Hi
I'm looking for ideas on how to implement a circuit that can measure the voltage of each cell in a multi-cell lithium battery.
The battery (24V) in question is made of 8 cells, each cell comprising a series of 38120 batteries.
The nominal voltage of each cell is 3.2V.
What I have in mind is to use a MCU (probably the PIC16F1503) to measure each cell as a sequence of operations, either directly (using its onboard 10-bit ADC) or a higher resolution connected ADC.
Finding a way of switching between each of the cells is proving to be a bit of a challenge for me though. My immediate thought was to use 2 (8:1) demultiplexers to switch the cell inputs, one to control the negative input and the other to control the positive input. After some testing, I realised that this will not work with the voltages involved. The MCU is driven using 3.3V so I will be using the same voltage to drive the demux. However, there will be over 24V potential between cell 1 and cell 8 which is way more than the demux can handle.
My next thought was to continue to use the demux, but to isolate the cell inputs from the DEMUX using optocouplers. In this way, the demux would be used to select the cell inputs (negative and positive) by turning on the appropriate optocouplers. The MCU would be programmed to drive the demux select lines so that at most there would only ever be one negative and one positive line active, and furthermore, the negative and positive lines would be connected to the same cell, therefore never exceeding the 38120 battery upper voltage level.
At this stage I am looking for feedback on the design and how effective it might be. Is there an alternative that I haven't thought of (another idea is to have 8 MCU's, each measuring just one cell, and feed the results to a controller). I am uncertain how accurately my design will be able to measure the cell voltage, given that the input to the ADC will be via an opto-coupler - does this drop the voltage like a diode does, or will the cell voltage be largely unaffected after passing through the opto-coupler?
I've attached a drawing that should make the above description a little clearer.
Thanks for your feedback.
I'm looking for ideas on how to implement a circuit that can measure the voltage of each cell in a multi-cell lithium battery.
The battery (24V) in question is made of 8 cells, each cell comprising a series of 38120 batteries.
The nominal voltage of each cell is 3.2V.
What I have in mind is to use a MCU (probably the PIC16F1503) to measure each cell as a sequence of operations, either directly (using its onboard 10-bit ADC) or a higher resolution connected ADC.
Finding a way of switching between each of the cells is proving to be a bit of a challenge for me though. My immediate thought was to use 2 (8:1) demultiplexers to switch the cell inputs, one to control the negative input and the other to control the positive input. After some testing, I realised that this will not work with the voltages involved. The MCU is driven using 3.3V so I will be using the same voltage to drive the demux. However, there will be over 24V potential between cell 1 and cell 8 which is way more than the demux can handle.
My next thought was to continue to use the demux, but to isolate the cell inputs from the DEMUX using optocouplers. In this way, the demux would be used to select the cell inputs (negative and positive) by turning on the appropriate optocouplers. The MCU would be programmed to drive the demux select lines so that at most there would only ever be one negative and one positive line active, and furthermore, the negative and positive lines would be connected to the same cell, therefore never exceeding the 38120 battery upper voltage level.
At this stage I am looking for feedback on the design and how effective it might be. Is there an alternative that I haven't thought of (another idea is to have 8 MCU's, each measuring just one cell, and feed the results to a controller). I am uncertain how accurately my design will be able to measure the cell voltage, given that the input to the ADC will be via an opto-coupler - does this drop the voltage like a diode does, or will the cell voltage be largely unaffected after passing through the opto-coupler?
I've attached a drawing that should make the above description a little clearer.
Thanks for your feedback.
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