Understanding Circulating Current in Parallel Battery and Transformer Systems

Circulating current is the unwanted flow of electrical current between parallel-connected power sources—like transformers, generators, or battery banks—caused by slight mismatches in voltage, phase angle, or impedance, rather than by the connected load. When you parallel two power sources to increase capacity, you expect them to share the load equally. Instead, if their output characteristics aren't perfectly matched, they end up pushing current into each other. This phenomenon changes the thermal profile of your installation, silently eating into your ampacity margins, wasting energy as heat, and in severe cases, triggering overcurrent protection or permanently degrading lithium cells.

The Physics of the Mismatch (and What It Isn't)

To understand circulating current, you have to look at the parallel loop itself. When two voltage sources are tied together on common busbars, they form a closed circuit with each other, independent of the main load. According to Kirchhoff’s Voltage Law, any difference in potential ($\Delta V$) between the two sources will drive current through the resistance of the connecting cables, lugs, and internal source impedance. Even a 0.1V mismatch across a 0.02Ω cable loop pushes 5A of dead current. This current does zero useful work; it simply generates $I^2R$ heat in your busbars and cables.

People commonly confuse circulating current with ground loops or eddy currents. A ground loop is typically a low-current, low-voltage issue where multiple grounding paths create noise in sensitive signal or data lines. Eddy currents are localized, induced current loops that form inside the solid iron cores of transformers and motors due to changing magnetic fields. Circulating current, by contrast, is a macro-circuit power delivery issue involving high currents flowing through the main DC or AC conductors between distinct parallel sources.

Worked Numeric Example: Paralleling 48V LiFePO4 Batteries

Let’s look at a common off-grid solar scenario: paralleling two 48V 100Ah server rack LiFePO4 batteries (such as the SOK or EG4 models) to feed a 5kW hybrid inverter. You wire them up, but Battery A has been sitting on a charger and is resting at 52.0V. Battery B was just pulled from a discharge cycle and is resting at 51.6V.

The total resistance of the loop connecting them includes the internal BMS MOSFET resistance, the 2/0 AWG copper cables, and the crimped lugs. Let's assume a realistic total loop resistance ($R_{loop}$) of 0.04Ω.

• Voltage Difference ($\Delta V$): 52.0V - 51.6V = 0.4V
• Circulating Current ($I_{circ}$): $\Delta V / R_{loop}$ = 0.4V / 0.04Ω = 10 Amps

Before the inverter even turns on, Battery A is dumping 10A into Battery B. Now, assume the inverter wakes up and pulls a 40A load. Ideally, each battery would supply 20A. But because of the 10A circulating current flowing from A to B, the actual current distribution becomes heavily skewed:

Battery A is doing 75% of the work. Its cables will run hotter, its BMS will experience higher internal heating, and it will hit its low-voltage cutoff long before Battery B is fully depleted. Over time, this asymmetric cycling drastically reduces the lifespan of the overworked pack.

Where You Meet This in Practice

Circulating current isn't just a battery problem; it shows up anywhere parallel power sources are used to scale capacity or provide redundancy.

• Solar and Off-Grid Battery Banks: As shown above, mismatched state-of-charge (SoC), degraded cells in one parallel string, or asymmetrical cable routing causes DC circulating currents. This is why Victron Energy's Wiring Unlimited guide heavily emphasizes symmetrical busbar wiring and top-balancing before connection.
• Parallel Transformers (Industrial/Commercial): When two AC transformers share a bus, differences in tap changer settings, percent impedance (%Z), or phase shift angles cause AC circulating currents. This wastes kVA capacity and can trigger ANSI 87T differential relays if not properly compensated.
• Parallel Inverters and UPS Systems: In dual-inverter setups, slight timing drifts in the PWM switching or output voltage calibration cause one inverter to push reactive power into the other, manifesting as AC circulating current that heats up the output chokes.

Mitigation Strategies and Hardware Fixes

Eliminating circulating current requires matching the electrical characteristics of the parallel paths as closely as physically possible.

For DC Battery Systems

1. Top Balance First: Never parallel lithium batteries with different voltages. Charge them individually to their maximum absorption voltage (e.g., 3.65V per cell for LiFePO4) and let them rest until they match within 0.05V before connecting the busbars.
2. Symmetrical Wiring: Avoid "daisy-chaining" cables from one battery to the next. Use a centralized busbar where the positive and negative cables to every battery are the exact same length and gauge (e.g., all 2/0 AWG, exactly 3 feet long). This ensures the cable resistance is identical for every parallel path.
3. Active Balancers or BMS Limits: For large banks, install a smart BMS that supports parallel communication (like Bantam Gold or EG4 Rev2) which actively throttles charge/discharge FETs to force equal load sharing, or use external DC-DC battery balancers.

For AC Transformers and Generators

1. Match %Z and Tap Settings: Ensure transformers have identical voltage ratios and closely matched percent impedances. If tap changers are used, they must be set to the exact same position.
2. Droop Compensation: In parallel generator setups, configure the Automatic Voltage Regulators (AVRs) and governors for "droop" mode rather than isochronous mode, allowing the machines to naturally share load without fighting each other's voltage setpoints.