Did the dealer perform a check of the charging system before telling you that? Sounds like your battery might be overcharging, which can be very bad. See if your batter is very hot after a drive or if it is swelling up or leaking. I'm surprised the CEL isn't on if that's the case. Take it back to the dealer. Tell them that you want it figured out and leave it there. That's complete BS that they just told you they are "not putting strong enough batteries in from the factory" and gave it back to you IMO.
2007 WRX sedan
Cobb Stage 2-protuned by P&L