| Electronics > Projects, Designs, and Technical Stuff |
| Please Criticise my Circuit and Layout |
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| Pentoad:
I did some tests on a breadboard today using just one of the comparators in the chip. I also put an LED across the load resistor. As it approached the balance voltage, I noticed the LED getting gradually brighter and the mosfet got roasting hot, probably because it was being switched at many kilohertz if not hundreds of kilohertz. With the mosfet fully on and not in this resonating zone, it stayed barely warm. I reckon the voltage drop within the breadboard was making the resonance a lot worse. I will just need to figure out resistor values for the hysteresis and that should fix it. |
| mikerj:
You will need considerable hysteresis on the comparator driving the MOFSET. All batteries have some internal impedance, so the voltage will drop as soon as you switch the MOSFET on and rise when you switch it off. You need to measure this for your application and then add suitable margin for cell aging and temperature effects. 10mv feels like it would be too small. Where does your 1mv accuracy spec come from? It seems rather pointless if you are balancing regular LiCoO2 cells. |
| Pentoad:
The 1mv accuracy is to keep this system as accurate as the management systems in electric cars. I made a system before which used a whole arduino pro mini per cell and that could only manage an 8mv accuracy as the ADC on it is only 10 bit. Another issue I've come up against is the power consumption of the module when not in use which could be a few milliamps. I was thinking of putting a small reed relay on each module which would be externally powered. The reed relay will just turn off the power supply to the comparator and potentiometers and as such will only need to handle a few ma of current. Will adding in hysteresis reduce the accuracy at all? |
| Berni:
Not if you add a large value capacitor in the positive feedback line in to integrate the error. |
| mikerj:
--- Quote from: Pentoad on October 17, 2019, 09:28:34 pm ---The 1mv accuracy is to keep this system as accurate as the management systems in electric cars. I made a system before which used a whole arduino pro mini per cell and that could only manage an 8mv accuracy as the ADC on it is only 10 bit --- End quote --- Are you certain that EVs require and achieve that level of accuracy? Renesas doesn't seem to think to. Remember that perfection is the enemy of good enough. --- Quote from: Pentoad on October 17, 2019, 09:28:34 pm ---Will adding in hysteresis reduce the accuracy at all? --- End quote --- It will stop it oscillating, so it will improve the accuracy. You aren't going to hit 1mV over temperature/life with your circuit anyway. |
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