drlava
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I've seen here and there posts about people having trouble with their batteries, some from DX (I've had issues, too) and other places, so now's a good time to post a simple test you can do to find the health of your rechargeable battery.
For an example I'll use a common 18650 li-ion cell. When fully charged these carry a potential of 4.2-4.25V and when discharged 3V. This is true of any cell that hasn't shut down whether it's new or old and tired. So what is the difference between a new cell and an old cell? Internally a battery consists of an electrolyte/electrode interface where the potential is generated by chemical reactions. As the reactions progress forward and are reversed during charging, the interface slowly degrades. This degradation results in not only a loss of capacity but also an increase in what is known as the batteries internal resistance.
High battery internal resistance is a BAD THING for a number of reasons. First, it makes the battery output voltage drop when the battery is being used. As soon as you take the battery out of the circuit to measure its voltage, the voltage rises and it seems OK again. Secondly, high internal resistance saps output power from the battery's already diminished capacity. This sapped power winds up as HEAT in the battery and can cause ugly issues and further degradation.
Measuring the battery's internal resistance is simple. All you need is a 4 ohm 5W power resistor or similar and a multimeter. Five common 1W 1 Ohm resistors in series would work. If you aren't measuring an 18650, choose a load resistance that will load the cell but not overload it. Capacity/3 should be ok.
1) First, measure the battery's voltage when charged. This is V1
2) Next, connect your multimeter leads to each side of the resistor and briefly connect the resistor across the battery. Note the voltage reading, this is V2.
3) Measure your resistor to get its precise resistance, this is R
4) The battery's internal resistance (Ri) is calculated with the formula:
Ri = (V1-V2)*R/V2
Example: cell measures 4.2V unloaded, 4.0V when connected to a 4 ohm resistor has an internal resistance of 200 mOhms (0.2 ohms)
A new high quality 18650 battery will have an internal resistance under 100mOhms.
A used up 18650 battery will have a resistance 400mOhms or more.
So, feel free to test a few batteries you have around and post what you find!!
For an example I'll use a common 18650 li-ion cell. When fully charged these carry a potential of 4.2-4.25V and when discharged 3V. This is true of any cell that hasn't shut down whether it's new or old and tired. So what is the difference between a new cell and an old cell? Internally a battery consists of an electrolyte/electrode interface where the potential is generated by chemical reactions. As the reactions progress forward and are reversed during charging, the interface slowly degrades. This degradation results in not only a loss of capacity but also an increase in what is known as the batteries internal resistance.
High battery internal resistance is a BAD THING for a number of reasons. First, it makes the battery output voltage drop when the battery is being used. As soon as you take the battery out of the circuit to measure its voltage, the voltage rises and it seems OK again. Secondly, high internal resistance saps output power from the battery's already diminished capacity. This sapped power winds up as HEAT in the battery and can cause ugly issues and further degradation.
Measuring the battery's internal resistance is simple. All you need is a 4 ohm 5W power resistor or similar and a multimeter. Five common 1W 1 Ohm resistors in series would work. If you aren't measuring an 18650, choose a load resistance that will load the cell but not overload it. Capacity/3 should be ok.
1) First, measure the battery's voltage when charged. This is V1
2) Next, connect your multimeter leads to each side of the resistor and briefly connect the resistor across the battery. Note the voltage reading, this is V2.
3) Measure your resistor to get its precise resistance, this is R
4) The battery's internal resistance (Ri) is calculated with the formula:
Ri = (V1-V2)*R/V2
Example: cell measures 4.2V unloaded, 4.0V when connected to a 4 ohm resistor has an internal resistance of 200 mOhms (0.2 ohms)
A new high quality 18650 battery will have an internal resistance under 100mOhms.
A used up 18650 battery will have a resistance 400mOhms or more.
So, feel free to test a few batteries you have around and post what you find!!
Last edited:
, came from a laptop). they would probably be several 100 milliohm anyway haha
