voltage measurement for lithium ion battery (Isolated/differential) - Page 1
Forum section: electronics / projects

author: Rasil1127
author_rank: Newbie
timestamp: August 28, 2023, 11:09:51 pm
content: Hi,I've been trying to develop my own BMS for quite a while but have failed a lot.. the main problem is isolated voltage measurement/differential (I'm not sure which one).I know several ways of doing it but not sure which would be the cheapest, most compact and very simple.Requirements are as follows:- possibly measuring over 20 lithium-ion cells (3.6v) in series with each other- has to be in a small footprint- Most importantly CHEAP!the ways I know of measuring this is withcommon mode differential amplifier,High Linearity Optocoupler,1 µ controller per cell,various battery monitors/fuel gauges,resistor dividers.I have a problem with all these methods, I'm just not sure which one I should go with or maybe someone knowledgeable knows a way I haven't listed here. the problems are listed below:common mode differential amplifier: very difficult to get the circuit correct in my experience, and expensive (considering I need 20 of them for a 20s battery) I tried lots of circuits never worked for me..High Linearity Optocoupler: Meh i guess they are cheap? probably takes up too much space, after searching for months I just cannot understand how I can make it work in my application...1 µ controller per cell: too expensive (maybe you guys know a very small µC that is very cheap.)various battery monitors/fuel gauges: been trying at the BQ7693003, no good working library for the ARDUINO framework I tried basically everything with that to no working circuit..resistor dividers: i can get this somewhat accurate but it becomes lots of resistors even for 10 series cells let alone 20. Maybe you guys know some clever ways of using this and transistors.Please help me decide how to do this I welcome anyone to answer

author: radiolistener
author_rank: Super Contributor
timestamp: August 28, 2023, 11:26:37 pm
content: just use separate wires for voltage measurement, current should not flow through these wires to avoid voltage drop down.Also note that li-ion batteries are very sensitive to overvoltage even for 0.05V and it requires very precise voltage measurement.Usual requirement for li-ion absolute voltage mesurement should be not worse than ±0.05V. Not any DMM allows so precise measurement.

author: Benta
author_rank: Super Contributor
timestamp: August 28, 2023, 11:36:43 pm
content: Do you just need to measure cell voltage?Or is eg, temperature, current etc. also needed?

author: Rasil1127
author_rank: Newbie
timestamp: August 28, 2023, 11:49:56 pm
content: Quote from: Benta on August 28, 2023, 11:36:43 pmRight now, the measurement of the voltages of each cell is needed the most. however current will be nice. and yes temperature is also needed but that's a simple circuit that i will add later on.

author: Rasil1127
author_rank: Newbie
timestamp: August 28, 2023, 11:51:40 pm
content: Quote from: radiolistener on August 28, 2023, 11:26:37 pmI don't understand, how will wires on each cell help with voltage measurement? i need it to be measured by a micro controller, but since each cell is referenced to its own ground and not the pack ground it poses a problem where i cannot measure each cell separately.

author: Benta
author_rank: Super Contributor
timestamp: August 29, 2023, 01:00:39 am
content: Here's a design idea for you. The transistor collector currents reflect the cell voltages.I'm going to bed now, be back tomorrow evening.

author: dobsonr741
author_rank: Frequent Contributor
timestamp: August 29, 2023, 01:09:17 am
content: What is not working with BQ76930?I'm puzzled, as you were not happy to use a professional battery monitor over I2C. Rather you would opt to roll your own hardware design, not even considering that will need software too.

author: Rasil1127
author_rank: Newbie
timestamp: August 29, 2023, 10:47:27 am
content: Quote from: Benta on August 29, 2023, 01:00:39 amThank you for this design, will look into this

author: Rasil1127
author_rank: Newbie
timestamp: August 29, 2023, 10:49:46 am
content: Quote from: dobsonr741 on August 29, 2023, 01:09:17 amMy first-ever choice was this monitor, however, I'm not really a big software guy, never have been but if I make the hardware and understand it usually I can write software for it. however, this monitor has CRC and a whole load of other stuff I really don't get and since there are no working libraries for this I'm really left here trying to write software from the ground up and just cannot due to my skillset. this is actually the cheapest/best method.