I've broken the clips of my Thinkpad's aftermarket battery.
These clips hold the battery in the laptop. Without those, it can slide out, so the battery is not much useful. They were very weak indeed (probabily because the battery is not an original one, but I don't know because this is the only one I disassembled).
I've fixed it with some stick-on velcro (that has glue on its other side).
This should be very cheap, and you are likely to find it in the nearest fabric store.
The velcro seems to keep the battery in the laptop quite stable.