LiPo Internal Resistance Meter
Available upgrades since last upload of documentation:
Further information in e-mail.
This device can be used to measure the internal resistance of Lithium Polymer batteries used in electric RC models. The power consumption of these models is quite high, and the batteries wear out in 100-200 recharge cycle. The internal resistance is a good measurement of the age of the batteries. New cells are below 5mohm, and the ones at the end of their life are above 20mohms. It also depends on the original quality.
The only problem is that I don't know how the internal resistance is defined, so I made my own definition, and I measure that: The battery is fully charged, cell voltage is about 4.2V. The unloaded cell voltage is measured. Than it is loaded with 1ohm for 2.5ms, and the loaded cell voltage is measured. The load current depends on the number of cells (N*4.2V/1ohm) and the time of loading. I experienced that the voltage falls immediately to a low value, than rises again in about 100us. Than falls exponentially to a lowest value in about 2.5ms. Than it remains constant for long enough time to measure all the cells.
In case of a 8 cell battery the voltage is about 33.6V, the load current is 33.6A so the 20W resistor is loaded by 1130W.
- 2-8 cell LiPo battery, (tested up to 4 cells)
- Accuracy of voltage measurement: estimated 1%
- Accuracy of Rin measurement: estimated 6% due to the accuracy of the load resistance and the RDSon of the FETs.
- Load time: 2.5ms + Ncells*110us
- ADC resolution: 16bit
For calibration an accurate 19.0V voltage source will be used that can supply about 500mA current.
Although the calibration is done at about 2/3 full scale there is about 1% inaccurancy at some voltages.
The PCB is hacked since the last pdf-print!