One holds voltage steady no matter what you connect to it. The other holds current steady instead. Neither ideal actually exists — and the gap between ideal and real is exactly why your headlights dim when you crank the engine.
An ideal voltage source maintains a fixed output voltage regardless of how much current the load draws — until it physically can't, because it's hit its maximum current capability. An ideal current source does the opposite: it maintains a fixed output current regardless of the load's voltage or impedance — until it hits its compliance voltage, the maximum voltage it can produce to force that current through. Real sources are neither. Every real voltage source has internal impedance that makes its terminal voltage sag under heavy load; every real current source has some finite output impedance too, which is exactly why the "ideal" version of each is a modeling convenience, not something you can buy off a shelf.
Plot terminal voltage on the vertical axis and output current on the horizontal axis — the standard "V-I characteristic" used to describe any electrical source. A voltage source's characteristic is a horizontal line: voltage stays flat as current changes, because that's the entire point of a voltage source. A current source's characteristic is a vertical line: current stays flat as voltage changes, because that's the entire point of a current source. Neither line extends forever — each source has a limit where its ideal behavior breaks down.
For a voltage source, that limit is a maximum current — draw more than the source can supply and terminal voltage collapses. For a current source, that limit is a maximum voltage — called the compliance voltage — ask the source to push its rated current through more impedance than it has voltage headroom for, and the current it can actually deliver starts to fall short.
Model a real voltage source as an ideal voltage source in series with a small internal (Thevenin) resistance. Draw more current, and more voltage gets dropped across that internal resistance instead of reaching the terminals — hence the droop. Model a real current source as an ideal current source in parallel with a large internal (Norton) resistance. Some of the source's current always leaks through that internal resistance instead of the load, and as load voltage rises toward the source's maximum output voltage, more of the set current gets diverted internally instead of delivered — hence the sag past compliance. One droop comes from a series impedance, the other from a parallel one, but the underlying story — an ideal source plus one real-world impedance that steals a share of what the ideal source produces — is identical.
Only approximately, and only at light load. Every real battery has genuine internal resistance — from its chemistry, electrode surface area, and internal construction — which behaves exactly like a small resistor in series with an ideal voltage source. At light load (a phone, a small sensor), the current drawn is tiny enough that the voltage dropped across that internal resistance is negligible, and the battery behaves almost exactly like the ideal voltage source it's usually treated as. Cranking a car's starter motor draws a very large current for a couple of seconds— hundreds of amps — and that current through the battery's internal resistance produces a real, measurable voltage drop at the terminals. That's the entire reason headlights dim when you crank the engine: the battery's terminal voltage genuinely sags under that heavy load, exactly as the drooping voltage-source curve above predicts, and the headlights are simply showing you the lower voltage now available.
Explains the difference between an ideal voltage source (constant voltage regardless of load current, until its current limit) and an ideal current source (constant current regardless of load voltage, until its compliance voltage limit) — and why every real source, including a battery, sits between these two ideals because of its own internal impedance.
Maintains a fixed terminal voltage no matter what load current is drawn, up to the maximum current the source can physically supply. Beyond that current limit, terminal voltage collapses because the source has run out of ability to hold voltage while sourcing more current. Modeled as an ideal voltage source plus a small series (Thevenin) internal resistance in any real device.
Maintains a fixed output current no matter what load voltage or impedance it sees, up to the compliance voltage — the maximum voltage the source can produce to force that current through the load. Beyond compliance voltage, output current falls short because the source has run out of voltage headroom. Modeled as an ideal current source plus a large parallel (Norton) internal resistance in any real device.
Every real voltage source (batteries, most power supplies at moderate load) has some internal resistance in series with its ideal output, causing terminal voltage to sag as load current increases. Every real current source (many op-amp current mirrors, constant-current LED drivers, some lab bench supplies in current-limit mode) has a finite compliance voltage and a finite (not infinite) parallel output impedance. Engineers pick whichever ideal model — voltage source or current source — better approximates the real device's dominant behavior over the range it will actually operate in.
The starter motor draws very high current (often hundreds of amps) for a couple of seconds, and that current flowing through the battery's own internal resistance produces a real voltage drop at the battery terminals. The headlights are directly showing you that lower terminal voltage — the battery is behaving like a real voltage source sagging under heavy load, not an ideal one.
Compliance voltage is the maximum voltage a current source can produce at its output terminals while still forcing its set current through the load. If the load impedance requires more voltage than the compliance limit to sustain that current, the source can no longer maintain it and delivered current falls below the set value.
A battery is designed and used as a voltage source — it is intended to hold a roughly constant terminal voltage across a wide range of load currents. It is not an ideal voltage source, however, because its internal resistance causes real, measurable voltage sag under heavy load, most noticeably during high-current events like engine cranking.
Because a real device usually behaves much more like one ideal type than the other over its intended operating range — a lab bench power supply behaves like a voltage source at light-to-moderate load and like a current source once it hits its current limit. Choosing the ideal model that matches the dominant behavior in the range you care about massively simplifies circuit analysis without meaningfully sacrificing accuracy.
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