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Car Forum / Driving, Maintenance, Tuning / Maintenance and Repair / February 2007Test for unknown current draw on battery?
Help please! I either have a marginally bad battery or I have an unknown current draw to cause a dead battery? I put a test meter on ohm setting at X10 between the pos and neg cables (battery removed) and I get a slight movement of the meter needle - I don't recall the actual reading. So I opened the side door to simulate for the dome light to go on and got a major jump of the meter needle. Is this a valid test to verify a current draw on a vehicles battery? Should there be any movement at all of the meter needle if there is zero draw? Or am I totally out of bounds on this? Thanks, Steve This is on a 1990 Ford F-150 with about a 5 or six year old battery - only the 2nd battery that's ever been in the truck since new - @130K miles. > Help please! I either have a marginally bad battery or I have an > unknown current draw to cause a dead battery? I put a test meter on [quoted text clipped - 3 lines] > light to go on and got a major jump of the meter needle. Is this a > valid test to verify a current draw on a vehicles battery? No. > Should there be any movement at all of the meter needle if there is zero > draw? Or am I totally out of bounds on this? It's really beyond the scope of a newsgroup to teach basic electricity... but with that said: You're using the wrong function of your multimeter. Measure the current drawn from the battery starting with the highest DC Amps range your meter has, and working your way down in ranges until you get a measurable reading in milli- or micro-amps with all electrical loads off, including the dome light, and report the reading back here. >You're using the wrong function of your multimeter. Measure the current >drawn from the battery starting with the highest DC Amps range your meter >has, and working your way down in ranges until you get a measurable reading >in milli- or micro-amps with all electrical loads off, including the dome >light, and report the reading back here. Keep in mind that most meters will go up in smoke if you don't have a high enough range. Don't try it if you don't have a 10A range. Don't do anything stupid like turn on the headlights or step on the brakes while the meter is in series. I can't imagine anything constructive happening when connecting an ohm meter across a batteries terminals, or even between 1 and ground. You'll probably damage the meter. To check how much current you have flowing while the engine is off, disconnect one of the terminals, then connect your meter between the disconnected terminal and the disconnected cable. The meter has to be set to read amperage, and this sometimes entails unplugging one of the test leads and plugging it into a different hole. Depends on the meter I guess. Not at all an unusual question for a newsgroup. Heck people learn a lot more complicated topics then that! If you still need help, e-mail me off list. > I can't imagine anything constructive happening when connecting an ohm > meter across a batteries terminals, or even between 1 and ground. > You'll probably damage the meter. Read the original post more carefully. He said the battery had been removed. He was essentially testing the resistance from the positive battery terminal to chassis ground with no battery in place. That wont damage any meter. >> I can't imagine anything constructive happening when connecting an ohm >> meter across a batteries terminals, or even between 1 and ground. >> You'll probably damage the meter. >Read the original post more carefully. He said the battery had been >removed. >He was essentially testing the resistance from the positive battery terminal >to chassis ground with no battery in place. >That wont damage any meter. I was replying to a post about using a current range. You must put the meter in series with the battery to use a current range and most cheap meters only have 0.5-1 watt shunt resistor. It doesn't take much to smoke it. "AZ Nomad" <aznomad.2@PremoveOBthisOX.COM> wrote in message news:slrnespia7.o72.aznomad.2@ip70- > >That wont damage any meter. > > I was replying to a post about using a current range. You must put the > meter in series with the battery to use a current range and most cheap > meters only have 0.5-1 watt shunt resistor. It doesn't take much to smoke it. Absolutely correct. His original test was okay as far as it went. It just didnt go far enough. If he found a low resistance, it would be indicative, but not diagnostic. Don mentioned the action of semiconductors, such as diodes, and he is correct about that too. While an ohmmeter might indicate, for example, a lamp active in a circuit, you would need to reverse the ohmmeter connections to be sure that it was a linear resistance and not a semiconductor, perhaps like the diodes in the alternator. I have a large diameter wire shunt (with clamps on each end) I put in series with the battery and cable . That way, I can hook it up and let everything stabilize with full battery voltage at whatever resting current develops. I can then use the voltmeter function to determine the current through the shunt. Keeps me from burning up meters. The voltmeter function (which has a high impedance) is practically immune to burnout when used in this way and does not significantly change the current draw. "You want the meter set to measure milliamps of current, not resistance." Rather start out at the largest ampere setting. In the event of leaky diodes in the alternator, a parasitic draw of close to 4 amps is possible. 