YACT: How do I test how much of the battery is being used when my car is off?

[Shawn]

I set my multimeter to amps and connected the positive to my car battery's positive and the negative to the chassis it just blew the fuse.

I then tried connecting it to my multimeter's unfused connection and it let out a huge spark when i touched it to the positive battery terminal followed by a burning smell.

I took my multimeter apart and it cooked the trace on my multimeter to the unfused connection. I can patch the trace with my soldering iron but I guess I'm doing something wrong. Either that or it's using more than 50 amps while off?

 

[The Stigenator]

shirted batt?

 

[Shawn]

Dunno, but it usually takes several tries to get my car started. It doesn't even turn over, it just clicks. Eventually it'll catch and it starts right up so the starter is fine.

After I drive it for a while I can start the car ok but once it sits overnight I have trouble starting it again. I suspect it's the battery but I just wanted to make sure something else isn't drawing a lot of current while the car is turned off. The battery is less than a year old.

 

[Howard]

Yep, you're shorting the battery. Ammeters by design have 0 (as close as possible, anyway) resistance. Put it in series.

 

[Shawn]

Originally posted by: Howard
Yep, you're shorting the battery. Ammeters by design have 0 (as close as possible, anyway) resistance. Put it in series.

How do I do that?

edit: Put the multimeter between the positive terminal on the battery and my car's positive connection?

 

[Zenmervolt]

Did you even bother to read the instructions with your multi-meter to know how to hook it up when using the ammeter?

"Mistakenly placing the ammeter in parallel with a circuit will blow the fuse, possibly damaging the meter and causing injury."

You fried your multi-meter when you used the un-fused connection. That means you're out at least $100 if you had a half-decent meter. Buy a new one and connect one side of it to the car's positive cable and the other to the battery's positive terminal (or do the same with the negative side).

ZV

 

[Shawn]

Originally posted by: Zenmervolt
Did you even bother to read the instructions with your multi-meter to know how to hook it up when using the ammeter?

"Mistakenly placing the ammeter in parallel with a circuit will blow the fuse, possibly damaging the meter and causing injury."

You fried your multi-meter when you used the un-fused connection. That means you're out at least $100 if you had a half-decent meter. Buy a new one and connect one side of it to the car's positive cable and the other to the battery's positive terminal (or do the same with the negative side).

ZV

Nah, I already fixed it.

 

[DarkKnight69]

Originally posted by: Shawn
I set my multimeter to amps and connected the positive to my car battery's positive and the negative to the chassis it just blew the fuse.

I then tried connecting it to my multimeter's unfused connection and it let out a huge spark when i touched it to the positive battery terminal followed by a burning smell.

I took my multimeter apart and it cooked the trace on my multimeter to the unfused connection. I can patch the trace with my soldering iron but I guess I'm doing something wrong. Either that or it's using more than 50 amps while off?

You are retarded, you need to break a connection to measure amperage...

Meaning u need to take one wire off and use the MM as the wire...

 

[Zenmervolt]

Originally posted by: Shawn

Originally posted by: Zenmervolt
Did you even bother to read the instructions with your multi-meter to know how to hook it up when using the ammeter?

"Mistakenly placing the ammeter in parallel with a circuit will blow the fuse, possibly damaging the meter and causing injury."

You fried your multi-meter when you used the un-fused connection. That means you're out at least $100 if you had a half-decent meter. Buy a new one and connect one side of it to the car's positive cable and the other to the battery's positive terminal (or do the same with the negative side).

ZV

Nah, I already fixed it.

You can fix the multi-meter but you don't know how to measure amperage?

ZV

 

[nineball9]

Originally posted by: Shawn

Originally posted by: Howard
Yep, you're shorting the battery. Ammeters by design have 0 (as close as possible, anyway) resistance. Put it in series.

How do I do that?

edit: Put the multimeter between the positive terminal on the battery and my car's positive connection?

As Howard noted, you shorted the battery. Functionally you did the same as connecting the positive and negative posts of the battery with a wire. Instead of a wire you used a meter which can not handle the several hundred amps a battery can put out, and even if it could, you'd mess up your battery.

Sheesh, I type slowly!

To measure current with a meter, you need to break the circuit and connect the meter in-line. That is, disconnect or cut a wire (or connection) and connect the meter to each end of the disconnected/cut wire or connection. This way, the meter is in series with the circuit and all the current flows through the meter.

That said, home meters do not have the capability of measuring the current an automobile battery can source unless you want to measure a low current device of which there are few in a car. Typical home meters can measure 2 to 10 amps DC (your meter will vary); a car battery can source many hundreds of amps.

Another method of measuring current is a clamp device which goes around the wire to be measured instead of having to cut the wire. Auto repair shops use these clamp leads to measure current as it's easier and their devices can measure 600 amps or more. You can purchase speciality clamp meters for DC and sometimes AC circuits but they tend to be on the expensive side especially if you want to measure currents found in automobiles.

