Jaguar Repair: recently purchased xj6, brands of cars, free batteries

Question
i recently purchased a 1986 xj6 that is in excellent condition. It has about 100,000 miles on it, and runs great.  The only problem we found with the car is with some start ups.  The engine spins, but will not crank.  You can put a battery charger booster on the car, and it will crank right up.  It doesn't seem to turn any faster, but the additional voltage seems to make a difference.  I've seen where this car has an ignition amplifier on it.  Any answers?

Answer
Hi Curt,
That is a classic symptom on the Ser.III XJ-6. The car requires a "Deep Charge" battery like a diesel or marine battery. Even though a standard battery seems to spin the engine over quite well, it will often not allow the car to start. The standard test is what you did by adding a jumper battery even though the engine was able to spin over quite well. Even when a Deep Charge battery gets aged it too will cause hard starting. The battery then would work for a good time in other brands of cars but not in the XJ-6.
I kept other cars in free batteries (core charge)for a long time from "bad" batteries from the local Jag dealer when the Series III was new.
Howard