function get_item_meta (string)
	if string.find(string, "return {") then
		return minetest.deserialize(string)
	else return nil
	end
end

function set_item_meta (table)
	return minetest.serialize(table)
end

and example usage:

read values from item's meta:
		item=itemstack:to_table()
		local meta=get_item_meta(item["metadata"])
		if meta==nil then return false end --tool not charghed
		if meta["charge"]==nil then return false end
		charge=meta["charge"]

save them back:
		meta["charge"]=charge
		item["metadata"]=set_item_meta(meta)
		itemstack:replace(item)