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I have a 2008 Mazda 6 that I purchased new. The first battery lasted 8 years (I only drive about 3K miles a year). The second battery lasted about 18 months. The third battery lasted about a month before issues started.

The car is driven infrequently. Sometimes after it sits for several days, I find the battery is drained/dead and it needs to be jumped. I dropped the car off at my local mechanic who's been good to me for 30 years...he kept if for 2 weeks, starting it occasionally to see if the battery was draining, but it didn't act up while he had it (there was no charge for babysitting!). We brought it home, I drove it a few more times without incident, and now it has drained the battery again.

I see several threads from 2-4 years ago where this/similar issues are discussed, with lots of recommendations but I don't see any definitive answers as to a common diagnosis. Has anyone had this problem and found a defect or easy solution? I am willing to pull fuses, but I don't see many non-essential ones to start with. I'd like to learn from your pain if you've had this issue and are willing to share what worked for you.

Nothing aftermarket except for driver's side seat heater, which has not been on this summer/fall.
 

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Thank you Talon!

Wondering if anyone else has thoughts, or even better, found a solution to a similar issue....
 
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Thank you Talon!

Wondering if anyone else has thoughts, or even better, found a solution to a similar issue....
In your case, I would highly recommend you get a trickle charger. You might want to unplug the battery to save it from dying if you know you're not going to drive for several days.

This pattern you're mentioning with each battery sounds bad. what is your mileage? I suspect your alternator is getting weak.
 

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We were watching the car and starting it every 3-4 days, and all was well. Then we left for vacation and came back after 12 days to a dead car (with the 2 month old battery).

Mileage is around 31K. I hadn't thought about unplugging the battery. We've got it plugged in to a charger in the garage right now. I am afraid to drive the car anywhere for fear of getting stranded, which is really frustrating. As you can imagine, with only 31K miles in over 10 years it's close to pristine.
 
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We were watching the car and starting it every 3-4 days, and all was well. Then we left for vacation and came back after 12 days to a dead car (with the 2 month old battery).

Mileage is around 31K. I hadn't thought about unplugging the battery. We've got it plugged in to a charger in the garage right now. I am afraid to drive the car anywhere for fear of getting stranded, which is really frustrating. As you can imagine, with only 31K miles in over 10 years it's close to pristine.
you've replaced the battery twice in 31K? there's def something wrong here. The only thing I can suggest is check to see if your alternator is bad but its unlikely.

You might need to take the car out for a good 30 minute, uninterrupted highway drive to let the alternator fully charge the battery. That could take an hour or even more if you're driving in the city, according to google. If you only do short distance driving with your car that's probably the issue.
 

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you've replaced the battery twice in 31K? there's def something wrong here. The only thing I can suggest is check to see if your alternator is bad but its unlikely.

You might need to take the car out for a good 30 minute, uninterrupted highway drive to let the alternator fully charge the battery. That could take an hour or even more if you're driving in the city, according to google. If you only do short distance driving with your car that's probably the issue.
Thanks for taking time to provide suggestions. Really appreciate it! The Mazda is our second car, and most of the driving is short distances. We can take it out for a long drive...but is there any chance that will resolve the issue? Without a "fix" I won't drive the car (because I don't want to get stranded somewhere) and I can't get rid of it because who wants a car with a mystery electrical problem? When I find it dead, it is DEAD, with the exception of the dash lights, radio, and security system. Ticking, no cranking when the key is turned. Plug it into a charger and it starts right up.

Wish there were some fuses I could pull for non-essential items (like trunk light) to see if that made any difference, but all the fuses seem to control something important.
 

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This is what I'm using:

does this stay in the car at all times and always on? looks intriguing and might be cheaper than having a mechanic search out what's causing the intermittent full drain of the brand new battery....can you tell me more?
 

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does this stay in the car at all times and always on? looks intriguing and might be cheaper than having a mechanic search out what's causing the intermittent full drain of the brand new battery....can you tell me more?
Yes, it stays connected to the car as long as I'm not using it. I connect this whenever I'm not using the car and leave it that way until I'll be using it. This battery charger is a bit expensive but it's a known brand.

This will not solve your problem with your battery if you have a parasitic load. This only helps in "extending" the battery life in such a way that, if you frequently use the car for short trips, that the alternator couldn't fully charge the battery, CTEK will charge your battery safely. Safely, because it's a smart charger. You can leave it without worrying about it, i.e, overcharging.

You can read more about it here:

 
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If you drive only short distances this is what is killing your batteries. If you make sure the battery is fully charged and then unplug it if you dont plan to use the car for at least a week, I dont think you will run into any issues should you need to use the car again.
 

