battery failed after subaru cleaned it few months ago from corrossion.will get a new battery this time or clean terminals and charge me?

SADLY< it is the MOST common for a BATTERY and the ALTERNATOR to fail at the SAME exact time so you need to replace them BOTH! The ALTERNAOTR these days have the VOLTAGE REGULATOR inside them! AAND SUABRU< like all the OTHER JAOANES and most ASIAN CARS<use DESNO as a supplier! DENSO starter, DENSO injection, DENSO alternator etc! OPIWER WASH your ENGINE using GUNK ORANGE original engine cleaner at a DIY car wash! GEt a FREE charge system test from AUTO+ZONE but do NTO buy their CHEAPO batteries EVER! I got 17 YEARS from the battery in my used BENZ! I WAS SHOCKED! The EURO and Asian batteries just Seem to last longest and are the MOST PURE with build quality! OTHER ONES seem to make MOE{NEY for the person SELLING  THEM! BATTERIES with REAL access caps are STL the best! you can SERVICE each cell! MAINTENANCE free batteries are ANOTHER name for CHEAPER TO MAKE batteries! The SEALED ONES usually and are MADE from THINNER casings, and CHEAP internals! YUCK! 

Hay Wesee:  Did they test the battery to see if the battery was good or not? They may need test your alternator as well, You may have a loose connection on the alternator that is causing it to put out a lower than needed recharge, or your belt mite be a little loose. OR it could be the battery. Go to an Auto-Zone and have it tested for free.  Good luck with that one!

Warranties rarely cover things like batteries and usually have a disclaimer about neglect (the whole reason why it got so far as to corrode and fail). The details of your extended warranty may say otherwise. Most batteries have a pro-rated warranty, but you have to go a retailer that sells the same brand (maybe the dealer since it's an OEM battery).

The battery warranty will pay for part of a new battery, but not the labor to install or remove.  Every month you wait, the pro-rated amount diminishes. No matter what, the terminals will need to be cleaned and the dealer will say that is part of the install/replacement tasks that they charge you for.  If it was me, I would use the pro-rated warranty to get a new battery (if it is a reputable battery manufacturer) and install it myself. I would clean the terminals if they are salvageable or replace them. I would also buy the insulating rings (color-coded of course) and spray-on coating to protect them. If the OEM manufacturer is not a major player, I'd take my money and buy the best 60 month battery I could find and install it myself. Don't forget to ask about the core charge. That is what they will refund you when you bring your old battery back for disposal. If they say they don't pay a core charge, run from there as fast as possible and find another retailer.

By the way, high levels of corrosion on the terminals are an indication of a couple of possibilities. One is the charging system is faulty and creating heat. That can be from an overcharging condition or the system is not putting out a clean D.C. signal from the alternator. I bet the OEM battery does not have removable caps, so the source is around the posts. Somebody may have whacked on the terminals too heavily, I've seen idiot mechanics do that while attempting to drive the terminals on the battery posts. That will definitely drive the terminal down, but it also will break the seal around the post as well as weaken the cell connection that will burn up prematurely.

Cleaning the terminal means that some battery acid bubbled up from one of the cells and escaped outside. That means that at least one of the cells became useless. There may have still been enough cells to keep producing 12V, but the overall capacity of the battery was down. Usually when one cell bubbles up, then the other cells aren't far behind, and the battery will need replacement soon enough. Since the car is still under warranty, have the dealership do the fix, and remind them that you just brought the car in recently and all they did was clean the battery when it should have been replaced. Bring the receipt with you to remind them that they just cleaned the terminals, so this time they better replace the whole battery, they may not even charge you for labour this time.

Do not waste your time.  Get a new battery and replace the battery terminals.  See a mechanic and get the car's charging system checked.

If the battery doesn't have a weak or dead cell, and cleaning the terminals helps it to start, they will probably just do that. If they have to replace the battery and it's near the end of the warranty, they might charge you a prorated price.