measuring a 12 volt battery with an oscilloscope  - Page 1
Forum section: electronics / beginners

author: Rhetro
author_rank: Newbie
timestamp: June 07, 2025, 12:04:54 am
content: hello all!a stupid question. I just don’t know enough about these things -yet. sorry.I measured a 12V battery with a probe and attached ground lead:probe to +ground lead to -read somewhere that 12 v is enough to ruinthe oscilloscope -can’t remember where..it’s isolated so i can’t see howthat can be true.thanks in advancei’m just paranoid

author: themadhippy
author_rank: Super Contributor
timestamp: June 07, 2025, 12:58:49 am
content: QuoteAll depends on the scope,if its maximum input level is 5v then it aint gonna be happy with 12v shoved up it,if however its maximum input level is 300v 12v aint gonna bother it

author: Doctorandus_P
author_rank: Super Contributor
timestamp: June 07, 2025, 01:04:45 am
content: Quote from: Rhetro on June 07, 2025, 12:04:54 amThank you for the warning.I'm allergic to stupid questions.

author: wraper
author_rank: Unknown
timestamp: June 07, 2025, 01:13:14 am
content: Any oscillscope will take way more than that just fine unless it's a pocket toy and not a real oscilloscope.

author: radiolistener
author_rank: Super Contributor
timestamp: June 07, 2025, 01:38:53 am
content: it depends on selected input impedance and frequency. If you selected 50 Ω then absolute maximum input is usually 5 Vrms and if you apply 12 V DC for 50 Ω input it will damage your oscilloscope input.For 1 MΩ limit is usually much higher, but very depends on frequency. For DC it can be even 300 V, so there is no issue to attach 12V DC for 1 MΩ input. But if you apply high frequency AC, this limit is decreased to about 40 V for MHz range and above

author: TimFox
author_rank: Super Contributor
timestamp: June 07, 2025, 03:14:49 am
content: Where was it written that 12 V would ruin an oscilloscope?As radiolistener pointed out, that can be true for a 50 ohm input, where 12 V DC would dissipate about 3 W, which is probably too high.However, typical oscilloscopes have 5 V/div maximum, which implies no problem up to 40 V pk-pk into the high impedance input.

author: Psi
author_rank: Super Contributor
timestamp: June 07, 2025, 03:29:07 am
content: It would have to be a very oddball scope to be damaged from 12V. They probably exist but youd very unlikely to come across one.It will say the max p2p voltage written on the input terminal, so you can always check there.It's usually between 200-400V peak to peak.Note that if it's something other than a scope, like a spectrum analyzer, then the max voltage with be MUCH lower than above.Probably more like 25-50V but could be lower

author: tautech
author_rank: Super Contributor
timestamp: June 07, 2025, 04:56:25 am
content: Quote from: Rhetro on June 07, 2025, 12:04:54 amConfirmed and welcome.We each must know the capability of each of our instruments which are normally outlined in their datasheets.Any half decent scope won't be hurt by 12V except in 50 Ohm input mode where the norm is 1M Ohm input.This presumes a direct connection whereas most scope use is with a 10:1 (10x) probe that forms a voltage divider for the scope to see only 1/10th of the voltage on the probe.A greater risk is of ground loops where the reference (gnd) lead is tied to protective mains ground earth so it is wise to know/find if the probe reference point is elevated above mains ground.Have fun and play safe.

author: BennoG
author_rank: Frequent Contributor
timestamp: June 07, 2025, 06:41:54 am
content: The only way you can and will damage your scope with a 12V car battery is put 1 ground clip on the - and the other ground clip on the +.If your are lucky you will only burn your probes but there is also a real change you will burn your scope.Benno

author: radiolistener
author_rank: Super Contributor
timestamp: June 07, 2025, 04:15:29 pm
content: Quote from: BennoG on June 07, 2025, 06:41:54 amNo. That's not true.Actually, if you are using a modern professional oscilloscope, it typically features a 50 Ω input mode designed for precise measurements with minimal noise. In this mode, the maximum allowable input signal amplitude is usually max 5 Vrms. Therefore, directly connecting a higher voltage source - such as a 12 V car battery to the oscilloscope input operating in 50 Ω mode can very likely cause serious damage or degradation to the input circuitry. It’s important to always consider the input mode and the specific oscilloscope’s specifications when connecting high-voltage sources.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: June 07, 2025, 11:31:57 pm
content: I agree with Radiolistener.If you have a modern scope and use a 10X probe, you will not have any trouble. Or for that matter any probe at 12 Volts.50 ohm input is only for measuring the output of something that has a 50 ohm impedance, and relatively low output, like your Signal Generator. You only use it when you are connecting directly to a 50 ohm coax,Modern scopes will not let you use 50 ohm input with the 10X probe.It does not matter if you connect the probe to pos or neg and the GND lead of the probe to the other battery terminal. You will read the battery voltage as pos or neg depending on your probe position.

