The internal resistance of a battery, expressed in milliohms (mΩ), describes the opposition to current flow inside a cell created by its metallic current collectors and tabs, the ionic resistance of the electrolyte and separator, and the electrochemical processes at each electrode interface. AC internal resistance (ACIR) and DC internal resistance (DCIR) are equivalent resistance or impedance results obtained from two different test methods: ACIR from a small 1 kHz alternating signal, and DCIR from the voltage response to a DC current step sampled at a defined time. For example, the SHS180-314Ah hybrid solid-liquid cell used across MegSolid's 314Ah storage products, including the MegSolid 314Ah Hybrid Solid-State Battery, lists 0.18 ± 0.03 mΩ AC resistance and 0.4 ± 0.1 mΩ DC resistance on its cell specification sheet, with ≥ 94.5% cell energy efficiency at 0.5P.
This guide uses 314Ah storage cells, 51.2V packs, and 832V 1P260S strings as reference points instead of small consumer cylindrical cells. It covers the calculation formulas, the electrochemistry behind the ACIR and DCIR gap, practical test methods, how to interpret readings, and the series and parallel consistency issues relevant to our Solid-State Energy Storage Systems.
How to Calculate Internal Resistance of a Battery (Voltage Drop & Two-Step DCIR Formulas)
Engineers searching how to calculate internal resistance of a battery or how to find internal resistance of a battery usually need one of two DC methods: a single load step referenced to resting voltage, or a two-step current method that reduces part of the open-circuit voltage relaxation error.
1. Single-Step Method: Open-Circuit Voltage to Loaded Voltage
R_internal (Ω) = (V_OCV - V_load) / (I_load)
- V_OCV: rested open-circuit voltage before the load is applied (V).
- V_load: terminal voltage at a defined sampling time after applying a constant discharge current, such as 10 s (V).
- I_load: discharge current (A).
Illustrative pack example (hypothetical readings, not a MegSolid test record): a 51.2V pack rests at 52.80V, then sags to 52.10V after 10 s at 100A:
R_10s = (52.80V - 52.10V) / (100A) = 0.0070 Ω = 7.0 mΩ
That pack-level figure includes all 16 cells plus busbars, terminals, the fuse, and the BMS current path. For reference, MegSolid's MEG-Solid-512314FL1 (51.2V / 314Ah / 16.07 kWh) datasheet lists internal impedance ≤ 15 mΩ, and the MG 512314 movable unit product page lists ≤ 80 mΩ assemblage impedance. The public pages do not state the measurement method behind either figure, so neither limit should be treated as a DCIR value until the test method is confirmed.
2. Two-Step Current Method: Reducing OCV Relaxation Error
The single-step method assumes V_OCV is stable, yet a cell that has recently been charged or discharged keeps relaxing for minutes to hours. Two-step methods reduce, but do not fully remove, this error by measuring the voltage at two different currents in quick succession:
DCIR (mΩ) = (V_1 - V_2) / (I_2 - I_1) × 1,000
Illustrative 314Ah cell example at 50% SoC and 25^°C (hypothetical readings chosen to sit at the 0.4 mΩ nominal DC value on the SHS180-314Ah sheet):
- Step 1: I_1 = 31.4A (0.1C), terminal voltage V_1 = 3.2950V at the end of step 1.
- Step 2: I_2 = 157.0A (0.5C), terminal voltage V_2 = 3.2448V at the defined sampling time in step 2.
DCIR = (3.2950V - 3.2448V) / (157.0A - 31.4A) × 1,000 = (0.0502V) / (125.6A) × 1,000 = 0.40 mΩ
Engineering boundaries on DCIR calculations:
- Standard reference: IEC 62620 (secondary lithium cells and batteries for industrial applications) is the most relevant performance standard for stationary BESS. It describes an AC method applicable to cells and a DC method applicable to both cells and batteries. IEC 61960-3 contains comparable methods but targets portable applications. IEC 62619 is a safety standard and should not be cited as the resistance test procedure.
- Always state the method and sampling instant: a DCIR value is only meaningful together with its current steps, the time at which each voltage is sampled, SoC, and cell temperature. Writing the sampling time into the label (for example DCIR_1s, DCIR_10s, or DCIR_30s) avoids confusion. As a general trend, a shorter sampling time reflects mostly ohmic and fast interfacial contributions, while longer sampling times include a growing share of diffusion polarization.
