Car Battery drains

Here's a copy of a reply I posted previously:

Testing for a drain can be a little tricky with all the computers on the car. Some of them can draw a high current for up to 20 minutes until they go to "sleep" mode. Anything you do to momentarily break the circuit starts that 20 minute wait period all over. That includes switching the range on your ammeter. If you start out on a higher scale you won't get the necessary accuracy. If you start out on a lower scale you can pop the meter's internal fuse. On top of that, some computers can lock up after the battery is reconnected but that doesn't seem to be a big problem yet in 2003 except on Volkswagens.

The secret is to disconnect one battery cable to insert your ammeter AND connect a jumper wire across the meter to in effect short it out. Some computers might "wake up" so let the meter and jumper sit there for at least 20 minutes, then remove the jumper wire. All current will go through the ammeter so it can be measured. If you need to switch to a lower scale, put the jumper wire back in place first, switch the meter, then remove the jumper again.

Unless the manufacturer specifies differently, the industry-accepted maximum is 35 milliamps to maintain the memory circuits in the radio and computers. At that rate a good battery will still start the engine after sitting for three weeks. Slightly more than that won't kill the battery overnight but you don't want to see 200 milliamps or more.

Typically when there is a drain, it's really a drain. You won't have to guess. You'll likely find an amp or more, especially if the battery goes dead overnight. Remove one fuse at a time to see if one makes the current drop to an acceptable level. That fuse's function will give you an idea of which component to look at.

First be sure the top of the battery is clean. If condensed acid covers it, the battery will self-discharge through that acid. That acid is caused by excessive bubbling in the battery which is about to fail in the next few months.

The voltage regulator is built into the generator. The circuitry in it could cause the battery to discharge but that would be very uncommon. There are also two sets of diodes which are one-way valves for electricity. One diode in each set would have to short to drain the battery but that would be a serious short that would blow the fuse or melt wires. That would also kill all output from the generator.

To see if the regulator is the problem, unplug the connector next to the bolted-on output wire. If that stops the discharge replace the generator or its internal voltage regulator.

35 ma or less is perfectly acceptable. At that rate, a good battery will still start the engine after three weeks. I would suspect the battery is getting old and its capacity has gone down due to the normal lead flaking off the plates. It should be maintaining 12.6 volts when it's fully charged.

There likely are computers on your car that go to "sleep mode" after the ignition switch is turned off. When the voltage from a weak battery drops enough over time, it is possible for those computers to "wake up" and draw heavy current which isn't showing up in your current drain measurements. I'd try a new battery first.

The cold cranking amps isn't really a big factor here. The typical starter today only draws around 150 amps so a battery with lower cold cranking amps will get the job done. More current is needed when the engine is real cold, and batteries lose some of their capacity when they're cold but their CCA rating is for when they're cold. Their capacity goes up in warmer weather. To be safe, your battery needs to have twice the CCA rating as your starter draws. You have well over three times the capacity so that is not an issue.

Even a battery with a high CCA rating might only be able to deliver that current for a short period of time. Many batteries with lower ratings can deliver their maximum current for longer periods. Your batteries all meet the needs of the starter and since they were new, there should have been no problem holding a charge for three weeks.

My best guess is something is turning on after the engine has been off for a while. That is one of the drawbacks of all the computers used on cars today. I've run into a radio amplifier that turned itself on after a few hours. That was found by the owner who noticed a slight hiss in the speakers that wasn't normal. I also had corrosion between two adjacent terminals in a Body Computer connector. One wire was hot all the time to park the windshield wipers even if the ignition switch was already off, and the next wire was for the parking lights. When humidity went up in the car, presumably when the fresh air stopped flowing from the heater, the corrosion conducted enough current through the tail lights to drain the battery in a few days. I found that one by accident after removing the connector and seeing the corrosion. I also have a '72 Challenger in which the factory radio would suddenly start playing a few hours after the ignition switch was turned off. Turning the switch on and back off stopped the radio for another few hours, then it turned on again. That turned out to be two overheated wires in the firewall connector that were melted and barely touching. A little current flow would warm the wires enough to expand and touch sending current from the battery circuit to the accessory circuit off the ignition switch. The radio would get enough current to start playing but it would turn off if I turned on the wipers. Not enough current could get through to run the wiper motor so the voltage would drop enough to stop the radio from playing. Simply bending those two wires away from each other solved that problem. Now imagine what can happen when you add a whole lot more computers and other electronics to a car.