author: golden_labels
author_rank: Super Contributor
timestamp: June 08, 2025, 12:50:40 am
content: Measurement instruments have their input limits printed near the connectors. Exactly for the purpose of avoiding such mistakes. Check that, done. Ask, if unsure or don’t know how to interpret the markings.

author: Rhetro
author_rank: Newbie
timestamp: June 08, 2025, 01:21:59 am
content: right, because it's atypical for a beginner to do that.Hey Doc: Why is an olympian such as yourself hanging out at the kiddie pool ?

author: Rhetro
author_rank: Newbie
timestamp: June 08, 2025, 01:45:11 am
content: Thanks for all the help folks.I just jumped in with a used cheap Hantek DS05102P. (start small)I should have been more clear:The battery I tested a 12V 7Ah alarm battery (it may have been misconstrued as a car batt. -hence the answered email that referred to me ruining it..)Ok. I did have the probe set to 10X and showing that it has a max of 300v RMS?I got the thing used with no literature. Jumped the gun (my mistake) and just figured that it was isolated (as a small batt) from ground mains (I know I know...I'm learning).Found out after the fact that I just need to connect the probe itself to the positive, correct?To all who helped:Thanks for your patienceback to the library (internet..)Cheers!

author: radiolistener
author_rank: Super Contributor
timestamp: June 08, 2025, 03:51:22 am
content: Quote from: Rhetro on June 08, 2025, 01:45:11 amThe 300 V rating applies to DC and low-frequency signals. At higher frequencies, particularly above 10–100 kHz, the allowable voltage significantly decreases, dropping to approximately 10-25 V at 100 MHz due to capacitive and dielectric limitations.You can find curve with voltage limit vs frequency in the probe manual

author: Terry Bites
author_rank: Super Contributor
timestamp: June 08, 2025, 05:58:47 pm
content: Why would you do that?

author: Rhetro
author_rank: Newbie
timestamp: June 09, 2025, 06:53:46 pm
content: my burglar alarm was making all sorts of racket. So I called up the alarm company and asked them what gives?They said that the 12V battery was bad and to get a new one -which I did; problems solved.But before doing so, I wanted to measure the voltage of the bad battery and check if it was still putting out 12V. And I suppose, see what the signal of a 'bad battery looks like' on an oscilloscope -rather than just a rating on a voltmeter.So. I aside from some arcing, wisps of smoke, and melting plastic -not that that happened (really, it didn't) How would I know if there's a problem with it? It turns on just fine. It seems normal. I looked at other smaller batts with it. AA, 9V. Just playing around since I just got the oscilloscope (again, it came with no literature).I figured checking the 12v was a good first experiment with it.I broke it, it's essentially useless -my fault entirely.If so, I'll take my lumps, and be more prudent next time.Live and learn.

author: bdunham7
author_rank: Super Contributor
timestamp: June 09, 2025, 07:23:33 pm
content: Quote from: Rhetro on June 09, 2025, 06:53:46 pmA battery will just show you a flat line, nothing interesting or any improvement over a meter. A battery connected to a charging circuit may show you something else and that may be interesting. If the battery is disconnected and sitting by itself, you just connect both terminals to the oscilloscope probes or connectors without any worries. If the battery is in a circuit, you need to make sure you know what part, if any, of that circuit is grounded. Standard Dave Jones EEVBlogvideo here:https://www.eevblog.com/2012/05/18/eevblog-279-how-not-to-blow-up-your-oscilloscope/What is broken and exactly how did you connect it to break it? I wouldn't expect any serious or unrepairable damage from a 12V battery.

author: Rhetro
author_rank: Newbie
timestamp: June 09, 2025, 09:56:06 pm
content: Hey Bdunham7Watched the video. Most helpful!!Ok. Floating circuit (widget). Got it! That was my intention. I didn't want anything connected to the battery for that reason (the whole earth mains scared me) .So if it's a charger connected to AC, the probe ground clip MUST be connected to the ground on the charger -and not ANY other point, or bang!What I meant to say earlier, was IF I broke it..I connected the probe to the positiveand the ground clip to the negative.Thanks again!back to the books (vids)cheers!!