The PP45s have two tension levels of contact available; I recommend making sure you get the higher level ones for this type of use. The others can be pulled loose from the weight of the cable pulling downward during a big bump, once they have worn a bit from repeated disconnections.
The Anderson SB50 will not disconnect by accident even once it is worn, but it will pull loose if yanked (like if you are separated from the bike in a crash), as long as the cable at the other end is secured. There are even specific cable clamps they make specifically designed to hold the wires to the connector shell to prevent strain on the crimps/etc when used in this manner (I see them on powerchair and UPS stuff now and then, too).
I had SLA 18Ah bricks on CrazyBike2 ages back, and in some of my early testing I just set them in baskets, and going off my driveway was enough of a bump to bounce one out, and the SB50 disconnected just fine, no cable or battery damage (other than scratches from it's landing).
As for magnetic connectors...for some higher current uses you could try the ones on the AC cords for deep-fryers, but you will probably need to replace the magnets with better ones, as they often don't even hold on very well just sitting there, and I doubt they'd hold together with any kind of vibration, much less road bumps or slight cable wiggles.
Might be easier to just build your own. There have been a few "backpack battery pack" threads, and I think at least one used a DIY magnetic connector, if you look around the forum (possibly in the skateboard section?)