This type of electrical problem can be real frustrating to find, especially if it doesn't act up all the time. Since you have a new battery already, the next thing I would suggest is using an auto-ranging digital meter to monitor the current drain over a few days. Check on it every half hour to see if the current suddenly increases.

You might also have your new battery load-tested again, but also have the generator load-tested with a tester that measures "ripple". Ripple is not displayed as a voltage; it is displayed as a bar chart from low to high. If ripple is high, suspect a defective diode in the generator. It will also only produce exactly one third of its rated capacity. That will take a real long time to fully charge the battery because one third of normal capacity is barely enough to run the electric fuel pump, ignition and fuel injection systems, radio, and a few lights with little left over to recharge the battery. That COULD explain why your battery only has 12.3 volts instead of 12.6 volts.

If I understand correctly, the engine is running when you measure 12.7 volts at the battery? That's way too low. It must be between 13.75 and 14.75 volts.

12.7 is fine after the engine is stopped.

Yup. 14.3 is perfect. The 12.7 with the engine off is actually just a fuzz high and is due to "surface charge". That's the free electrons in the electrolyte that haven't found their way into the plates yet. When mechanics perform a load-test on your battery, they will draw a high current for a few seconds before doing the actual 15-second test. That gets rid of that surface charge and makes the test more accurate by only measuring the current that was able to be stored in the plates.

I think you'll find if you measure the battery voltage a few minutes after stopping the engine, it will read 12.6, (once the drain problem is solved). Regardless, 12.7 volts is fine.

Voltage isn't the key here. As the battery discharges, the voltage will remain fairly steady. This can be real hard to explain without speaking with a live person using hand gestures and gauging audience response, but the technical reason is due to an increase in "internal resistance". That's a good hour-long discussion in my Automotive Electrical class. If you really want to know more about it, visit the bottom of this page:

http://randysrepairshop.net/basics-of-batteries.html

I suspect you would find the same readings if you disconnected the battery right after stopping the engine. There will be that surface charge I mentioned earlier causing the voltage reading to be a little over 12.6 volts. As some of the free electrons settle into the plates and leave the electrolyte, the voltage from that surface charge will go down, ideally to 12.6 volts. If you find 12.49 12 hours later, that would be acceptable too.

It's the current that is the concern. That is what should be monitored after the engine is stopped, but that setup takes a little more work. I can describe that but since you have a new battery, lets wait and see if that takes care of the dead battery issue.

This is easier to describe with pictures and drawings so if you give me a few days, it's worth adding to my web site, but in the meantime, to clarify how an amp meter works, what you described is how a voltmeter is connected. Voltage is electrical pressure just like pressure in a water pipe or compressed air line. Nothing goes through a pressure gauge to read the pressure, and nothing, (very little), goes through a voltmeter to read voltage, so when you connect one meter lead to the battery positive post and the other lead to the car body, which is connected directly to the battery negative post, no current flows through the meter, but it does measure the pressure, in your case 12.47 volts.

Amp meters measure electrical current flow similar to a municipal water meter inline with the water pipe coming into your house from the street. All water goes through the meter with very little resistance, or opposition, to that flow. In an amp meter, all electrical current goes through the meter. The better the meter, the less effect it has on the circuit and current flow. It should look like it's not even there. If you install an amp meter like you described, from the battery's positive post to the negative post, or car body, you'd have a direct short, smoke, melted wires, and lots of excitement.

