jchapura

I had to replace my 997.1 S's battery yesterday and did so with a Optima YellowTop.

Much of the time, I left my old battery on a CTEK battery maintainer and will do so with the new battery (the maintainer has an AGM mode).

When it's not on the maintainer, I notice a characteristically small but consistent voltage drop. I have an AntiGravity Battery Tracker which makes for easily tracking the voltage over time.

For example, after a drive yesterday with the new battery, at 1600, by midnight it had dropped 0.11v. Today, between 0001 and noon/1200, it has dropped 0.05v. The old battery seemed to drop between 0.05v and 0.10v almost every day it wasn't on the maintainer.

Is this amount of voltage drop typical or should I be looking for the source of an excessive or atypical current draw?

r_towle

Without a way to measure I will say this
most/all cars now have a constant current drain
keeping memory in the ECU (s) takes power

you may also have a problem.

finding it may require you to remove all fuses
measure
add one fuse at a time back
measure

given the age I would suspect under hood lights first

jchapura

I'd like to calculate the total power consumed by the car in a 24 hour period only knowing the battery's starting and ending terminal voltages.
Will it make things easier if we assume that all processes are linear across the 24 hours?
Is there a way to calculate the power used in the 24 hours if the starting voltage is 13.05v and the ending voltage is 12.95v?
Once the power consumption is calculated I think the average amp draw could be calculated. I've read that the typical current draw is around 50 milliamps (I assume this is an instantaneous reading).

ADias

These cars have a sitting draw of about 40mA or about 1Ah battery capacity loss per day.

964hans

Hi,
I think this does not work. Battery voltage changes with temperature. I am not quite sure but for every degree C temperature drop the battery voltage rises about 0,02 Volt.

A better method is to measure the current draw. This can be done by measuring the voltage across a shunt resistor or inserting a Ampere meter between one of the battery connections as described in the annex.

regards
Hans

groovzilla

One battery draw issue that not many 997 owners are aware of is an issue with the front trunk light shorting out.
I had this issue several years ago on one of my 997's. Battery would be down to 11V only after a week in garage.
I assume the problem stems from the constant opening/closing of frunk and somehow loosening the connection..

Just wanted to mention to make sure you check for this possible issue.

jchapura

Thank you for the diagnostic procedure PDF. Agreed that temperature influences measured voltage.
But I think that lead acid batteries exhibit voltage change when discharging. See chart below.

Since voltage is easier to measure, I was hoping for some typical voltage drop per day values to compare with what I see.

jchapura

How can one tell if the light is on all the time? Can one see it in a darkened garage? Or just by current measurement when the frunk is closed, or some other electrical measurement?

ADias

Drop a cell phone with video on and close the lid... Wait a minute open the lid, check the video.

jchapura

Ingenious.

jchapura

Using the video capture method, the frunk light goes out and stays out after closing the frunk lid.

964hans

I still think it is a unreliable method but if you want to carry out the measurement ensure that the ambient temperature is stable within <1 degree C during the measuring period

rileyracing1

I have a battery voltage tracker on my 2yr old Odyssey AGM and when the trickle charger kicks in and bumps the battery back up to 13v it takes about 3.5 days with the car locked to get the battery voltage down to 12.5v and where the tender will kick in and bump the car back to 13v. I have had the car off the tender for a month where the battery voltage was in the low 12v and the car still started without any issues . Our cars do like to suck juice when sitting no doubt unlike my 1989 sl500 which sat for almost a year and still started without any issues !

jchapura

This decrease averages to 0.006V/hr.
My decrease averages to 0.0031V/hr. So, we're in the same ballpark.

Based on Hans' current draw chart and my car's options, I should be drawing ~19 mA. ADias suggests 40mA or 1Ahr per day loss is expected/normal. So, we're in the same ballpark.

The YellowTop has a C20 capacity of 80 Ahr. At a 1 Ahr draw for 1 day for this battery, the capacity decrease is 1.25%. If I look at a discharge graph for this battery type and these other conditions (and estimate), it seems that a 0.1V-0.2V decrease is plausible, especially at the beginning of the discharge cycle after the battery has been fully charged. The discharge curve flattens out as the depth of discharge progresses until the end when the slope becomes more steep again.

patentguy

Until I read this thread, I didn't have the battery reading displayed on my screen. My car has its original AGM battery, approaching three years. Never had a battery problem, drive daily, don't use a battery tender.
So I turned on the battery display out of curiosity. It reads 12.1 V with engine off. The other day, it sat at about 13.5 V while driving, with bumps up to 15.3 V when braking, as discussed above. Today, on a half hour drive in the morning, it sat at 14.7 V for half the drive and then dropped down to 13.5 V. Coming home in the afternoon, it was pegged at 14.7 V the whole way.
Is all of this normal, or should I have the dealer look at it before the warranty runs out?