Instantly wakeup after suspend when on battery

Hi, I have Hp mini 311c notebook.
When on AC with battery plug in, I can suspend normally and wake it up when I want by opening the lid or pressing a key.
But when the notebook is on battery only, I can go to suspend but it instantly wake up without me doing anything.
I would like to trace down what wake up my notebook when on battery. It must be pm-utils or laptop-mode-tools but I don’t know where to begin with ?
Anyone know how to catch what signal wake up the pc ?