Battery Full charging

hi i ave a problem with my nexus 5... recently when device is charging my battery doesn t increase in % but decrease or is equal... now i don t understand if the problem is the battery or charger... is use paranoid full F2FS uber kernel with undervolt.. during charging i don t use many app just like chrome youtube or facebook..i use wifi or data, my signal is bad (2g) the call is bad.. metallic.. for this reason i use wifi ever.. help me please