EEVblog Electronics Community Forum
Products => Test Equipment => Topic started by: sparrow321 on August 29, 2023, 05:57:55 am
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I've been thinking about simple testing battery internal resistance/voltage under load.
I realized that the oscilloscope have 50ohm impedance high load mode, so as long as we don't burn it its perfect to test IR by just switching the input impedance and comparing.
Is it such a bad/stupid idea to connect a battery directly to oscilloscope probe?
I plan on using zt-702s built-in scope for this.
Constructive criticism is welcome ;)
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Is it such a bad/stupid idea to connect a battery directly to oscilloscope probe?
I plan on using zt-702s built-in scope for this.
Constructive criticism is welcome ;)
Welcome to the forum!
Nothing terribly bad will happen, but keep in mind that only high-end oscilloscopes have 50-ohm terminated inputs (zt-702s doesn’t). You’ll have to add your own 50-ohm terminator (then remove it). Also, the ADC in the vast majority of oscilloscopes is only 8-bit, much worse than in any digital multimeter. Whatever you measure will be in 256 steps of the input selected range.
With these limitations taken into account, it should work :)
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The ZT-702S is a combo handheld 10K-count multimeter + oscilloscope. Dave's review is here:
https://www.eevblog.com/forum/blog/eevblog-1540-the-%2480-zotek-zt-702s-osciloscope-multimeter-review/ (https://www.eevblog.com/forum/blog/eevblog-1540-the-%2480-zotek-zt-702s-osciloscope-multimeter-review/)
You'll get a much more accurate voltage readings using the multimeter than you will trying to read a voltage off of an oscilloscope trace. The ADC used by the oscilloscope in the ZT-702S is the MS9280 which is only a 10-bit ADC whereas the 10K-count multimeter gives you at least 18 bits of resolution.
What kind of batteries do you want to test the internal resistance of? In general there is no problem connecting a battery directly to an oscilloscope probe as long as you observe the max voltage input of the scope.
I realized that the oscilloscope have 50ohm impedance high load mode ...
I couldn't find a reference to that in the user manual:
https://www.dropbox.com/s/03cvx7b5zk33wr4/ZT-702S%20EN.pdf?dl=0 (https://www.dropbox.com/s/03cvx7b5zk33wr4/ZT-702S%20EN.pdf?dl=0)
but in general such a feature is used for impedance matching of high frequency low-voltage signals, not power dissipation. Note that a 50 ohm load for a 3.7V Li-ion battery is only 75 mA -- a very light current draw.
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I appreciate the informative answers very much.
You raised some very good points that i overlooked..
1. Most important of all is the *nonexistent 50ohm input mode*
I'm so used to high-end scopes in the lab my brin just assumed any scope will have this option..
So using the dmm to measure without fiddling without any external connections to a resistor.
2. Another great point is the fact you want to actually draw a load not just use the resistor value as reference for the Voc-Vcc formula..
The batteries I'm testing some old nicd nimh i found.
Some of them went down to zero and i read that with some tlc they can actually be pretty much revived.
But i want to make sure all battery specs are up to par once i got them holding voltage
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Why don't you just use a DMM with a way to switch in and out a resistor of 50 ohms (it doesn't have to be exactly 50 ohms as long as you know exactly what the resistance is). You would have better accuracy as most DMM's have their best accuracy in DC voltage measurement.
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The Clarostat 240C decade power resistor box has been discontinued, but is readily available on eBay and test-equipment vendors.
See https://www.digikey.co.il/htmldatasheets/production/1934264/0/0/1/240c-resistance-decade-box.html (https://www.digikey.co.il/htmldatasheets/production/1934264/0/0/1/240c-resistance-decade-box.html) and note the maximum current for each switched range.
A modern replacement is made by IET https://www.ietlabs.com/hprs-series-high-power-resistance-decade-box.html (https://www.ietlabs.com/hprs-series-high-power-resistance-decade-box.html) that goes down to 1 m\$\Omega\$ per step (the Clarostat goes down to 1\$\Omega\$ per step)
It beats connecting power resistors manually to banana plugs when you need a load resistance.