In battery testing, the measuring instrument must not significantly alter the quantity being measured. A voltmeter therefore requires very high internal resistance so it draws negligible current when connected in parallel. An ammeter requires very low internal resistance so it introduces minimal voltage drop when connected in series, allowing the battery’s true current output to be measured.
The ideal voltmeter behaves like an open circuit, while the ideal ammeter behaves like a short circuit. Their practical resistance values are chosen to minimize measurement error caused by loading and burden voltage.
Why Voltmeters Need High Internal Resistance
A Voltmeter Is Connected in Parallel
A voltmeter measures the potential difference between two points, so it is connected directly across the battery terminals or another circuit element.
Because parallel branches share the same voltage, the voltmeter creates an additional current path. Its current is approximately:
[ I_V = \frac{V}{R_V} ]
where (V) is the measured voltage and (R_V) is the voltmeter’s internal resistance.
High Resistance Prevents Measurement Loading
If the voltmeter’s resistance were low, it would draw significant current from the battery. This extra current would change the battery’s operating condition and could produce an inaccurate voltage reading.
A high-resistance voltmeter draws only a very small current, so it measures the battery voltage while causing minimal disturbance.
High Resistance Matters Especially for Battery Cells
A battery can be modeled as an ideal electromotive force (E) in series with internal resistance (r). When the voltmeter draws current, the measured terminal voltage becomes approximately:
[ V_{\text{measured}} = E - I_V r ]
A larger (R_V) produces a smaller (I_V), which reduces the artificial voltage drop (I_Vr).
This is particularly important for cells with relatively high internal resistance, where even a small measurement current can create a noticeable voltage error.
Why Ammeters Need Low Internal Resistance
An Ammeter Is Connected in Series
An ammeter measures circuit current, so it must be placed in series with the load or battery-testing circuit.
The entire circuit current flows through the meter. Any resistance inside the ammeter therefore becomes part of the circuit.
Low Resistance Minimizes Burden Voltage
The voltage drop introduced by the ammeter is:
[ V_A = I R_A ]
where (I) is the measured current and (R_A) is the ammeter’s internal resistance.
This drop is called burden voltage. If (R_A) is too large, the ammeter reduces the voltage available to the load and changes the current that the battery would otherwise deliver.
Low Resistance Preserves the True Operating Current
A battery’s current under load depends on the total circuit resistance, including the battery’s internal resistance and all external resistances:
[ I = \frac{E}{R_{\text{load}} + r + R_A} ]
Keeping (R_A) very small makes the measured current closer to the current that would flow without the instrument.
This is essential when evaluating discharge performance, power capability, efficiency, and battery internal resistance.
How Instrument Resistance Affects Battery Test Results
The Voltmeter Can Alter the Open-Circuit Measurement
An open-circuit voltage measurement is intended to determine the battery’s electromotive force with essentially no external load.
A finite-resistance voltmeter creates a small load, so the result is technically a terminal-voltage measurement under the meter’s load rather than a perfectly unloaded E.M.F. measurement.
The Ammeter Can Alter the Load-Line Measurement
During a discharge or pulse-current test, the ammeter adds a small series resistance.
That resistance can cause a small extra voltage drop and reduce the measured current, especially at high current levels. Precision battery testers therefore specify ammeter burden voltage and measurement accuracy.
The Errors Become More Significant in Certain Tests
Instrument effects are more important when:
- The battery has high internal resistance.
- The test current is large.
- The measured voltage is small.
- The required accuracy is high.
- The test uses sensitive pulse, impedance, or low-current measurements.
The measurement system should be selected relative to the battery and test conditions, not only by its nominal voltage or current range.
Understanding the Trade-offs
Resistance Cannot Be Treated as the Only Specification
A high-input-resistance voltmeter is desirable, but accuracy also depends on input leakage, resolution, noise, bandwidth, and input protection.
Similarly, a low-resistance ammeter is desirable, but its shunt accuracy, thermal behavior, current rating, and burden voltage are also important.
Very High Resistance Can Increase Sensitivity to Noise
High-impedance voltage measurements can be more susceptible to electrical noise, leakage currents, cable contamination, and electromagnetic interference.
Good battery test systems therefore combine high input resistance with appropriate shielding, filtering, guarding, and stable wiring.
Very Low Ammeter Resistance Creates Practical Constraints
Reducing ammeter resistance lowers burden voltage, but the sensing element must still withstand the measurement current and dissipate the associated heat:
[ P_A = I^2 R_A ]
At high currents, even a small resistance can produce significant heating and measurement error.
Do Not Confuse Battery Internal Resistance with Instrument Resistance
Battery internal resistance is a property of the cell and its construction. Instrument resistance is an added measurement-system parameter.
Accurate testing requires the instrument’s contribution to be negligible or precisely characterized so it is not mistaken for a battery characteristic.
How to Apply This to Your Battery Test
The correct instrument behavior depends on whether the test measures voltage, current, or both.
- If your primary focus is voltage or E.M.F.: Use a voltmeter with input resistance much higher than the battery and measurement circuit resistances so its current draw does not load the cell.
- If your primary focus is current output: Use an ammeter or current-sensing method with very low burden voltage so the sensor does not materially reduce the battery current.
- If your primary focus is internal resistance: Account for all instrument, cable, fixture, and contact resistances, because each can influence the observed voltage response.
- If your primary focus is high-current performance: Verify the ammeter’s burden voltage, thermal limits, and calibration at the actual test current.
- If your primary focus is low-current or open-circuit behavior: Use a high-input-resistance voltage measurement system and control leakage, noise, and settling time.
Choosing instrument resistance correctly ensures that the test system observes the battery’s behavior rather than creating it.
Summary Table:
| Instrument | Connection | Ideal Resistance | Why It Matters | Impact of Wrong Resistance |
|---|---|---|---|---|
| Voltmeter | Parallel | Very high (open circuit) | Draws negligible current, avoids loading the battery | Low resistance causes voltage drop and inaccurate readings |
| Ammeter | Series | Very low (short circuit) | Minimal burden voltage, preserves true current | High resistance reduces current and alters test results |
Optimize your battery testing with precision instruments from KINTEK. Our high-resistance voltmeters and low-resistance ammeters ensure accurate measurements across R&D and production. Contact our specialists today to enhance your testing workflow—get in touch and upgrade your lab's performance.