Li-Ion battery State of Charge estimation

[AimHigh]

Hi,

Good Day!

Statement of Problem: In our application, we are using a 3S2P Li-Ion battery(12.65V). BMS is built using BQ24XXX TI's Li_ion controllers (Which does not have State of Charge estimation). So we need to measure the state of charge of Li_Ion battery on our application using this nRF52832 SoC. 

Query: 

1. For 12.6 V system voltage, We used a simple voltage divider to scale down the voltage which can be supported by the ADC on Chip. Is this a feasible way to measure voltage?

2. We also have a current sense resistor value of 10 mOhm,  Would we be able to measure the current range from 10mA ~ 8 Amperes (which is 100 micro Volt ~ 80 milli Volt drop) using nRF52832 ADC? If not what will be the measurable range or could it be any suggested solution here?

Thank you

Aim High.

[Dmitry]

Hi,

1. Yes, it's a usual way to measure the voltage.

2. As long as you take a differential signal directly from your current sense and don't forget to calibrate ADC offset periodically. Keep in mind that you have to measure a positive (charging) and negative (discharging) voltage drop, so you need an opamp in any case. IMHO a gas gauge chip like BQ34110 would be a simpler and more reliable solution.

[AimHigh]

Hi Dmitry,

Thank you for your response and suggestion. 

We have designed the bidirectional using TI's op-amp to sense the current. 

I have got one more question here, So what I understood is we have to gather the data of Voltage vs State of Charge for our application and then use that data to predict the state of charge for the battery or please correct me If I got it wrong.

Again for the above method, We did not consider temperature variation, So any insights on how SoC estimation would differ with temperature and other parameters?

Thank you

[Dmitry]

It's rather a complex task if you need an accurate results - you should take into account discharge rate, cell temperature, self-discharge, battery aging, etc. That's why I suggest to use a gas gauge chip. If you need only a rough estimation, you can monitor battery voltage and convert it to a state of charge with discharge curve found in specification for your battery.

Also take a look at this article that explains how it's implemented in Maxim chip.

[AimHigh]

Dmitry thank you for all the information and suggestions. Our application where the accurate state of charge is not required, Sure we will go ahead with the approximation for the SOC estimation.