Further to my last post which was regarding the calculation of Rd for a 6V battery I have tried to calculate Rd for a 12v battery using the same formulas - but have run into a problem.
In the posts regarding an EVM spreadsheet guide (version 11) by Poh Ng for a 12V battery Rd was eventually shown by Chris Glaser to be 412K Ohms - but using the bq24450 formula as I did for the 6V battery I get a value for Rd of 28.6K
I enclose the workings below.
Where is my mistake? Is wolframalpha wrong ? Or is there an error in the spreadsheet ?
Vboost = Vref * (Ra+Rb+Rc//Rd) ÷ Rc//Rd
Where // denotes resistors in parallel and the R equivalent is ( R1 * R2) ÷ (R1 + R2)
For a 12V battery in the EVM board calculation Rc =46000 and Vboost is 14.4
14.4 = 2.3 * (92800 +46000//Rd) ÷ (46000//Rd)
14.4 = 2.3 * (92800 + ( (46000 * Rd) ÷ (46000 + Rd))) ÷ ((46000 * Rd) ÷ (46000+ Rd))
If Rd = Z
14.4 = 2.3 * (92800 + ( (46000 * Z) ÷ (46000 + Z))) ÷ ((46000 * Z) ÷ (46000 + Z))
And using www.wolframalpha.com
Z = 28611
Rd = 28.6K
