In renewable energy systems and energy storage applications, it’s common practice to connect multiple battery packs in parallel to increase overall capacity and output power. However, users often encounter an issue: the current is not equally shared between the packs during charging or discharging. Why does this happen—and how can it be resolved?
Let’s explore the causes in detail and discuss effective solutions.
Part 1: Intrinsic Differences Between Battery Packs
1. Inconsistent Battery Capacities
Batteries from different manufacturers, batches, or with varying usage histories can have different actual capacities. A battery with a higher capacity tends to discharge or charge more aggressively, naturally carrying a larger portion of the current.
Example:
If one pack is 100Ah and another is 80Ah under the same load, the 100Ah pack will supply more current, leading to imbalance.
2. Differences in Internal Resistance
Internal resistance significantly affects current flow. Batteries with lower internal resistance allow current to pass more easily, while those with higher resistance restrict it.
Case Study:
Two batteries with internal resistances of 0.1Ω and 0.2Ω connected in parallel will not share current equally. The 0.1Ω battery will carry a higher current load.
3. Aging and Degradation
As batteries age, their capacity decreases, and internal resistance increases. Older batteries naturally lag behind newer ones in performance and will contribute less to the total current.
Part 2: External Connection Factors
1. Unequal Cable Resistance
Different lengths or thicknesses (gauges) of connection cables cause unequal voltage drops, which results in uneven current flow.
Real-World Example:
If one connection cable has a resistance of 0.05Ω and another 0.1Ω, the lower-resistance line will allow more current to pass, overloading that battery.
2. Poor Terminal Connections
Loose terminals or oxidation buildup increases contact resistance. This disrupts normal current flow, leads to heat buildup, and reduces system efficiency.
Part 3: Environmental Influences
1. Temperature Variations
Battery performance is sensitive to temperature. Higher temperatures improve conductivity and reduce internal resistance, while lower temperatures restrict current flow.
Example:
A battery pack operating at 30°C will have significantly higher discharge capability than one at 15°C.
2. Uneven Load Distribution
If the system's load isn't well-balanced across all battery packs, some packs may be overburdened, resulting in uneven wear and performance.
Part 4: Solutions — How to Ensure Equal Current Sharing?
To improve current distribution and overall system performance, consider the following practical steps:
✅ Use a Common Busbar
Connect all battery packs to a central busbar for both positive and negative terminals. This ensures each pack has an equal opportunity to contribute current to the load.
✅ Standardize Cable Lengths and Gauges
Use equal-length and same-gauge wires for all connections. This minimizes differences in line resistance and ensures fair current sharing.
✅ Match Battery Packs Carefully
Whenever possible, use battery packs with similar capacities, state of health (SOH), and age. Avoid mixing old and new batteries in the same system.
✅ Monitor and Balance Temperatures
Ensure uniform ambient conditions for all packs, or install thermal management systems to prevent temperature-induced imbalances.
Conclusion
While connecting battery packs in parallel is a straightforward way to scale up your system, inconsistent current sharing is a real challenge that can shorten battery life and reduce system efficiency. By understanding the root causes—from internal battery differences to environmental conditions—and applying smart engineering practices, you can build a safer, more reliable, and better-performing energy storage system.
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