We've been towing my 2009 Mini Clubman S behind our RV while we travel the western states. I often put it on a trickle charge after a days drive to top off the battery from what little the auxiliary braking system draws if we're not going out in it right after setting up camp. Until two days ago, it's always started quickly. We drove it all morning, then towed it for about 3 hours. When we went to unhook it, the battery was dead. Borrowed some jumper cables, got it started and drove around a bit. Hooked it back up and drove another 3 hours for our campsite for the next couple of days. It barely started this time so I let it run for awhile, then put the trickle charger on it for the night. Next morning it was dead. Since the auxiliary braking system is disconnected I assumed the battery developed an internal short.

We jumpstarted it again and spent the day sightseeing, never shutting it off. After returning to our campsite, I disconnected the negative terminal with the intention of giving it a few hours and check the discharge state. At rest it measureed 12,8 which is normal. I also chosse to check for an drain on it by measuring between the positive post of the battery and the disconnected negative cable and was surprised to see it read 1.4 volts! I don't have an ammeter with me but I know that represents too many amps. I am at a loss to understand what it might be. I started trying to trace it down by first pulling relays then fuses and nothing affected it. While I was doing this, the voltage was slowly dropping and when I stopped, it was down to 4 tenths. An hour later it was zero BUT when I touched the negative cable to the battery, there was a healthy spark and now the potential between the positive post and negative cable is again 1.4 volts.

Any ideas?

 

What you're measuring here is called a "floating voltage" and it is a completely meaningless/worthless measurement. Ignore it, move on, and track down an amp meter to properly measure current draw on the battery.
If you charge up the battery and leave the negative cable disconnected for a few days what happens? Does the battery still lose its charge? If so, then the battery is defective.

 

The battery holds voltage just fine when disconnectsd. It was 12.84 yesterady afternoon and is 12.78 this morning. I doubt that 1.4 volts is simply a floating voltage if it can drain a battery in a matter of hours which it has done 3 times now.

 

I think you're misinterpreting my description of floating voltage. Here's how wikipedia describes it:

"Conductors are also described as having a floating voltage if they are not connected electrically to another non-floating (grounded) conductor. Without such a connection, voltages and current flows are induced by electromagnetic fields or charge accumulation within the conductor rather than being due to the usual external potential difference of a power source."

So, when you disconnect the negative cable from the battery, it's no longer connected to the non-floating negative terminal of the battery and therefore its voltage is now floating with respect to the battery voltage. The 1.4V you measure is nothing more than a random accumulation of electrons, it's telling you nothing about what the battery is doing.
If you want to diagnose the parasitic draw on the battery you will need an amp meter.

 

Thanks for the definition but I still doubt what I'm reading and experiencing is that based on the arc that occurs when I reconnect the negative terminal. A standing voltage has no current behind it so as you suggest, to correctly assess what's going on will require an ammeter. I have a couple back home but I"m a thousand miles away and don't care to purchase another just for this purpose. I'll certainly bring one on my next trip but for now I'm simply disconnecting the battery between uses.

 

I'm back home with my ammeter and the drain is 1.2 amps with a fully charged battery. As I related earlier, I removed every fuse and relay I can see one by one with no change in the drain. I am having a performance issue that I thought was my high pressure fuel pump but after replacing it without solving the problem, I'm beginning to suspect the main engine computer and now wonder if that's not the drain as well.

Any thoughts?

FWIW, the performance 'issue' is I lose power intermittantly. No miss, no stumble, just no power. It will return as mysteriously as it disappears. 2009 Clubman S

 

I suppose you are measuring the drain when the car went to sleep?

Did you also remove fuses in the engine compartment? I think there are also fuses on the underside of the engine compartment fuse block.

Also, there are certain things wired to +12v, like the starter. Maybe something funky going on there, with maybe having a short?

 

Thanks for responding. I guess the car is awake initially as I have the battery cable disconnect before I put the ammeter in series between the negative post and the negative cable which likely wakes the car up.

I did remove each fuse and each relay in the engine compartment fuse block. I have a hard time imaging fuses located on the underside of the module, that's where the wiring harness comes into the assembly.

Yes, the starter is wired directly without a fuse so will have to check that out in addition to the main engine computer. Unfortunately I have to set it aside for the time being as I have to prep for another extended out of town trip coming up but if you think of anything else, please post. I will be watching this thread while I'm gone.

 

Closing out this thread with a bit of final information - the voltage 'drain' appears to just be the car waking up with the application of power. As I noted earlier, it will continue for a period time before dropping to millivolts. That apparently is the car going back to sleep as 'cpmetz' mentioned. It was apparently simply a failing battery that had me believing there was some other problem. Case closed.