- Current limits apply: test currents must stay within the cell or pack datasheet limits. The SHS180-314Ah sheet rates cycle life at 0.5P/0.5P, so a 1C test step on that cell should only be used if the manufacturer's test procedure permits it.
ACIR vs. DCIR: Why the Two Battery Internal Resistance Values Differ
On the SHS180-314Ah sheet, the nominal DC value (0.4 mΩ) is about 2.2 times the nominal AC value (0.18 mΩ). A simplified Randles-type equivalent circuit helps explain the gap:
Z = R_Ω + ≤ft[ (1) / (jω C_dl) ∥ ≤ft( R_ct + Z_W ) ]
In this simplified model, the double-layer capacitance branch (1/jω C_dl) sits in parallel with the series combination of charge-transfer resistance and Warburg diffusion impedance, and the whole interfacial network sits in series with the ohmic resistance. Real cells are often fitted with more elaborate circuits (for example, separate SEI and charge-transfer arcs, or constant-phase elements in place of ideal capacitors); the appropriate model should be selected from Electrochemical Impedance Spectroscopy (EIS) data for the specific cell rather than assumed universally.
- 1. Ohmic resistance (R_Ω): the near-instantaneous resistance of the current collectors, tabs, welds, separator pores, and bulk electrolyte. A 1 kHz AC milliohm meter reports a value dominated by this high-frequency ohmic response, because the double-layer capacitance offers relatively low impedance at that frequency. The 1 kHz reading is therefore close to, but not strictly equal to, R_Ω; inductive effects, contact geometry, and residual interfacial contributions can all influence it.
- 2. Charge-transfer and SEI resistance (R_ct): the kinetic barrier for lithium ions to shed their solvation shell, cross the Solid Electrolyte Interphase (SEI), and intercalate into the electrode. A DC step sampled after the interfacial time constant includes a large part of this term.
- 3. Diffusion impedance (Z_W, Warburg element): under sustained current, lithium concentration gradients build up inside active material particles and the electrolyte. Longer sampling times capture more of this polarization, which is why DCIR_30s usually reads higher than DCIR_10s on the same cell.
Using DCIR for Simplified Voltage Drop and Loss Estimates
DCIR is useful for first-order estimates of load voltage drop and resistive loss:
Δ V ≈ I · DCIR ; P_loss ≈ I^2 · DCIR
At 0.5C (157A) on a cell at the 0.4 mΩ nominal DC value, the estimated voltage drop is 157 × 0.0004 = 0.0628V and the estimated resistive loss is 157^2 × 0.0004 = 9.86 W per cell.
These are simplified equivalent-model estimates, not measured heat generation. Because a measured DCIR value contains polarization contributions that change with sampling time, and because real cell heat also depends on SoC, temperature, dynamic polarization, and reversible entropic heat, I^2 × DCIR should not be used as a substitute for a calibrated cell thermal model or calorimetry data. Likewise, a voltage-drop percentage at a single operating point cannot be used to derive the full charge-discharge energy efficiency of a cell; the ≥ 94.5% efficiency on the SHS180-314Ah sheet is a separately tested specification.
Test Internal Resistance of a Battery: What Is a Good Milliohm (mΩ) Reading?
For anyone searching test internal resistance battery what is a good reading, the short answer is: a good reading falls inside the manufacturer's datasheet tolerance when measured with the same method, SoC, temperature, and sampling time, and it sits close to the readings of sister cells in the same string. There is no universal "good" milliohm value across cell sizes, because resistance scales inversely with capacity and electrode area.
