Reference voltage/current draw

[832ark]

Hi

Potentially have an issue with either charging or parasitic drain. Fitted a new battery a couple of weeks ago after I thought it had failed. Yesterday battery was flat again. 
 

I’m wondering if anyone can tell me what voltage they see across the battery at idle please? I’m expecting 14.3/14.5v-ish which I see after a cold start but seems to drop to 13.8v once warmed up.

Next, I’m wondering what current draw people are seeing with the engine off and the car unlocked, frunk open. I haven’t taken a reading yet but will be trying over the next few days.

Lastly, how intelligent is the charging system on a 987.1? I couldn’t find any reference to needing to code the battery to the car - is that correct? 

Thanks in advance! 
 

 

 

[Menoporsche]

I don't believe the battery needs coding, but on previous discussions I think you'll hear conflicting answers on that depending on "can get away with it / want to optimise all systems". 

Re your frunk open - try immediately after off and 30 mins/1 hour after off. There is something about powering down after 20 mins, so you might get two different readings. I can't remember how long before courtesy lights switch off, for example.

[832ark]

20 minutes ago, Menoporsche said:

I don't believe the battery needs coding, but on previous discussions I think you'll hear conflicting answers on that depending on "can get away with it / want to optimise all systems". 

Re your frunk open - try immediately after off and 30 mins/1 hour after off. There is something about powering down after 20 mins, so you might get two different readings. I can't remember how long before courtesy lights switch off, for example.

I was wondering about that, will the car go to sleep if it’s unlocked and the frunk open?

[Paul P]

@Menoporsche is right - no coding required on the 987.

high 13's seems right for running - much over 14 would start be in "over charging" territory I think.

Various circuits power down over time 

Usual candidates are frunk and boot lights (put your phone on video record and shut the lids to see if the light goes out)

After that the PCM is another known issue - pull fuse d9 and d10 ( d9 is the head unit, d10 is the amp) to isolate.

Re the measurement ( teaching my granny to suck eggs potentially - so sorry if thats the case) - current has to be measured inline - so you have to interrrupt the circuit to attach the meter - have seen it done with meter connected then battery terminal disocnnected but its a faff.

There is a method for finding parasitic draw by measuring voltage drop across fuses - have a google and see if you can find a reference....

[Menoporsche]

1 hour ago, 832ark said:

I was wondering about that, will the car go to sleep if it’s unlocked and the frunk open?

I hope so as mine is always unlocked in the garage (not with frunk open though). It's on a trickle charger through the cig socket though which adds another factor into the equation.

[Paul P]

1st google hit I found for the "voltage drop across fuse" - 1st part uses an "amp clamp" to be sure of the drop he was looking for - but I figure - you can jsut use the measurement across the fuse and the chart to give you an idea of circuits that aren't "as sleep"

 

This doc gives a baseline of 40ma for a significant enough dain on a single circuit.

https://www.autonerdz.com/yabbfiles/Attachments/HowtodoParasiticdraintestacrossfuses.pdf

 

 

[BBB]

Answering the question a different way: what have you changed on the car? If you’ve recently changed or installed something then that’s likely to be the cause of the flattened battery. 

[832ark]

5 hours ago, Paul P said:

@Menoporsche is right - no coding required on the 987.

high 13's seems right for running - much over 14 would start be in "over charging" territory I think.

Various circuits power down over time 

Usual candidates are frunk and boot lights (put your phone on video record and shut the lids to see if the light goes out)

After that the PCM is another known issue - pull fuse d9 and d10 ( d9 is the head unit, d10 is the amp) to isolate.

Re the measurement ( teaching my granny to suck eggs potentially - so sorry if thats the case) - current has to be measured inline - so you have to interrrupt the circuit to attach the meter - have seen it done with meter connected then battery terminal disocnnected but its a faff.

There is a method for finding parasitic draw by measuring voltage drop across fuses - have a google and see if you can find a reference....

Thanks Paul, done some more testing this eve. I seem to be getting high 13s in terms of voltage when at idle and 2k rpm. I’m planning to use the series method to measure the parasitic current. 

[832ark]

3 hours ago, BBB said:

Answering the question a different way: what have you changed on the car? If you’ve recently changed or installed something then that’s likely to be the cause of the flattened battery. 

I haven’t, otherwise that would be my first port of call. The only electrical gremlin I’m aware of is the boot light not working (not the bulb). It does tie in with the colder/damper weather. No obvious water ingress but maybe the moist air is just enough to short a failing component on a board somewhere? 

[Paul P]

56 minutes ago, 832ark said:

I haven’t, otherwise that would be my first port of call. The only electrical gremlin I’m aware of is the boot light not working (not the bulb). It does tie in with the colder/damper weather. No obvious water ingress but maybe the moist air is just enough to short a failing component on a board somewhere? 

“Danger Will Robinson, Danger”

double check under the passenger seat for signs of water.  Not the surface of the carpet. The foam underneath.  Poke aprons under the seat and get your fingers down to floor level.  
 

One of the modules under there is the rear control unit.  It does boot light and latch, brake and rear lights, spoiler and roof.  It doesn’t like getting wet.  
 

if all good then great but be sure before you chase anything else 

[832ark]

16 minutes ago, Paul P said:

“Danger Will Robinson, Danger”

double check under the passenger seat for signs of water.  Not the surface of the carpet. The foam underneath.  Poke aprons under the seat and get your fingers down to floor level.  
 