4 amps = 4,000 milliamps "He was essentially testing the resistance from the positive battery terminal to chassis ground with no battery in place. That wont damage any meter." No, it won't. And unless I'm reading it wrong, it won't register anything at all, regardless of any situation. Maybe you meant between the disconnected negative battery cable and ground. You should get 0 ohms, unless there's corrosion. They're supposed to be 2 common points. But no I didn't read the original post very carefully. But regardless, resistance measurements aren't what's needed here. > No, it won't. And unless I'm reading it wrong, it won't register > anything at all, regardless of any situation. Maybe you meant between > the disconnected negative battery cable and ground. You should get 0 > ohms, unless there's corrosion. They're supposed to be 2 common > points. Thanks for all the help! I definitely did get a meter reading on ohms with the battery removed and taken between the pos and neg cables. I didn't say I knew what I was doing....... ;-} My multitester only has DCmA up to 250. I didn't use the meter anymore for fear of wrecking it. I put in a good, charged, spare battery that I had from another vehicle and when I connected the terminals I immediately heard something like the EFI cycle on and off?? I don't remember that happening before when disconnecting and reconnecting the battery?? The engine started and ran fine. The guage in the dash looked normal - needle about center - not discharging at all When I shut the engine off I just disconnected the neg cable until I can figure it out better. Any ideas on the EFI cycling on and off when reconnecting the battery would surely help?? I have the dead battery charging now and have to wait to see if it takes a good charge. Thanks again! Steve >Any ideas on the EFI cycling on and off when reconnecting the battery >would surely help?? It is normal for it to make some noise when connecting a battery back up. >I have the dead battery charging now and have to wait to see if it >takes a good charge. . Earlier you stated the battery was five or six years old correct? If it is then you have most likely found the culprit. I would just hook up the new battery and see what happens... chances are way in your favor that it will be fine. If that battery goes dead too then go get a good dvm that has a 10amp setting on it so you can see what is going on. Steve B. "Chris" <chrism3667@yahoo.com> wrote in message > That wont damage any meter." > [quoted text clipped - 3 lines] > ohms, unless there's corrosion. They're supposed to be 2 common > points. Nope, bubba. You should get infinitely high ohms, not zero. If it is zero, then you will get an infinitely high current flow. And if you read what he said, he was measuring between the positive cable and ground (with the battery removed), which might indicate a short circuit, a lamp active, etc. >"Chris" <chrism3667@yahoo.com> wrote in message >> That wont damage any meter." [quoted text clipped - 11 lines] >and ground (with the battery removed), which might indicate a short circuit, >a lamp active, etc. IT INDICATES NO USEFUL INFORMATION WHATSOEVER! He is measuring the resistance of all sorts of solid state electronic devices. The reading could be quite low depending on the ohmeter's polarity. Impose a backwards voltage on an alternator and it is essentially a short circuit to anything above the voltage required to make the diodes conduct. Even if the polarity of the ohmmeter correctly imposes + voltage on the + battery cable, the resistance reading is still totally meaningless and unpredictable because none of the circuits -- such as courtesy light timer to mention just one -- can reach a normal condition unless provided 12 volts and allowed to time out. NO conventional ohmeter imposes anywhere close to 12 v on the circuit it is measuring. Don www.donsautomotive.com "Don" <don@NO-SPAMdonsautomotive.com> wrote in message > IT INDICATES NO USEFUL INFORMATION WHATSOEVER! He is measuring the > resistance of all sorts of solid state electronic devices. The [quoted text clipped - 11 lines] > Don > www.donsautomotive.com Have it your way, Don. "Steve" <kskenter@2z.net> wrote in message Should > there be any movement at all of the meter needle if there is zero > draw? Or am I totally out of bounds on this? > Thanks, > Steve You are not totally out of bounds. If you can get a valid resistance reading, you can estimate* the current draw using Ohms law. For example, if you found the resistance to be 990 ohms to ground, and E= I*R or (13.5=I*990) you can assume the resting current might be near 14 milliamperes. As another posted mentioned, it is often better to use the ammeter function in series with one of the battery leads. In this way you can read the current draw directly. Be careful, or you can ruin a meter this way. Not all systems shut down immediately when you turn the key off. Sometimes you have to wait a few minutes before they hibernate. Remember this when you put the ammeter in series with the battery cable. Now, what is normal? Varies. 