IR Meter

The ESR meter is designed to measure the Internal Resistance of a LiPo pack or cell within a 2S to 6S pack.  ESR1 is a modified meter with an extra JST connector that allows you to power it up with a 2S to 6S pack so you can measure the IR of single cell. The Full Analysis ESR meters are now being sold instead of the regular version.

 

OPERATING NOTES

 

(a) Temperature

The ESR of any LiPo is dependent on temperature; the lower the temperature, the higher will be the ESR.  When comparing two packs therefore it is essential that you do so at the same temperature.  Leave the two packs together for an hour or two to ensure this.

To demonstrate this, take a cell reading of cell 1 on a pack. Hold the palm of your hand against the flat side of cell 1 for only 15 seconds and take another reading.  You will see that the ESR has fallen slightly.

The ESR of lipo packs can vary by 30 to 90% for a temperature change of 10⁰ to 30⁰C  (50⁰ to 85⁰F).

(b) Winter Flying

The above demonstrates why it is important to warm your LiPos before use in cold conditions.  If you launch with cold LiPos the voltage and power level is much lower, often to the point where the ESC will shut down on undervoltage.  The current through the LiPo will heat it up (and damage it!), reduce the ESR and the power will gradually rise.  This is why so many LiPos fail in cold weather.  Therefore pre-warm your LiPos in winter.

(c)  Voltage Readings

In Voltage mode the meter will read the pack or cell voltage, dependant on mode.  It is not intended as a precision voltmeter as the resolution is limited, but it is accurate and consistent enough for general use and cell voltage comparison purposes.

 (d)  Error Warnings

(i) If you try to measure a pack which has an ESR >250milliohms or a cell of >35milliohms, the meter will display an “Over range” warning.

(ii) If any cell is less than 3.20V, the meter will display “Cell Voltage Low” and the meter will not measure ESR.  Pressing the button will display the actual cell voltage for 3 seconds and then default to the low cell warning.  Note that the meter will only read cell voltages down to 1.5V as a minimum voltage is necessary to change the meter to Cell mode.

 Disconnect the search lead and toggle the meter to return to normal.

The lowest voltage at which a battery pack which can be measured is 7.0V, so that a very poor or tired  2S LiPo may give a false reading. Even a good 2S pack needs to be fully charged to ensure that the voltage does not fall too low during the current measurement pulse.

(e)  “Max I”  and “αC”  readings

These readings are calculated using the Lipotool  which is a rule of thumb, albeit a surprisingly accurate one. Approximations within the digital maths processor will result in the same “Max I” for slightly differing ESR values – This is not a processing error.

“αC” is a practical real value of C as it is understood by users  ie  it is the number you can multiply the pack capacity by, in Ah, to give a safe maximum current which will not have a deleterious effect on pack life. Read more details below under (g) The LiPotool.

 

(f) State of Charge

The ESR of a LiPo is largely independent of its state of charge but  a fully charged pack will be 5 – 10% lower than one at storage charge. I would recommend measurements at the fully charged state for consistency; essential for other reasons with a 2S pack. (See note above under “Error Readings”).

  It is possible to connect the meter in parallel with a working load (ESC + Motor) and monitor the ESR as the discharge proceeds.  This will clearly demonstrate the self heating effect and consequent fall in ESR as the battery temperature rises.

(g) The “LiPotool”

The original ESR meter was designed and built as an aid to my testing of LiPo batteries several years ago as the IR of a cell tells more about its performance capabilities than any other parameter. At the time I was involved in full capacity constant current discharge testing for several LiPo stockists in an attempt to directly compare the performance of various pack brands and 'C' ratings.

 

After much testing and comparing results, it became obvious that there was a close correlation between the maximum current delivery capability of a lipo cell and its IR.

  In high power discharge testing, it was temperature rise which dictated the max current that could safely be drawn from a lipo pack.  This temperature increase is caused by the load current passing through the effective resistance of lipo and the heat dissipated within the pack  is a square law.

 

In conjunction with Mark Forsyth and John Julian (both qualified engineers and practising model flyers)  the “ LiPotool” was generated which specifies a maximum initial heat dissipation limit within the LiPo pack and calculates a maximum continuous current which may be drawn from the pack without exceeding this limit.

 

We are not claiming that the LiPotool is a precise and exact tool, more a rule of thumb, but it has proved to be a surprisingly accurate guide when compared with the results of full discharge constant current testing of LiPo packs.

 The current at which the packs showed obvious signs of overstress (excess temperature rise and voltage sag-and-recovery) invariably correlated with the predicted maximum current produced by the LiPotool.

It is somewhat conservative; aiming at a max continuous current which should ensure that your LiPos have a good lifespan.  In real flying the LiPo will be subjected to surges and a practical safe surge is probably about 40% greater than the safe continuous current for the region of 10 seconds or so. This represents doubling the heat dissipated within the pack for the duration of the surge.

 

Where cells in a pack produce different maximum current or “C” readings, you should always rate a pack at the lower value as this is the weakest cell in the pack. (The weakest link in the chain.)

 

 Readings of Real “αC” values from your meter may well disappoint, but this is generally because they have been so wildly exaggerated by so many suppliers that they have become a joke.  Some LiPos, generally in the 20C and 25C categories are genuinely rated but I have never yet tested a LiPo capable of a genuine 40C continuous discharge without dangerously excessive temperature rise.  Be suspicious of any claims of >35C.

 

 

REMEMBER: FOR ACCURATE RESULTS YOU MUST TAKE ESR AND CURRENT READINGS AT 22 - 25C (72 - 77C)

 

 

 

 

 

 

 


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