Safe Operation Guidelines for Direct Parallel Connection of Two Batteries

In battery applications, battery parallel connection is a common method to increase capacity, but directly paralleling two batteries with inconsistent parameters (e.g., voltage, internal resistance, capacity, age) can cause serious safety hazards. This article analyzes the hazards, fundamental principles, and correct operation methods to help users avoid battery damage or even risks like fire and explosion.

I. Fundamental Principle: Voltage Difference Triggers Uncontrolled Circulating Current

If there is a voltage difference (even as small as 0.05V~0.1V) between the open-circuit voltages of two batteries, the higher-voltage battery will "backfeed" current to the lower-voltage one, creating a circulating current. Due to the extremely small internal resistance of batteries (usually only a few milliohms), the circulating current can reach tens or even hundreds of amps, equivalent to the higher-voltage battery "short-circuit charging" the lower-voltage one. Without any protection mechanism to limit the current, it is completely out of control, which is the root cause of all hazards.

II. Specific Hazards: From Battery Degradation to Safety Accidents

1.Ultra-high circulating current causes instant high temperature: Unrestricted circulating current leads to severe battery heating, with connecting wires and terminals becoming hot or even burning, easily melting terminal posts or causing fires. For example, when two lead-acid batteries have a voltage difference of 0.1V, the circulating current can exceed 50A, and the instant heat generated is enough to burn the battery case.

2.Mutual backfeeding accelerates lifespan decay: The higher-voltage battery continuously discharges externally (overdischarging), leading to overloaded aging; the lower-voltage battery is forced to charge rapidly with high current (overcharging), causing water loss and gas evolution (for lead-acid batteries) or heating and cell aging (for lithium batteries). Both batteries damage each other and eventually become scrap together.

3.Lithium batteries without protection are prone to thermal runaway: Lithium batteries have smaller internal resistance, so the circulating current caused by voltage difference is larger and the heat generated is more intense. Without a protection board to limit the current, the internal separator may be damaged by high temperature, triggering thermal runaway and eventually causing fire or explosion. Even lead-acid batteries will bulge or become scrap due to overcharging.

4.Mixing new and old batteries is the most hazardous: When mixing new and old batteries, new batteries have higher voltage and smaller internal resistance, while old batteries have lower voltage and larger internal resistance. After paralleling, the new battery will frantically feed current to the old one, causing the old battery to bulge severely due to overcharging, and the new battery to age rapidly due to continuous discharging. Both batteries' lifespans are halved.

III. Special Risks of Lithium Batteries

Lithium batteries (especially ternary lithium) are more sensitive to voltage difference and circulating current. Even a voltage difference of only 0.03V can cause the circulating current to exceed the battery's rated current, leading to a sudden rise in internal temperature. Without a protection board for isolation, the risk of thermal runaway increases exponentially, potentially causing an explosion. Therefore, lithium batteries are strictly prohibited from direct parallel connection without protection.

IV. Extreme Conditions for Bare Parallel Connection (Rare Cases)

If bare parallel connection is absolutely necessary, all the following conditions must be met simultaneously:

1.Same brand, same model, same capacity, same batch;

2.Close factory time and consistent age;

3.Completely consistent open-circuit voltage after standing (error <0.02V);

4.Basically consistent internal resistance;

5.Both batteries are charged to the same voltage before paralleling.

Even if these conditions are met, professional practice still recommends adding a voltage balancing module for isolation protection, and bare parallel connection is not recommended.

V. Correct Safe Operation Guidelines

Install protection devices: A protection board + voltage balancing module must be installed to achieve current limiting, voltage balancing, and isolation, preventing circulating current and overcharging/overdischarging.

Charge first, then parallel: Charge each battery separately, then parallel them after the voltage is consistent when standing for 2~4 hours.

Avoid mixing: Absolutely prohibit mixing new and old batteries or paralleling batteries with different capacities (e.g., 10Ah and 20Ah batteries).

Regular inspection: After paralleling, regularly measure the voltage of each battery. If a voltage difference (>0.05V) occurs, disconnect immediately and inspect.

In summary, direct parallel connection of batteries seems simple, but it hides huge risks. Understanding key factors like voltage difference, circulating current, and internal resistance, and following the principles of "adding protection, charging first, and not mixing" are essential to ensure battery safety and extend lifespan. For lithium batteries, protection boards and voltage balancing modules are absolutely necessary—never take chances.