Can a Makeshift Battery Be Used to Start a Recreational Vehicle’s (RV) Engine? – Part of The Chemistry of Breaking Bad

Author: Falk Harnisch, Tunga Salthammer

Can a Makeshift Battery Be Used to Start an RV’s Engine?

In order to build an electrochemical battery, Walter asks Jesse to bring him the RV’s brake pads and various galvanized metal objects and finds dish-washing sponges and KOH electrolyte solution from their chemical stocks. He uses the sponges and KOH electrolyte solution to make an ion conductor to connect the two half cells of the electrochemical cell, i.e., the anode (negative electrode) and the cathode (positive electrode).

Let’s take a closer look at the two half-cell reactions:

Anode:

Walter uses galvanized metal objects, such as screws, coins, and nuts. The corresponding idealized electrode reaction is, therefore, the oxidation of zinc according to

Zn → Zn2+ + 2e

with a standard potential, i.e., the maximum voltage achievable under ideal conditions, of

EøZn/Zn2+ = -0.76 V

Cathode:

Walter uses the RV’s brake pads. These are graphite blocks coated with mercury oxide (HgO). The graphite simply serves as an inert electrode material (electron conductor). The electrochemical reaction takes place with HgO. We assume here (for reasons of simplicity) that mercury is present as Hg2+ and reacts according to

Hg2+ + 2e → Hg

The standard potential is

EøHg2+/Hg = 0.85 V

Cell reaction:

The total reaction of the galvanic cell can now be shown in simplified form as follows:

Zn + Hg2+ → Zn2+ + Hg

The cell voltage can then be calculated according to

Ecell = EøHg2+/HgEøZn/Zn2+ = 1.61 V

Interestingly, the electrochemical cell used in Breaking Bad is similar to the mercury oxide batteries commonly used as button cells for watches. These have an operating voltage of 1.35 V. The operating voltage means the voltage that can effectively be delivered in use.

But what maximum voltage could we expect (even theoretically) from the electrochemical battery seen in Breaking Bad?

Walter and Jesse did not just use one electrochemical cell – they stacked six cells together. This increases the available voltage (when connected in series) to a maximum of 6 x 1.61 V = 9.66 V. This 9.66 V maximum is available in an ideal scenario, disregarding voltage drops during the electricity flow, side reactions, etc.

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