Multimeter to check car battery current

Question

short background, my car battery will often be too flat to start the car if I don't use it for about 48 hours. Worst it happened the other day after leaving it at airport car park for 4 days and come 10pm ready to go home it was flat, AA came and it was reading 7 volts. Took car to Kwik Fit next day for a battery check and came back no problems.

So, given the battery's health seems to be fine, after doing some reading I thought I would get a multimeter to check how many amps the battery is using when the car isn't running, as my assumption is there must be a drain from somewhere. I managed to use it to check the volts late last night (12.56) and this morning, it was around 12.26. Following some guides (https://www.youtube.com/watch?v=zdIKNnwEjIs | http://www.popularmechanics.com/cars/how-to/a5859/how-to-stop-car-battery-drains/ | http://www.wikihow.com/Find-a-Parasitic-Battery-Drain) I tried to check the amps but when I do it, I believe I see a 3.something come up on the multimeter but when putting the test probe on the end of the black cable onto the negative battery post, it creates a spark, and so naturally I just took the probe off it straight away as to not cause any (further) damage.

This is the multimeter I bought - http://www.argos.co.uk/static/Product/partNumber/7015603.htm

Car is a 2004 Subaru Impreza GX Sport.

When checking the amps I put the black cable in the COM port and the RED cable in the 10A port and set the dial to A_ (DC) 20m/10A (just to the bottom left of the dial)

Just don't know if those settings are right and why there would be sparks? Don't know if it is a problem with the car/battery terminals/posts or just the settings on the multimeter.

Any help would be greatly appreciated as I really want to just be able to measure the amps so I can find a paristic battery drain.

Answer 1

To check the battery voltage a meter is connected in parallel. This involves touching the black lead to the negative battery terminal and the red lead to the positive battery. In this configuration the meter has a very high resistance (usually over 2 million ohms).

Checking current is done differently, in series. If you did the procedure above you may have damaged your meter or blown the meter fuse. To check current disconnect the negative battery cable. Connect the black lead to the now naked battery post. Connect the red lead to the battery cable. Current will now flow through the meter allowing it to be measured. In this configuration the meter has a very low resistance (usually just a couple of ohms).

Set the meter to the 10A range. After the battery is disconnected and the meter is connected there will be a sudden inrush. If you use the 20mA setting the meter fuse may be blown during the inrush. After the inrush allow the car to fall asleep. This may take some time, sometimes upwards of an hour. A rough rule of thumb is less than 25mA of draw when the car is fully asleep.

The absolute best way to check it is to use a battery disconnect. Install this between the negative terminal and the negative battery post. Drive the car for a day or two. In the morning before going somewhere connect the meter across the green nob. Make sure that the leads are well connected and don't fall off. Now twist the green nob to disconnect the battery and read the current consumption.

Answer 2

The safest way to test if you're not SURE of what you're doing (and to avoid other issues like bad lead connections) is to use a clamp meter. These are capable of measuring AC and DC voltage and amperage by simply clamping the wire. You don't have to disconnect anything for this to work. Not to mention that measuring this ONE wire coming right off of the battery probably isn't going to tell you much... even if you do determine that there is a reading of several amps coming off of your battery, you still have to track that down. The clamp meter would be better suited for the needle-in-a-haystack search than your traditional probe meter anyway.

Here's an example of one such meter: http://www.northerntool.com/shop/tools/product_200460552_200460552

If you go this route, make sure you buy a clamp meter advertised to measure amperage on both AC AND DC. Some of them are AC only, for some reason.

EDIT: After I answered this I started thinking about why some clamps can't read DC amperage and looked it up. This is somewhat anecdotal as it does not answer the question, but it might be interesting to some people. Apparently, clamp meters use electromagnetic induction to read AC amperage, but this is not possible on DC. The clamps are constructed slightly different to make this possible. See this link for a more in-depth explanation: http://www.kew-ltd.co.jp/en/support/mame_02.html

Answer 3

I suggest removing fuses one by one to measure current, which will find a vampire load. Perhaps someone has even made a fuse-shaped current probe.