What happens if I draw too much current from a home made battery?

Question

If I try to make a battery like one of these: https://www.wikihow.com/Make-a-Homemade-Battery And then I short the two terminals, what will happen?

Disregarding anything getting hot and burning up, I'm curious what happens to the chemical reaction. I assume there is a max speed at which the reaction can occur because it seems unlikely that shorting the wires would give "infinite" current and use up the reaction in an instant.

My assumption is that the current will max out and the voltage will drop to basically nothing. Is this true? And if so what is actually limiting the rate of the reaction? Is this the same chemical mechanism as a store bought battery or are they different somehow?

Answer 1

Yes, you're right. There will a maximum speed that is determined by the kinetics of several parts of the system.

Electronic Resistance: Pretty straight-forward. All the parts have some inherent resistance. Though this tends to be minial compared to some of the other causes.

Migration: At high currents, the ions become depleted (or in excess) at the electrode's surface. This means that the reaction will not run at the same potential until new ions diffuse from the bulk.

Adsorption: In order for the redox reaction to occur, there is often a step needed where the ions adsorbs to the metallic surface. This is a sort of chemical reaction, so has a rate constant associated with it.

Chemical Reaction: Some batteries can also involve a chemical reaction to form an activated reactant.

Electron Transfer: This is the actual redox part itself. Transferring an electron from the metal to the reductant, or vice-versa.

All of this will play a role, but I mostly think of migration effects when I think of kinetic limitations. Once that surface layer is depleted, all of the other processes stop too.

This is most likely to be worse for a home-made battery. The electrical resistance will probably be higher because there are longer wires, less reliable construction, etc. Distances between the electrodes is longer, so in the long run there's more distance for ions to travel to equalize a charge imbalance. Your electrode surfaces are likely to be dirtier, causing slower absorption.

The book Electrochemical Methods by Alan Bard and Larry Faulkner is a good resource if you want to do a deep dive.

Answer 2

This may be more of an electrical engineering question than a chemistry question.

A battery can be modeled as an ideal voltage source in series with its internal resistance. If the load goes to zero ohms the voltage across the load would measure zero volts. The current would be limited by the internal resistance of the battery.

Both the load and battery will have resistive heating. The temperature will rise until the amount of heat generated equals the amount of heat dissipated. This heat dissipation may cause anything from an imperceptible rise in temperature to the dangerous "thermal runaways" which are known to cause cell phones to explode.

Though the internal resistance of a battery is not usually discussed due to most load resistances being several orders of magnitude higher than the internal resistance of a battery, all practical batteries have an internal resistance. What affects the internal resistance will differ greatly between the home-made vs store bought batteries. Store bought batteries are engineered to have a lower internal resistance.

Home-made batteries will likely have a very high internal resistance. Even when shorted, those batteries may have limited current discharge. The chemical mechanism on a homemade battery may not change much when a homemade battery is shorted.

(Disclaimer:: Shorting any kind of battery is dangerous and should not be tried at home.)