Table 1: Published Resistance Data & Theoretical Series Examples for 314Ah Storage Products
| Unit & Configuration | Published or Calculated Value | Test Method Stated? | Source & Status | Interpretation Note |
|---|---|---|---|---|
| SHS180-314Ah cell (3.2V, 314Ah, 1,005 Wh; used in MegSolid 314Ah products) | AC 0.18 ± 0.03 mΩ (0.15–0.21); DC 0.4 ± 0.1 mΩ (0.3–0.5) | AC and DC stated; DC pulse details to confirm | Cell specification sheet | Confirm DC current steps, sampling time, and SoC in the original datasheet and batch reports |
| MEG-Solid-512314FL1 pack (51.2V, 314Ah) | Internal impedance ≤ 15 mΩ | Not stated (not confirmed as AC or DC) | Official product datasheet | Includes cells, busbars, terminals, and BMS path |
| MG 512314 movable unit (51.2V, 314Ah) | Assemblage impedance ≤ 80 mΩ | Not stated (not confirmed as AC or DC) | Official product page | Upper limit for the assembled unit |
| 16S cell-only sum (SHS180-314Ah × 16, as in 51.2V packs) | AC 16 × 0.18 = 2.88 mΩ; DC 16 × 0.40 = 6.40 mΩ | Arithmetic only | Calculated from SHS180-314Ah sheet | Cell contribution only; excludes busbars, fuse, contactors, and BMS shunt |
| 260S cell-only sum (SHS180-314Ah × 260, as in Energon 1P260S) | AC 260 × 0.18 = 46.8 mΩ (39.0–54.6); DC 260 × 0.40 = 104 mΩ (78–130) | Arithmetic only | Calculated from SHS180-314Ah sheet | Cell contribution only; not a published Energon string resistance specification |
How to read Table 1:
- Cell-only sums are not system values: the 16S and 260S rows add up the SHS180-314Ah cell resistance only, so they are comparable with field readings only when taken under the same method, SoC, temperature, and sampling time. A complete pack or string also includes module busbars, inter-module cables, fuses, contactors, and current shunts, which add further resistance, and cell-to-cell values vary within the datasheet tolerance.
- Trend matters more than a single reading: record a Beginning-of-Life (BOL) baseline at commissioning under a fixed method (same current steps, sampling time, 50% SoC, and stable cell temperature), then track the percentage change over time.
- Empirical inspection approach: some operators use a DCIR rise of roughly 50% above the BOL baseline as a prompt for closer investigation, and a rise approaching 100% as a signal to assess remaining life. This is an empirical screening approach, not a universal industry replacement standard. Alarm and replacement decisions should follow the manufacturer's documentation, warranty terms, and a combined review of capacity (SOH), resistance trend, and cell-to-cell spread.
- Spread inside one string: a single cell reading well above its neighbors, measured under identical conditions, is often a stronger fault indicator than a uniform rise across the whole string.
How to Test Internal Resistance Correctly: 4-Wire Kelvin Measurement
Standard 2-wire multimeters cannot resolve a 0.18 mΩ cell, because the voltage drop across the test leads and probe contacts can be many times larger than the value being measured. Reliable testing uses 4-terminal Kelvin sensing:
- 1. Separate force and sense leads: one pair of leads injects the test current, while a second pair measures voltage at the cell terminal or weld pad. This mainly removes the voltage drop across the test leads and their associated contacts. The position of the sense points still affects the result, because any terminal, weld, or busbar section between the sense points is included in the reading. Keep probe placement identical between tests.
- 2. ACIR: use a 1 kHz AC battery milliohm meter with Kelvin probes. Readings are fast and repeatable, which makes ACIR useful for incoming inspection and cell sorting.
- 3. DCIR: use a programmable cycler or electronic load with Kelvin voltage sensing, logging voltage at defined sampling times after each current step.
- 4. Control the conditions: fix SoC, let the cell rest to thermal equilibrium, and record cell temperature with every reading. Resistance rises noticeably as temperature drops.
- 5. Respect voltage ratings: measuring modules or strings at hundreds of volts requires instruments, probes, and personal protective equipment rated for the full DC voltage. Many bench battery testers are limited to a few hundred volts and must not be connected across an 832V string.
How Internal Resistance Inconsistency Contributes to the BESS "Barrel Effect" & Cluster Circulating Current
In a large stationary battery, the absolute internal resistance of the battery cells matters, but so does how consistent it is from cell to cell and from cluster to cluster.
1. Series String Barrel Effect in a 260S Configuration
In the 1P260S string of the 261.24 kWh Energon cabinet (260 × SHS180-314Ah cells, 832V nominal, 676–936V DC), all 260 cells carry the same current. Consider an illustrative 157A (0.5C) current and one aged cell at 0.80 mΩ DCIR beside neighbors at 0.40 mΩ (hypothetical values for illustration only):
- 1. Possible earlier discharge cutoff: in the simplified model, the aged cell drops an extra 157 × (0.80 - 0.40) × 10^(-3) = 0.0628V under load. Near the steep end of the LFP discharge curve, that extra drop can bring it to its low-voltage limit before its neighbors.
- 2. Possible earlier charge cutoff: during charging, the same cell's terminal voltage sits about 0.0628V higher (V_term ≈ V_OCV + I · R), so it can reach the upper limit first.
