@Smoke_Maker
I've got an alignment of 10ohm 100W with 10 parallel = 1ohm.
Its a very stabel alignment so I think I'll give it a go.
Yep for the voltage drop I need to see whats going on.

I'm using an prescaler for reading the voltage from a battery (with an ADC). prescaler: 0 - 3V, ADC: 12bit (removing 2 LSB (interference) = 2^10 = 1024.
30V/1024 = 29mV per step.

How do I now what the maximum resistance of an prescaler may be?
beacause a very high resistor (Mohm) whoulden't charge the capacitors inside an ADC right?
Should I put an filter before the ADC for the 50-60Hz noise?

Any thoughts, comments and help would be greatly appreciated.

greets,
Leek

 

Sorry, but I've sort of lost your strategy here. It sounds like you're going with a static resistive load for testing your batteries, although you started this thread asking about constant currents. Both can be meaningful but obviously they're not the same.

It comes back to a question I asked early on: What are you really wanting to accomplish? Have you abandoned your idea of using a constant current?

 

@wayneh
Sorry for the late reply, was this weekend away.

Yes I abandoned the constant current idea.
I was trying to change my starting post put I can't change it anymore??

The plan:
Using the microcontoller to change the resitor value with relays.
Reading the voltage and the current draw from the battery with an ADC and trasfer this data to the PC.



and my last post:

I'm using an prescaler for reading the voltage from a battery (with an ADC). prescaler: 0 - 3V, ADC: 12bit (removing 2 LSB (interference)) = 2^10 = 1024.
30V/1024 = 29mV per step.

How do I know what the maximum resistance of an prescaler may be?
beacause a very high resistor (Mohm) whoulden't charge the capacitors inside an ADC right?
Should I put an filter before the ADC for the 50-60Hz noise?

Any thoughts, comments and help would be greatly appreciated.

greets,
Leek

 

I think you should check the rating of the battery and try to improve the performance of the battery so that you can get the required current.....

 

I'm using an prescaler for reading the voltage from a battery (with an ADC). prescaler: 0 - 3V, ADC: 12bit (removing 2 LSB (interference)) = 2^10 = 1024.[/I]
30V/1024 = 29mV per step.

How do I know what the maximum resistance of an prescaler may be?
beacause a very high resistor (Mohm) whoulden't charge the capacitors inside an ADC right?
Should I put an filter before the ADC for the 50-60Hz noise?

Any thoughts, comments and help would be greatly appreciated.

greets,
Leek

Leek,

Pick your ADC first and the data sheet will tell you what the input range will be for current and voltage, the data sheet might have a suggested prescaler in it so check some different ADC data sheets, also look for a instrument grade ADC it will drift less with closer data on the PC to what the battery is doing.

As for the filter you will not need a lot of filtering because it is a resistive load.

 

@Smoke_Maker
I'm using the microcontroller with internal ADC STM32F100RBT6B on the STM32VLDISCOVERY.

This is a 12bit ADC, 16 channels, Vref is internally connected to Vadc (connected to VDD = 3,3V) I wan't to measure between 0 -30V splited in 2 serperated voltage dividers (because the asked me to).

Battery voltage: 24V (2*12V).

Thank you for helping me out!
I'm getting stuck...

My plan:
create 2 voltage dividers for reading the battery voltage.
1: 0 - 15V
2: 15 - 30V
If I wan't to measure the current I need to put a resistor in serie with the load so I could readout the voltagedrop over the resistor.

for the filtering, your right but I need to make sure I don't get noise from the net right?

Shall I start a new topic where I could explain a bit better whats the plan now?

 

My plan:
create 2 voltage dividers for reading the battery voltage.
1: 0 - 15V
2: 15 - 30V
If I wan't to measure the current I need to put a resistor in serie with the load so I could readout the voltagedrop over the resistor.

for the filtering, your right but I need to make sure I don't get noise from the net right?

Shall I start a new topic where I could explain a bit better whats the plan now?

I think this thread is fine, but you may want to jump over to the "programmers corner" for you microcontroller help, but it is time to start posting your schematic.

To read current you are correct use a shunt (resistor).

I'm not sure what you mean by noise from the net, but you circuit board needs to have a clean power supply voltage powering the MC (microcontroller) and you should have some filtering for the ADC input pin of the MC, I think your MC will do that for you on the ADC pin by averaging the voltage.

Your MC data sheet looks like the ADC input pin will sink and source 25ma and the voltage range is 2.4V to 3.6V but not higher than Vcc voltage. So design your prescaler to meet the data sheet.