Battery
Turnigy 130 Blue F Turnigy 130 Blue H
Turnigy 130 Red (new) Turnigy 130 Green (new) Turnigy 130 Yellow (new)
No Load Volts 4.08 4.11 4.20 4.20 4.20
Single Battery Tests Loaded Volts
3.00 3.20 3.98 3.98 3.98
Current 0.34 0.31 0.41 0.41 0.41
Delta 1.08 0.91 0.22 0.22 0.22
Resistance 3.18 2.94 0.54 0.54 0.54
Determining IR
To get the IR for the cell expressed in ohms:
1. Write down the description of the battery (cell).
2. Hook up the cell under test to the switch box.
3. Read the no load voltage and write it down.
4. Throw the switch on and wait 10 seconds.
5. Read the voltage under load and write it down.
6. Read the current in amps and write it down.
7. Turn off the switch.
8. Subtract the loaded voltage from the no load voltage.
9. Divide this delta by the current and write it down.
10. Multiply by 1000 if you prefer milli - ohms.
trolyte at low temperatures, and it may ex- plode if charged in this condition. Never charge a battery below 10 degrees Celsius. A Li-Po pack can be considered as a stable voltage source in series with a small series resistance: its ESR, or Equivalent Series Re- sistance, also known as IR or Internal Re- sistance. The IR is measurable and one of the best ways to monitor a Li-Po battery’s condition. Most decent higher capacity and higher discharge rated Li-Po cells will have roughly 2 to 6 milliohms (0.002 to 0.006
4.2 4.1 4
3.9 3.8 3.7
0 20 40 60 160 mAh Discharge Curve Volts
Volts Amps
The new cells in this table have about half an ohm resistance. The two well-abused blue dot cells with about 3 ohms were trashed after this test. Stew find cells with over an ohm of resistance not worth using.
ohms) of internal resistance at room temper- ature when brand new. Lower discharge rat- ed packs and small capacity packs will gen- erally have higher internal resistance readings.
It is not unusual to measure internal re- sistance numbers in the region of 500 mil- liohms on smaller 100 to 200 mAh micro park flyer Li-Po packs when they are brand new for example. To calculate the total inter- nal resistance of a series wired pack, you add the IR of each cell together. The reason you don’t see this IR value quoted for a cell is that it is not constant, but a negative func- tion of temperature. The colder the cell, the higher the IR. That’s the reason your battery seems to poop out a low temperatures. Li-Pos are particularly susceptible to this effect, and the small single-cell batteries we use in micro fliers all the more so due to their low thermal mass. As a battery ages, the IR increases and apparent capacity decreases. Because the battery itself creates current, you cannot simply hook up an ohmmeter and measure resistance. Instead you must measure the no-load voltage, then apply a known electrical load (I use a light bulb). Then re-measure the voltage under load and also measure the amperage. Ten sec- onds is long enough for the voltage drop to be stable.
The internal resistance is the voltage drop divided by the amperage. It turns out it re- ally doesn’t matter what the state of charge is as long as it’s above the end voltage of 3.2 volts. I prefer to have it above 3.7 volts per cell. For my < 200 mAh micro park flyer Li- Po cells, a 6-volt flashlight bulb load will draw about 400 mA, about the same as a ParkZone 6mm Vapor motor.
As long as the measurements all take Secs80 100 120 140
0.75 0.6 0.45 0.3 0.15 0
4
3.75 3.5 3.25 3
2.75 0
Data taken with the Medusa Power Analyzer of a moderately used cell under test with the light bulb load. IR (half an ohm or so in this case) is not calculated by this Analyzer (above left) and the results need to be exported to a spread sheet for this
FLYING MODELS 200 400 Secs 600 800 1000 1200
operation. A complete discharge of a 160 mAh Li-Po recorded (above right) on a West Mountain Radio CBA. This really didn’t do the cell any good as the discharge should have stopped at 3.25 volts. The CBA warned of this, but was ignored.
41
place at the same temperature, you can use them as a figure of merit for the cells. Take a reading when the cell is new and write it down. After some usage take another read- ing, the IR will have increased. Compare your cells, the bad ones will have high val- ues of IR. My 160 mAh losers had over an ohm of resistance.
The critical measurement is current. Most
digital ammeters need to use the 20-amp range to measure this. Since we are drawing less than half an amp testing small batter- ies, the error is rather large on the order of 10%. (Instruments derive the current value by reading the voltage drop across a shunt resistor. This is typically a few millivolts.) Within this limitation the comparisons are valid. You can certainly tell which cells are outliers and see the degradation of cells over time. When my mini cells get much over one ohm it’s time to pitch them.
Some Li-Po chargers can automatically show an internal resistance reading for each call in a pack. Since most power ana- lyzers require an external power supply to read the low voltage of a single cell, I nor- mally use two Radio Shack VOMs which have internal batteries. One is set on 20 volts and the other is set on 20 amps. I have a switch box connected to these and a load bank of flashlight bulbs. I can switch in up to four in parallel to increase the current for larger capacity cells.
For batteries of two or more cells I use a Medusa Research Power Analyzer Pro. There is the option to send the results to my computer but, since the resistance is not computed, I need to transfer the results to a spreadsheet for this calculation. I find it eas- ier just to write the numbers down and do the math on a calculator.
Complete Discharge of 160 mAh Cell At 500 mAh
Amps
Volts
Volts
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