Lestraveled
- Joined May 19, 2014
- 1,946
I don't have a "widget" that I built to measure the internal resistance of a battery. If I need to measure the IR of a battery, I pull out a Mosfet switch assembly, hook it up to my pulse generator, assemble the right load resistors and measure the IR. A very hands on approach from a very hands on kind of guy. (Give me some flint, sulfur, a wire tie, and a oscilloscope and I can discover the meaning of the universe..... it's 42, by the way.)
I realized from the conversations with nsaspook and Julia, that in the data processing world, you do not need to measure precisely at certain points in the transition to determine IR, you just have to make a good educated guess. What do I mean by this?
The micro controller decides when A/D samples are taken and when the second load is applied. So, take several samples before the load is applied to determine the slope. Apply the second load then take samples (throwing away the samples affected by inductance) and determine the new slope. Take the data from before the transition and interpolate a line forward. From the data after the transition, interpolate a line backwards. At the time when the load was applied, derive a delta between the two projected voltages and you have the data required for an accurate IR measurement. A problem I see with this method is the inaccuracy of using a big contactor as to when it actually closes. A Mosfet switch would eliminate this issue and would require less to control.
Anyway, remember, the internal resistance of a battery is independent of the current the measurement is taken at. If you take two IR measurements at significantly different current levels and they agree, then you have done it right.
I realized from the conversations with nsaspook and Julia, that in the data processing world, you do not need to measure precisely at certain points in the transition to determine IR, you just have to make a good educated guess. What do I mean by this?
The micro controller decides when A/D samples are taken and when the second load is applied. So, take several samples before the load is applied to determine the slope. Apply the second load then take samples (throwing away the samples affected by inductance) and determine the new slope. Take the data from before the transition and interpolate a line forward. From the data after the transition, interpolate a line backwards. At the time when the load was applied, derive a delta between the two projected voltages and you have the data required for an accurate IR measurement. A problem I see with this method is the inaccuracy of using a big contactor as to when it actually closes. A Mosfet switch would eliminate this issue and would require less to control.
Anyway, remember, the internal resistance of a battery is independent of the current the measurement is taken at. If you take two IR measurements at significantly different current levels and they agree, then you have done it right.