Battery re twice in 18 months. What is causing that?

Short trips.

Hello, it is unusual that you would need to replace a battery so frequently. The battery itself is likely not the root cause. One major common concern these days is that the electronics of the vehicle can be pretty hard on a battery, especially if the vehicle accessories are continuing to be used while the engine is not running. Simply put - ALWAYS run your vehicle when running accessories such as a radio, lights or mobile phone. Another major contributor to a premature battery drain is infrequent short trips being driven. If you don't drive enough or frequently enough, you can expose the weakness of an otherwise good battery. Evaluate your driving habits and operation of vehicle accessories and you may easily solve your concern. Besides that, the vehicle should be checked for signs of parasitic draw, such as if an electrical accessory is staying powered when it should have shut off. Check for the presence of any aftermarket components such as an alarm -- even if it's dormant. This can draw power and drain your battery too.

I had the same problem in a Dodge van. Had the charging system checked several times and was told there was no problem. I finally did extensive testing myself and found the alternator was putting out almost 15 volts. It was burning up the battery. When the alternator went out, the replacement output was around 13.8 volts. It had over 200,00 miles when I traded it in and I never put another battery in it.