Electric Vehicle Battery Modification Warning: Series Connection of Two 72V LiFePO₄ Packs Harms Lifespan

Owners of electric scooters, motorcycles, and low‑speed EVs often try to combine battery packs to increase range or power. However, using the wrong connection method can drastically shorten battery life and create safety risks. A recent technical analysis by CHALONG FLY addresses a common question: "Is it safe to connect two 72V 60Ah LiFePO₄ battery packs in series?" The answer is clear – series connection significantly damages battery longevity and introduces serious hazards. For the same final specifications, using two 144V 30Ah packs in parallel or a single custom 144V 60Ah pack is far superior.

Series vs. Parallel: One Word Makes All the Difference

• Series increases voltage (voltages add, capacity stays the same).

• Parallel increases capacity (capacities add, voltage stays the same).

If you connect two 72V 60Ah packs in series, you get a 144V 60Ah system.
If you connect two 144V 30Ah packs in parallel, you also get 144V 60Ah.
Although the final numbers are identical, real‑world performance and safety are worlds apart.

Three Critical Flaws of the Series Solution

1. The “Weakest Link” Effect Is Amplified

In a series circuit, the same current flows through all packs. The overall performance is limited by the weakest pack. Even if two packs start with similar parameters, they will drift apart over time due to differences in internal resistance and capacity degradation. The weaker pack will be over‑discharged or over‑charged first, accelerating the deterioration of the other pack – a vicious cycle.

2. LiFePO₄’s Flat Voltage Plateau Makes Matters Worse

LiFePO₄ cells have a very flat discharge voltage (≈3.2V per cell). From full to nearly empty, the voltage changes very little. That means two 72V packs in series cannot be balanced simply by monitoring voltage. During charging, one pack may become dangerously overcharged while the other is still not full – a condition that can lead to thermal runaway and even fire.

3. BMS and Charger Incompatibility

Standard Battery Management Systems (BMS) are designed for a single pack. They cannot balance two packs connected in series. You would need an expensive custom 144V charger and a multi‑pack BMS system, which is technically complex and costly. Most aftermarket modifications lack this capability.

Why Parallel Is Much Better: Self‑Balancing Is Key

When you connect two 144V 30Ah packs in parallel:

• Automatic balancing – In a parallel circuit, the pack with slightly higher voltage naturally discharges to the lower‑voltage pack, like communicating vessels. This greatly reduces the matching requirement.

• Fault tolerance – If one pack fails or its BMS trips, the other pack still provides 144V, allowing the vehicle to continue at reduced power instead of stopping dead.

• Simple charging – Use a standard 144V charger directly on the parallel bus – no complex coordination needed.

CHALONG FLY's Professional Recommendations

For users who truly need a 144V 60Ah system, CHALONG FLY recommends the following priority:

1. Best: Order a single custom 144V 60Ah LiFePO₄ battery pack – No matching issues, one unified BSM, simplest and safest.

2. Second best: If you must use two packs, choose two 144V 30Ah packs in parallel – And be sure to:

   • Ask the supplier to match the two packs (similar voltage, internal resistance, and capacity).

   • Confirm that each pack’s BMS supports parallel operation (some do not).

   • Use a 144V charger for the parallel combination.

3. Strongly NOT recommended: Two 72V 60Ah packs in series – This approach brings constant balancing headaches, severe imbalance after only tens of cycles, and real risks of over‑charge fire or over‑discharge failure.

Safety Reminder from CHALONG FLY

Battery modification involves high voltage and large currents. Never attempt such changes without professional guidance. Always consult your battery supplier or a qualified electrical engineer to ensure compatibility between the packs, BMS, and charger. Even though LiFePO₄ is more stable than NMC, improper connections can still cause accidents. CHALONG FLY is committed to promoting safe and intelligent energy solutions for electric mobility.