HV Battery Fusing
- Taiping Li
- Adam Marchand
Summary
| Application | Nominal Current | Peak Current | Fuse | Supplier |
|---|---|---|---|---|
| HV battery pack | 50 - 100 A | 170 A | 175 A: EV25-175 | Eaton Bussmann |
| HV Charger | 23 A | 23 A | 50 A: EV10-50B | Eaton Bussmann |
| Solar Master | < 5 A | 7 - 8 A | 12 A: KLM-12 | Eaton Bussmann |
| Motor Controller | TBD | TBD | ||
| DC-DC | < 3 A | 5 A | 5 A: BK/PCE-5-R | Digikey |
Battery Pack Fusing
Fuse Calculations
Using the calculations specified in this application guide from Eaton, the DC current required for our HV system is as follows:
Nominal and Maximum Current
Our motors are rated to 3.75 kW of nominal power and a maximum of 7.5 kW each. This means that with a nominal cell voltage of 3.635 V, the pack voltage is 130.86 V, and this gives a current of 57.3 A and 114.6 A. Assume that the maximum current drawn is with both motors at full power when the pack voltage is the lowest, so (7.5kW + 7.5kW) / (36 cells * 2.5V/cell) = 166.7 A = 167 A. Adding the maximum power drawn by the LV system, 1.67A assuming the lowest voltage and 150 W drawn by the DC-DC converters, the nominal current is around 116 A and the maximum expected current draw from our pack is approximately 169 ADC.
Temperature Derating
Assume that the ambient temperature is the highest operating temperature of the cells at 60 degrees C. The temperature correction factor is therefore 0.8.
Temperature Connection Derating
Type 1 bus bars (i.e. fishbones between cells) are 138mm long, with a cross section that is 1.5875mm (1/16") thick and 11mm wide.
Type 2 bus bars (i.e. connections between fishbones and modules) are 132.5mm long, with a cross section of 1.5875mm (1/16") thick and 6.8mm wide.
Since the bus bar sizes haven' been determined, assume that this factor is 1.
Cooling Air Correction
The battery box will be vented, but it is hard to determine the speed of the air flowing through the fuse link. A rough estimate with a 120mm fan at 50 CFM is 0.5m/s, so we take the 1 m/sec value of 1.05 for safety purposes.
High Altitude Derating
The highest part of a route by bike between Omaha, NE and Bend, OR on google maps is 3000 m above sea level. Using the formula given, the altitude correction factor is 0.95.
Enclosure correction Factor
The battery box will be fairly well ventilated, so this factor is not used.
Cyclic Loading
The recommended safety margin is 1.3 in EV applications as given by the datasheet. However because we're expecting a smoother drive profile compared to normal EVs with less loading, a value of 1.2 will be used in this case.
Total Current
The derated nominal current calculated by using the given formula is 86 A for 3.75 kW, 173 A for 15 kW, and an absolute maximum of 254 A. Going off the nominal value of 173 A, the next available fuse is a 175 A. The EV series by Eaton Bussmann is specially designed for high voltage battery and charging systems.
Checking the I2t curve of the fuse to make sure Imax < 50% * It, the maximum current the fuse can safely handle without melting is 175 A for 30 s, 263 A for 4 s, and 350 A for 0.4s.