Which Battery Tender/Charger

[OlyS4]

1) Those of you currently using a battery tender or trickle charger to avoid the dreaded dead battery, which do you personally recommend? I am ignoring the fact that BMW sells one and hoping instead to find an equally safe and reliable one for half the price.

2) Anyone that is choosing to use the BMW tender, is there an advantage I should know about?

3) Ours (4.8 with CA and most the other goodies) is driven shorter distances throughout the week and lower than average miles overall, thinking I would put the tender on it during the weekends to get a full charge unless we take a road trip. How many hours does it typically take on the charger to reach a full charge again with the charger you're recommending?

Thanks!