EGO ST12 - Help Required :)

Hello all, I am looking for some basic advice. I have an ST12 EGO scooter that recently started to smell and make a hiss sound when charging. It will not function correctly - the throttle delivers power on/off intermittently. I assume that one of the batteries has died so how would I go about testing this? More importantly how do I access the battery compartment?? Apologies for the basic questions but I am new to all of this. Thank you so much!