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Thanks for taking time to provide suggestions. Really appreciate it! The Mazda is our second car, and most of the driving is short distances. We can take it out for a long drive...but is there any chance that will resolve the issue? Without a "fix" I won't drive the car (because I don't want to get stranded somewhere) and I can't get rid of it because who wants a car with a mystery electrical problem? When I find it dead, it is DEAD, with the exception of the dash lights, radio, and security system. Ticking, no cranking when the key is turned. Plug it into a charger and it starts right up.

Wish there were some fuses I could pull for non-essential items (like trunk light) to see if that made any difference, but all the fuses seem to control something important.
I am suffering thru this same problem right now with my 2010 6 at 180K. Here is what i've done:

  1. Taken current motomaster battery for warranty included battery test. Load test turned out fine. Battery is 2yrs old.
  2. Measured parasitic draw current to be 150mA (0.150A) at 12.4V. Did extensive fuse pulling from hood box and inside box (BCM). Isolated to "Room Fuse" inside. Once i removed the room fuse, the 0.15A went down to 0. I personally dont think 0.15A is a lot of Amps for new-ish cars. I have a feeling its the battery. Currently using the car with Room fuse pulled. No power locks, keyless entry, radio, clock or TPMS.
  3. Ordered Odyssey AGM battery from Amazon. Will arrive on Friday Dec 6 and i will test with that over the following few days.
Does anyone here know if 0.15A of standby draw is abnormal for a 2010 Mazda 6 GS 2.5?
 
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I am suffering thru this same problem right now with my 2010 6 at 180K. Here is what i've done:

  1. Taken current motomaster battery for warranty included battery test. Load test turned out fine. Battery is 2yrs old.
  2. Measured parasitic draw current to be 150mA (0.150A) at 12.4V. Did extensive fuse pulling from hood box and inside box (BCM). Isolated to "Room Fuse" inside. Once i removed the room fuse, the 0.15A went down to 0. I personally dont think 0.15A is a lot of Amps for new-ish cars. I have a feeling its the battery. Currently using the car with Room fuse pulled. No power locks, keyless entry, radio, clock or TPMS.
  3. Ordered Odyssey AGM battery from Amazon. Will arrive on Friday Dec 6 and i will test with that over the following few days.
Does anyone here know if 0.15A of standby draw is abnormal for a 2010 Mazda 6 GS 2.5?
It's not the battery. What about your alternator?

If you do only short distance driving, your battery is probably not getting a full charge.
 

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It's not the battery. What about your alternator?

If you do only short distance driving, your battery is probably not getting a full charge.
The alternator (and regulator) is charging the battery and working fine as per mechanic. I drive it 40-50mins 1-way to work/school and back, this should charge the battery completely. I leave it overnight, the next morning its fine; however, when i leave it two consecutive nights, it discharges completely. I have tried disconnecting the battery overnight two to three days consecutively and starting it after the 2-3 day mark and it works fine. Either something is draining the battery or the battery is not able to sustain the 0.15A draw.

It's not the alternator.
 
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The alternator (and regulator) is charging the battery and working fine as per mechanic. I drive it 40-50mins 1-way to work/school and back, this should charge the battery completely. I leave it overnight, the next morning its fine; however, when i leave it two consecutive nights, it discharges completely. I have tried disconnecting the battery overnight two to three days consecutively and starting it after the 2-3 day mark and it works fine. Either something is draining the battery or the battery is not able to sustain the 0.15A draw.

It's not the alternator.
Your ROOM Fuse is causing the battery drain. You have to check out your wiring diagram and figure out exactly what it is (one of the things controlled by the room fuse) that is causing the drain.
 

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I have a 2008 Mazda 6 that I purchased new. The first battery lasted 8 years (I only drive about 3K miles a year). The second battery lasted about 18 months. The third battery lasted about a month before issues started.

The car is driven infrequently. Sometimes after it sits for several days, I find the battery is drained/dead and it needs to be jumped. I dropped the car off at my local mechanic who's been good to me for 30 years...he kept if for 2 weeks, starting it occasionally to see if the battery was draining, but it didn't act up while he had it (there was no charge for babysitting!). We brought it home, I drove it a few more times without incident, and now it has drained the battery again.

I see several threads from 2-4 years ago where this/similar issues are discussed, with lots of recommendations but I don't see any definitive answers as to a common diagnosis. Has anyone had this problem and found a defect or easy solution? I am willing to pull fuses, but I don't see many non-essential ones to start with. I'd like to learn from your pain if you've had this issue and are willing to share what worked for you.

Nothing aftermarket except for driver's side seat heater, which has not been on this summer/fall.
I just had my BCM replaced due to the same battery drain problem. 4 batteries I've gone through and Mazda couldn't find a problem. Mechanic my husband knew who works on race cars found it after 8 hours of diagnostics. Just got it back today so we'll see if that's the resolution. I definitely believe there's a defect and Mazda won't own it. I'm sending them an email about it as well.
 
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