Internal resistance gives an indication of how efficient a cell (or battery) will be in giving back what you put in .
If 1WHr of electricity is put into two separate cells, the one with low resistance will give back more of that energy ... the one with higher resistance will waste more in heating up the cell and give less back in useable electricity.
How much will be the difference will depend on the charging and discharging currents .... the lower the currents the less will be lost ..
Let's consider 2 cells , one with 100mOhm ...another with 300mOhm Int res.
charging at 1A the loss in the cell is I squared R = 100mW and 300mW ... this is lost in charging and discharging.
If the cell voltage is 3.7 , thenpower going into the cell @1A is IxV = 3.7W
So If 3.7W is put into both cells the one with
100mOhm int res. gives back 3.5W....5.4% islost...
300mOhm cell gives back 3.1W.....16.2% islost ... this lost energy is what heats up the battery pack ...
At 0.5A charge and discharge the % loss is halved to 2.6% and 8.1%
At 0.25A charge and discharge the % lost is ...............1.3% and 4.05%
The higher the temperature the lower the internal resistance , ... You may measure the internal resistanceat room temp , 25*C, but if the cell is kept outside and cycles at 10*C the resistance will be double and so will the losses.
So if energy has a high value to you, reject cells with high resistance .... if you have excess solar capacity and sometimes powergoes to waste , then keep all but the very worst cells.
I'f you're a real perfectionist you could put those with high resistance on the outside of the pack where the heat can get out more easily ...
Although high cell temperature does increase efficiency (reduces heat in cell) it is stressful for the cell ...temps below 10*C should be avoided when cycling (not for storage) as resistance is high.
If 1WHr of electricity is put into two separate cells, the one with low resistance will give back more of that energy ... the one with higher resistance will waste more in heating up the cell and give less back in useable electricity.
How much will be the difference will depend on the charging and discharging currents .... the lower the currents the less will be lost ..
Let's consider 2 cells , one with 100mOhm ...another with 300mOhm Int res.
charging at 1A the loss in the cell is I squared R = 100mW and 300mW ... this is lost in charging and discharging.
If the cell voltage is 3.7 , thenpower going into the cell @1A is IxV = 3.7W
So If 3.7W is put into both cells the one with
100mOhm int res. gives back 3.5W....5.4% islost...
300mOhm cell gives back 3.1W.....16.2% islost ... this lost energy is what heats up the battery pack ...
At 0.5A charge and discharge the % loss is halved to 2.6% and 8.1%
At 0.25A charge and discharge the % lost is ...............1.3% and 4.05%
The higher the temperature the lower the internal resistance , ... You may measure the internal resistanceat room temp , 25*C, but if the cell is kept outside and cycles at 10*C the resistance will be double and so will the losses.
So if energy has a high value to you, reject cells with high resistance .... if you have excess solar capacity and sometimes powergoes to waste , then keep all but the very worst cells.
I'f you're a real perfectionist you could put those with high resistance on the outside of the pack where the heat can get out more easily ...
Although high cell temperature does increase efficiency (reduces heat in cell) it is stressful for the cell ...temps below 10*C should be avoided when cycling (not for storage) as resistance is high.