Battery won't stay charged, alternator is fine
 

So I've been having this issue for about a year where i would have to drive my RX-8 at least twice a week, otherwise it won't start if it sits for over a week. Recently I had to jump it to get it started and after letting it run for 10 minutes, i shut it off and tried to start it but it was completely dead. Also i've probably gone through about 3 batteries within a year, so it wasn't that the battery had a bad cell or anything...

I went to the shop to get the alternator tested and they said it was completely fine, so i have no idea what the issue could be. I also changed/tightened the battery clamps and that wasn't it either :/ anyone know what could be going on?

Sounds like a big drain on the system. Do a search on dark current; should be around 20ma.

This is almost always the battery cables and/or clamps.

However, if you are positive that that's not it (and have completely removed, cleaned, remounted and reseated the clamps), then...

Get a volt meter, turn off the car completely, keys out. Disconnect the negative battery cable. Set the voltmeter on amperage / current, and clip the red end to the negative battery cable and the black end to the battery's negative terminal. If you measure more than 10-15 milliamps across the meter, you have a dark current draw even if the car is off. With someone watching the current readout on the voltmeter, start pulling fuses. Start with fuses in the kickpanel in the driver's footwell. Keep pulling fuses there and then in the engine bay's fuse box. Any fuse that makes the current drop when pulled is fusing a system that is draining your battery.

Now you have an idea of where to start to figure out what's drawing power.

If it's not the grounds for the battery (which keep the battery from re-charging), then the most common culprits of the very small slice left is aftermarket audio or aftermarket alarm. A very very very rare chance of an OEM component failing and allowing a constant draw.

So if it runs 10.5 ma is that normal? I took out a fuse and now its around 9.10 ma, sometimes down to 8...

There are things that draw current from the battery normally and are ok. Examples are the ECU (to retain stored fuel trims, etc...), audio system (clock, keeps time, presets, etc...).

If you are sitting at 10.5, then you don't have a dark current problem. The problem HAS to be that you are draining the battery via the starter when you crank up, and that charge isn't getting replaced by the alternator. Most common problem is the connections, specifically the negative connection. You tested your alternator as fine, and that is the only other item it could be. If you double, triple, quadruple check all the connection points and they are CLEAN and SECURE (both are important!) then you might have an alternator failure that isn't detected by the test.

Lead acid batteries do not like to be deeply discharged

You can easily kill a relatively new battery by draining it to nothing ( get yours checked :) )

I suggest you get a trickle charger that can keep the battery topped up..or disconnect the neg battery cable if you are leaving it for more than 3-4 days without driving it

Since our 8 is going to be more of a weekend car and will probably be a 2x a week driver, I bought a battery tender plus. I hard wired the leads to the battery and have the pig tail up near the AC filler port.

Whenever the car is parked in the garage the tender gets plugged in.

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OP What year is your car? I would get the battery checked out to see if it is capable of holding a charge first. THEN, if it checks out fine, start checking the car.

He has been through 3 batteries within a year. Getting a new battery or setting up a trickle charger isn't going to solve his problem. It will just keep fixing symptoms. He needs to address why his battery isn't being charged in the first place.

I somehow missed that line in the original Post.

In light of that I agree with your Post #5, he needs to track down the draw.

It would not hurt, in the meantime to put a trickle charger on it, just to stave off the inevitable.

Back to killing batteries.......I have seen them last 2-3 months when they are severely discharged and left flat....

If the charging system is working..and there re no huge draws past what the alternator can keep up with....you are left with the battery or wiring. He says there are no parasitic draws to speak of.....

I would carefully check the wiring from the alternator to the battery, but they should have checked that when they checked the charging system