Controllable battery charger based on max712
February 18, 2010
Well here it is!! My attemps at building my own battery charger using a sample of the max712, here is the original post that inspired me, http://letsmakerobots.com/node/9272 and special thanks to ezekiel181 !!!
Although I will build a prototype on a breadboard just to test I have already started drowing a circuit using diptrace. The final version will be a programmable battery charger using a picaxe 40x2, a keypad from Sprkfun to set the outputs and an LCD to display remaining time, elapsed time and the
- Gather electrical components for the breadboard prototype
- Finish designing pcb with all corrections discovered from prototyping.
- Build prototype using all components of finished design (relays, keypad ...)
- Find a suitable power source ( laptop supply from dealextream maybe)
- Coding to understand keyboard input, do necessary calculations, set relays to correct position, display info on LCD.
- 12 or maybe 6 relays depending on how they are configured
- Blocking diode
- 1 external power PNP transistor
- 3 resistors( well thats what the datasheet says but I clearly see 5 in the diagram)
- 3 capacitors ( same as before, I see 5 in the typical operating circuit)
- A keypad to input info
- An LCD to display info
- A picaxe to drive set the relays depending on cell number, fast charge time...
Well the only bit of work I can put up is the box I am planning to put this all in, it may seem a bit large but this way the laptop power supply, keypad, LCD will all fit in perfectly!
The phones is there for size reference.
Well my first idea on how to control the connections of pgm0,pgm1,pgm2,pgm3 was by using transistors as a switch, but since I don’t know how much current and in which direction it would be travelling meant that I would be safer off using relays.
I still need to do some more research to get some points strait but for now I am pretty optimist on this project.
There has been a change in my plans, voodoobot suggested using one of these http://www.sparkfun.com/commerce/product_info.php?products_id=9117 , which does seem like an interesting option, 1 of these with the select option would allow me to set all the needed variables, it would also be useful because it only requires 2 pins instead of the 7 that the keypad require. There is only 1 inconvenience, the coding, I have absolutely no idea how to read or even count the pulses, sure, a post here on LMR would helpJ but, since I have 2 samples might consider that part for my second battery charger.
Well know for the work;
For this I will need to input variables via the keypad into the picaxe, have it do a few simple calculations then have it set its outputs high or low to set the relays. The problems come when dealing with R1, a resistance that need to be changed depending on the batteries being charged, to solve this I have 3 possible solutions; 1 use a variable resistance condoled by the picaxe, 2 use a variable resistance that I will adjust by hand, 3 use a 8mA current limiting diode (from the max713 ev kit diagram)