New battery caused 'Battery" warning light to light? alternator?

I also have an ES300 and have replaced the battery. There are a few things you can check first. If your battery was really that corroded you might want to replace the battery terminal connectors with new ones. If they are loose(able to turn them with your hand, even if it requires a lot of force you should not be able to twist them with your hand) it could affect the alternators ability to charge the battery if its not making complete contact with the battery terminals. If you tighten the connectors all the way and can still hand twist them, then its time to replace. If the old terminals are fine you would at least need to clean them thoroughly due to the severity of corrosion from the previous battery. If its not charging the battery at all you might want to check the alternator fuses. Thirdly, a loose alternator belt can also cause the alternator to put out less volts, therefore not supplying enough power to run the necessary accessories causing the battery to be used while the car is running instead of recharging it from the jolt of juice it lost starting the car.. If its loose you can have it adjusted and tightened, if the belt is cracked or worn, have it replaced. Make sure to check that the wiring is all sound and not broken or corroded that connect to the battery. Make sure the ground wire is still grounded. If these things fail to solve the, it might be time for a new alternator. If you have autozones in your area they will run a free alternator check for you. Just go in and ask for one and they'll wheel out the machine to your car. (P.S. If your running any kind of additional electronics like an aftermarket amplifier or any other high drain device you'll have to install a high output alternator to accommodate the extra demand. A new alternator putting out the factory level voltage won't be enough even though it's brand new. Those can be obtained off of eBay.)