Battery about 1 year old, but losing a lot of charge overnight after a 50 mile run, and the engine won't turn over . With no electrical systems switched on the battery is losing about 90mamps, which seems excessive to me. As far as I can tell the charging system is operative, though I have not checked the out put of the alternator as yet.

So is the battery a dud? Is there a minor short somewhere? Is it the alternator?
Thanks in anticipation.

 

90 milliamps is not that large of a draw, although about twice what the factory would recommend, most cars should see no more than about a 50 milliamp draw while all systems are asleep. even at 90 it would take over a week to drain a fully charged battery with the car parked and not driven.

but you said it drains while you are driving? this would mean the charging system isn't working and not a draw so you are looking in the wrong area.

if it is starting fine after a drive and only will not turn over after sitting overnight recheck the draw, it would need to be about 5-10 amps(not milliamps) to discharge a battery after sitting for half a day. the only common thing on these cars that draws that much power is the trunk ajar lights/system since the BCM does not sleep with the trunk ajar and the car will pull over 6 full amps if it is not securely closed.

 

What is the brand of the battery?

Do you have any after market electronics (radio, radar, security system, Xon lights) wired into the vehicle?

 

 

Thank you for all your comments. I've I've gone back and recheck the current draw from the battery and it went down to 25 mA today but then later went back up to 90 again. However after taking out the fuses labelled BTN and Fan, the current dropped down to about 10 mA and when I put the fuses back in it stayed at that! This seems pretty bizarre. The alternator seems to be giving about 15 volts which I would assume is adequate for battery charging, so maybe it's a dud battery after all.

 

the BTN fuse powers just about everything inside the cabin of the car, it is mainly used as an off draw circuit while shipping the cars from the factory to prevent the batteries from being discharged, the fuse is installed by the technicians after the cars arrive at their dealer.

so if something isn't going to sleep properly inside the car then the draw will be high, you can continue to dissect the draw further by removing fuses powering circuits after they split from the BTN fuse which powers the inside fuse box.

disconnect the door ajar switch so the interior lights do not function with the driver door open so you can test the fuses at the inside panel if the problem recurs.

 

Thanks Karack. I had already removed all the under the hood, (note my use of US lingo buddy) fuses and it was only the BTN and the fan ones that reduced the current draw. I am wondering if the fault lies in the central locking, or a short somewhere, as there are no lights on in the interior. I'll keep looking.

 

i still feel like the draw doesn't match the numbers though if it is still having the issue and draining the battery completely overnight.

first i would just try checking the trunk by lifting it and seeing if it is ajar. the RX8 trunk is horrible by design that it looks closed even when it is not and the draw when open is exceedingly high, since the BCM does not time out the lights when left open for a period of time. why it draws 5 times as much power as all the interior lights is also a mystery to me.

 

Conform the German Workshopmanual of the RX8 the dead current is:
Cars with immobilizer.....25-45 mA (general average of approx. 30 mA).
Cars without immobilizer...30 mA, or less.
The manual defines dead current as the constant current drain (for audio, clock, PCM, etc. etc.) if the ignition switch is in the LOCK position and the key removed.
---------------------------------------------------------------------------------------------------------------------
Ruhestrom*2
(Fahrzeuge mit Wegfahrsperre)
(mA)
25-45
(Der Durchschnittswert gibt ca. 30 an.)
Ruhestrom*2
(Fahrzeuge ohne Wegfahrsperre)
(mA)
30 oder weniger

*2 :
Ruhestrom ist die konstante Stromaufnahme (für die Audioanlage, Uhr, PCM, usw.) wenn der Zündschalter auf LOCK steht und der Zündschlüssel nicht im Schloss steckt.

 

thanks for the clarification, i didn't specify but the 50ma limit i set for cars usually includes aftermarket piggyback alarm systems and audio systems which may have more memory draw than the factory unit.

the difference is minimal though, even a 50ma draw would take nearly a month to totally discharge a factory cca rated battery. if it is seeing less than a 1amp draw and dying completely overnight then the battery is most likely faulty.

 

The battery is a dud. On charge 48 hours and it could only put out 6v. It was a cheap one but I have bought such before that have lasted over 3 years. The make was Silverline. No guarantee with it.

 

I've got a 2004, purchased new in '05. I took out the trunk light bulb in 2006 because I or my wife kept pressing the trunk unlock button somehow on the remote without meaning to, and wouldn't notice that the trunk was ajar. Tons of dead batteries.

 

One item to note and a help request - I've noticed that when my hood is up (and after a full Battery Tender charge), my 2010 GT drains the battery much faster (~ 30 days) than when the hood is kept closed (~ 90 days) as noticed this Winter. Has anyone else noticed that? Any way to test the amperage draw on the battery in both Hood Up and Hood Down cases?

 

If you have alligator clamps on your multimeter, you could probably leave them connected, lay them out of the front of the car and close the hood. The rubber seal won't damage them. Or, you could fiddle with the hood position switch, which is part of the alarm and simulate having it open or closed. I imagine closed completes the circuit, but who knows. I just don't know if it has any sort of anti-tamper measures.

 

I assumed the alarm mode is the likely culprit. I reviewed the wiring diagrams but it's hard to tell whether it's on/off. Judging by other comments from the Miata owners, it appears that the Mazdas of 2009-2012 cycle through a systems check of which the alarm may be part of the check. (Without the code, I assume it tracks the number of cycles that the hood is open and stores this and other useless info in state counters for the Mazda shop/corporate techops).