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BATTERY INTERNAL RESISTANCE
Cell temperature
For many cell chemistries the resistance of the pack decreases initially
from room temperature to a certain level (perhaps 45C) after which the
resistance of the pack will increase again. Lithium cells in particular
suffer poor cell resistance near 0C.
Cell charge
Generally the closer to a full charge a cell is the lower the internal
resistance.
Cell age
As cells become older their internal resistance tends to increase.
The i-Meter has been built to allow cells to be tested under a known
set of loads which will facilitate comparisons.
Let it be stated that there is no perfect test for cell internal resistance.
Due to the nature of batteries, cells exhibit characteristics of resistance,
capacitance and inductance of which all contribute to the measured
internal resistance. Internal resistance can vary considerably even
within the same cell, this makes the task of determining the internal
resistance a lot harder.
Factors effecting resistance readings:
Contact corrosion
While not actually part of the cell itself, having dirty contacts to the cells
can make a noticeable difference in the resistance. Wiping clean the
contacts can improve contact resistance by 10 or more milliohms.
Once we have an idea of the internal resistance of a cell we can then
use that to determine how efficient our power setup is as well as
knowing how much power is being turned to heat in the battery pack
and additionally know how much the voltage can potentially drop under
load.
B A T T E R Y C H E C K E R
I N T . R E S I S T A N C E
W A T T M E T E R
S E R V O T E S T E R
T A C H O M E T E R R P M
T E M P E R A T U R E
T H R U S T C A L C . . .
S E T U P
I n t . R e s i e t a n c e
R e s : 3 9 m o h m
