3.6 mA Parasitic Drain - starter/alternator line

[Eddystone]

The battery in my 1999 S70 T5 has been dying.

With everything off, I am measuring a 3.6 mA current in series between the negative battery post and the negative cable. Pulled all fuses and relays in the main fuse box with no effect. Looked at the recently discussed terminal coming from the battery to the main fuse box. Looks fine, has 0 ohms resistance and there is no voltage drop across that line.

Disconnected the main red power line to the fuse box from the positive battery cable, and I am still measuring a 3.6 mA drain with presumably just the starter and alternator connected.

So, am I looking at bad alternator diodes? Any suggestions? Soon would be good...

[Eddystone]

Update: Checked the battery with a hydrometer and one cell is bad. Was reminded to check that by a post in this forum. So, new battery, and we'll see where we stand...

[yanga001]

3.6 mA seems to be too small to cause the battery to die in any manner of noticeable time. Normally batteries are rated for 50 Ah + meaning that it would require a couple thousand hours to discharge at that rate. My money would be on a dead battery causing you trouble (as you already found out). It might be more cost then its worth to try to eliminate a draw that small if it is even possible as the cars dash, anti-theft, and other features would likely draw an amount close to that in an off state.

[abscate]

Anything under 50mA is considered normal, som3.6mA is really small

[wizechatmgr]

3.6mA could just be the radio keeping settings...

[RickHaleParker]

At 3.6mA it would take 278 hours to discharge just 1 Amp-hour of charge.

3.6mA @ 12V is only 43 mW. Flea power.

[Eddystone]

Thanks to all who responded. I know 3.6 milliamps is a very small current (although 10 microamps through and indwelling catheter can kill you) but this car sometimes sits for a week, and I would STILL be curious to know what down that alternator/starter cable causes a constant current leak and WHY. I'm not going to bother troubleshooting it unless my new battery starts going dead. I'm not that curious, and I have other fish to fry.

The battery that came with the car when I bought it (which went bad) has a manufacturing date of 2/18 on it. It is an AutoZone Autocraft 48H6 battery with a 3-year replacement warranty, but they won't replace it without the original receipt, which I don't have. I'm sort of shocked that one cell out of six would simply go dead, but presumably that would give a 2 volt drop or even effectively be a load on the battery. When fully charged, I noticed that it had a charge of 11.82 volts on it after stabilizing. I had had the cover off of the battery to check the electrolyte levels like I did in the 60's but didn't think to hunt down my old hydrometer/tester until I read another post. If I hadn't just noticed it in the basement the other day, I never would have found it. Sometimes, Old School just plain works.

So far, battery seems to be maintaining charge... +++ XXX

[Eddystone]

3.6mA could just be the radio keeping settings...

Not down the alternator/starter cable.

Radio is fed from the fuse box.

[yanga001]

If you are interested in finding the draw then i guess you could try and find a sensitive clamp meter and check the direct lines to the alternator or the starter. Have you tried disconnecting lines to the starter and alternator.

That kind of current draw could be any number of things, for an alternator it could be the draw of the regulators as you said. Otherwise it could be higher resistance in a line, a minor short in the starter or alternator windings, minor faults in the regulators (as you mentioned), or just some unknown system in the electronics (99's had more than the 98's).

Working backwards, assuming you have a voltage of around 13, and 3.6 mA load then you are looking at a resistance of roughly 3.6 ~ 3.7 K ohms which would be very healthy for any kind of automotive system at rest.

From a solid state physics perspective it is likely the regulators as mentioned earlier. The regulator is likely a bridge rectifiers made from heavy diodes. These diodes operate by having a positive region, a "depletion region", and a more negative region by having materials that are pre charged in each of these regions. If we try to polarize the diode one way then the positive can become more positive and negative becomes more negative and charges can jump across this depletion region with ease. However, operating them in reverse will block the flow of charges (ie current) thus resulting in a "open circuit". This block is not perfect as a small "leakage current" will flow as a result of imperfections in the materials thus showing a small drain.

In regards to your situation, we want to look at these systems at rest as the alternator is not in a charging or excited state so we can eliminate that. Looking at a regulation circuit, we would hope a diode or mosfet is there to ensure that the battery does not continously go through the regulator if the car is not on (ie the alternator is not charging). This circuit will still take some power and could result in the 3.6 mA you see across the alternator. Think of it as a circuit put in to ensure your alternator charging circuit does not have current going in the wrong direction.

If anyone knows more on this then feel free to correct my logic above. I am not an automotive engineer or a full electrical engineer, however this is my best guess as to the scenario he sees above.

[abscate]

Most voltmeters won’t reliably measure mA of current so don’t base those measurements as being too reliable. You can get surface leakage in the battery from moisture at this level too, so you can really chase your tail on this