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

[BShanahan14rulz]

Dr. Lava,

If we wanted to test capacity of our cells, could we use just a high power resistor and watch the voltage? If so, what voltage range should be considered inside of the capacity discharge? Start at fully-charged 4.2V, do I go to the minimum safe state, ~2.7V, or do I stop at 3.6V?

 

[drlava]

You would want to use a constant current drain, and you would discharge to about 2.7V.

 

[Elektrotechniker]

Thanks for this but why do we need to know the internal resistance of a battery?

 

[HIMNL9]

^ internal resistance says you if your battery is in good condition, or if is "near to death" ..... or also just the quality of the cells you buy ( for different brands, usually, lower internal resistance = better quality battery) ..... and, lower internal resistance, also = more stored energy and more current capacity, or better said, more ampere you can draw from it without overheating and explosion risks .....

 

[hakzaw1]

Very deserving for a sticky-=-<(*U*)>-=-

Good info we all need to know
+rep for sharing

TY Andrew--



hak

 

[nkgamer1990]

can sum1 test an aw 18650, just curious about the quality

 

[LarryDFW]

can sum1 test an aw 18650, just curious about the quality

Since they have the added resistance of the series protection circuit,

they will have ~60 mOhms additional "internal resistance" + the "internal resistance" of the 18650 cell itself.

LarryDFW

 

[elite ares]

I'd like to see some photos of your test rigs. I have a small selection of power resistors and would like to test my selection of 3.6V CR123A Ebay batteries.

 

[elite ares]

Here's my set up and results. An earlier post by LarryDFW showed a chart for 18650 internal resistances. The two CR123As I've tested so far are nearly identical. I don't know what they should be but they work very well in my yobresal builds. Using drlava's formula "Ri = (V1-V2)*R/V2" where 4.22 - 3.82 * 5 / 3.82 = .524 Ohms internal resistance. Comments, criticisms, and suggestions are welcome.

Juice 3.6v CR123A rechargeable photo lithium battery and paralleled Ohmite 10 Ohm 5 Watt resistors for 5 Ohms.

Resistor leads twisted and bent as such give decent tension for connecting to the battery.

Juice 3.6v CR123A rechargeable photo lithium battery purchased from Ebay fully charged @ 4.22 volts.

Voltage after connecting resistors. It is 3.82v. Yes, the resistors got warm and dropped in voltage after a short time.

 

[LarryDFW]

Here's my set up and results. An earlier post by LarryDFW showed a chart for 18650 internal resistances. The two CR123As I've tested so far are nearly identical. I don't know what they should be but they work very well in my yobresal builds. Using drlava's formula "Ri = (V1-V2)*R/V2" where 4.22 - 3.82 * 5 / 3.82 = .524 Ohms internal resistance. Comments, criticisms, and suggestions are welcome.

Elite;

Thanks for posting the photos of your test setup.
That should help others to understand the procedure.

The internal resistance of 524 milliohms is quite high.

The "IR" should be slightly higher than an #18650,
but I would expect the IR to be ~200-250 milliohms in a used RCR123A battery.

It just points out the need to use quality #18650 batteries ...
in lasers that draw heavy currents.

With a low IR Sanyo 18650, the battery with the same load
would drop from 4.22 VDC to 4.15 VDC (compared to your drop to 3.83 VDC).

A 2% voltage drop with Sanyo 18650,
vs 9% voltage drop with the Juice RCR123A.

LarryDFW

 

[Rafa]

I have measured an OLD (maybe 12 years old) SONY Energetic #18650 extracted from a Laptob battery.

3.85 V1 (It's not fully charged.)
3.65 V2
4.45 R

243 mOhms
I don't think this is bad for being THAT old.

 

[Elektrotechniker]

What's the lowest mAh in a cell for which i can use a 4 W and not do cap/3 ?

 

[LarryDFW]

Silvershot;

If you are asking which battery capacity can supply a 4W laser diode,

that load would be 1 amp for an IR diode (only 4W we use).

You would want a little over twice that 1000ma in capacity.

I would use a "2800mah or higher" "brand name" cell.

LarryDFW

 

[qumefox]

Was wondering to build an instrument, just for the curiosity, but the double cycle reading present some problems, for analog instruments (analog sample and hold is a PITA ) ..... maybe is much more easy with a microcontroller ..... must be enough one with a pair of A/D input and an output ..... and a high current relais, maybe a mercury-wetted reed relais, or a power hexfet, taking in consideration also its RdsOn (also if, for some models, it can be ignored, being less than 10 milliohms ..... i have still some IRF3205 with 8 milliohms of RdsON) ..... For precision maniacs must be took in consideration also the temperature of the battery, i know (Ri increase when temp decrease), but at our level, i suppose it can be ignored .....

Actually I have just such an instrument already prototyped in hardware. It's PIC controlled constant current sink capable of reading both voltage and temperature and when done it will be capable of testing both internal resistance, and capacity at varying current draws... However.. I suck at teh code. So while the hardware was easy for the most part.. It's taking me forever to write the code to do it, since i'm having to learn C in the process.. Though since I plan on selling these, I refuse to farm out the code work.

 

[anselm]

I have tested all my 18650s from several batches (laptop battery) the other day.
Here is a table with data, if anyone cares...

 

[HIMNL9]

Actually I have just such an instrument already prototyped in hardware. It's PIC controlled constant current sink capable of reading both voltage and temperature and when done it will be capable of testing both internal resistance, and capacity at varying current draws... However.. I suck at teh code. So while the hardware was easy for the most part.. It's taking me forever to write the code to do it, since i'm having to learn C in the process.. Though since I plan on selling these, I refuse to farm out the code work.

LOL, we are in the same situation ..... easy to build circuits, but not easy programming

I was in the same condition, almost ..... started the hardware part, then stopped cause can't develope a software part good enpugh for it.

Perhaps we need a programmer (now, let me see who of the programmers here we can kidnap ..... j/k )