 

[Shawn]

OK, I measured the amps and it's using 65 mA while the car is off. Does that seem normal?

edit: the repair manual says 20mA max for my car. I added an aftermarket stereo and alarm so that may be what's taking up the rest.

 

[DarkKnight69]

Originally posted by: Shawn
OK, I measured the amps and it's using 65 milliamps while the car is off. Does that seem normal? I think I read that as long as it's under 300 it's ok.

Why are you doing this experiment? What kinda car?

That sounds about correct to me...

 

[Shawn]

Originally posted by: DarkKnight69

Originally posted by: Shawn
OK, I measured the amps and it's using 65 milliamps while the car is off. Does that seem normal? I think I read that as long as it's under 300 it's ok.

Why are you doing this experiment? What kinda car?

That sounds about correct to me...

Read the 3rd post. It's a 94 miata.

 

[Shawn]

With my alarm armed the car is drawing 85mA. Guess I have a bad battery then?

 

[DarkKnight69]

To see if u have bad battery, you should be testing voltage, not amperage.

If it is below 11 volts (or so) off, you likely have a bad battery...

 

[Evadman]

Originally posted by: Shawn
OK, I measured the amps and it's using 65 milliamps while the car is off. Does that seem normal? I think I read that as long as it's under 300 it's ok.

65 is high, though it may be in spec for your car. 300 is deffiantely too high. 25 used to be concidered the norm, but the more electronics in your car, the more standby power will be pulled. If a battery is 44 amp/hr that means a 300ma draw would kill it in 44/(1/.300) = 146 hours or about 6 days. So if you left your car off for 6 days it would not start. My truck pulls either 19 or 20 ma, I forget which, it was a while ago that I checked. Sounds like you are correct, this is either your battery or the cables. The 'trying to start' is a good indication of a poor connection or corrosion.

A optima red top is only 44 amp/hours IIRC, which is a good standard.

Originally posted by: DarkKnight69
To see if u have bad battery, you should be testing voltage, not amperage.
If it is below 11 volts (or so) off, you likely have a bad battery...

<bzzzzt> wrong. But thanks for playing. Please enjoy this copy of 'Armchair Mechanic' for your home enjoyment.

 

[TheGizmo]

Originally posted by: Shawn
Dunno, but it usually takes several tries to get my car started. It doesn't even turn over, it just clicks. Eventually it'll catch and it starts right up so the starter is fine.

After I drive it for a while I can start the car ok but once it sits overnight I have trouble starting it again. I suspect it's the battery but I just wanted to make sure something else isn't drawing a lot of current while the car is turned off. The battery is less than a year old.

sounds more like the battery terminals aren't making contact all the time. usually if you hear clicking its a sign that your battery is dead or not connected, then if all of a sudden it works its a sign that the battery is not connected properly. though it could be all kinds of other things, but thats my half-assed guess

 

[DarkKnight69]

Originally posted by: Evadman

Originally posted by: Shawn
OK, I measured the amps and it's using 65 milliamps while the car is off. Does that seem normal? I think I read that as long as it's under 300 it's ok.

65 is high, though it may be in spec for your car. 300 is deffiantely too high. 25 used to be concidered the norm, but the more electronics in your car, the more standby power will be pulled. If a battery is 44 amp/hr that means a 300ma draw would kill it in 44/(1/.300) = 146 hours or about 6 days. So if you left your car off for 6 days it would not start. My truck pulls either 19 or 20 ma, I forget which, it was a while ago that I checked. Sounds like you are correct, this is either your battery or the cables. The 'trying to start' is a good indication of a poor connection or corrosion.

A optima red top is only 44 amp/hours IIRC, which is a good standard.

Originally posted by: DarkKnight69
To see if u have bad battery, you should be testing voltage, not amperage.
If it is below 11 volts (or so) off, you likely have a bad battery...

<bzzzzt> wrong. But thanks for playing. Please enjoy this copy of 'Armchair Mechanic' for your home enjoyment.

You do relize what is being drawn from the batt has little to do with whether or not the batt is good or not...

Voltage is a good way to see if you have failing cells...

 

[Shawn]

The battery is 12.43v.


I did a little experimenting and I found out that my car without the amplifiers and after market trunk light hooked up only uses 20mA.

The 2 amplifiers draw 30mA alone and my after market trunk light draws 10mA even when off. WTF?!

 

[Shawn]

Alright now I know why the trunk light was drawing so much. I was following where the trunk light wires went and discovered/remembered that the trunk light is wired to the alarm, so not only will the light turn on when the trunk is opened but the alarm will go off if it was armed.

 

[So]

Wow. It's all been covered in the thread, but the image of someone ignorantly connecting an ammeter in parallel and being shocked (pun!) when sparks fly is hilarious to me.

<--- EE

Nothing against you, I can definitely understand how someone totally ignorant of electrical education could make that mistake, but I'm so used to it.