Already some time ago I built this prototype of a battery pack:

It can hold two 14.8V 5Ah LiPo batteries wired in parallel. I did some tests with one battery earlier and estimated from that this should be enough to mow at least 1.5h non stop.

Apart from connector orientation the design is mirrored. The front face with the two connectors will sit on the robot with the left connector providing the amps and the right one reporting individual cell voltages. The connectors on the other side (facing upwards when battery pack is mounted on robot) do exactly the same. The idea is that the battery changing station can grab the pack and connect to it in one step. The changing station will be a big disk that can rotate, move up/downwards and hold up to 3 battery packs (see sketch on system design)

The biggest problem on a mechanical point of view is how to craft the docking mechanism. The problem is that this has to be very accurate to work smoothly, but my most advanced tool is a cordless drill... Fine adjustment must be possible via screws with washers in bigger holes or something.

This are the electronics inside the battery pack. There are two of these simple boards in one battery pack:

As you can see rather simple, simply a bunch of connectors and solder pads to join everything in parallel and provide mounting options. The PCB is homemade via toner transfer (probably should do a video on my selfmade laminator and etching machine..)

I also purchased the mowing plates and hood that will be laser cut from aluminium:

As you can see blades of different sizes and two or four blades per plate can be mounted.

All in all it's 65€ and the parts will arrive ~ 10. Oct. I purchased at http://cutworks.de/ an their website/tools are really great, will see how the parts turn out.