I don't know what resistance your dew heater is, but as an example, lets say it's going to draw 0.5A = 500mA.
Your 9800mAh battery will last for 9800/500 = 19 or so hours from fully charged to flat. The 7Ah one will last 7/0.5 = 14 hour or so.
This example shouldn't be a problem if you recharge after each night's session, but you need to do these calculations for the heaters you have built.
Al.