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Low cost parametric cell monitoring development update.
I have been developing an idea on how to make a low cost cell monitoring system. I would like to share some of my design requirements and progress to see if the product might meet the needs and desires of others. I am only at the prototyping stage of the cell circuits themselves and software control is very crude at the moment. I have only confirmed some of my design goals so far.
Design Goals and Strategy:
1. Voltage of each cell communicated to head controller for all control decisions, graphical display, and data logging. Control algorithms to include over and under cell voltage conditions at various loads and temperatures. Software development on Android tablet for cell voltage graphical display and data logging. Having the over and under voltage decisions made in software should have configurability advantages over solutions that use hardware trimming that must compromise when dealing with heavy current loads and very cold temperatures.
2. At least 2% cell voltage accuracy without software calibration, Better than 1% with software based correction files.
3. Average Less than $2.00 per cell circuit including PCBs. My current prototype is distributive in nature but groups 3 cells per circuit board. The PCB and surface mount components are less than $6.00 for 3 cells so on target with $2.00 average per cell goal. However, $1.00 per cell may be achievable in volume.
4. Less than 100micro amp quiescent current load on each cell. I don't want the distributive cell circuits to significantly discharge or imbalance the cells they are monitoring. I figure 10,000 hours to discharge the cell 1Ah is acceptable and any long term imbalance would only be a fraction of this low supply current.
5. Single wire daisy chain communication between cell circuits to reduce assembly efforts and cost. My current prototype uses two wires but I have ideas on making it a single wire.
My approach would be considered a digital solution but it is simply using a capacitor charge that sequentially propagates through the cell chain. Each cell circuit produces a digital pulse upon respective comparator voltage threshold crossings of the charging capacitor. This data pulse propagates to the end of line monitoring point which is opto isolated from the head controller board. The head board measures the time interval between data pulses to decode the voltage of each cell. My first prototype suffered from inaccuracies of resistor and capacitor values and their temperature coefficients. I since added an additional reference pulse for each cell group that allows the head board to cancel out these component value variations using the ratio of time intervals in software.
Future Cell Balancing Concept:
I also have ideas on how to make it have dissipative balancing using the same data bus to select and hold a bleed resistor for a particular cell for a period of time. For example if a single cell is always first to full SOC during charging then it may be desirable to bleed an Ah or so from it to bring it in line with the rest. This additional latching circuitry will be considered on the next version in order to keep the focus on low cost monitoring for now. This dissipative balancing concept is different from other shunting solutions that rely on shunting a portion of the charging current and thus need to use high wattage resistors. Also, having the balancing done in this case upon command might also provide a level of safety while being supervised.
Thanks
Jeff
I have been developing an idea on how to make a low cost cell monitoring system. I would like to share some of my design requirements and progress to see if the product might meet the needs and desires of others. I am only at the prototyping stage of the cell circuits themselves and software control is very crude at the moment. I have only confirmed some of my design goals so far.
Design Goals and Strategy:
1. Voltage of each cell communicated to head controller for all control decisions, graphical display, and data logging. Control algorithms to include over and under cell voltage conditions at various loads and temperatures. Software development on Android tablet for cell voltage graphical display and data logging. Having the over and under voltage decisions made in software should have configurability advantages over solutions that use hardware trimming that must compromise when dealing with heavy current loads and very cold temperatures.
2. At least 2% cell voltage accuracy without software calibration, Better than 1% with software based correction files.
3. Average Less than $2.00 per cell circuit including PCBs. My current prototype is distributive in nature but groups 3 cells per circuit board. The PCB and surface mount components are less than $6.00 for 3 cells so on target with $2.00 average per cell goal. However, $1.00 per cell may be achievable in volume.
4. Less than 100micro amp quiescent current load on each cell. I don't want the distributive cell circuits to significantly discharge or imbalance the cells they are monitoring. I figure 10,000 hours to discharge the cell 1Ah is acceptable and any long term imbalance would only be a fraction of this low supply current.
5. Single wire daisy chain communication between cell circuits to reduce assembly efforts and cost. My current prototype uses two wires but I have ideas on making it a single wire.
My approach would be considered a digital solution but it is simply using a capacitor charge that sequentially propagates through the cell chain. Each cell circuit produces a digital pulse upon respective comparator voltage threshold crossings of the charging capacitor. This data pulse propagates to the end of line monitoring point which is opto isolated from the head controller board. The head board measures the time interval between data pulses to decode the voltage of each cell. My first prototype suffered from inaccuracies of resistor and capacitor values and their temperature coefficients. I since added an additional reference pulse for each cell group that allows the head board to cancel out these component value variations using the ratio of time intervals in software.
Future Cell Balancing Concept:
I also have ideas on how to make it have dissipative balancing using the same data bus to select and hold a bleed resistor for a particular cell for a period of time. For example if a single cell is always first to full SOC during charging then it may be desirable to bleed an Ah or so from it to bring it in line with the rest. This additional latching circuitry will be considered on the next version in order to keep the focus on low cost monitoring for now. This dissipative balancing concept is different from other shunting solutions that rely on shunting a portion of the charging current and thus need to use high wattage resistors. Also, having the balancing done in this case upon command might also provide a level of safety while being supervised.
Thanks
Jeff
