Propeller ADC Calculations

Cluso99

I will be using a Propeller to monitor the voltage on a 24V Lithium Battery Bank. I want to monitor each cell in the bank as well as a shunt to measure current (either charging or discharging).
There are 8 cells in a 24V bank.

Here is my circuit (taken from Parallax's ADC Sigma-Delta App Note). I am using multiple copies of this circuit.

Here are the data calculations. Note the formula shown for the Low Voltage value. The High Voltage formula has "+" instead of "-".

And here is a graph of those values/ranges.

Here is the spreadsheet (you will have to remove the ".txt" extension)
Propeller_ADC_Calculations.xlsx.txt

Now for some explanations...

Each battery cell is ~3.2V. To keep it simple, let's assume it is 3V.
Therefore I need to read values around 0V, 3V, 6V, 9V, 12V, 15V, 18V, 21V, 24V. I will require 8 ADC circuits for this.
Typically the feedback resistor (from the prop pin) R3=100K.
As seen from the data calculations, I get a bigger range if I use an input resistor R1=500K.
R2=150K gives a range of -1.100V to 15.400V
R2=56K gives a range of 8.132V to 24.632V
R2=33K gives a range of 18.400V to 34.900V

So, with just 3 variations, I can read my 8 cells.

However, if the voltage drops dramatically overall, I will not be able to read the overall voltage. So I am going to add a 9th ADC circuit to also read the 24V value.
This time, R1=500K, R2=56K and R3=47K. This gives a range of -1.171V to 33.935V.

For the shunt (in the -ve lead from the battery to ground), I need to read a value between +/-150mV and 0V.
R1=33K, R4=33K, R3=100K gives a range of -0.5V to +0.5V (note the use of R4 and not R2).

evanh

Cluso: I gather you've heard that "bottom balancing" is the best way for Lithium-Phosphates?

MJB

Cluso99 wrote: »

I will be using a Propeller to monitor the voltage on a 24V Lithium Battery Bank. I want to monitor each cell in the bank as well as a shunt to measure current (either charging or discharging).
There are 8 cells in a 24V bank.

Here is my circuit (taken from Parallax's ADC Sigma-Delta App Note). I am using multiple copies of this circuit.

Here are the data calculations. Note the formula shown for the Low Voltage value. The High Voltage formula has "+" instead of "-".

you are sharing the Py pin of all the SigmaDelta units, to save pins?

DigitalBob

I would use a MCP3208 ADC that has eight channels. The common would connect to the -Neg. of the pack then you could use each channel to measure the Positive of each cell (cells in series). I built an 8 channel board using the MCP3208 with volt dividers and buffer amps. The LF411 works great because of its low noise add a 1uf ceramic cap. with the feedback resistor.

Regards

Bob

Cluso99

Yes, I used a common feedback pin to save pins as there are no time constraints here.

Why use an external ADC chip? The prop is quite capable on its own. An external chip still requires 2 resistors as a voltage divider, and caps to stabilise are a good idea. That only leaves the feedback resistor(s) that can be saved vs a dedicated chip. Plus I would require 2 ADC chips since I need 9 channels.

I have laid out a pcb with 12 channels (wip). Currently I am not sure about what else to include... MicroSD, LCD, LEDs plus an external interface (serial ?), push buttons or a rotary/switch.

evanh, I need to google bottom balancing. I am using a smart charger, but I want to ensure a cell doesn't fail.

JohnR2010

Great project here is the document Cluso is referring too https://www.parallax.com/sites/default/files/downloads/AN008-SigmaDeltaADC-v1.0.pdf.

Mainly wanted to mark this thread with my input so I can find it easier later on. Thanks for sharing!!

MJB

Cluso99 wrote: »

Yes, I used a common feedback pin to save pins as there are no time constraints here.

Why use an external ADC chip? The prop is quite capable on its own. An external chip still requires 2 resistors as a voltage divider, and caps to stabilise are a good idea. That only leaves the feedback resistor(s) that can be saved vs a dedicated chip. Plus I would require 2 ADC chips since I need 9 channels.

I have laid out a pcb with 12 channels (wip). Currently I am not sure about what else to include... MicroSD, LCD, LEDs plus an external interface (serial ?), push buttons or a rotary/switch.

evanh, I need to google bottom balancing. I am using a smart charger, but I want to ensure a cell doesn't fail.

this would be a nice project to write in TACHYON ;-)

jmg

Cluso99 wrote: »

I am using a smart charger, but I want to ensure a cell doesn't fail.

A simple go/no-go you can probably get, but differential cell voltages are going to be harder to get precision on.
Especially across a divider ratio change, so I would add more channels so you can avoid trying to measure a cell difference across a divider ratio.
Better may be to use analog muxes into a single differential ADC.

Domanik

An opamp subtracter connected across individual cells will provide the difference (ie 3.2V), level shifted to a ground-propeller reference. Opto isolators would be used to switch the opamp inputs between cells.

Cluso99

I have ADC 12 circuits on my board. They will all be referenced to the ground of the device which is a 150mV shunt which is then connected to the -ve of the 24V battery.

The first ADC circuit connects to the shunt (-ve battery). This measures the current in/out of the battery. It's range is +/-0.5V.

Then each battery cell is connected to each of 8 ADC circuits. So, I am measuring successive voltages, and need to subtract the previous voltage to obtain the actual cell voltage.

It is a monitoring circuit to advise the user if something is wrong (cells are out of balance) and to trip a contactor to remove the charger/battery to prevent damage.

I see no need to complicate matters by adding lots of extra circuitry.