Create a custom Yoast SEO Variable

functions.php

//
// Add a Custom Yoast Variable
//
add_action('wpseo_register_extra_replacements', 'add_yoast_supercharge_variables');
function add_yoast_supercharge_variables() {
    if (function_exists ('wpseo_register_var_replacement')){
		wpseo_register_var_replacement('%%BRAND%%', 'supercharge_yoast_brand', 'advanced', 'Supercharge Brand' );
	}
}


function supercharge_yoast_brand() {
	$post_id = get_the_ID();
	$this_brand = '';
	if ( get_post_type($post_id) == 'battery' ) {
		$brands = get_the_terms( $post_id, 'brands' );
		if ( $brands ) {
			if (count($brands) > 0) {
				$this_brand = $brands[0]->name;
			}
		}
	}
	
	return $this_brand;
}