Alternator not keeping up

I have a 16 hp Vangaurd with a 12 volt Burner
When I run the burner it works fine even running almost non stop for hours, but the next time I go to start the machine the battery is low and does not have enough power to start the engine. I have to jump it from another battery.
When I do NOT run the burner the battery stays charged up fine and battery has good charge when I go to restart the engine.
So the alternator must be charging but not as well as it used to?
What should I test to figure out what is going on?
This is on a Hydrotek SS Skid.

I would think the battery is shot. I buy the biggest one I can fit.

I agree, it sounds like maybe a dead cell in the battery. I'v e always had good luck with Everstart batteries and you can get them at Wal-Mart.

Burner-Battery problems

Battery is first bet, but bad connections are the next suspect..
there are a few common wiring errors made on 12vDC burners..
turning on the burner with a 15amp switch eventually (lke..sooner than later) partially scorches its contacts,
and slowly kills the ignitor with extra current-draw.. in between, the ingitor draws more and more current, till it pops.
..
an ADC burner draws almost exactly 16 amps..
On a 16amp Briggs charging system.. any more currernt drain kills the battery.

make sure the power switch is rated for 20 to 25 amps,
make sure the burner has 10 gauge wire connecting it all the way to the battery,
NEVER use a pressure switch to turn on & off the HV ignitor, (use a 70 to 80 amp rated relay, and a FLOW switch),
..and carry a multi-meter in your tool bag.. you need to monitor battery voltage.
..it helps "predict" failure.

good information