P3006 and P1259 but no loss of MPG. How will it delcine?

Check Engine light is on but car seems to be running OK (getting 40mpg). I do notice that the car seems to rely on the Gas more than the battery than it used to. My question is, will MPG start to drop gradually until one day the Traction Battery is just dead or will the car go from running well to not running well in minutes.

My shop says it needs a battery but it's not in the buget at the moment so I need to see if I can psotpone it until the new year.

2003, 145k, just bought it 3 months ago

 

correction, C1259, and MPG is dropping slowly and Battey has wild swings from 100% to 50%. Gas engine is doing so much work now. Even when going downhill on the freeway it's very hard to get the batt only to maintain the speed. It either wants to use the gas or both, or charge the batt even when it's at 100%.

New HV time I guess