Battery Fundamentals
Part of Energy Storage & Batteries
Before building any battery, you need to understand the electrochemical principles that make them work — why certain metals produce voltage, how electrolytes enable current flow, and what determines a cell’s capacity and lifespan.
Why Electrochemistry Matters
A battery converts chemical energy into electrical energy through controlled chemical reactions. Without understanding these reactions, you are just stacking metals and hoping for electricity. With understanding, you can design cells from whatever materials are available, predict their voltage and capacity, troubleshoot failures, and even invent new combinations suited to your specific resources.
Every battery — from a lemon cell to a lead-acid car battery — operates on the same fundamental principle: two different metals (or compounds) immersed in a conducting liquid, where one metal wants to dissolve more than the other. That difference in “wanting to dissolve” is what produces voltage.
The Electrochemical Series
The electrochemical series ranks metals by their tendency to lose electrons (oxidize). Metals higher on the list dissolve more readily and become the negative electrode (anode). Metals lower on the list resist dissolving and become the positive electrode (cathode).
| Metal | Standard Potential (V) | Role in Battery |
|---|---|---|
| Lithium | -3.04 | Anode (strongest) |
| Magnesium | -2.37 | Anode |
| Aluminum | -1.66 | Anode |
| Zinc | -0.76 | Anode (most practical) |
| Iron | -0.44 | Anode |
| Nickel | -0.26 | Either |
| Tin | -0.14 | Either |
| Lead | -0.13 | Either |
| Hydrogen | 0.00 | Reference |
| Copper | +0.34 | Cathode |
| Silver | +0.80 | Cathode |
| Carbon (graphite) | +0.74* | Cathode/collector |
*Carbon’s potential varies with surface chemistry and is used primarily as an inert current collector.
Predicting Cell Voltage
The voltage of any cell equals the difference between the two electrode potentials. A zinc-copper cell produces approximately 0.76 + 0.34 = 1.10V. A zinc-carbon cell produces approximately 1.5V. The farther apart two metals sit on the table, the higher the voltage.
How a Cell Works
The Anode (Negative Terminal)
The anode metal dissolves into the electrolyte, releasing electrons into the external circuit. For a zinc anode:
Zn → Zn2+ + 2e-
The zinc atoms leave the metal surface and enter the solution as zinc ions. The electrons left behind flow through the wire to power your device.
The Cathode (Positive Terminal)
At the cathode, something accepts those electrons. In a copper cathode cell, copper ions from the solution plate onto the electrode:
Cu2+ + 2e- → Cu
In cells using carbon cathodes with manganese dioxide, the MnO2 accepts electrons:
2MnO2 + 2H+ + 2e- → Mn2O3 + H2O
The Electrolyte
The electrolyte is a conducting liquid (or paste) that allows ions to move between electrodes, completing the internal circuit. Common electrolytes include:
- Saltwater (NaCl solution): Weakest, but universally available
- Vinegar (acetic acid): Mildly acidic, produces moderate current
- Sulfuric acid (dilute): Strong, used in lead-acid batteries
- Potassium hydroxide (KOH): Alkaline, used in nickel-iron cells
- Ammonium chloride paste: Used in dry cells
Electrolyte Safety
Sulfuric acid causes severe burns. Potassium hydroxide destroys organic tissue. Always add acid to water (never water to acid), wear eye protection, and keep clean water nearby for flushing spills.
Cell Voltage vs. Current vs. Capacity
These three properties determine what a battery can actually do:
Voltage
Voltage is determined solely by the electrode chemistry — the metal pair and electrolyte. Making electrodes bigger does not increase voltage. To increase voltage, connect cells in series (positive of one to negative of the next).
| Configuration | Voltage | Current Capacity |
|---|---|---|
| 1 cell (1.1V) | 1.1V | 1x |
| 3 cells in series | 3.3V | 1x |
| 3 cells in parallel | 1.1V | 3x |
| 6 cells (3S2P) | 3.3V | 2x |
Current
Maximum current depends on electrode surface area, electrolyte conductivity, and internal resistance. Larger electrodes and more conductive electrolytes produce higher current. To increase current capacity, connect cells in parallel (all positives together, all negatives together).
