Circuit for li-Ion cell capacity measurement?

[wari]

Hey

I need a simple circuit for measuring the capacity on li-Ion cells (18650)

I want to do this as a school project (it would be my thirst electronics project)

It has to be computer controlled so I will use LabJack UE9 for computer connection, and will probably use a variable resistor (operated by computer controlled small motor).

So this is my starting point, but as my English is not so fluent I couldn't find a good circuit to base my project on (not so good at googleing).

All the measurements in this forum seem to be taken with commercial products, does anyone know of a place where I should dig?


Thanks in advance,:thinking:

 

[linterno]

Hi wari and welcome to CPF.

Based on Li-Ion battery voltage you could determine remaining capacity.

This is what is usually written here about Li-Ion batteries remaining capacity:

Resting voltages (no load connected) are a good indicator of state of charge. These figures are approximate:

4.2V = 100%
4.1V = about 90%
4.0V = about 80%
3.9V = about 60%
3.8V = about 40%
3.7V = about 20%
3.6V = empty
<3.5V = over-discharged

You could use a microcontroller with ADC (Analog to digital converter) connected to your computer's serial port to receive the voltage reading.

Please look at PICAXE. This could be all you need for your project. I would recommend you PICAXE-08M.Read Datasheets Sections 1, 2 and 3 first.

 

[Mr Happy]

Hey

I need a simple circuit for measuring the capacity on li-Ion cells (18650)

I want to do this as a school project (it would be my thirst electronics project)

It has to be computer controlled so I will use LabJack UE9 for computer connection, and will probably use a variable resistor (operated by computer controlled small motor).

So this is my starting point, but as my English is not so fluent I couldn't find a good circuit to base my project on (not so good at googleing).

All the measurements in this forum seem to be taken with commercial products, does anyone know of a place where I should dig?


Thanks in advance,:thinking:

The capacity of Li-ion cells is measured the same way as the capacity of any other battery: starting with a fully charged cell, you discharge it down to some pre-determined end voltage (let's say 3.0 V for Li-ion), and measure the current and/or voltage continuously during the discharge.

If you integrate the discharge current over the discharge period you will get the capacity in amp-hours. If you integrate the power (volts x amps) over the discharge period you will get the capacity in watt-hours.

Since you say this is a school project, you may not be familiar with what "integrate" means. However, do not worry, it is just a simple process of adding up. For instance, suppose you take measurements of current at intervals of 6 minutes, which is 0.1 hours. Then the sum total amp-hours is:

capacity = first reading x 0.1 + second reading x 0.1 + third reading x 0.1 + .... + last reading x 0.1

(note: the first reading would be taken after the first 6 minutes have passed)

Another good way of visualizing this is to plot all the readings on a graph and then the capacity is given as the area under the discharge curve. You may have done that sort of thing already in math class.

The single most important thing you will need for the experiment is a logging voltmeter and/or current meter. If you use a fixed known resistance as the discharge load you can measure the voltage across the load and deduce the current from I = V/R. That way a single meter will give you all information you need.

 

[wari]

Tanks Mr Happy

I think what you sayed will be very useful when Im starting to program the LabJack.


this is what I'm puting together now. (sry for posting such a crapy picture)


this is what worked for me in the end - very simpel (exept for the LabJack part (expencive) and Phyton programming with PID controller)