Can I mix new and old batteries in a parallel circuit?
The fresh batteries will attempt to 'charge' the older batteries, since there will be a voltage difference.
Depending on the type of battery, and the differences in voltage, the results may be anything from &quot;It works just fine&quot; to bulging and/or leaking and/or overheating batteries.  The latter because these are not rechargeable.
With rechargeable batteries, the batteries with the higher voltages will charge the lower voltage batteries until they equalize.