If the battery is not the sealed type does it have a breather pipe to vent the gases outside.
if an open type have you checked the fluid level is not low and below the level of the plates they will overheat and fry as they say, giving off the kind of smell you describe.
Also have a sniff around the charger, over heating electrics usually smells like fish.
If the tops can be removed, top up with distilled water & fully charge. If it holds charge then battery is ok. If you have not charged battery all winter & let it go flat then it has probably had it. If you are not using van you need to charge batteries once a month.