Or perhaps someone is curious enough to justify buying lots of batteries to run through their paces. In any case, here are a few ideas of other questions that could be examined.

  • This experiment used a constant resistance circuit-- how do the lifetimes compare in a constant current application?
  • Is the behavior seen here consistent for other sizes of battery? (Be aware that discharging a D-cell through even 33 Ω may take a while...)
  • Do batteries discharge evenly? What happens if two otherwise identical batteries are placed in series? Measure the voltage at both batteries-- ideally the center voltage should be half of the overall voltage at all times. Is it? Is there any difference if you mix brands together?
  • My measurement of the energy capacity of the two brands shows them as being similar, but the Energizer capacity was lower than the Duracell in both cases. Two measurements is not enough for the difference to be statistically significant-- measuring a larger number of batteries could tell you if Duracell does indeed have just a little more capacity. How many batteries would you need to measure to show that?
  • In this experiment, the circuit was constantly on. What happens if the current is only on intermittently? How much does a battery recover after being used for a while?
  • Both runs were done at room temperature. How much of an affect does temperature have on the battery? Be aware that the temperature difference will change the resistance as well, so that should be accounted in the results.
  • (For those with a lot of time to spare:) People claim that batteries last longer in storage if they are chilled. How quickly does a battery sitting on a shelf decay? How about a battery being stored in a cool environment?
I'm sure there are plenty of others; feel free to add any ideas to the comments!

Disclaimer: Electricity can be dangerous, and you should be aware of the hazards of working with even common alkaline batteries. For example, mixing a drained (or even partially drained) battery with a fresh battery can result in leakage, or possibly fire. The materials inside an alkaline battery are not something to mess with if you're not sure of what you're doing-- so be careful, and if you see anything that looks wrong, stop. If you're young, get help from a parent or teacher. Dispose of waste properly and responsibly.

Any experiments operated using this blog as a basis are done at your own risk and responsibility; I am not liable for any damages that may be caused by duplicating this work or running any experiment derived from this work.  You and you alone are accountable for any consequences of your actions, so if you are unsure about anything, find someone who can help. 

1 comment:

AndreVC said...

Congrats on this very interesting project.
Im thinking of using it to profile some rechargeable batteries.
Any ideas on how to compare the energy used([P=V.i]*time spent) with the mAh written on the batteries? I mean, can we consider the battery is empty when V < 1Volt?
best regards