GregsGarage wrote:It is possible to dissipate small amounts of balancing current through the 6802, but I have not connected the balancing pins. This board will only monitor cell voltages. I did this for a couple of reasons. First it makes layout much easier and gives you a much smaller board. Second, daily balancing shouldn't be necessary for most packs. I haven't done any balancing on my pack for more than 6 months, and other reports also suggest similar experiences. So balancing will be moved off-board of the vehicle and be done periodically when needed. I am thinking this will be some sort of individual cell charger to bring all cells up to the same SOC rather than holding all cells at a high voltage for extended periods while dissipating current from the highest cells.
Another goal for this design was to make the master and slave buses more robust and resistant to interference. What I have found in my Matiz, where the battery boxes are spread out from the front to the back of the car, is that communications aren't as robust as they should be. I get some glitches during hard acceleration and as the pack gets discharged. So I have moved the isolation to the pcb, both the master and slave bus are fully isolated from the traction pack. This allows me to increase the voltage levels and so improve communications (I hope
).
Hi greg, I got the schematic, but I am a bit confused now,
Does your 6802 Slave works with peter V2 master board or are you planing to make it work with V3 using your own code?
I would do some radical changes into the BMS itself, starting with the Master.
Right now all the calculations are taking place on the master which has to receive all the (useless) data from the slaves. This is fine as originally designed by peter for less than 100cells, but it is a problem once this number goes up.
My idea is to use the slave PIC to communicate with the LTC6802 and a fuel gauge/monitor and carry out all the operations for the 12Cells on the built in PIC. The master would only know:
Temperature of the pack
Capacity Status of the pack (Remaining/used/instant consumption)
Lowest cell on the pack
Highest cell on the pack
HVC (to stop or reduce Regen)
LVC (To warn the master the pack voltage is low or disable the pack)
This is much less data that before, with much less work for the master!!!!
The build in VC would allow the pack to be used independently with a charger and the charger would know what the capacity of the pack is. The only thing the master would need to do is reading data and display it on a screen or give a command to enable the slaves or master relay (for the immobilizer).
I was also looking at the Datasheet for the 6802 and given that you can daisy chain them, will it be possible to make a PCB say for 48Cells (4 6802 and only one PIC, Opto, etc) instead of using the pics to communicate with each other every 12 cells, thrus reducing components and complexity.
I have been looking into MAXIM DS2438 as one of the possible options for the fuel gauge IC
It has a inbuilt temperature sensor for the pack, costs about the same as the I2C temperature sensor used on V2 and has advanced features like compensation for aging of the pack and build in time meter that could be used to disable the charger after a preset time or capacity is reached, for security.
http://www.maximintegrated.com/datasheet/index.mvp/id/2919?ver=E&t=al&utm_expid=50713806-2&utm_referrer=http%3A%2F%2Fwww.google.co.uk%2Furl%3Fsa%3Dt%26rct%3Dj%26q%3Dmaxim%2520ds2438%26source%3Dweb%26cd%3D1%26ved%3D0CDUQFjAA%26url%3Dhttp%253A%252F%252Fwww.maximintegrated.com%252Fdatasheet%252Findex.mvp%252Fid%252F2919%26ei%3DF4_WUMemHqaZ0QX67IDQAw%26usg%3DAFQjCNHij3KU16SHY6u5GYpOL8T2b11U-A%26bvm%3Dbv.1355534169%2Cd.d2kBalancing:
I agree 200% with you about the balancing
My sugestion is to place a header on the Main PCB with the balancing lines from the 6802. This can be used to atach a second board with the MOSFET's, resistors and indicator LED's. This board could be left permanently or it could be used once in a few months only, since it is removable.
Since it is only a header the PCB design would be very simple as well.
I would like to see the schematic of the 6802 board to fiddle a bit with it, if you dont mind.
Thanks, Carlos