no resistance between battery terminals - Page 1
Forum section: electronics / beginners

author: waspinator
author_rank: Contributor
timestamp: September 20, 2017, 04:19:17 pm
content: I'm trying to figure out if my battery is faulty or if I'm doing something wrong. Measuring continuity across the terminals without anything connected is showing a short. I've tried with another multimeter without a continuity tester, in the ohms range and the screen turns off. The voltage across the terminals reads 12.4v. it's a 12v lead acid battery.What kind of problem would have these symptoms?

author: stmdude
author_rank: Frequent Contributor
timestamp: September 20, 2017, 04:24:45 pm
content: Ow, that's not how you use a multimeter in the ohms-range / continuity, and it might break your meter.Hopefully your multimeter has enough protection to have survived it.What are you trying to do? "Faulty" is a broad definition.Fully charged, the battery should measure around 12.4V, so that checks out. That's about as far as you can check with a just a multimeter. If you want to check the internal resistance or the ability to deliver current, you'll need a load of some sort. If we're talking about a car battery here, you could hook up a headlight (for a car) to it, and see how much the voltage drops. For a regular headlight, it (more or less) shouldn't drop more than 0.1V or so.

author: waspinator
author_rank: Contributor
timestamp: September 20, 2017, 04:35:18 pm
content: Okay, thanks. I didn't realize I couldn't do that. I was investigating a short in my system (not a car), and I thought I'd disconnect everything and work my way up from the battery to see where it was. I tried this test on another 12v battery from a cordless drill and it should infinit resistance.What's happening when I'm measuring the resistance across terminals?

author: stmdude
author_rank: Frequent Contributor
timestamp: September 20, 2017, 04:47:41 pm
content: Quote from: waspinator on September 20, 2017, 04:35:18 pmWell, when measuring resistance, the multimeter will apply a small voltage to what you're measuring, and measure the voltage-drop across whatever you're measuring. Lets assume that the voltage it's applying is 1V.When you put the probes on the terminals, it got 12V into it instead of something that should be less than 1V. Thats quite a difference, and it'll break crappier meters.

author: Monkeh
author_rank: Super Contributor
timestamp: September 20, 2017, 04:49:38 pm
content: Quote from: waspinator on September 20, 2017, 04:35:18 pmIf you're getting 12V out of the battery how the hell could it be shorted?

author: oldway
author_rank: Super Contributor
timestamp: September 20, 2017, 04:59:10 pm
content: You can't measure the internal resistance of a battery with a multimeter, there are special tools for this, like Fluke battery analyser serie 500.http://www.fluke.com/fluke/uken/products/battery-analyzers.htmSome ESR meters with dc insulation can also be used...like this one:https://www.eevblog.com/forum/projects/esr-meter-adapter-design-and-construction/

author: kalel
author_rank: Frequent Contributor
timestamp: September 20, 2017, 04:59:16 pm
content: Quote from: waspinator on September 20, 2017, 04:35:18 pm(Disclaimer: Newbie here).From what I understand, a multimeter will have to use electricity in order to measure resistance.E.g., if you have a piece of wire, and want to measure the resistance of the piece, a multimeter will have to put current through the wire in order to measure the resistance.The more resistance there is, the more the voltage will drop. So, when the multimeter checks the voltage, the lower the measured voltage, the higher the resistance of the wire is (which caused the voltage to drop).When you connect a multimeter to the terminals of a battery, the multimeter will try to put current through the battery in order to measure the resistance.In this state, the multimeter will set itself in an "open to easily receive current" position (or low resistance position), which is fine when measuring the wire because the maximum the multimeter can receive is what it produced itself (the wire will not add any current).But a battery can produce a lot of current when attached to a low resistance. For example, if I was to connect a short piece of wire (very low resistance) directly between the battery terminals, the wire would heat up extremely and possibly burn out (better not to do this or do it extremely carefully to avoid burns or anything catching on fire).When you are connecting a multimeter in the "resistance measure" mode or "current measure mode", in both cases the resistance of the multimeter is very small. It will act almost like that short piece of wire (get very hot) and something inside may burn out (some multimeters have decent protection against this, but others do not). That is why you should never put the multimeter in resistance, continuity, diode check or current modes across a battery terminal (especially not mains, that could cause a multimeter to explode and injure you or worse). There may be additional modes that have low resistance on some multimeters that I didn't mention (I can't guarantee that there are not).Measuring voltage on a small battery directly is safe (as long as you don't short the terminals with a single probe, then the same or similar thing would happen as with a single piece of wire), because the multimeter is then in a high resistance mode. High resistance mode means a very small amount of current will flow through the multimeter (if the voltage measured is within the range the multimeter is supposed to measure), so nothing should heat up or be damaged.

author: stmdude
author_rank: Frequent Contributor
timestamp: September 20, 2017, 05:01:25 pm
content: Quote from: Monkeh on September 20, 2017, 04:49:38 pmDude, this is the beginners forum..

author: Zero999
author_rank: Super Contributor
timestamp: September 20, 2017, 05:29:32 pm
content: As everyone has said, a multimeter isn't suitable for measuring the internal resistance of a battery, which should be very low.The battery is reading the correct voltage.What's the problem? Do you just want to know if the battery is good or not? If so, connect a load to it, such as at car headlight bulb and measure the voltage. If it falls below 12V, whilst the load is connected, then the battery has a high resistance. It's most likely dead but may just need recharging. If the voltage stays at around 12V, with the load connected, then that means it's good.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: September 20, 2017, 05:45:00 pm
content: The answer you get when you hook a multimeter on the ohms scale will be wrong for the reasons explained above. But how it is wrong will depend on your meter and how you connect the leads. If the voltage applied by the meter is the same polarity as the battery the meter will observe some positive voltage that depends on the meter impedance, the voltage the meter is applying and the voltage of the battery you are measuring. In all cases it will be relatively high, often out of range and will be interpreted as open or very high resistance.Reverse the leads on the battery and the voltage will tend to be negative. Different meters will respond differently to this, either showing zero due to the designers interpretation that negative numbers are impossible, or some other weird response.This is possibly why you got the different results from your drill battery and the battery in your car. But it is also possible that the battery pack had the outputs disconnected. Some of the tool batteries have multiple terminals and use the extra terminals to sense correct connection before powering the supply terminals.

author: waspinator
author_rank: Contributor
timestamp: September 20, 2017, 11:15:23 pm
content: Thank you everyone for the great answers, I see now how foolish it was to try to measure for a short circuit at the battery terminals. It turned out a boost converter's inputs had shorted, which caused the initial problem. Luckily both my meters seemed to have survived my naive experiments.