On modern power panels the glow plug current is controlled by a current regulator IC that has an adjustable pot. There is also a meter on the power panel that shows you what is happening. The power panels also include a reversable switch for your electric fuel pump and connector jacks for your starter. You can plug the 12 volt charger into the starter jacks to recharge your field box battery. That being the case, I would just bite the bullet and buy one. They may look expensive but they do a lot of things and are reliable.