Charging current tests done and I’ve got results.
I used a simple test setup shown below. I’m supplying current from the BMS’ charger side with my bench top configurable power supply and on the battery side of the BMS is a configurable DC load. I don’t need to use an actual battery in the test setup like this. Voltage is not a critical variable in this test as I’m interested only in the thermal performance in relation to the current.
The tested currents were: 2, 3, 4, 5, 6 and 7 Amps. For each current test the BMS was left to run for couple of minutes, allowing the thermals to stabilize after which a FLIR image was taken with FLIR ONE. a FLIR image was taken from the bottom side of BMS on currents 5 and 7 Amp.
I also measured the voltage from the following points (shown below) to measure and calculate the losses happening specifically in the MOSFETs for each test current and after letting thermals stabilize.
Test current: 2 A
Voltage drop: 53 mV
Resistance: 26,5 mOhm
Power loss: 106 mW
Test current: 3 A
Voltage drop: 82 mV
Resistance: 27,3 mOhm
Power loss: 246 mW
Test current: 4 A
Voltage drop: 115 mV
Resistance: 28,8 mOhm
Power loss: 460 mW
Test current: 5 A
Voltage drop: 151 mV
Resistance: 30,2 mOhm
Power loss: 755 mW
Test current: 6 A
Voltage drop: 196 mV
Resistance: 32,7 mOhm
Power loss: 1176 mW
Test current: 7 A
Voltage drop: 260 mV
Resistance: 37,1 mOhm
Power loss: 1820 mW
Here are the results of all the current tests collected to a google sheet and a couple of charts drawn from them.
From the charts can be seen the effects of the increasing temperature alongside increasing current. Voltage vs. Current and Resistance vs. Current should technically be linear, but as the mosfets heat up their Rds(on) resistance increases, therefore producing more voltage loss as the current increases. This is then magnified even more in the Power Loss vs. Current, as it already rises in the square of the current.
Conclusion:
With these MOSFETs, I wouldn’t personally go above 5 Amps for the charging current, as things start to get real toasty above 5 Amps. Far cry from the 10 Amps I envisioned in the beginning. So what’s the game plan? I’ll throw a poll at the end of this post to get some feedback on what the forum members think is high enough as the supported charging current.
To achieve higher charging current the MOSFETs should be optimized for as low as possible Rds(on) resistance, to minimize conduction losses -> minimize heat generation. Switching speeds are not critical as the MOSFETs are not run with PWM, but are rather just closed or opened for extended periods. Other 2 important attributes then are package size and cost.
Once I get some votes on the poll below and see what people would want, I can then start to check possible options for the mosfets and then discuss about the cost with them. One consideration though is how high of a charge current should the Lite version support, if it’s meant to be the lighter, smaller and cheaper option.
How many amps do you think the BMS should support as a maximum charge current? (Lite version)
- 5 A is enough
- 6 A
- 7 A
- 8 A
- 9 A
- 10 A
?
