Im trying to figure out what the baseline is for a battery reading on an X300.

At rest with nothing on my battery tester is telling me that the battery is about 12.20 volts, but it slowly drops by about a hundredth or so v every couple of seconds as I test it.

With the car running I am showing 14.42v initially, but after about 45 min when the coolant temp reads 185 and my car begins to stutter a bit I show 14.10 and dropping at the battery.

Im looking for what these figures should be and possibly what I am seeing here.

 

You didn't say how old the battery is. Four years is a long life for a battery, but I have gotten 7 out of a Panasonic car battery once. A car battery can hold 2.2 volts per cell for a total of 13.2. However, they will start heading down towards 12 volts. A hydrometer will tell you what condition that your batterry is in if you have removeable lid(s) on the top of the battery. Do the test after trying to start the car but not letting it run. All cells should be in the green and pretty much equal. If one or more is below the green, she's a goner.

The test for an altenator after making sure that the battery is good is to hook up the voltmeter, run her engine at 1500 RPM and note the voltage. It should be 13.5 to 14.5, but 15 isn't unheard of (Fords). Hold that RPM and turn on lights and accessories, one at a time. The voltage should not drop more than about half a volt.

 

Thanks GLENDORAMIKE I should have specified. The battery is brand new, right off the shelf. I needed to know why a battery might look as though it was dropping when the car was sitting still and also what it should show with the car running.

Thank you.

 

Jack,

A battery "disconnected" should be about 12.7v and stable.

When connected to a car it has a drain, memory/clock etc, and it should still be 12.5 and stable under normal "sleeping mode". It will drop after quite some time, days, not hours. Under normal conditions it should be able to "sleep" for about 3+ weeks before the drain is sufficient to prevent starting the engine.

With the engine running 13.8 - 14.2 is considered within spec.

The temp thing may still be a voltage regulator failing as it gets hot, they are thermal sensitive things at the best of times, and it also could be leaking to earth whilst the car is "sleeping", which would explain your dropping voltage as you describe.

My suggestion would be to dis-connect the +ve cable from the alternator, insulate it from shorting, reconnect the battery, and observe the readings again. If they stabilise, fault is more than likely the alternator.

To give you a start of numbers etc:

My 3,2 X300 12/95 build
3 year old battery DIN100 size.
At sleep after sitting for 5 hours = 12.86v SOLID.
Engine running at idle NO accesories on = 14.33/14.34v SOLID.
A/C, headlights on HI, engine idling = 14.15v SOLID.
Engine OFF, sat for 5 minutes = 12.97v and after 30 minutes =12.86v and SOLID.

I will do an amperage draw tomorrow, and repost my findings tomorrow night. Too dark here now and I might bugga something up.

I have done this on the XJ-S and it "sleeps" with 0.2 amps being drawn, which is the radio memory/clock, and the trip computer memory/clock. It has sat for 8 weeks now, battery connected (I am basically lazy), and it fired straight up tonight, and sat at 14.2v idle, no accesssories ON, and 13.9V at idle A/C & both electric fans ON, went back to 12.6v after 30 minutes from shutdown. It has an N50 lead acid battery of 3 year old.

 

Thanks Grant, that tells me why sometimes you can't get the engine started whereas all the things work

 

I believe that the cause/effect relationship on these two points appears backwards. The engine stutter lowers the rpm, which reduces the charge voltage rather than the other way around.

 

SPARKENZAP,

I didn't intend to assign a cause and effect relationship. If I could demonstrate a specific cause and effect I believe I would be closer to isolating the problem. I was just reporting observations.

I do understand that the lowering of belt speed is going to lower what the output of the alternator is. But it's like a chicken or the egg question.

Is the faulty CTS causing havoc with the ECM which is changing engine performance which causes the engine and speedo stutter which causes the lower readings and the eventual engine fail at idle?

Or

Is the alternator leaking to ground once the system gets to peak temp causing the engine and speedometer to stutter and the engine to eventually fail at idle?

Or

Is there a loose ground somewhere? etc....


I should also state that the spark plugs and the coils are all less than a year old. However I have read enough recently to make me wonder if there could be some oil leakage at the bottom of the coil/plug wells causing some sort of miss or backfeed.

Thank you too Grant for the readings. I will do mine again and make sure of what I am seeing.

 

Jack,

As promised.

Woke up missy, disconnected the battery. connected the amp meter between the earth cable and battery, and had 0.49amps draw, and stayed that way for 6 hours whilst I did an oil change, trans fluid change, washed the engine bay, and tickled her fenders.

 

Check this video out it shows you how to check for battery drain..

http://www.youtube.com/watch?v=yFCT-YZbU5o