ProgressiveRC ESR - Manual (Page 2)

OPERATING NOTES
(a) Temperature
The ESR of any lipo is dependant 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 10 to 30 deg.cent (50 to 85F).
(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) Interpreting Values
“Pack” Readings
In this mode the meter measures the total resistance of the cells + the resistance of the links between cells +
resistance of the leads + the resistance of the connector. This is the effective resistance in the circuit in real
conditions. You can assume that leads plus a good 50A connector have a resistance of about 4 - 6 milliohms.
Try to take readings at the same temperature, say 20deg.cent so that you know what is a good reading for a
particular pack size. Eg a good 3S 2250 20C pack will be about 15
– 20 milliohms whereas a poor pack will
40 milliohms +. Some packs have a higher ESR but also a high temperature coefficient so that their initial
voltage drop is higher but reduces due to self-heating. They may finish up hot with a similar performance to
the lower ESR pack, but they are hotter, more highly stressed and likely to have shorter lives.
“Cell” Readings
In Cell mode the meter reads only the ESR of the cell corresponding to the two adjacent pin positions on the
balance connector. The reading is taken as a 4-Wire Kelvin connection so that only the cell is measured and
very accurately. No other wires or contacts are included in this value so that you can now accurately compare
all the cells in the pack. Some makers claim that their cells are “Resistance matched” and this can be
checked.
When a pack is damaged or dying, it is invariably one cell which is deteriorating and this will be shown in the
ESR value rising, so you can trace the degeneration of your lipos and know which are the better ones.
(d) Voltage Readings
In default mode the meter will read the pack or cell voltage, dependant on switch position. 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.
(e) Current Operating Level
The unit uses a 16 Amp constant current pulse to measure the ESR of a pack. This is large enough to
facilitate an accurate measurement of a large capacity pack but small enough to be acceptable to a small lipo
without damage.
(f) Error Readings
If the main input is reverse connected to a pack the unit will not be damaged, but the display will not appear.
Reversing the cell measuring search lead (much more likely) will again cause no damage but will result in a
reading of about 0.6V.
If the search lead is left open circuit it will read zero voltage and about 40milliohms (Over-
range) if the „Read
ESR‟ button is pressed.
The lowest voltage at which a battery pack which can be measured is 6.0V, so that a 2 cell LiFe (A123) is
marginal although a 90% discharged 2S lipo is OK.
(g) State of Charge
The ESR of a lipo is largely independent of its state of charge unless the pack is completely exhausted, which
is unwise in any case. 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.