Battery Gauge Meter

[QuanLe8191]

Deciding between voltage and diode dropper: Voltage divider: lost of the accuracy of 28-30%

Right now go with the diode dropper that is stated in the doc Schematic will have both so we can go through and see both of the option

[kammce]

There is another option that I proposed before. There is a single cell voltage measurement IC that can be used to query the voltage and give use a estimated state of charge without the need of a current shunt. Its based on a mathematical model of voltage over time and the expected charge within the battery. I think this can simplify the design tremendously.

The idea would be to connect the balance connector and the main connector of the battery to drive and arm boards, then just sample the lowest cell in series using the charge expectation IC. Since all other cells should reflect each other, we can know when the whole back is discharged when the lowest cell is discharged.

[kammce]

The idea comes from this device the MAX17043G+U

We would get high accuracy from the device (at least higher than our MCUs doing the sampling) and the design would just require an i2c bus and a voltage measurement. And we wouldn't need to calculate a shunt because this device does not require a shunt resistor to do its work.

[kammce]

Here is a version of the chip from AliExpress that is cheaper: https://www.aliexpress.com/item/4000007759641.html

[nathaniel-nac]

@kammce That clears things up actually. When I was going about implementing the said chip from Sparkfun, I had initial thoughts of using the diode- voltage dropper at the input of the LiPo Gauge to match the gauge's maximum input voltage. Then when I went to reference your initial post in slack, you mentioned using a resistor divider to scale the voltage, which sparked the initial questions at the beginning of this thread.

I like the idea of your other proposition, but to clarify, when you say to connect the balance connector and main connector of the battery, you mean connecting them to a BMS right? And then from there, we can bring out the connections of the lowest cell(or I guess any cell) and the battery ground to be connected to the LiPo gauge.

[nathaniel-nac]

Actually, scratch that part about using a BMS, I think I just figured out what you meant.

[nathaniel-nac]

Actually now that I think about it again, would it actually be a good idea to add a BMS, just to ensure that all cells do discharge at somewhat the same rate?

[QuanLe8191]

Nate can you put a link to the circuit schematic in here for easy access?

[nathaniel-nac]

Yes of course.

Battery Management Circuit Schematic: https://easyeda.com/editor#id=2b864c16d7eb4353bfe6344c9ca3e8e3

[kammce]

Actually now that I think about it again, would it actually be a good idea to add a BMS, just to ensure that all cells do discharge at somewhat the same rate?

Just so you know, very few BMS system controls the discharge of each cell during the discharge phase of a battery. Meaning when you power something using the BMS, the BMS's job is usually to turn off power in over current situations or in over temp situations. Fancier BMS do a bunch more but these are the basics. BMS DO handle discharging cells during the CHARGING of the pack to make sure that other cells do not get overcharged. So the cell balancing only happens when you are charging up the battery pack.