Measuring voltage of lipo battery

[Blisk]

I have lipo battery with 8 cells so total voltate is 33.6V
Now I need to measure each cell and when first one drops to 3.8V when discharge relay will turn off.
Problem is now how to measeure each cell because they are serial connected.
I thought to use Cmos 4097 which has 2 x 8 channels to switch between cells.
problem is because power voltage of 4097 is 12V and battery from first to last cell is 33.6V.
That will burn 4097.

Any idea how to solve this?

 

[KlausST]

Hi,

Any idea how to solve this?

two resistors in series give a voltage divider...

Klaus

 

[Blisk]

Is it possible to solve this with some other chip or other way?

 

[Audioguru]

A rechargeable lithium cell voltage is from 3.2V when the load should be disconnected to 4.2V when the charger should be disconnected when the charging current drops low.
3.8V is higher than the 3.7V average voltage of a discharging cell and most of the charge still remains. Why stop the discharge at such a high voltage? A cell is normally stored at 3.7V for long durations.

 

[KlausST]

Hi,

Is it possible to solve this with some other chip or other way?

Yes, if you want it difficult and expensive.

--> there are selection guides at manufacturers and distributors for selecting analog switches, analog mux and ADCs.

...like ADG5208. But where is the benefit?

Klaus

 

[Blisk]

Ok. I thought if there is not so dificult and expencive way to do it. If not I will use resistor divider.

 

[Blisk]

It can be setup how low you will discharge I just put value of 3.8 it can also be 3.7.
Problem is how to switch woltmeter input from one to another cell to measure it??

 

[KlausST]

Hi,

Problem is how to switch woltmeter input from one to another cell to measure it??

I´m confused. What´s wrong with 4097?
***

About voltmeter: You didn´t give any information about it.

Klaus

 

[Audioguru]

A voltage divider will not let you measure the voltage of each and every cell. Instead it will allow measuring the total voltage of all cells, one cell or any number of cells that are connected together.

 

[KlausST]

Hi,

A voltage divider will not let you measure the voltage of each and every cell. Instead it will allow measuring the total voltage of all cells, one cell or any number of cells that are connected together.

Many ADCs - even the ADC inside AVR microcontrollers - are able to perform differential measurement.

So with the 4097 it is possible to select each cell individually.

Klaus

 

[Blisk]

Hi,



Many ADCs - even the ADC inside AVR microcontrollers - are able to perform differential measurement.

So with the 4097 it is possible to select each cell individually.

Klaus

Yes I know that AVR micro controllers can do that but I have no knovledge of AVR and programming it that's why I look at 4097.

- - - Updated - - -

I will use voltmeter to check voltage of 8 cells in battery. Chip will go from one cell to another every fev seconds and at output of chip like 4097 I will measure voltage, when voltage drops to 3.8 It will stop discharge.

 

[Audioguru]

The input voltage of a CD4097 or an ADC must be within the power supply voltage range of it. So you can measure the first cell with one input at the battery ground which is also the CD4047 ground.
You can measure the 2nd cell with one input at the anode and the other input at the cathode and the voltmeter can be differential. But the 3rd cell's anode might be 12.6V so it needs to be reduced with something that messes up its cathode voltage.

Maybe the 3rd cell can use a voltage divider for its anode and another voltage divider for its cathode and measure the voltage differentially then amplify the resulting voltage back up to normal. Then the 4th, 5th, 6th, 7th and 8th cells also need two voltage dividers each and amplification.

 

[Blisk]

Yes this is idea now how to do it.
I have also found another chip which is not expencive and I think may work and it works to 40V.
It is cmos DG408.
The best will be if I could to that with some microcontroller.

 

[alijamali74]

I have a suggestion.
You can use tow analog multiplexer and address them to connect desired ends of any cell to its output.any output should have one resistor it means tow resistor for tow multiplexer.now you can switch between tow head of any cell and measure it's voltag.by measuring voltage from tow head of any cell and subtract them you have voltage of a cell.