- 3. Higher local loss: in the simplified I^2R estimate, the aged cell's resistive loss is 157^2 × 0.0008 = 19.7 W versus 9.86 W for its neighbors. Higher local temperature can accelerate SEI growth and further resistance increase.
Whether a higher-resistance cell actually triggers protection first depends on more than resistance. Each cell's actual capacity, SoC, and OCV position, the BMS balancing capability and strategy, cell temperature, and the operating current all determine which cell reaches a voltage limit first. A lower-capacity cell with normal resistance can limit the string just as much as a high-resistance cell. The usable energy of a series string is governed by the combined spread of capacity, SoC, and resistance, which is why cell grading, balancing, and thermal uniformity all matter.
2. Parallel Cluster Current Sharing and Circulating Current
When battery clusters are paralleled directly on a common DC bus, a simple resistive model illustrates two effects:
- Unequal current sharing: with equal open-circuit voltages and purely resistive behavior, current divides in inverse proportion to resistance (I_A / I_B = R_B / R_A). If Cluster A measures 104 mΩ and Cluster B measures 130 mΩ (illustrative values), Cluster A carries 130 / 104 = 1.25 times the current of Cluster B.
- Circulating current: if cluster voltages differ when contactors close, current flows from the higher cluster into the lower one, approximately I_circ = Δ V / (R_A + R_B). An illustrative 5V difference across 104 + 130 = 234 mΩ gives about 21A between clusters, even with zero load.
In practice, current distribution also depends on each cluster's OCV and SoC, dynamic polarization during load changes, cable and protection-device resistance, and the control architecture (for example, whether each cluster has its own DC/DC stage or PCS, and how pre-charge and contactor sequencing are managed). The resistive model above is a starting point for analysis, not a full system simulation.
For MegSolid systems, the Energon cabinet is specified with a 125 kVA AC rating and supports parallel operation of up to 10 units. These published figures do not by themselves define the final multi-cabinet connection topology, so cluster coupling and current-sharing design must be confirmed from the project single-line diagram and system documentation.
How MegSolid Addresses Internal Resistance at Cell, Pack & System Level
Table 2 separates what MegSolid publishes from what buyers should request in project documentation.
Table 2: Published MegSolid Data vs. Items to Verify in Procurement
| Engineering Layer | Published MegSolid Data | What to Request or Verify | Why It Matters for Internal Resistance |
|---|---|---|---|
| Cell | SHS180-314Ah (used in MegSolid 314Ah packs, Energon, and 5000INTL): 0.18 ± 0.03 mΩ AC, 0.4 ± 0.1 mΩ DC, ≥ 94.5% energy efficiency @ 0.5P | Original cell datasheet, DC test method and sampling time, and batch test reports | Low baseline resistance reduces voltage sag and resistive loss |
| Cell grading | Grading criteria not published | Sorting parameters (capacity, OCV, ACIR, self-discharge K-value, DCIR) and allowed spread per string | Tight matching reduces the series barrel effect |
| Pack | MEG-Solid-512314FL1 internal impedance ≤ 15 mΩ; MG 512314 ≤ 80 mΩ assemblage impedance (methods not stated) | Pack resistance test method (AC or DC), sense-point locations, and delivery acceptance limit | Captures busbar, weld, and terminal quality |
| Thermal | Energon liquid cooling, -20^°C to +55^°C (derated above 45^°C); cell temperature difference target of 1^°C to 3^°C to be confirmed | Controlled specification and test conditions for the temperature-difference figure | Even temperature helps keep resistance aging uniform across cells |
| System | Energon 261.24 kWh / 125 kVA, 1P260S, parallel up to 10 units; 5000INTL 5,015.9 kWh / 2,700 kW, smart liquid cooling | Cluster topology, single-line diagram, DC bus arrangement, and BMS resistance-trend monitoring | Determines exposure to cluster current imbalance |
Practical takeaways for EPC and O&M teams:
- 1. Ask for cell-level resistance data together with the test method and sampling time, not a single headline number.
- 2. Record ACIR and DCIR baselines at commissioning, then trend them under identical conditions.
- 3. Review outlier cells together with capacity, SoC, and balancing data rather than relying on average string resistance alone.
FAQ
Does the internal resistance of a battery change with temperature?
Yes. In the short term, lithium cell resistance rises as temperature falls, because electrolyte viscosity increases and charge-transfer kinetics slow down. Below 0^°C, DCIR can reach several times its 25^°C value, depending on chemistry, SoC, and test method. Warming a cell lowers its resistance temporarily, but prolonged operation at high temperature accelerates SEI growth and can permanently raise the baseline. Always compare readings at a stated temperature.
