In over my head!!! Battery/alternator problem

With the engine off, disconnect the negative terminal from the battery, and install the voltmeter in-line. That means run the red wire from the negative post of the battery, and run the black wire to the cable. This will give you a reading on how much of a drain you have on your battery. You could just install a battery tender in your car, and not worry about the drain. Two weeks idle is not that bad. My cars battery drain was overnight. My car was diagnosed with a bad alternator, and since it's been replaced, my car can sit for months and the battery will not drain.
In order for you to take out the alternator, you have to remove the right aft tire, and remove the inner fender. Then drain the coolant, and remove the coolant pipe that runs in front of your alternator.

 

Steve, I think you said that a little backward -- if you remove the battery completely and measure the resistance between the "+" cable end and the "-" cable end, ideally you'd like to measure an infinite resistance, not 0 -- so getting a "reading" is normal.

As Jim said (although it would be with the "voltmeter" in the ammeter mode), a maybe more meaningful measurement is to measure the current drawn from the battery when it is still in place, but everything possible is turned "off" -- i.e., place a large-capacity ammeter between one battery post and its cable end (while the other cable end is still hooked up to the battery). Something like .050-.100 A shouldn't be a problem -- if it's something like 0.5A, this is trouble.

 

Reading voltage across the batteryless car is simply reading residual charge (coulombs) left in filter capacitors -- which, as you noticed, drains away quickly under the meter's load.

Reading resistance across the batteryless car would show "admittance" (inverse of resistance) only for things still turned on (drawing power) on an "off" car: Alarm system, Lo-Jack, radio memory, external stereo amp, clock, etc.

If everything is off, you'd show infinite resistance (zero admittance). But such a reading isn't very reliable, because it depends on the voltage battery your ohmmeter uses (typically 1.5v or 9v) ... which may not be sufficient to bias diodes in a 12v system, so there may be load your ohmmeter wouldn't read.

The best check, as people have told you, is to measure current draw from the battery with the car off. To measure current, you have to place an ammeter in series with the battery (car to meter to battery to car). A small multimeter is sufficient to read residual current with the car off. Just be sure not to hit the starter with the meter inline, unless you want to see the meter peg (and smoke come out of your meter).

If you don't have an alarm or recovery system hidden away somewhere, then from the amount of cap charge you're getting in an "off" car, I'd guess this comes from the stereo.

3X8 Ferraris are odd, in that the radio wiring is not switched with the ignition key. Most cars only power the stereo/radio when the key is in "on" or "accessory", but the F-cars don't have an "acc" position (only "avv": start). (Can you turn your radio on when the car is off?)

You might have a massive noise filter on the radio power line. Such a filter can draw a lot of power, and is typically only used on switched circuits, rather than on "always-on" leads.

That, or your external stereo amp is always on. The previous owner installed a hidden switch to remotely power the stereo and external amp on my 328. The consignment house couldn't figure out why the battery was draining all the time, because they didn't know to turn it off. These days, most amps have a sense line from the radio that switches the amp on or off. Depending on the amp, it might default to on if the sense line is interrupted. (It may depend on how old your stereo installation is.)

 

Um, not exactly.

An alternator is a "delta"-pole three phase alternating current (AC) device, with the voltage regulator controlling the current in the armature field winding. The three stator windings each use diodes to rectify the generated AC current to direct current (DC). (If that helps. ) The AC "alternator" became standard in cars, because it produces electricity at lower engine RPMs than was possible with older DC "generators", and because the variable field winding allows the power (and engine load) to be regulated at the source, rather than afterward.

Lots of shops talk about replacing the alternators, in reponse to any battery problem. Caveat emptor. If you're getting 15 or more volts across the battery while it's charging, then your alternator is probably OK. If your battery is bad, you get false voltage readings from the alternator, so many shops replace both, "just in case" (and because it's coming from your pocket, not theirs).

You need to track down your current load. You can measure static ("off") current, and then pull fuses, one at a time, to see when the current load drops. I'd start with any inline fuses wired to the battery terminal clamps, because that's probably an aftermarket gizmo.

 

Need to revive an old thread here....Similar issue with battery drain on 308. Made a bunch of measurements to get some understanding. Also replaced the battery and ended up with no difference.

I read about 125 mA of current draw with the car off. This is a bit higher than I expected, but with a 70 amp-hour battery, it should be good for a more than a week of sitting. After charging the battery with the shop charger I measure about 12.25 Volts before starting the car. After start and before bringing the rpm's over 2K, the reading is 11.75 volts. After the 2K rev-up, the voltage reads 12.6 at 900 RPM. (Strange issue with the undervoltage before running up the engine, but have seen this same feature with several other Italian cars so I assume it is normal) At higher RPM the voltage increases to 13+ volts - Did not take any readings above 2500.

Numbers don't seem great, but not so far out of line to indicate loss of one of the bridge diodes or other voltage regulator or alternator problems. Do these readings sound "normal"???
Carl

 

When was the last time your radio was pulled out? I am just wondering how that problem developed.

 

Thanks Steve - After thinking about the problem, I realise that the draw when the car is off would completely drain a fully charged new battery in 23 days. I think that rather than look at the charging system, I should look into the reason for the high discharge with the car off.

 

Set up your test gear, and pull fuses to track down the likely culprits. I would guess it is either an aftermarket alarm or stereo gear (amplifier). Sometimes the installers fail to put the amps on a switched power lead, so they remain always on.