My daughter has a 97 Lexus 300. When the car sits for more than a few days, and the night temperature is 40s or lower, the battery drains. I’ve hooked up and ammeter, and the standby current is around 100ma. Is this normal? The battery is only 18 months old. I suppose it could be defective.
You think the battery has excessive drain because…? It’s possible that there are periods when something might power and cause more drain. If you set up an ammeter, set it up where you can view it without opening the hood, or a door, or anything which will cause (even temporarily) a power use increase. From time to time, view the ammeter, look for lights, and listen for anything electrical running. If you see, or hear, something, investigate further.
I have had the same battery problems with my previous 1997 SC 400 and in my current 1999 SC 400. The problem is most likely not with your battery or the alternator.
I have found by replacing the factory cable connectors to the battery with Marine type LEAD connectors that are painted, except for the areas in contact with the battery post and cables will correct the problem!
I discovered after numerous dead battery situations in both cars that the alternator was putting out the required 14.2 V (the nominal value), but the battery was not charging. Therefore the only problem could be that the charge was not getting to the battery. I replaced the connectors with new factory connectors that were flimsy aluminum. This did not solve the problem.
I finally got a good conduction current into the battery after replacing the battery connectors with plain old lead type connectors that can be purchased at any pep boys for less than five bucks.
I have since utilized the Marine connectors in my 1999 SC 400, because they are painted on any surface other than connection to the cables and the battery terminal to prevent shorts.
Hopefully this will help correct your problem as well.
PS: my brother who is a certified mechanic actually uses spray paint or grease on the battery post after tightening the connection in order to prevent oxidation of the connection.