The IQbuds2 MAX and charging case have built-in, fully rechargeable, lithium-ion batteries.

These are not replaceable but the user.  As per the product and safety guide - 

• ##Do not attempt to replace the bud batteries or the charger accompanying battery yourself. You could damage the battery which could cause overheating, fire or injury.  Batteries in this product are only serviceable by Nuheara or an authorised service agent and should be returned to Nuheara for service or replacement (a cost will apply if outside the warranty period).