Terminal voltage is the actual voltage you can measure across the two terminals of a battery or power source when it’s connected to a circuit and current is flowing. It’s almost always lower than the battery’s rated voltage because some energy is lost inside the battery itself. A fresh 1.5V AA battery, for example, might only deliver 1.4V or less to your device under load. Understanding why this gap exists comes down to one concept: internal resistance.
Terminal Voltage vs. EMF
Every battery has an electromotive force, or EMF, which is the maximum voltage it can produce when nothing is drawing current from it. EMF is determined by the chemistry inside the battery, the specific combination of metals and electrolyte. If you measure a battery sitting on a shelf with no load attached, the reading on your multimeter will be very close to the EMF.
The moment you connect the battery to something that draws current, the voltage at the terminals drops. This happens because the battery has internal resistance: the materials inside it (electrodes, electrolyte, connections between cells) naturally resist the flow of current, and that resistance uses up some of the battery’s energy as heat. The voltage that’s left over, the portion actually delivered to your device, is the terminal voltage.
The Formula
Terminal voltage follows a straightforward equation:
V = EMF − I × r
- V is the terminal voltage (what you measure at the battery’s terminals)
- EMF is the battery’s full potential with no current flowing
- I is the current flowing through the circuit
- r is the internal resistance of the battery
The product I × r represents the voltage “lost” inside the battery. Two things make that loss bigger: higher current draw and higher internal resistance. A small flashlight pulling a modest current might barely dent the terminal voltage. A car starter motor pulling hundreds of amps will cause a noticeable drop.
If a load resistance (the device you’re powering) is connected, the total resistance in the circuit is the load resistance plus the internal resistance. The current flowing is then EMF divided by that total resistance. This means a battery with very low internal resistance can supply more current to the load and maintain a higher terminal voltage while doing it.
Why Terminal Voltage Drops Under Load
You’ve probably noticed lights dimming briefly when an air conditioner kicks on, or a car’s dashboard flickering when you turn the ignition. That’s terminal voltage dropping in real time. The sudden surge of current multiplies against internal resistance, pulling the terminal voltage down. Once the motor reaches running speed and current demand stabilizes, the voltage recovers.
This effect is sometimes called voltage sag. Common triggers include starting electric motors, energizing transformers, and any situation where a device briefly demands much more current than it will need during steady operation. Refrigerators, furnace fans, and power tools all cause momentary sags when they start up.
What Affects Internal Resistance
Internal resistance isn’t fixed. It changes over time and with conditions, which means terminal voltage changes too.
Battery age and wear. As a battery ages, its electrolyte loses water, the internal plates corrode, and chemical buildup (sulfation in lead-acid batteries) accumulates on electrode surfaces. All of these raise internal resistance. An old car battery might have several times the internal resistance of a new one, which is why aging batteries struggle to crank an engine even when they still hold a charge.
Temperature. Cold temperatures increase internal resistance. Chemical reactions inside the battery slow down, the electrolyte becomes more resistive, and the terminal voltage under load drops. This is why car batteries are more likely to fail on cold winter mornings: the engine needs high current to start, but the battery’s internal resistance is at its worst. Warm temperatures reduce internal resistance, though excessive heat accelerates long-term degradation.
Overcharging and deep discharging. Repeatedly overcharging a battery causes water loss and accelerates corrosion of the electrode grids, thinning them out and increasing resistance. Deep discharging to very low voltages causes heavy sulfation on the electrodes, which also raises internal resistance and can permanently reduce the battery’s ability to deliver current.
A Real-World Example: Your Car Battery
A fully charged 12V car battery at rest (engine off, no load) reads about 12.6 volts. That’s close to its EMF. When you start the engine and the alternator begins charging the battery, the voltage at the terminals rises to roughly 13.5 to 14.5 volts, because the alternator is now the voltage source pushing current into the battery.
During cranking, the opposite happens. The starter motor draws an enormous current, sometimes 200 amps or more. Even with the low internal resistance of a healthy car battery, multiplying that current by internal resistance produces a significant voltage drop. The terminal voltage can fall to 10 volts or lower for the few seconds it takes to start the engine. If the battery’s internal resistance has increased due to age or cold weather, the terminal voltage drops even further, and the engine may not start at all.
Charging vs. Discharging
The formula flips direction when a battery is being charged. During discharge, internal resistance causes the terminal voltage to be lower than the EMF. During charging, an external source has to push current into the battery against that same internal resistance, so the terminal voltage is actually higher than the EMF. The charger has to overcome both the battery’s natural voltage and the I × r loss going the other direction.
This is why battery chargers operate at voltages above the battery’s rated voltage. A 12V car battery charger typically outputs 14 to 15 volts to force current back into the cells. Charging stops (or switches to a trickle) when the terminal voltage reaches a set maximum, the charge cutoff voltage, to prevent damage from overcharging.
How to Measure Terminal Voltage
You can measure terminal voltage with any basic digital multimeter. Set the dial to DC voltage (marked with a solid line and dashes, or “DCV”). Insert the black probe into the COM jack and the red probe into the jack marked V. Touch the black probe to the negative terminal and the red to the positive terminal. Most modern multimeters will detect polarity automatically, so reversing the probes will just show a negative sign rather than causing damage.
To see the resting voltage (close to EMF), measure with no load connected. To see the terminal voltage under real conditions, measure while the battery is powering its normal load. The difference between those two readings tells you how much voltage is being lost to internal resistance at that particular current draw. If the gap is large, internal resistance is high, which often signals an aging or damaged battery.