Most inexpensive digital volt-ohm-amp meters have internal fuses to protect the meter on the 2 amp and lower scales but very often there is no fuse for the 10 amp scale. The amp meter has to be connected in series, or inline, with where you want to measure current flow. In this case it's the current flow through the battery we're interested in so we would remove a battery cable, then connect one meter lead to the cable clamp and the other lead to the battery post it came from. Since current stays the same in a series circuit, whatever current leaves one battery post is the exact same amount that comes back in on the other post so the amp meter can be connected to either post / cable. We typically insert it in the negative cable only because if the wrench should hit anything metal on the car while it's touching the cable clamp's bolt, no sparks will occur.

This setup would work fine on older cars without computers and on cars with computers up to about the early 1990s, but there's a clinker that showed up around the mid 1990s that makes measuring "ignition-off-draw" more complicated. Some computers take up to 20 minutes after the ignition switch is turned off to go to "sleep" mode. During that 20 minutes the electrical system can draw up to three amps. You must wait and make your measurements after that 20 minutes but anything you do to open the circuit, even for an instant, will wake those computers up again for another 20 minutes. You have to break the circuit to connect the amp meter. That means another 20 minute wait. If you start out on the 2 amp scale, 3 amps would blow the internal fuse so now you again have an open circuit. If you start out on the 10 amp scale, most meters require you to unplug one meter lead and move it to a different jack for the lower scales needed to get the necessary accuracy after the decimal point. That opens the circuit again and makes for another 20 minute wait.

To really confuse the issue, if you start out on a higher scale, then after 20 minutes you're able to switch to a lower scale for better accuracy, amp meter switches are always "break-before-make" styles meaning as you turn the knob, the circuit is broken first to the setting you're leaving, then it makes the connection to the new setting you're switching to. That very short break in the circuit from just switching ranges is enough to wake the computers up again and potentially blow the fuse in the meter.

The secret on these cars is to remove the battery cable, then connect a small jumper wire from the cable to the battery post. Current can flow freely now for the computers to go to sleep mode after 20 minutes. During that 20 minute wait you connect the amp meter to the same two points, battery post and battery cable. You can start out on the lowest scale without fear of blowing the meter's fuse because all current will be flowing through the easiest path which is through the jumper wire.

Once you've waited for the 20 time-out of the computers, remove the jumper wire. Now any current has to flow through the meter and can be measured. If you should have to switch ranges on the meter, simply reconnect the jumper wire first, switch to a different scale, then remove the jumper wire again. In that way the circuit was never broken so the computers will stay in sleep mode.

The magic number you're looking for once the computers have timed out is.035 amps. Most cheap digital meters have a 200 milliamp scale meaning they will read anything from 0 to 200 milliamps. You can also use the 2 amp scale but you'll lose one place after the decimal point. 35 ma. Is the industry standard maximum unless the manufacturer specifies differently. Some allow up to 50 ma. (.050 amps). At 35 ma, Chrysler says a good battery will still start the engine after sitting up to three weeks.

The clamp-on amp meter you mentioned won't work for this problem. Instead of measuring current flow THROUGH the meter, they work by measuring the strength of the magnetic field around a wire. The one important characteristic that makes all motors, generators, and ignition coils work is that whenever current flows through a wire, it sets up a magnetic field around that wire. More current equals a stronger magnetic field. Clamp-on meters measure that magnetic field and convert it into a current reading. They work great for large currents like the 150 amps to run the starter motor or the high current leaving the generator (alternator). They lose their accuracy on very low currents such as what you'll be looking for. Also, I was unpleasantly surprised to find my clamp-on meter only works in AC circuits so all I can do is tell if my electric hot water heater is working! The lowest scale is 20 amps, not even close to being accurate enough for automotive use even if it could read DC current. The advantage to a clamp-on meter, besides not having to break the circuit anyplace, is there's no internal fuses to blow. It will simply read over range if the current is higher than the meter can display.

For what most people need, you can find a dandy digital meter at Harbor Freight Tools for less than ten bucks. They often go on sale for $2.99. They even have the batteries installed already.

I'll try to get some drawings up on my web site to better show the connections for measuring your current drain. I've typed this description enough times that sending you to one page would be faster and more effective.