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Anik , 06-28-2024, 09:40 AM

Hi,
I am trying to design a battery gauge for 16 cell battery pack with MAX14921ECS+T IC. But on the EVM schematic I dont understand why buffer amplifier and other circuit needed here. The datasheet and EVM schematic attached.
Thank you
Anik

Anik , 06-28-2024, 10:41 AM

Also why they are isolating the MCU and battery gauge part with transformer. Is it necessary>?

Mini , 06-28-2024, 11:15 AM

Had a quick look, gotta read through datasheets, but it seems to be because they are isolating PC from battery pack. This evaluation board is made to be connected to PC. I think there are 2 reasons. First to isolate high voltage(up to 65V) from PC low voltage. For safety reason. Second it seems that they are powering ADC from battery so it's noise free. USB is quite noisy. You want your analog part to be separate from digital.

Anik , 06-28-2024, 11:21 AM

and what why there are buffer amplifier on the first image?

Mini , 06-28-2024, 11:31 AM

I assume you mean U4. It's reference voltage buffer. You have external ADC(U3) and reference voltage for it(U2). The reason why you need buffer for reference voltage is quite simple - reference voltages usually don't have much current capability. They want high impedance. Also there are few which don't like much output capacitance and so on. Since it seems to go to some other circuitry it's buffered. Op amps have very high input impedance which wont affect reference voltage source. Following circuitry can be quite low impedance. It's actually quite common to use reference voltage and buffer after that if you need current capability.

Mini , 06-28-2024, 11:35 AM

Just a side note one should be aware that having op amp after reference voltage might ruin your design. Since op amps introduce many extra properties like offset voltage and so on. I personally try to avoid op amp after reference voltage.

Anik , 06-28-2024, 11:44 AM

ok got it.
But sorry for my lack of knowledge. I am confused about this full circuit . Why we need this??

Mini , 06-28-2024, 11:49 AM

To be honest I'm not sure why they use external ADC there. Not familiar with big battery gauges. Have to read datasheet for that.

Mini , 06-28-2024, 11:53 AM

Mini , 06-28-2024, 11:53 AM

Actually your question about isolation is answered in the datasheet. I wonder why people don't read datasheets through.

Anik , 06-28-2024, 11:58 AM

I was reading the main datasheet and focusing on it, sorry I missed that even I have not read the description of EVM yet.

Mini , 06-28-2024, 11:59 AM

If you have a question about EVM always read their datasheet. I have got many answers from EVM datasheets/descriptions.

Anik , 06-28-2024, 12:01 PM

Maybe I got it.

Anik , 06-28-2024, 12:01 PM

Yes, Thank you.

Mini , 06-28-2024, 12:03 PM

Well yes I saw that. Your external ADC seems to be measuring total battery voltage. I was wondering why they didn't include it already inside the chip since they already have SPI connections anyway.

Mini , 06-28-2024, 12:06 PM

Mini , 06-28-2024, 12:07 PM

Typical schematic. But still question stays. Why not include that external ADC already inside. Maybe somebody can speculate about it.

QDrives , 06-28-2024, 07:10 PM

Do note that what is missing in the evaluation board is the battery protection circuit.
These circuits often use N-Channel MOSFETs on the low side to switch on/off. When they are switched off, you do not want your current to go through the USB connection. That is why the battery and communication is galvanically isolated.

Anik , 06-28-2024, 07:42 PM

I have done the design like this. I will request to take a look on it. I only need cell voltage measurement. No need the mosfet side( Cell balancing)
"When they are switched off, you do not want your current to go through the USB connection. That is why the battery and communication is galvanically isolated." is that mean the voltage from the battery canbe come through the CV or BA pins ??

Mini , 06-28-2024, 09:35 PM

You can't have any current flowing backwards if you have battery pack GND or positive disconnected. For the current to flow you need closed circuit.

Anik , 06-29-2024, 10:48 AM

Yes, I understand.

Anik , 06-29-2024, 10:49 AM

I will request to take a look on the schematic and leave some comments for improvments

QDrives , 06-29-2024, 01:08 PM

How do you charge and discharge the battery?

Anik , 06-29-2024, 01:11 PM

I am making this for a company.
They need to read the each cell voltage through ethernet.
They told they can not share more about it outside. So that I dont know.
I am using esp32 as microcontroller and W5500 ethernet chip.

QDrives , 06-29-2024, 01:17 PM

Do they provide power for your components?
Does it come from USB? You have ethernet...
If you do not know the bigger picture, you cannot make a proper design.

Anik , 06-29-2024, 01:20 PM

They are making a 16 cell 100A lithium battery pack.
They just told me they need a device which will have 16 wire for cell voltage measurement and need each cell 16bit analog value. And send this data through ethernet.

QDrives , 06-30-2024, 07:15 PM

And where does your power come from?
16bit analog? 16bit is digital, but you say analog???

Anik , 06-30-2024, 07:18 PM

Maybe I am not clear about your question, but if,
My designed circuit will run from a 12V supply. Its a another supply of power. And the work of the circuit is just measure a external lithium pack which has 16 cell.

QDrives , 07-01-2024, 08:42 PM

Your measurement is relative to the battery. If you get 12V supply that is not to the battery (ie FET in minus of battery for protection) than you need the galvanic isolation like shown in the evaluation board.
That is why I stated that you cannot make a proper design if you do not know the bigger picture.

Anik , 07-02-2024, 01:52 PM

Sorry again, If you can explain little more. Can I not use this without galvanic isolation?? If I dont use this isolation what could occur??

QDrives , 07-02-2024, 07:37 PM

If you do not have isolation, connect to other system(s) and have protection circuit that disconnects Gnd (minus of battery), the current may flow through your communication circuit (ie. USB cable).
I am not saying this WILL happen it COULD. As I do not know how your (and your clients) system is.

In case you did not know most battery protection disconnect the minus, but leave the plus connected. Your measurement system **IS** connected to the minus, so could your communication be the Gnd connection.