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How healthy are your batteries? (How to measure internal resistance)

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!!
 
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AUTO XX

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Handy thing to know if you are buying batteries on fleabay and need to leave +/- feedback.
Thanks drlava! :gj:
 

Toke

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Good description/procedure, and much safer than a calipher and tong-amp. :D
 

hugo999999

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this is really handy to know, i'm just scared to try it on my batteries (they are pretty old and would probably explode :), came from a laptop). they would probably be several 100 milliohm anyway haha
 

drlava

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Blord, with that resistor you are pulling about 2.4 amps from the battery during test which can be stressful for it. However it looks like it worked ok for you.
 

LarryDFW

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Dr. Lava;

I just tested a new 2600mah Sanyo #18650 cell @ 70° F:

V1=4.09 VDC No Load (Right off Charger - should allow an hour)
V2=3.97 VDC Loaded
R = 3.1 ohms
Internal resistance : 94 mOhms @ 1.28 amps load (about 1/2 C load rating)

According to 18650 Battery Testing Experts @ Cadex :

75-150 mOhm - Excellent
150-250 mOhm - Good
250-350 mOhm - Marginal
350-Up mOhm - Bad

LarryDFW
 
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SHIN

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Thank you. drlava.
Now I can differentiate my batteries!:D

SHIN
 

LarryDFW

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Dr. Lava;

Some new high power lasers are demanding quite a bit of power from these #18650 cells.

One of my customers is outputting 700mw from his Green PGL.

Most 18650 batteries struggle to deliver enough current to drive the inefficient IR diode @ ~3-4 amps.

This is where internal resistance is a critical parameter to output full power.

I just tested one sample of some new Sanyo #18650 cells @ 70° F:

V1=4.20 VDC No Load
V2=4.10 VDC Loaded
R = 3.1 ohms
Internal resistance : 76 mOhms @ 1.32 amps load (I test @ 1/2 C load rating)

According to the respected 18650 Battery Testing Experts @ Cadex :

75-150 mOhm - Excellent
150-250 mOhm - Good
250-350 mOhm - Marginal
350-Up mOhm - Bad

mOhm stands for milliohm or 1/1000 of an ohm.

LarryDFW
 
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drlava

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Most 18650 batteries struggle to deliver enough current to drive the inefficient IR diode @ over 4 amps.

This is where internal resistance is a critical parameter to output full power.
Exactly right, this and also 18650 s with bad internal resistance may have trouble supplying very high current 445nm diodes. This is the main reason I thought now would be a good time to post this. Thank you for the condition chart!
 

wbp

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This is *very* helpful - I have a collection of 18650's, I'm off to get some 1 ohm 1W R's today!
 

drlava

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Glad you noticed. The reason I said 5 not 4 1 ohm 1W resistors is because their power rating could be exceeded with a fresh good li-ion battery with just 4. It likely wouldn't cause a problem, but it would be a flaw in the recommendation.
 

Hemlock_Mike

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This is good info for cell safety. Keeping the test current at a "normal" condition might be necessary with protected cells since the impedance of the pass devices may show in the readings. Normally, the pass devices are of mosfet technology having very low ON resistance but it might affect calculations.
Andrew --- Did you test both protected and unprotected cells?
At rated current, I would assume there to be slight differences.

HMike

I'm going to test my cells because: 1: I don't want old dangerous cells around here and 2: a weak cell can affect power output graphing results.
 
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