Can Lithium Battery Packs be Used in Parallel?
Many people have this question when using lithium battery: If a device's battery life is insufficient, can two lithium battery packs be connected in parallel to increase capacity and extend usage time? The answer is clear—lithium battery packs can be used in parallel, but absolutely not arbitrarily.
Many people think that parallel connection is simply "positive to positive, negative to negative," a simple connection, but they ignore the characteristics of lithium battery. At best, this can damage the battery and shorten its lifespan; at worst, it can cause overheating, bulging, or even thermal runaway and other safety hazards. Today, Xindun Power will explain the core points of parallel lithium battery pack connection in simple term.
What is the purpose of parallel lithium battery packs?
The core function of parallel lithium battery packs is only one: constant voltage, combined capacity. Simply put, it's like placing two buckets of different capacities side by side. After connecting the bottoms, the total water volume (capacity) is the sum of the two, but the water level (voltage) remains the same.
For example, connecting two 12V 100Ah lithium battery packs in parallel can result in a 12V 200Ah battery system, maintaining the same voltage while nearly doubling the power supply time. This method is particularly suitable for scenarios requiring long range and low voltage, such as RV energy storage, solar backup power, and electric vehicle range extenders. It easily improves range without modifying equipment interfaces.
The safety hazards of parallel lithium battery packs mostly stem from "parameter mismatch." For safe parallel connection, the following three core conditions must be met, missing any one will result in failure.
First, the parameters must be perfectly matched. This is the most crucial requirement, including voltage, capacity, internal resistance, discharge rate, and even brand and batch consistency. Before parallel connection, the voltage difference between each battery pack must be controlled to ≤0.1V, the capacity difference to no more than 5%, and the internal resistance difference to ≤5mΩ. Otherwise, a situation of "high voltage battery recharging low voltage battery" may occur. For every 1V increase in voltage difference, the current may surge by tens of amperes, damaging the battery management system (BMS) and even causing danger.
Mixing old and new battery, or battery from different brands, is strictly prohibited. It's like putting an elderly person and a young person to work at high intensity together, not only is it inefficient, but they can also hinder each other and accelerate battery aging.
Second, protective devices must be installed. When connected in parallel, a BMS (Battery Management System) or current limiting module with parallel protection must be used to prevent excessive mutual charging current. While using diodes may seem simple, it can lead to voltage drop and overheating, posing a long term risk. Additionally, it's recommended to install a fuse on each parallel branch to prevent cascading failure from a single battery group.
Third, the connection method must be standardized. A star connection is recommended, concentrating the positive and negative terminals of all battery groups at single point before leading out the main line. This avoids uneven current distribution and localized overheating caused by different wire lengths or poor contact. Connection points must be securely welded or crimped tightly, with proper insulation to prevent loosening and contact resistance.
Avoid these pitfalls: 3 things you absolutely must not do
1. Do not mix battery packs with different chemical system, such as lithium battery and lead-acid battery. Even if the voltage is the same, their different charging and discharging characteristics can damage each other and cause safety accidents.
2. Do not connect battery in parallel without a protection board. Without BMS protection, voltage imbalance will quickly damage the battery and may even cause thermal runaway.
3. Do not connect too many battery in parallel. In ordinary civilian scenarios, it is recommended not to exceed 3 parallel groups. If more are needed, a customized solution from the manufacturer is required to avoid current distribution imbalance.
Daily Maintenance: 2 things to extend battery life
Maintaining parallel lithium battery packs is also crucial. Check the connection terminals monthly for oxidation or loosening, and clean and tighten them promptly. Measure the terminal voltage of each battery pack under no load conditions. If the deviation exceeds 0.1V, troubleshoot the problem immediately. Also, avoid charging and discharging in high-temperature (>50℃) or low-temperature (<-20℃) environments to prevent battery performance degradation. Parallel connection of lithium battery packs is feasible, but the key is "parameter matching, adequate protection, and standardized connection." If you simply want to extend the battery life and meet the above conditions, parallel connection is a cost effective option, however, if the parameters are mismatched or there are no protection measures, it's better not to connect them than to risk operating them.
Xindun Power not only specializes in inverter but also lithium battery. Our lithium battery support 15 units operating in parallel, have a cycle life of 6000 cycles, feature intelligent lithium battery BMS protection, and boast an ultra-wide operating temperature range of -20℃ to 60℃. For more information about lithium battery and inverter, remember to follow us!