One of the modules under there is the rear control unit.  It does boot light and latch, brake and rear lights, spoiler and roof.  It doesn’t like getting wet.  
 

if all good then great but be sure before you chase anything else 

I will check it. It has been replaced previously due to water ingress. The boot light hasn’t worked since I got in March, maybe the rear control module has been on its way out for a while and this is a new symptom? There’s no strange codes logged related to that module and all other related functions work correctly. 
 

Thanks again!

[832ark]

13 hours ago, Paul P said:

“Danger Will Robinson, Danger”

double check under the passenger seat for signs of water.  Not the surface of the carpet. The foam underneath.  Poke aprons under the seat and get your fingers down to floor level.  
 

One of the modules under there is the rear control unit.  It does boot light and latch, brake and rear lights, spoiler and roof.  It doesn’t like getting wet.  
 

if all good then great but be sure before you chase anything else 

I had a really good feel round the rear passenger carpet. It’s dry on top and underneath. Is the RCM under the seat itself? I didn’t find it under the carpet. 

[832ark]

And a follow up question - what kind of current draw would be normal when it’s asleep?

[Paul P]

3 hours ago, 832ark said:

I had a really good feel round the rear passenger carpet. It’s dry on top and underneath. Is the RCM under the seat itself? I didn’t find it under the carpet. 

It’s under the seat I think you can get to it without removing the seat but it’s a fiddle.   
 

if the foam feels (put your hand under the seat - feet the modules then feel under the carpet around there) then you are probably fine.  
 

I will have a look and see if I can find anything on “sleep current” for a 987.  I have the 986 numbers somewhere but the 987 has approx twice as many modules than the 986 so not comparable.  
 

 

[832ark]

23 hours ago, Paul P said:

It’s under the seat I think you can get to it without removing the seat but it’s a fiddle.   
 

if the foam feels (put your hand under the seat - feet the modules then feel under the carpet around there) then you are probably fine.  
 

I will have a look and see if I can find anything on “sleep current” for a 987.  I have the 986 numbers somewhere but the 987 has approx twice as many modules than the 986 so not comparable.  
 

 

Thanks Paul. Managed to get reading on parasitic draw today. Watched the meter for a good 10 mins and it stayed between 50 & 60mA. That seems to be a reasonable reading?

[BBB]

1 hour ago, 832ark said:

Thanks Paul. Managed to get reading on parasitic draw today. Watched the meter for a good 10 mins and it stayed between 50 & 60mA. That seems to be a reasonable reading?

This article is a good quick read, and suggests that 60mA is normal for a modern vehicle.

https://www.optimabatteries.com/experience/blog/what-is-normal-for-parasitic-draw

[832ark]

2 hours ago, BBB said:

This article is a good quick read, and suggests that 60mA is normal for a modern vehicle.

https://www.optimabatteries.com/experience/blog/what-is-normal-for-parasitic-draw

That’s what I thought 

[832ark]

Would be grateful if someone could get a voltage reading on a warm car at idle if possible please? I’m expecting 14.3v-ish. I get 14.2v on a cold start which drops to 13.9v as it starts to warm up.

[½cwt]

Is it possible your new battery is defective?

[832ark]

4 hours ago, ½cwt said:

Is it possible your new battery is defective?

That’s definitely something I’m considering. Going to have it tested today.

Voltage reading when I parked it last night, 12.3v - voltage reading now, 11.9v

[½cwt]

26 minutes ago, 832ark said:

That’s definitely something I’m considering. Going to have it tested today.

Voltage reading when I parked it last night, 12.3v - voltage reading now, 11.9v

The other possibility is your alternator may not be behaving (which could have killed the first battery) although they tend to work or not work rather than cause a drain.

[832ark]

Battery tested fine, although the earth test showed high resistance so I need to clean my earths up I think. My friendly mechanic said normally he’d expect to see a charging voltage north of 14v so wonders perhaps alternator. Be interesting to see what others are reading at idle?

[832ark]

Not had chance to take a look at the earth yet. Drove the car on Saturday eve and all was well, came to start it Monday lunchtime and all I get is a “click”. 
 

If the battery’s not faulty and the parasitic drain isn’t an issue I’m not sure where to go next. 
 

Would someone be able to measure their charging voltage at idle please with the car warmed up?

[Paul P]

I found this..... https://drive.google.com/file/d/1_oPk77tLOMxDnkE8ijZ8XR1VXNOEleJ3/view?usp=sharing

Its cut from a 3rd party manual for the 987 but is the only reference I can find.

I gives the expected draw from various modules and how long from closed until they sleep - the summary being - it takes 60 mins until it reaches it's "low" and that low should be approx 15 to 20 mA - also says to trick the bonnet into thinking its shut.

Whilst 60mA will take a while to flatten the battery - its more than spec - and it seems sleep should be after 60 mins - don't know if it helps but thought I would share.

 

[832ark]

2 hours ago, Paul P said:

I found this..... https://drive.google.com/file/d/1_oPk77tLOMxDnkE8ijZ8XR1VXNOEleJ3/view?usp=sharing

Its cut from a 3rd party manual for the 987 but is the only reference I can find.

I gives the expected draw from various modules and how long from closed until they sleep - the summary being - it takes 60 mins until it reaches it's "low" and that low should be approx 15 to 20 mA - also says to trick the bonnet into thinking its shut.

Whilst 60mA will take a while to flatten the battery - its more than spec - and it seems sleep should be after 60 mins - don't know if it helps but thought I would share.

 

Thank you, that’s really useful. I’ll do another parasitic load test and let it soak longer