My wife and i just switched and i bought her 2 auto batteries and myself a manual one. Is there any danger if i accidentally over fill hers or will the battery just stop working? Thanks.
You'll be fine. It'll just stop working. Dormouse probably has some good suggestions on not overfilling. Honestly all I do is count 15 drops and check to make sure no juice is leaking out of the bottom of the carto. If juice is leaking I dry it with a paper towel and vape away. Everyone says auto batteries are super sensitive to juice, but after several months, I haven't ruined a single one with juice. By the time you ruin it, it'll most likely be time to get a new one anyway.
After recharging the slim batteries for 2 or 3 months they will start to lose their ability to hold a charge. I use Vapeshot.com stuff now and they send you a free battery after you get 20 boxes of cartos anyway. Other manufacturers have warranties that may replace the battery as well.
If you want the battery to last longer get something bigger like an eGo. You should note that all Li batteries start to lose their strength after 150 or so charges. The larger batteries last longer simply because you don't have to charge them as much (in my opinion). Others will say that there is less strain on the bigger batteries which is true but it is a simple fact of life that Li batteries - no matter how big - can only be recharged so many times.

Learned my lesson.