Electrical Issues? '13 Avalon
My 2013 Avalon Limited was purchased in Dec 2012. It is my 3rd one - have always loved them. But THIS one has been a disappointment. It seems that every 10k miles the battery completely loses its charge and the battery must be completely replaced or fully charged - RIDICULOUS. I only have 30k miles on it! Has anyone else ever had this issue? Is this a known issue with Toyota? I have gone from feeling completely confident in my car to not knowing whether it will start up when I need it!
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Under 3 miles?
3-5 miles?
5-8 miles?
8-10 miles?
More ???
It's essentially a usage issue, not a vehicle problem so it isn't broken and cannot be "fixed" by changing any parts. If you could reduce the electrical load on the car that would help. But that means giving up specific creature comforts and AC in the summer is the first one that comes to mind. That of course would not be a popular solution even with me (I love my AC). If my regular commute was going to deplete my battery as yours does, I'd probably install a battery tender to restore the batteries state of charge overnight when the car isn't in use.
It would take a lot of time to explain all of the reasons why this cannot be fixed by just kicking up the output of the charging system at idle. But I can mention a few of them and the first one has to do with cooling the alternator when its providing a lot of current. Another problem has to do with maintaining the engine idle speed and keeping the engine running correctly when the alternator would be putting a heavy load on the engine. On top of that, trying to charge at a high rate with a low engine speed causes electrical noise that can interfere with computer input signals and communication waveforms between computers in the car. All of this (and more) are why the battery becomes the primary power source at idle. If you could alter your commute to get away from the stop and go traffic that would be the best choice, otherwise you will go through batteries on a regular basis.
While there may be issues as mentioned in previous posts, there is no way that your current driving would result in the battery draining as you've described.
You may have left something turned on, however you say you are careful to be sure you don't, so I would take the next step that the next time this happens have it towed to the dealer, and when they charge the system have them tell you what they found still turned on that caused the battery drain. I had a Chrysler back in early 2000 that had a similar problem and after all else failed they performed a "jiggle" test where the jiggle the wires and found a ground short that was draining the battery but only when the wires "jiggled"a certain way. Always consider that with any mass produced product there will be problems with a certain % of product and you may have unfortunately received one of those.
Unfortunately this isn't accurate and there are TSB's with multiple manufacturers including this Avalon that describe the above condition. It simply takes exactly the right circumstances and the problem occurs. You tried to use the argument of your usage is limited as well but hasn't caused any issues yet and that doesn't mean you aren't depleting your battery as well. It only means that the symptoms haven't occurred yet, if they are ever going to.
I'll try and explain this a little further. It takes a given amount of energy to start your engine. A fully discharged battery that has no reserve capacity left that can still start an engine. The reserve capacity is the amount of time that a fully charged battery can put out twenty five amps of current, and still have enough power left to start the engine. I'll repeat this, It's important to understand that a "discharged" battery at this point can still start an engine. Once the reserve capacity is all used up the battery typically remains in a constantly discharged state and begins to sulfate and fail. A "sulfated" battery is a condition where the sulfur that is in the electrolyte starts to crystalize in the lead dioxide plate rendering it no longer useful for further charge/discharge cycles. As long as the sulfur doesn't crystalize the chemical reaction is reversible through charging and the sulfur can be removed from the plate and re-enters the electrolyte.
Your car(s) are likely still functioning with some level of the reserve capacity available but not at a full state of charge, should you happen to deplete all of the reserve capacity you will end up in the same condition as the O.P. It's a balancing act where your usage for now manages to replace most (but not all) of the energy pulled from the battery. Tip that scale even slightly and the results for you could be very different.
Follow the thread on the start/stop functions on the 2015's and try to imagine just what kinds of problems we will be growing through with those, especially if someone does encounter a lot of stop and go traffic.