Author Topic: Different internal resistance readings from BT-3100 & a SM8124A (advice please)  (Read 216 times)

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Offline luddite

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HI

I have been happy with my Opus BT-3100 v2.2 charger but wanted a dedicated (battery powered) resistance tester for when I am out and about.
I yesterday received the SM8124A which according to reviews is ok.

However I noticed the readings are way off between the two devices so I need help please.

On the Opus using the "quicktest" setting it showed a resistance of 197
On the SM8124A it returned 102.

This is happening on all the ones I am testing, roughly twice the amount between them.

Is there an easy way to find out which unit is closest to being accurate please without (if possible) doing Ohms law)

Thanks for reading
 

Offline CaptDon

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What are you testing? Some meters use a 1ma current source and convert voltage to ohms, others do it differently. This makes a huge difference when measuring things like diodes and transistors.
Senior Master Captain 140 passenger boat.
Private Pilot S.E.L. / CDL Class A / Motorcoach
Locomotive Electronics Engineer.
 

Offline luddite

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  • Country: gb
Thanks for reply, sorry for not being clearer.

18650's but it seems that I am not going get a consistent reading with the Opus anyway.

I tested in all four slots & got wildly different readings 

Battery 1  40, 44, 64, 42
Battery 2  65, 47, 96, 37
Battery 3  53, 55, 69, 41
Battery 4  77, 43, 80, 54
Battery 5  70, 45, 76, 55

So I need to ignore the opus and get a reading from something else to compare to the readings I am getting on the dedicated impedance meter (SM8124a)

I am really good at maths, even algebra seems easy but I just struggle with Ohms hence needed a comparable device. 
 

Offline CaptDon

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What are you trying to measure? The internal resistance of a battery? That is surely going to be a speculative number at best. How would you begin to even evaluate that resistance? It would change with state of charge, temperature and battery age. What will the number tell you? A battery with 40 milliohms of internal resistance could possibly still generate more current flow under heavy loads than one with a resistance of 30 milliohms because the one with 40 milliohms of resistance could be of better construction with an ability to give up more electrons at a faster rate. Using internal resistance as a measure of what a battery is capable of isn't really the factor to judge by. Back in the days of finicky NiCads I would give a bank of them an equal charge overnight at the recommended C/10 and then measure current output and rate of falloff into a heavy load of 10 milliohms total using a current clamp meter. This would give me sort of a baseline for each battery. Then let them sit at room temperature for a week and repeat the test. Very different results!!! Some of the ones that looked good during the first test just didn't do crap a week later!!! Ohm's law is exceedingly simple, if you are confused you are probably applying it wrong. Remember ohm's triangle, The Indian sees the Eagle over the Rabbit....E over I, E over R, I times R. Ohm's law is not even algebra, it is simple math (Well, the power equations may border on alg).
Senior Master Captain 140 passenger boat.
Private Pilot S.E.L. / CDL Class A / Motorcoach
Locomotive Electronics Engineer.
 


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