10-50 milliamperes is not abnormally. As you approach 100 ma or more, you are getting into the realm where resting current becomes more of a concern. >"Steve" <kskenter@2z.net> wrote in message Should >> there be any movement at all of the meter needle if there is zero [quoted text clipped - 4 lines] >You are not totally out of bounds. If you can get a valid resistance >reading, That's a big "if." Any modern car has solid state electronic devices that remain connected with the key off. Some can take up to 1/2 hour to time out to a reduced draw. If they don't have 12v supplied they may never time out. Also the resistance of semiconductors is varies enormously depending on voltage supplied and polarity. For any vehicle with modern electronics an ohmmeter test is totally useless to calculate parasitic draw. I am not aware of any normal ohmeter that supplies a constant 12v to the resistance it is measuring. Don www.donsautomotive.com > you can >estimate* the current draw using Ohms law. [quoted text clipped - 18 lines] >more, you are getting into the realm where resting current becomes more of a >concern. > Help please! I either have a marginally bad battery or I have an > unknown current draw to cause a dead battery? I put a test meter on [quoted text clipped - 11 lines] > only the 2nd battery that's ever been in the truck since new - @130K > miles. Its a valid test, and it sounds pretty normal. With a battery that old, I'd expect it to be the battery and not an excessive current draw. >> Help please! I either have a marginally bad battery or I have an >> unknown current draw to cause a dead battery? I put a test meter on [quoted text clipped - 14 lines] > Its a valid test, and it sounds pretty normal. With a battery that old, > I'd expect it to be the battery and not an excessive current draw. Whoops, read to quickly. I thought you were in the AMMETER mode. You want the meter set to measure milliamps of current, not resistance. Leave the positive battery cable in place, remove the negative. Then put the ammeter between the negative battery post and the neg. battery cable (put the meter's positive lead on the cable). Then you will be directly measuring the current draw. Don't turn on any big loads (not bigger than a dome light, anyway) or you'll blow your meter's internal fuse. >> Help please! I either have a marginally bad battery or I have an >> unknown current draw to cause a dead battery? I put a test meter on [quoted text clipped - 13 lines] > >Its a valid test, Its 100% meaningless. See my post above. The OP didn't even consider the polarity of the ohmeter leads which will have a huge effect on draw from any solid state device. But, in any case, the test is 100% meaningless because the resistance of solid state circuits varies enormously according to voltage applied, time out functions etc. Absolutely all he did was confirm that the door switch works. Don www.donsautomotive.com > and it sounds pretty normal. With a battery that old, >I'd expect it to be the battery and not an excessive current draw. > Help please! I either have a marginally bad battery or I have an > unknown current draw to cause a dead battery? I put a test meter on [quoted text clipped - 11 lines] > only the 2nd battery that's ever been in the truck since new - @130K > miles. That just means the light bulb and door switch is working in the dome light, nothing else. You need the battery in place with everything off and the negative battery cable only removed. You then put the meter between the negative cable clamp and the negative post to see what the draw is. As someone else mentioned, less than 100 ma is best, but the draw depends on how many things have a memory, like a clock or radio station memory set on a stereo. Just having the connections on the battery get normally dirty after all these years can cause it to not get charged up fully so a good clean of the connections is always a good start. If you can open the caps on the battery, do so to check the fluid level. As they get old the fluid evaporates which causes low battery charge. It can be topped up with distilled water if low. Mike 86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 88 Cherokee 235 BFG AT's Canadian Off Road Trips Photos: Non members can still view! Jan/06 http://www.imagestation.com/album/pictures.html?id=2115147590 (More Off Road album links at bottom of the view page) |
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