2005 e320cdi

recently taking little longer to crank/turn over---suspected battery

replaced both batteries... now the CEL is on -- stays on

Yesterday-- same slow start (temp around 28 f) ... and for the first time
the Glow plug stayed lite stayed on for around 30 seconds

Is this a GP Controller issue? or Glow plugs? -- I thought w/ bad GP the car would actually indicate GP # in the Info messages?

what does the jury think?