Need simple ideas of monitoring cell voltage of multi cell battery

Thread Starter

bigjoncoop

Joined Feb 1, 2019
73
Hey fellas

I build DIY battery packs and in trying to find a cheap and easy solution to monitor cell voltages. This could range from 4 cells all the way up to a hundred cells if possible or more.

I'm thinking about using an Arduino but I would have to use ADC ic. There are so many on the market and I've never tried to do this before besides just using an Arduino to monitor one voltage at a time.

If anybody knows how to do this or can Point me to a solution that would be great.

I would like to use an Arduino so I can create an Android app and connect via Bluetooth can you get a graphical display of every cells voltage. I need to at least monitor each series group of batteries but ideally it would be nice to monitor each individual cell of the entire pack so if one cell is starting to have a problem it's easy to pinpoint exactly which one it is.

I know there are smart / Bluetooth BMS's
Available but these are only going to measure each series connected battery pack not each individual cell. Besides using a batrium or similar BMS which is very expensive but that is exactly what I'm trying to DIY a system is similar to those high-end BMS is with individual cell monitoring and graphical interface.

I have came across a few very similar projects people have made but no instructions or anything besides just showing off it working.

I am currently getting everything ready to build a 3kwh 18650 Powerwall which will be modular in design so I can add-on battery packs later and bring it up to maybe 5 or 6kwh and one of the purpose-built BMS has to do what I'm trying to do is very expensive.

Again I do not need to make a BMS I just want to make the voltage monitoring side of it so I can monitor just the individual cell voltages. Not control them just monitor them. I already have BMS.

This seems like it shouldn't be too difficult I just never used ADC ic's before. I came across a bunch of different ones one in particular I am looking at some LTC*** ADC IC'S, but there are a lot of similar ADC I just don't know which one to use and exactly how to implement them
 

ElectricSpidey

Joined Dec 2, 2017
927
The coding for a project like this will be the easy part, what I see as the challenge will be switching in and out the cells to be read, and keeping track of such.

Any thought on that?
 

MrSoftware

Joined Oct 29, 2013
1,676
So many ways to do this.. What's your budget? You essentially need a bunch of ADC's on a bus or network. You can potentially reduce the number of ADC's by using some analog multiplexers:

https://www.sparkfun.com/products/9056

Side note - I believe most (all?) arduinos have ADC functionality built in

But you're going to need to work out addressing and how to shuffle the data around. So first task is design requirements. Draw up your rigid requirements.

  • budget
  • how many cells
  • power consumption limits
  • what level of precision
  • how frequently do the voltages need to be checked
  • anything else you can think of?
 

Reloadron

Joined Jan 15, 2015
5,140
Where you run into headaches with doing this is when your A/D device be it an Arduino or other uC (micro-controller) type device is what is known as "common mode" most devices have common mode voltage range or limits. Arduino uses a single ended input referenced to Ground so that won't work. Even if we add a A/D which has differential inputs to our Arduino we still have common mode to deal with.

A suggested work around was using analog output optical couplers which I never had much luck with as to them being linear.

Analog Devices’ family of multicell, high voltage battery stack monitors are complete battery monitoring ICs that include 16-bit ADCs, precision voltage references, a high voltage input multiplexer, and a serial interface. Parts can be connected in series, without optocouplers or isolators, to allow the monitoring of every cell in a long string of series-connected batteries. Applications include electric and hybrid electric vehicles, high power portable equipment, backup battery systems, and high voltage data acquisition systems.

Analog Devices has done quite a bit of development with measuring cells in a stack. You may find something helpful in there.

Ron
 

ElectricSpidey

Joined Dec 2, 2017
927
My first thought would be to have a DPST mechanical relay at each cell with their output connected to a single buss connected to a single ADC, and use a digital MUX matrix to select the relays. (with load drivers)

You could use analog MUXs but you would have to use them in pairs, because you need to sample both leads.

I’m pretty sure the PSoC 5LP boards have the ADC to do the job (differential).
 

Reloadron

Joined Jan 15, 2015
5,140
My first thought would be to have a DPST mechanical relay at each cell with their output connected to a single buss connected to a single ADC, and use a digital MUX matrix to select the relays. (with load drivers)

You could use analog MUXs but you would have to use them in pairs, because you need to sample both leads.

I’m pretty sure the PSoC 5LP boards have the ADC to do the job (differential).
Differential is not the problem. I can hang an ADS1115 on an Arduino and have increased 16 bit resolution, programmable gain and other nice to have features like I2C interface. The problem will always come down to common mode voltage. That has always been the issue with measuring individual cells in a series configuration battery. Multicell stackable monitors were developed for doing just this. Earlier I mentioned Analog Devices but TI (Texas Instruments) is also into making solutions for doing this. Here is a good example from TI and these can be daisy chained, each capable of monitoring 16 cells. The bq76PL455A-Q1 16-Cell EV/HEV Integrated Battery Monitor and Protector is a solution based on need with Electric and Hybrid Electric Vehicles. This is one of those things which at first glance looks like it should be easy till you get into doing it.

Ron
 

dendad

Joined Feb 20, 2016
3,329
I agree with Ron.
Go with the already developed parts, otherwise you are in for a lot of work.
This is a case of do not try to reinvent the wheel. Unless you really want to have a go that is.
 
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