Checkbox validation with errorPlacement and hidden field

Question

I have multiple checkbox inputs that I am trying to create validation for. At least one checkbox has to be selected for the form to submit. What I did was add a hidden input. Then with a change function, there is a join function that combines the checkbox values. Under that there is a variable that checks for the checkboxes' length.

In the jQuery validate, I require the field and have a min of 1 setup.

Currently, when I hit submit, the form goes through without any of the checkboxes being selected.

Does anyone see what I am doing wrong?

HTML

<div id="interestError"></div>
<div class="equipmentWrap">
    <label class="equipmentOptions">Walker</label>
    <input type="checkbox" name="usedMowerOption" value="Walker">
</div>
<div style="display: none">
    <input type="hidden" name="interestHidden" id="interestHidden">
</div>

JS

//Interest length creation for validation
    var interestCheck = (newSelPush || usedSelPush);
    $('#interest').val(interestCheck);
    //Hidden interest input value
    var checkLength = interestCheck.length;
    console.log(checkLength);
    $('[name="interestHidden"]').val(checkLength);

//Validation
    $('#notifSubmit').on('click', function(event) {
        $('#prodNotifForm').validate({
            onfocusout : true,
            rules: {
                interestHidden: {
                    required: true,
                    min: 1
                }
            },
            messages: {
                interestHidden: {
                    required: "Please choose at least one interest",
                    min: "At least one interest needs chosen"
                }
            },
            errorPlacement: function (error, element) {
                if ($(element).attr('name') == 'interestHidden') {
                error.insertBefore(element.parent('div'));
                } else {
                    error.insertAfter(element); // <- default
                }
            },
            submitHandler: function(form) {
                //Variables for the form ids


                $.ajax({
                    url: 'subscriberNotifSend.php',
                    type: 'POST',
                    data: {
                        'notif_first_name': notifFirstName,
                    },
                    success: function(data) {

                        }
                    }
                });
            }
        });
    });

Update - code from the change function:

var showMe = '';
var checkLength = '';
$('#interest).val(checkLength);
console.log(checkLength);
var newStatus = '';
//Telling whether new is selected   
var usedStatus = '';
//Telling whether used is selected
var newSelPush = '';
var usedSelPush = '';
$('.equipmentOptionCont').change(function() {
    var newSel = [];
    var usedSel = [];
    //Get new mower options 
    $("input:checkbox[name=newMowerOption]:checked").each(function(){
        newSel.push($(this).val());
        newSelPush = newSel.join(', ');
    });
    //Get used mower options
    $("input:checkbox[name=usedMowerOption]:checked").each(function(){
        usedSel.push($(this).val());
        usedSelPush = usedSel.join(', ');
    });

    //Interest length creation for validation
    var interestCheck = (newSelPush || usedSelPush);
    $('#interest').val(interestCheck);
    //Hidden interest input value
    checkLength = interestCheck.length;
    console.log(checkLength);
    $('[name="interestHidden"]').val(checkLength);
});