Different internal resistance readings from BT-3100 & a SM8124A (advice please) - Page 1
Forum section: electronics / beginners

author: luddite
author_rank: Newbie
timestamp: March 21, 2021, 09:43:08 pm
content: HII 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 197On 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

author: CaptDon
author_rank: Super Contributor
timestamp: March 22, 2021, 03:26:13 pm
content: 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.

author: luddite
author_rank: Newbie
timestamp: March 23, 2021, 01:08:33 am
content: 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 readingsBattery 1 40, 44, 64, 42Battery 2 65, 47, 96, 37Battery 3 53, 55, 69, 41Battery 4 77, 43, 80, 54Battery 5 70, 45, 76, 55So 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.

author: CaptDon
author_rank: Super Contributor
timestamp: March 23, 2021, 01:51:32 pm
content: 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).