Capacity
Capacity (measured in ampere-hours) depends on the mass of active material in the electrodes. A cell with 100 grams of zinc can deliver more total energy than one with 10 grams, even though both produce the same voltage. When the anode is fully dissolved, the battery is dead.
Internal Resistance
Every cell has internal resistance caused by:
- Electrolyte resistance: Ions moving through liquid encounter friction
- Electrode surface area: Smaller surfaces create bottlenecks
- Polarization: Gas bubbles or reaction products coating electrodes
Internal resistance causes voltage to drop under load. A cell with 1.1V open-circuit might only deliver 0.8V when powering a load. Minimizing internal resistance means:
- Using concentrated electrolytes
- Maximizing electrode surface area (corrugated or porous electrodes)
- Adding depolarizers (chemicals that consume gas bubbles, like MnO2)
- Keeping electrodes close together
Testing Internal Resistance
Measure the cell voltage with no load (open circuit), then measure it while driving a known current. The difference divided by the current gives internal resistance. Example: 1.1V open, 0.9V at 0.5A means R = (1.1-0.9)/0.5 = 0.4 ohms.
Practical Cell Design Principles
When designing a cell from available materials:
- Choose the widest electrode gap on the electrochemical series that your materials allow — zinc and copper are the most practical pairing
- Maximize electrode surface area relative to cell volume — score, roughen, or corrugate metal surfaces
- Use the strongest electrolyte you can safely handle — dilute sulfuric acid outperforms saltwater by 10x
- Keep electrodes close but not touching — 5-15mm gap is ideal for most cells
- Include a depolarizer if available — manganese dioxide packed around the cathode absorbs hydrogen gas
- Seal the cell to prevent electrolyte evaporation and air contamination
Building Your First Cell
The simplest practical cell uses zinc and copper in saltwater:
- Cut a zinc strip and a copper strip, each approximately 50mm x 100mm
- Clean both strips with sand or vinegar to remove oxide
- Dissolve 3 tablespoons of salt in 500ml of warm water
- Place both strips in the solution, approximately 10mm apart, not touching
- Connect a voltmeter — you should read approximately 1.0-1.1V
- Connect a small LED through a 100-ohm resistor to verify current flow
This cell will produce useful current for several hours. The zinc will slowly dissolve, the copper will darken with deposited material, and the electrolyte will become cloudy with zinc salts.
Common Mistakes
- Letting electrodes touch: This short-circuits the cell internally, producing heat instead of useful electricity. Always use a separator or maintain a physical gap.
- Using identical metals: Two pieces of the same metal produce zero voltage. You need two different metals on the electrochemical series.
- Ignoring polarization: Hydrogen bubbles on the cathode insulate it from the electrolyte, killing output. Stir the electrolyte, add a depolarizer, or periodically clean the cathode.
- Expecting too much voltage from one cell: No aqueous cell produces more than about 2.1V (lead-acid). For higher voltage, connect cells in series.
- Using contaminated electrolyte: Dissolved metals from previous use change the chemistry unpredictably. Start with fresh electrolyte when building new cells.
Summary
Battery Fundamentals -- At a Glance
- All batteries work by pairing two different metals in an electrolyte; the voltage equals the difference in their electrochemical potentials
- Zinc-copper in saltwater is the simplest practical cell (~1.1V)
- Voltage is set by chemistry alone; current depends on electrode area; capacity depends on electrode mass
- Internal resistance drops voltage under load — minimize it with large electrodes, close spacing, and strong electrolyte
- Series connection increases voltage; parallel connection increases current capacity
- Always include a depolarizer to prevent hydrogen gas from killing cathode performance