How to prevent Circulating Current in Parallel Battery Banks

When engineers tie two lithium battery banks with slightly different voltages onto a shared bus, a current that does no external work starts looping between the banks — the industry calls it “circulating current.” It rarely trips a fault alarm, yet acts like a slow poison: the higher-voltage bank is forced to discharge, the lower-voltage bank is force-charged, and all the energy turns into I²R heat.

Field data shows that paralleling two 48V lithium banks with a 0.3V gap and a combined 30mΩ internal resistance can sustain a circulating current above 10A; heat dissipated in internal resistance alone exceeds 3W, and busbar losses add more visible temperature rise. When circulating current stays above 10% of rated current long-term, cells rack up shallow charge-discharge cycles and can lose 10%–15% of cycle life. More silently, the high bank “never fully charges” while the low bank “never fully discharges,” quietly shrinking usable capacity.

The closing transient is worse: slamming a full bank onto a depleted one can spike circulating current past 200A. Real cabinets have seen BMS MOSFETs blow and busbar insulation melt on contact. China’s GB/T 34131-2023 now mandates that BMS for paralleled clusters must flag circulating-current overrun and open the circuit within 0.3 seconds.

The industry is shifting from “passive balancing” to “electrical decoupling”: each bank hangs on the DC bus through its own DC/DC, or uses ideal-diode MOSFETs to block backfeed, cutting the DC-side loop at the root. For field crews, matching voltage within 0.1V and resting same-batch banks at the same temperature for two hours before paralleling is still the cheapest insurance.

The core function of parallel battery series blocking diodes is to use unidirectional conductivity to prevent circulating current (backflow) between batteries due to voltage differences, avoiding reverse charging of low-voltage batteries, overdischarging of high-voltage batteries, and energy loss. ‌‌