Range Drop and an Early Charger Cut-Off
A three-year-old 60 V e-rickshaw used for school transport has covered about 28,000 km. Its owner reports that range has fallen from about 90 km to 45 km and that the charger now switches to green after only 40 minutes, although the dashboard shows 100%. The pack is a 20-series LiFePO4 unit with a Bluetooth BMS. During a test run, the BMS log shows one cell group at 3.05 V under load while the other 19 groups sit between 3.30 V and 3.36 V. At rest, the low group recovers to 3.22 V. The pack case is not swollen. The charger nameplate says 73 V, 10 A. The owner has read online that a “BMS reset” or a higher-voltage charger can restore range, and he asks the workshop to make the pack accept more charge before the school season starts. The workshop has a compatible load bank but no cell-level replacement module in stock. A replacement pack costs about ₹55,000; the owner wants a cheaper fix.
1. What is the most likely explanation for the early charger cut-off and reduced range, and what tests would you perform before recommending a repair?
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The pattern points to one weak/high-resistance cell group, not a charger fault. During charge, the other 19 groups reach the BMS high-voltage cut-off first, so the charger stops before group 7 is full. During discharge, group 7 collapses early under load and the BMS reaches low-voltage cut-off while the other groups still have charge. The recovery to 3.22 V at rest is typical of a high-resistance group, not proof that it is healthy.
Tests: inspect the BMS sense harness and connector for a loose pin or corrosion; measure each group at rest with a calibrated multimeter and compare it with the BMS readings, because a sense-wire error can mimic a bad cell; perform a controlled full charge with the correct 73 V charger and log group voltages; then run a load-bank or road test and log group voltages, temperatures and cut-off point; check charger output and termination. If the sense wires are correct and group 7 diverges, perform a capacity test and replace the module or pack per OEM procedure. Do not charge a group below the OEM minimum.
2. Why does the dashboard show 100% when the pack is clearly not full, and how would you verify state of charge for the customer?
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Many dashboards estimate SOC from pack voltage, and LiFePO4 has a very flat voltage curve. After the charger stops, surface charge and a BMS reset can make the display jump to 100%. It is not a coulomb-counted SOC.
To verify, charge with the correct charger until the BMS terminates, rest the pack for 1–2 hours, measure and record all group voltages, then use the BMS’s accumulated Ah/coulomb count or perform a controlled discharge at a known current and time it. Compare usable Ah with the 60 Ah rating and note which group reaches cut-off first. Record ambient temperature and load because they affect results. Explain to the customer that 100% on the display means “voltage looks high,” not “60 Ah available.” If the BMS has a calibration or full-charge reset procedure, follow the OEM procedure; do not reset SOC blindly, because that hides the weak group and may leave him stranded.
3. The owner asks you to fit a 72 V charger or “boost” the pack to get more range. How do you respond, and what options do you offer?
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Refuse the 72 V charger and any BMS bypass. The pack is 20-series LiFePO4 with a full-charge voltage near 73 V; a higher-voltage or unregulated charger can overcharge groups, trip the BMS, damage the controller or start a fire. The charger must match chemistry, series count, maximum voltage, current, connector and BMS communication, and it should be a certified unit. “Boosting” by bypassing the BMS removes cell-level protection and is unsafe.
I would explain the diagnosis, show the group-voltage log, and offer three honest options: (1) supervised balancing and a capacity test to see whether the weak group can recover; (2) replace the faulty module or pack with an OEM-compatible unit, with a written warranty; (3) if the customer declines, document the advice and do not release the vehicle as fit for school transport. I would also confirm the existing charger is the correct 73 V unit and that the BMS is not misreporting because of a sense-wire fault.