A 12v bulb will need a 7.2 1400 mah battery to go for 3 hours, if your trying to jury rig this you should go with a rechargeable Drill battery of at least 16v level that way you have plenty of reserve juice
i think it extremely is an electro-hydraulic capacity steerage pump on that one, so it sounds very similar to the alternator isn't producing right. Assuming the force belt is there and not slipping, i might get your alternator output checked. could be a unfastened cord, could be a failing alternator.