More Data
The new BMS setup is working very well and is a great simplification from the previous setup. The new chargers (2 x 600w 48v chargers in series) deliver about 12A of charging which equates to approximately 7.5 miles of charge per hour. Additionally, the Bluetooth connection from the BMS monitoring Arduino UNO is reliable and fast.
Currently, there are three levels of protection for the battery:
- Chargers - The chargers should terminate if the pack voltage exceeds about 115 (32 x 3.6v), but they do not monitor individual cells, so if there is a cell imbalance, it possible that some cells could be overcharged while others are low.
- BMS - The chargers are routed through the BMS which is set to 3.5 volts for the High Voltage Cutoff. If a cell exceeds this threshold, the BMS will turn off the Charging FETS, disconnecting the chargers from the pack.
- BMS Monitor Arduino Uno - This Arduino communicates with the BMS over Bluetooth (HC-05), requesting every 2 seconds, a voltage level for all 32 cells. If any cell reports greater than 3.495V, this Arduino will turn-off a contactor that physically disconnects the chargers. Only a power cycle will permit the charger contactor to be reenergized. The Arduino also has an 8-second watch-dog timer running along with a 30 second timeout timer for communications with the BMS. If the BMS stops responding for 30 seconds, the Arduino will shut-down charging and turn on a buzzer.
Currently, the 220v charging power is on a wall-timer that is manually set based on the SOC of the pack (1-4 hours).
The BMS Arduino is monitoring the cells for low-voltage events. If any cell drops below 2.5v, the buzzer will sound and the IGN red indicator on the Dash will light, alerting the driver. The digital dashboard display can also show the status of the back - High and Low cell number, and max delta voltage across the cells, total pack voltage, temperature of the BMS. The BMS is set to not permit charging below 5℃ as an added protection measure.
The BMS enters into a balancing routine after the charging is complete, attempting to bring the cells to within 10mV of each other. A series of LEDs on the BMS flash during the balancing phase. So far, the pack reports below 10mV following charging, so the balancing appears to be working as expected.
Test Drive
With the system fully operational, I headed out around town to test the pack. Running now in second gear, Jane accelerates great up to over 40MPH. After 21 miles, it took right at 3 hours to charge at 1200W, equating to 3600Wh for 21 miles or about 170 Wh/mile efficiency. The pack is now 26 x 60 AH cells & 6 72AH cells at 3.2v each --> 6.1 KWH (total), so using an 80% safe usable capacity, the theoretical range is 29 miles. Another longer drive will be needed to confirm this.
The other benefit of being able to stay in 2nd gear for most roads around town, is the motor current draw is only high (>200A) for a few seconds during acceleration, resulting in a cooler running motor. So far the motor has not risen about 70℃ and stays below the 5,000 RPM spec when driving around town. Regenerative breaking is always very strong in 2nd.
