Like Turtle says and like Ive said dont run these two different batteries together,,YOU WILL FRY ONE OR BOTH,,,
a voltage regulator regulates voltage not amperage,,amperage is the thing that you need for loonger run times,,,a 1500mah battery has half the life of a 3000mah battery,,and mixing ratings on the batteries will cause one to get blown or melted,,period,,,if you have the room for two packs to get higher amps then you have the room for one that is rated higher to begin with..
more voltage gives more power and more mah's gives more time,,if you have a problem because you are burning through your batteries fast then it sounds like you are undervolted too,,,you need to check how much voltage your motor will handle and volt up your entire rig,,,if the esc is rated for 9 cells then go to 9 cells at a higher mah rating,,,if you are running the steering servo from the receiver then you need to add an external bec to the system so that the servo can work more efficiently,,power is wasted if it does not give enough for the components it serves...the motor will run warm or hot if undervolted and so will the steering servo,,heat in these rigs is our worst enemy...heat in an electrical circuit means wasted power.
