low idle.....

[charlie_the_tuna]

so I go out to start my truck yesterday and there's not enough battery to turn it over, so I throw it on the charger for a half hour and it starts right up. only now it's idling at like 400 rpm. what gives??? should I raise the idle or will it come back?? ferm? spare?
2002 gmc Yukon xl 6.0