Building a Custom Electric Bike battery pack often involves navigating the specifics of lithium-ion cells. A common question arises when sourcing components: can you mix 18650 Li-Ion batteries with different mAh (milliamp-hours) ratings? This article explores the implications of mixing battery capacities in your custom electric bike project, drawing from real-world experiences and expert insights to help you make informed decisions.
One e-bike enthusiast encountered this very issue while upgrading their battery for increased range. Initially, they constructed a 10s2p battery pack using 3.6v, 2200mAh 18650 cells, paired with a compatible 10s 36v Battery Management System (BMS). While functional for charging and discharging, the range proved limiting. To enhance capacity, they expanded the pack to 10s3p, incorporating 10 additional 3.6v, 2400mAh cells alongside the original 20x 2200mAh cells.
Upon recharging, the BMS performed as expected, cutting off at 40v. However, during subsequent rides, an unexpected issue emerged: the battery voltage dropped below the anticipated 36v cutoff, reaching as low as 34.8v. This hadn’t occurred before the introduction of the 2400mAh cells, prompting concern about whether mixing cells of different capacities was the root cause.
The user initially suspected cell imbalance. After recharging to the 40v cutoff, they sought advice before further testing, questioning the safety and effectiveness of this mixed configuration. Online research revealed conflicting opinions regarding mixing cells of different mAh and even brands, highlighting the need for clarity on this topic for custom electric bike builders.
While seemingly straightforward to increase capacity by adding higher mAh cells, mixing different capacities in series or parallel configurations can lead to complications. Cells with lower mAh will reach their discharge limit sooner than those with higher mAh. In a mixed parallel setup, this can lead to the lower capacity cells becoming over-discharged if the system continues to draw current, potentially reducing their lifespan and overall pack performance. Conversely, in a series connection, the pack’s capacity is limited by the lowest capacity cell group.
For optimal performance and longevity of your custom electric bike battery, it’s generally recommended to use 18650 cells of the same mAh rating, from the same manufacturer, and ideally from the same batch. This ensures more balanced charging and discharging across the entire pack, maximizing efficiency and safety for your custom electric bike. If you are considering mixing cells, thorough research and careful monitoring are crucial to mitigate potential risks and ensure a reliable power source for your electric bike.
