I haven't tried this so not entirely sure if possible. On old BMW airheads the battery is fairly big and easily replaced by smaller modern gel equivalent. Could you use shell of old large battery and place the smaller gel battery inside but use the surplus space as a hidden storage area? Would look like a regular battery if done properly with the connections wired up inside the shell.
You would have to make sure the internal connections are rock solid as a failed earth is a common fault on bikes and rectified by cleaning up the earth connection, which would be a fools errand if the real earth connection problem was inside your fake battery case.
|