I have a battery tender jr and it has been good.

FWIW, my PCA pres has the oEM Porsche one and just toasted 2 of his batteries. It wasn't shutting off properly.

 

Just a little food for thought. The guys over at North Hollywood Speedo warned me not to charge the car with the battery cables connected(even using the cigarette lighter Porsche charger). They told me it could damage the circuitry in the instruments.

Since then I simple remove the battery cables and hook up the battery tender until I see the green solid light.

 

I sure hate to dispute what they're saying and they may well have some example/s to back it up but if there's fused 12v. available at the cig. lighter, then applying 12v. to it should do no harm. Ask them where the instruments are connected to the cig. lighter. Also, consider the fact that, every time you disconnect your battery, you are requiring your OBDII vehicle to "re-learn" it's memory. If you're due for an inspection anytime soon, you're in for a surprise!