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Internal resistence

Internal resistence

03.Nov 2017
| Blog - GridFree


 

The value of the internal resistance

Question: In your promotional materials you mention several times that the internal resistance of the LTO cells is “very low”“lower than for the LFP cells”, is “only 1 mOhm and less”. However we have tested the internal resistance and the value is more than 20 mOhm. We think your information is not correct.

Answer:

First, note that the internal resistance of the cell is not a constant value. The internal resistance increases when the cell is fully charged and also increases when the cell is getting discharged. To measure the internal resistance for reference value, you should measure within DOD of 80% to 20%.  This means after you charge the cell to full, you let the cell discharge to 80% first and only after that you perform the tests.

Second, please check the quality of the terminal connection. The actual internal resistance of the cell is very low. However if you have a poor cabling and you measure the resistance, you will get a higher value, because the resistance of the cabling will also be measured. Please use high-precise cabling and clips to measure the resistance. (See the example of the test results above - with the cabling the test result is 25 mOhm, with the direct connection to the cell the result is on 18 mOhm)

Third, the most important: the internal resistance is always dependent on the capacity of the cell. If you have a 2 AH cell, 20 AH cell, and 200 AH cell of the same technology, the smaller the cell, the higher the internal resistance. For example the 200 AH cell will have only 0.2mOhm, the 20 AH cell will have 2mOhmand the 2 AH cell will have 20 mOhm. So in order to compare the internal resistance of the cells, you need to compare the internal resistance in relation to the capacity.

Comparative internal resistance

For large cells, the reference internal resistance is usually related to the cells with the capacity of 100AH or 200AH. For example the tests of a 1.3 Ah cell with LTO technology show the internal resistance of 18 mOhm. After recalculating the internal resistance by the ration of capacity (100Ah to 1.3 Ah = 77 times) you get the comparative internal resistance 0.23 mOhm. This result shows that the internal resistance of the LTO cell is very low. 

 

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