40vern

Well I wanted to post a picture of my meter but my phone want let me.
I set the meter on 12v and unhooked the negative dattery terminal
I checked meter and I get 12
Does this mean I have a bad short?

rustypigeon

The Multimeter does not care if your battery terminal is hooked up or not. If your battery is charged to 12v, then the meter will read 12v, assuming you have the meter terminals on the battery posts.

irab88

iTrader: (4)

no.

if there is something draining the battery, the voltage will read slightly lower. the dmm will read lower than what the battery is actually capable of

if the battery is fully charged, it should read above 12.5 volts (closer to 13) when un-hooked from the car/truck.

scope103

The voltage of a lead-acid battery is temperature dependent, so a chart of voltage vs. percentage charge is very approximate. If you actually care about percentage charge you need to use a hydrometer.

But since a multimeter is easy to use, it's useful for telling whether your battery is dead vs charged.

The ESR (Equivalent Series Resistance) of a good car battery is so low that you really can't detect parasitic drains by measuring voltage. For instance, I doubt you could detect whether the dome light is on or not even with a good-quality 3.5 digit multimeter.

Last, the most important part of measuring voltage is that it is always "between" two points. So the state-of-charge discussion is for the voltage between posts of the battery. If 40vern were measuring the voltage between the negative post of the battery and the disconnected negative lead, that's something completely different (so he was wise to try to include a picture.)

snobdds

Simply put, if you only have 12v on the multi meter, then your battery is bad.

Replace an move on...

40vern

I left battery unhooked all night and I started this morning

40vern

I was trying to check the volts.between battery terminal and.battery cable

scope103

What that tells you is that there is "some" connection through the primary circuit. Like the clock.

If you're trying to tell whether you have a "parasitic current draw" you need to measure the current through the battery. Set your multimeter to read current, and start with your highest scale (probably 10amps, by moving the leads to a different jack). Connect that between the battery terminal and the cable. The meter itself is now a "dead short" so it is electrically like re-connecting the cable, but now you're measuring the current flowing when everything is off. It should read less than 0.100 amps (100ma). If you turn on the dome light it should come up to something closer to 1amp. Headlights will be more like 5-10amps.

Don't try to crank it; that takes somewhere in the neighborhood of 100amps and you'll incinerate your meter.

James Woods

iTrader: (1)

I couldn't follow why the argument over voltage was going on when all he had to do was measure the draw.

Once you get your reading, if it is higher then it should be, start pulling fuses and it should drop a small amount for each fuse but when you get the big drop is where your draw is going to be coming from. Depending on the circuit and your need for it, you can then just leave the fuse out or chase down the problem.

blake.nemitz

wait a minute, only twelve volts, is this running? if you only had 12 running then youve got bigger problems than your battery however if that is the reading when shut off then ok. as far as to where it is going i have no idea where to start and have nothing constructive to add

scope103

He said he disconnected the ground cable. Probably not running.

Plus, he said he got "12" volts. Not 12.00. Maybe he's using an analog voltmeter, and just read off the nearest whole number. (I think he was expecting "0" from the negative terminal to the ground cable.) But as many have suggested, reading all 3 (or 4) digits can be useful information.

40vern

Ended up being the alternator
I wanted to use the meter to check for a draw but I don't think I had the meter on the correct setting
I unhooked the negative cable on my Ford and I got the same readings as I did on my Toyota
I had the meter set on 12 volts

Any way I did a search and read the check the battery first then the alternator

Well since I left the battery unhooked all night and it held a charge then I checked the alternator at orileys

Seems to run allot better and maybe it is helping with the t p s and all that good stuff

Thanks to all

scope103

Thanks for the followup. We often never hear of the resolution.

40vern

No problem
When I do search I find allot of people with the same problems but most leave you hanging cause they never tell you what solved their problem.

40vern

started having problems again
battery draining over night
the picture is of the meter connected to the negative battery cable and the negative terminal
i was wondering if the reading is good or bad?
thanks again

osv

throwing away a car battery simply because it's showing 12v could result in throwing away a perfectly good battery, that only needs to be charged up.

I imagine that you meant something different there, but i'm not sure what.

the way to handle it is to unhook a terminal on the battery, then charge the battery, disconnect the charger, check the voltage, then maybe leave it overnight, check the voltage again, to see if it's holding the same charge.

then put it in service, and see where you are.

osv

voltage across, current through... all you are showing there is some current drain, which may or may not be an acceptable amount... I can't see the scale that the meter is on.

typically there will always be some kind of current drain, so that reading may never be zero... in my rig, there is a digital clock that's lit up all night, for instance.

we know that if you disconnect the truck battery, it resets the ecu... so I also wonder how much, if any, current drain is required to maintain the ecu... on your pc or laptop, there is a small battery that keeps the voltage up, but I don't think that you'll see that type of battery on the ecu in your truck... so I wonder if it's not using a small amount of current to maintain it's state?

at any rate, the battery could be failing, if it can't support a very small current drain... measuring parasitic current like you are doing there is helpful for troubleshooting, but it's not how you test whether or not the battery is good.

osv

here is a detailed procedure for testing your battery:

http://autorepair.about.com/library/.../aa101604a.htm

scope103

If the meter is set up correctly (usually the 10A scale requires plugging the leads into different jacks), then you're showing .003 amps or 3ma. Which will run a 110amp-hour battery down in about ... 3 years. (Don't to get too excited; self-discharge rate of a lead-acid battery is faster than that.) Point is, if your meter is set up correctly that draw will not discharge a good battery in any reasonable length of time. 3ma is about what the clock and radio together draw.

You could, now, switch to a ma scale to get a more accurate reading (your .003 is probably +/- about .020). But even if it's really 45ma that's not draining your battery.

I'm guessing a bad battery. Make sure it's charged, then disconnect it and leave it overnight. Hook it up in the morning and try to start it (better yet, hook it up and measure the voltage while someone is trying to start it).

40vern

thanks i just replaced the battery this weekend
when i get back home i will look at it again