A source of electricity is any device or system that converts stored, chemical, or kinetic energy into electromotive force (voltage) to push electrons through a closed circuit. When makers and electricians ask what are some sources of electricity, they are usually looking past the macro-level utility grid and focusing on the practical power supplies sitting on their benches or in their solar sheds. Whether you are pulling 5A from a USB-C PD brick or 200A from a lithium battery bank, understanding the true behavior of your power source is the difference between a reliable build and a melted terminal lug.
The Core Definition: What Makes a Power Source?
In circuit theory, we often talk about an 'ideal' voltage source—one that maintains a perfect, unwavering voltage regardless of how much current you draw. In reality, every physical source of electricity has internal impedance. This internal resistance dictates exactly what the source changes in a real circuit: it governs voltage sag under heavy loads, limits the maximum available short-circuit current, and directly determines how you must size your fuses and breakers for overcurrent protection.
According to the U.S. Energy Information Administration (EIA), while the macro grid relies on massive kinetic generators (turbines driven by steam, wind, or water), the micro-sources we use in DIY electronics rely on electrochemical reactions (batteries), photovoltaic effects (solar), or switched-mode power conversion (bench supplies).
Where You Meet This in Practice: Bench, Home, and Grid
To choose the right source for your project, you need to look at the internal resistance and maximum continuous current ratings. Here is how the most common sources of electricity stack up on the workbench and in the home.
| Source Type | Nominal Voltage | Typical Internal Resistance | Max Continuous Current | Best Use Case |
|---|---|---|---|---|
| AA Alkaline Cell | 1.5V | ~0.150 Ω (150 mΩ) | 0.5A | Low-draw sensors, clocks, remotes |
| 18650 Li-ion (e.g., Samsung 30Q) | 3.6V | ~0.020 Ω (20 mΩ) | 15A - 20A | E-bikes, high-power flashlights, power walls |
| 12V LiFePO4 (100Ah Drop-in) | 12.8V | ~0.005 Ω (5 mΩ) | 100A (BMS limited) | RVs, off-grid cabins, marine house banks |
| Bench Power Supply (e.g., Rigol DP832) | 0-30V (Adjustable) | Near 0 Ω (Active regulation) | 3A - 5A per channel | Prototyping, testing, lab debugging |
| Utility Grid (US Residential) | 120V / 240V Split | Micro-ohms (Transformer dependent) | 200A+ (Service entrance) | Whole-home appliances, HVAC, heavy tools |
Key Benchmark: A healthy, high-drain 18650 lithium-ion cell should have an internal resistance below 30 milliohms. If your multimeter's milliohm function reads above 80 mΩ, the cell is degraded and will suffer severe voltage sag under load.
Worked Numeric Example: Voltage Sag and Internal Resistance
Let's look at how internal resistance changes circuit behavior using a real-world component. As detailed in All About Circuits' DC theory textbook, a real voltage source is modeled as an ideal voltage source in series with an internal resistor.
The Setup: You are building a DIY high-lumen e-bike headlight powered by a single Samsung 30Q 18650 cell. The LED driver pulls a hard 15A.
The Numbers: The cell is fully charged to 4.2V. The datasheet lists the internal resistance ($R_i$) at 0.020 Ω (20 mΩ).
The Calculation:
- Voltage Drop ($V_{drop}$) = Current ($I$) × Internal Resistance ($R_i$)
- $V_{drop}$ = 15A × 0.020 Ω = 0.3V
- Terminal Voltage = 4.2V - 0.3V = 3.9V
At 3.9V, your LED driver operates perfectly. Now, imagine you swap in a cheap, no-name 18650 cell with an internal resistance of 0.150 Ω. The voltage drop becomes 15A × 0.150 Ω = 2.25V. Your terminal voltage plummets to 1.95V. The LED driver hits its undervoltage lockout, the light shuts off, and you are left in the dark despite the battery technically being 'full'.
Real-World Scenario Walkthrough: The Off-Grid Inverter Brownout
Theory is clean; the jobsite is messy. Here is a scenario that plays out constantly in off-grid solar installations.
1. The Setup
A DIYer wires a 2000W pure sine wave inverter to a single 100Ah AGM lead-acid battery in a cabin. They use 4 AWG copper wire for the 10-foot run between the battery and the inverter. The user plugs in a 1500W microwave and hits start.
2. The Numbers
A 1500W microwave actually draws closer to 2000W of DC input power due to the inverter's 85% efficiency curve at that load.
Current Draw ($I$) = Power / Voltage = 2000W / 12V = 166A.
The AGM battery has an internal resistance of roughly 0.015 Ω. The 4 AWG wire (20 feet round trip) adds another 0.005 Ω of resistance.
3. The Outcome
The moment the microwave magnetron kicks on, the inverter beeps loudly and shuts down, displaying a 'Low Voltage Cutoff' (LVC) error on the LCD screen. The battery monitor still reads 85% State of Charge (SoC).
4. What Went Wrong
Let's calculate the total voltage drop at 166A:
Total Resistance = 0.015 Ω (battery) + 0.005 Ω (wire) = 0.020 Ω.
Total $V_{drop}$ = 166A × 0.020 Ω = 3.32V.
The battery's resting voltage of 12.6V drops to 9.28V at the inverter's terminals. Most 12V inverters have an LVC hard-coded at 10.0V to protect the battery from over-discharge. The source wasn't empty; its internal impedance and the undersized wiring choked the voltage under high current.
The Fix: Parallel a second identical AGM battery (halving the battery internal resistance to 0.0075 Ω) and upgrade the cabling to 2/0 AWG welding cable to drop the wire resistance to near zero.
Frequently Asked Questions
Is a solar panel a voltage source or a current source?
A solar panel is technically a current source that is limited by voltage. Under full sun, it outputs a relatively constant current regardless of the voltage, until it approaches its Open Circuit Voltage ($V_{oc}$), at which point the current drops off a cliff. This is why Maximum Power Point Tracking (MPPT) charge controllers are required to constantly adjust the impedance match and pull maximum wattage from the panel.
Why does the utility grid never sag like a battery?
The grid is backed by massive, multi-ton spinning alternators at the power plant, stepped down through utility transformers with incredibly low internal impedance. The 'source' at your wall outlet can deliver thousands of amps of fault current before the voltage sags significantly, which is exactly why a dead short in your home wiring will instantly trip a 20A breaker or blow a fuse rather than just dimming the lights.
Can I wire different sources of electricity in parallel?
You can parallel identical voltage sources (like two matching 12V batteries) to double your Amp-hour capacity and halve your internal resistance. However, you must never parallel mismatched sources—such as a fully charged LiFePO4 battery and a depleted AGM battery. The higher-voltage source will violently dump current into the lower-voltage source, bypassing the load, generating massive heat, and potentially causing a lithium thermal runaway fire.
