retepsnikrep
Senior Member
I'm fairly new to the picaxe but not new to basic programming or electronics. I'm an EV and hybrid vehicle enthusiast. My current project is to build a picaxe based bms for some large Lithium cells in a series high voltage (144v 50ah) pack. I'm familiar and happy with all the risks of HV battery projects.
My ideas are as follows.
Use an individual 08M pic on each cell to monitor
1) Cell voltage
2) Cell temperature
the 08M will also control an individual bypass resistor so that when the cell reaches a set voltage say 4.2 it turns on and bypasses a set current.
The individual cell pics should communicate with a host picaxe monitor and display system via an opto isolated bus.
Now the bus is the issue for me. I have ben reading the very interesting slave master serial project on this site, that may be one option.
But I don't need full bi-directional data comms.
I was thinking master pic could send an command signal on 08M input 3 (opto isolated) and all in parallel so the command initiates a data send routine in all the cell pics.
The individual pics could then respond on a simple opto isolated serial interface in sequence using a simple timing loop (different for each pic)
For example
Master sends data request via the opto interface and this triggers a simultaneous interupt on all 08M pics.
Cell 1 transmits identification number and data immediately,
Cell 2 waits X micros seconds and then transmits it's cell ID and data,
Cell 3 ditto and so on.
This data is stored in a simple array/scratch pad until all cells have sent data then master can process as reqd. Any failure to receive a cell's data triggers an alarms as reqd.
So I'm happy with the interupt triggering the data send routine on the 08m pics and the timing, but what about the serial output, how best to construct an opto isolated interface so all can send on same connections.
Anyone offer any help on this? Circuit ideas?
Another issue is that the as each 08m pic's power will be supplied by it's own cell, it has an issue measuring it's own cell V. I was thinking use a 1.25v prescion refrence and then do some simple calcs to take into account fluctuating supply voltage. But this will use another ADC input.
Also as the cell minium voltage is about 2v under heavy load what is the mimimum operating voltage for an 08M device. Should I use a charge pump dc-dc converter on each cell to maintain a steady 5v supply for each pic, this would increase cost considerably. A simple capacitor and diode on the + side of the chip supply isolating it from the drain would maintain the voltage for a few seconds, will that do it? 5v miniature backup power supply capacitors are pretty cheap, what's the running drain of an 08M say at 4v?
Lots of questions, I'm happy with most thing and digesting all the great advice on this site but extra comments are welcome. I'll of course make all my stuff available on the site inc pcb's software etc.
Peter
My ideas are as follows.
Use an individual 08M pic on each cell to monitor
1) Cell voltage
2) Cell temperature
the 08M will also control an individual bypass resistor so that when the cell reaches a set voltage say 4.2 it turns on and bypasses a set current.
The individual cell pics should communicate with a host picaxe monitor and display system via an opto isolated bus.
Now the bus is the issue for me. I have ben reading the very interesting slave master serial project on this site, that may be one option.
But I don't need full bi-directional data comms.
I was thinking master pic could send an command signal on 08M input 3 (opto isolated) and all in parallel so the command initiates a data send routine in all the cell pics.
The individual pics could then respond on a simple opto isolated serial interface in sequence using a simple timing loop (different for each pic)
For example
Master sends data request via the opto interface and this triggers a simultaneous interupt on all 08M pics.
Cell 1 transmits identification number and data immediately,
Cell 2 waits X micros seconds and then transmits it's cell ID and data,
Cell 3 ditto and so on.
This data is stored in a simple array/scratch pad until all cells have sent data then master can process as reqd. Any failure to receive a cell's data triggers an alarms as reqd.
So I'm happy with the interupt triggering the data send routine on the 08m pics and the timing, but what about the serial output, how best to construct an opto isolated interface so all can send on same connections.
Anyone offer any help on this? Circuit ideas?
Another issue is that the as each 08m pic's power will be supplied by it's own cell, it has an issue measuring it's own cell V. I was thinking use a 1.25v prescion refrence and then do some simple calcs to take into account fluctuating supply voltage. But this will use another ADC input.
Also as the cell minium voltage is about 2v under heavy load what is the mimimum operating voltage for an 08M device. Should I use a charge pump dc-dc converter on each cell to maintain a steady 5v supply for each pic, this would increase cost considerably. A simple capacitor and diode on the + side of the chip supply isolating it from the drain would maintain the voltage for a few seconds, will that do it? 5v miniature backup power supply capacitors are pretty cheap, what's the running drain of an 08M say at 4v?
Lots of questions, I'm happy with most thing and digesting all the great advice on this site but extra comments are welcome. I'll of course make all my stuff available on the site inc pcb's software etc.
Peter