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Battery Internal Resistance
The TP820CD offers a quick and relatively effective means to measure the approximate Internal
Resistance (IR) of LiPo/LiIon/LiFe, NiCd/NiMH and Pb/lead-acid batteries as well as the individual
cells of LiPo/LiIon/LiFe batteries when the balance connector is connected to the balancer. However,
it’s important to note that while IR measurement from the TP820CD and other equipment readily
available in the industry can provide valuable insight into the performance of a given battery, the data
is not absolute. Even when using lab-quality, calibrated equipment to measure the IR of
cells/batteries there are many variables that will affect the IR of a battery at any given time including:
State of Charge (SOC): The IR will read higher when the battery is discharged versus partially
charged versus fully charged. As a result you should always measure the IR of batteries at the same
SOC, per your preference, in order for the results to be relative and valuable to reference. We
suggest checking at full charge as this is the easiest SOC to achieve consistently with nearly all
batteries.
Cell/Battery Temperature: The IR will read higher when the cells/battery are cooler versus warmer.
This in mind you should always measure the IR of batteries when they are at the same temperature if
you would like to compare the relative results directly.
Physical Condition/Age: The IR will read higher in cells that are physically damaged, swollen and/or
considerably aged and deliver less voltage and/or capacity under load accordingly. We recommend
checking the IR of batteries when they are new and tracking their relative IR over time as the battery
ages. In the case of cells such as those featuring the Thunder Power RC exclusive Generation 6
chemistry, the IR can actually decrease throughout the first 25-100+ cycles while often stabilizing for
hundreds more cycles after.
Resistance of the Wire Leads/Connector: Because the IR is being measured through the battery
and charger leads, it is possible for them to effect the measurement especially if the gauge of wire is
relatively small and/or there are poor connections that contribute to increased IR. We suggest using
the same charger leads/connectors, ensuring solid connections at all times, when making IR
measurements to compare relative results. This can be especially impactful when checking the IR of
individual cells of LiPo/LiIon/LiFe batteries through balance connectors as those
connectors/connections can often vary greatly in resistance.
And again, due to the myriad of possible variables, IR measurement is not absolute and entirely
indicative of actual battery performance. However, if you take the time to ensure you check all
batteries at the same SOC, temp, with the same leads/connectors, etc. it is indeed possible to achieve
relative results that can be compared effectively.
Here is some additional information, including a few generic reference examples, regarding
approximate IR measurements/values:
A 3S 11.1V 2250mAh 45C battery will have approximately ½ the IR of a 6S 22.2V 2250mAh 45C of
the same make/model, condition, age, etc. This is because the 6S battery has twice the number of
cells, and approximately twice the amount of IR as a result. This is not exact as there will be some
differences in the exact IR due to additional welds between tabs, etc., however, it is a good general
reference that higher cell count batteries (of the same capacity/C rating) will have higher IR than lower
cell count batteries.
A 3S 11.1V 2250mAh 45C battery will have approximately double the IR of a 3S 11.1V 4500mAh 45C
of the same make/model, condition, age, etc. This is because the 4500mAh battery has twice the
capacity and approximately ½ the IR as a result. Again, not exact but a good general reference that
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