Connecting batteries in parallel is a common technique used to extend the operational duration of a power system. This arrangement involves linking the positive terminal of one battery directly to the positive terminal of the second, and similarly connecting all the negative terminals together. The primary goal is to increase the total energy reserve available to the load without altering the required system voltage. Understanding the physical and electrical consequences of this specific connection is crucial for successful and safe implementation.
Impact on Voltage and Total Energy Capacity
The most fundamental result of connecting two batteries in parallel is that the system’s terminal voltage remains unchanged. For example, if two fully charged 12-volt batteries are connected in parallel, the total potential difference across the combined terminals will still be 12 volts. This is because the voltage is determined by the specific chemistry and cell design, not the number of units connected this way.
Conversely, the total energy capacity of the system is the sum of the individual capacities. Capacity is typically measured in Amp-hours (Ah), which indicates how much current a battery can supply over a specific period. If two 100 Ah batteries are placed in parallel, the resulting system possesses a total capacity of 200 Ah.
This additive capacity directly translates into an extended run time for the connected load. A device that normally draws 10 amperes from a single 100 Ah battery could theoretically operate for twice as long from the combined 200 Ah parallel bank. This increase in the total stored charge is the main functional benefit driving the use of parallel battery configurations.
The total energy stored, measured in watt-hours (Wh), also becomes additive, calculated by multiplying the system voltage by the total Amp-hour capacity. The parallel configuration essentially creates a single, high-capacity power source capable of delivering the required current for a longer duration at the original voltage.
Understanding Current Flow and Internal Resistance
A parallel connection significantly increases the available current delivery capability to the external load. Each battery contributes a portion of the total current required, and the sum of these contributions equals the total current drawn by the external circuit. This arrangement means that the load is shared, which reduces the electrical stress and heat generation on any single battery unit during periods of high current demand.
The precise manner in which this load is shared is governed by the internal resistance (IR) of each battery. Internal resistance is an inherent property that opposes the flow of current within the battery itself. When batteries are connected in parallel, the unit with the lower internal resistance will naturally supply a disproportionately larger share of the total current demanded by the load.
Differences in internal resistance, even slight ones, can lead to uneven discharge rates between the batteries over time. This uneven sharing causes the battery with the higher resistance to be underutilized and potentially age prematurely. Furthermore, if the batteries are not perfectly matched in state of charge or open-circuit voltage, a circulating current, often called a balancing current, can flow directly between the batteries themselves.
This circulating current flows from the higher-voltage battery to the lower-voltage battery until their voltages equalize across the parallel bus. If the initial voltage difference is too great, the resulting current flow can be substantial, generating heat and wasting stored energy. Minimizing the differences in internal resistance and ensuring batteries are closely matched is paramount for maximizing the lifespan and efficiency of a parallel bank.
Safety Considerations for Parallel Connections
Connecting batteries in parallel introduces safety considerations, particularly regarding the matching of the individual units. A hazard arises when attempting to connect batteries with different nominal voltages, such as a 6-volt battery and a 12-volt battery. This fundamental mismatch results in an uncontrolled current surge from the higher-voltage battery into the lower-voltage one, causing rapid heating.
Connecting batteries of different chemistries, such as a lead-acid battery and a lithium-ion battery, should never be done. These chemistries have widely disparate charging requirements and discharge characteristics, and the resulting current conflict can lead to thermal runaway or severe overheating. Even batteries of the same type and voltage must have a similar State of Charge (SoC) before the parallel connection is made.
A large difference in SoC means one battery will attempt to rapidly charge the other upon connection, generating excessive heat and potentially damaging both batteries permanently. To mitigate these risks, proper wiring and fusing are required for any parallel setup.
Fusing and Cabling
Each battery’s positive terminal should include a correctly rated fuse to isolate it from the bank in case of a short circuit or an external overcurrent event. Fuses protect the wiring and the batteries from catastrophic failure by interrupting the circuit before excessive heat can build up. Using appropriately sized cables is also necessary to safely handle the increased maximum current capacity of the combined battery bank.