Why is ACIR lower than DCIR in a lithium battery?
ACIR uses a 1 kHz signal, and the result is dominated by the high-frequency ohmic response because slower interfacial and diffusion processes contribute little at that frequency. DCIR applies a sustained current step and samples the voltage after a defined time, so it adds charge-transfer, SEI, and diffusion polarization. On the SHS180-314Ah cell sheet, the nominal DC value (0.4 mΩ) is about 2.2 times the nominal AC value (0.18 mΩ).
Can I measure battery internal resistance with a multimeter?
Not for large-format storage cells. A 314Ah cell's resistance is a fraction of a milliohm, far below the lead and contact voltage drop of a 2-wire meter. Use a 4-wire Kelvin AC milliohm meter for ACIR, or a cycler or electronic load with Kelvin sensing for DCIR, and keep sense-point placement consistent.
At what internal resistance should a BESS cell or module be replaced?
There is no universal resistance threshold for replacement. Some operators use a DCIR rise of around 50% over the commissioning baseline as a trigger for closer inspection and a rise approaching 100% as a prompt to assess remaining life, but this is an empirical screening approach rather than an industry-wide standard. Replacement decisions should combine resistance trend, capacity fade (SOH), cell-to-cell spread, and the manufacturer's warranty criteria.
How do you calculate DC internal resistance using one load step?
Measure rested open-circuit voltage, apply a permitted constant discharge current, then record loaded terminal voltage at a defined sampling time. Calculate R = (V_OCV − V_load) / I_load. In the article’s illustrative 51.2V pack example, (52.80 − 52.10)V ÷ 100A = 0.007Ω, or 7.0mΩ at 10 seconds.
How do you calculate DCIR with two current steps on a 314Ah cell?
Take two voltage readings at defined currents and matched sampling conditions, then use DCIR (mΩ) = (V1 − V2) / (I2 − I1) × 1,000. The article’s hypothetical example uses 3.2950V at 31.4A and 3.2448V at 157.0A, yielding 0.40mΩ. This is an illustration, not a factory test record.
What is a good ACIR or DCIR reading for a 314Ah battery cell?
A good reading matches the exact cell datasheet tolerance at a comparable SoC, temperature and test method. MegSolid’s SHS180-314Ah cell sheet lists AC resistance of 0.18 ± 0.03mΩ and DC resistance of 0.4 ± 0.1mΩ. These values must not be applied as universal thresholds to other cells or complete battery packs.
Why should ACIR and DCIR be measured at a fixed SoC and temperature?
Internal resistance changes with temperature, SoC and the time after a current change. Comparing a cold cell at low SoC with a warm, rested commissioning reading can create a false aging signal. Keep current steps, sampling time, Kelvin probe position, cell temperature and SoC consistent when trending battery health.
Can you multiply cell ACIR or DCIR by 260 to obtain Energon cabinet resistance?
Multiplying a cell value by 260 gives a theoretical cell-only series sum. It excludes busbars, wiring, current shunts, fuses, contactors and other hardware. The source gives 46.8mΩ AC or 104mΩ DC for 260 nominal SHS180-314Ah cells as arithmetic illustrations, not published Energon cabinet resistance ratings.
How does one high-resistance cell affect a series BESS string?
One higher-resistance cell can show greater voltage sag during discharge, higher terminal voltage during charge, and more local resistive loss. It may reach a BMS limit earlier than neighboring cells, reducing usable string energy. The actual outcome also depends on capacity, SoC, temperature and balancing—not resistance alone.
Source 314Ah Cells & Storage Systems from MegSolid
Lower and more uniform battery internal resistance across the cells of a high-voltage string generally helps limit resistive loss, cooling load, and capacity mismatch, although the size of any efficiency improvement must be calculated for the specific operating current, temperature, and duty cycle. MegSolid provides cell-level AC and DC resistance data for the SHS180-314Ah cell used across its 314Ah products, pack-level impedance limits for its 51.2V residential units, and project documentation for its liquid-cooled C&I and utility systems.
Contact our engineering team for datasheets, test methods, and integration support:
- For 314Ah Hybrid Solid-State Cell Specifications: Review cell parameters and request resistance test documentation on our MegSolid 314Ah Hybrid Solid-State Battery page.
- For Liquid-Cooled C&I and Utility Storage: Explore cabinet and container platforms on our Solid-State Energy Storage Systems hub.