dillond666 wrote:Holy smoke, this project has really grown legs!............the 18x has grown into a 28x
I guess the picaxes are so low priced it is as well to have a degree of future proofing in the hardware.
I was wondering if a 2W load resistor is meaty enough, I like bobc's idea of using a logic level FET for switching on the load, a 30A version is only 28p from rapid electronics, so the option is there to use a larger external load if necessary. I suppose it depends on the charger cutback arrangement. Do you still use the zivan? If so, is it possible to interface it with the master board?
I used to be a happy, relatively normal bloke before this thread started! Now I'm sneaking away to caress and fondle the pages of the Picaxe book....ooh the shame of it
Yes changing to the 28X1 did add a few more inputs, but I did it mainly because it has a timeout function on the slave data serial input, so the Master software does not become stuck waiting for a slave to respond if there is a hardware failure on a particular cell. It can now detect that condition and report appropriately. Vital to the whole system.
I choose the 2w 15R balancing load as that has been proven to work over the last four years on Cedric's cell controllers. The balancing load of about 250ma was suffcient to keep 200AH cells in balance during that time so I'm happy with that. On smaller cells it will be even more effective.
I will be using a Zivan NG3 and the same Charger cutback arrangement. The Picaxes don't do DAC output in the conventional sense ASFAIK, you have to use a PWM output but this drives an opto through an RC smoothing circuit so in effect the opto led changes brightness smoothly in response to changes in PWM Mark/Space ratio. I haven't decided wether to pull charger voltage up or down with opto control yet. Pulling it up gives a slightly more fail safe arrangement I think.
The Master schematic incorporates this PWM control function for both Charger and Controller cutback
Many, many years ago as a callow youth I used to sneak about with a copy of Mayfair
Now I skulk about, collar turned up, in an old fleece, with a carrier bag full of Schematics, Software printouts, Pic data sheets and the like
How times have changed!
I've just ordered the 08M Picaxes to start building up the 50 slaves I need when the pcb's are finished. I have also ordered a PIC assembler level programmer so I can look at converting the Slave software to assembly and program it direct into bare pics once it's all tested. This will make the slaves a bit cheaper but owners won't be able to re-program themselves like they can with the Picaxe versions.
One of the Picaxes main attractions is the ease of re-programming
with a simple serial/usb cable and the free software it means as We/I release upgraded Slave/Master software people can just download it to there system in a few minutes. The free 32mb Picaxe basic editor is below, download it and tinker around!
http://www.rev-ed.co.uk/software/bas805.exe
The current sensor I will be utilising is based on the Allegro ACS754LCB series, which gives an 0-5v Output covering -100 to + 100A range.
5v output = +100A
2.5V output = 0A
0v output = -100A
Resolution will be about 200ma with the Pic 28X1 10bit adc
This will be mounted off the master board to reduce interference.
http://www.farnell.com/datasheets/75747.pdf
Other sensors can be used but conditioning of the signal will be reqd to ensure signal is within 0-5v for the Pic ADC input.
If you were using a current sensor with a positive/negative voltage output (quite common) then this example circuit will condition signal, opto isolate it and bring it to 0-4vdc output with a centre (2.0v) 0 amp position.
www.solarvan.co.uk/CurrentSensorCircuit.jpg
I will be building my battery pack up into 10 cell subpacks for testing which will only be connected together for final install/bench test.
I can thoroughly bench test each 10 cell subpack before install and it's a lot safer at <40V per block, the entire pack charged voltage will be nearer 190V
a bit too dangerous for tinkering with for my liking. Still got to be careful though I don't want any expensive smoke or singed dignity.
Back to work now!!