The lead acid battery in our 928 needs to be fully charged to cope with the power demand of starting the 8 cylinder engine.
It takes a battery of 70+ Ah and a starting current of over 600 Amps to turn the cold engine over. This is especially important in wintertime when the battery capacity is lower than in summer, and the engine requires more force to turn due to thicker oil in the various passages and bearings. So sort of a worst case scenario.

How do we keep the battery in condition?

• Charge the battery if not used for more than a 5 days. The 928 slowly drains the battery (clock, ecu's, alarm etc consume power).
• Do not start the engine when the battery is low. You drain the battery even more.
• Try to avoid discharging the battery for more that 20%. A flat battery with almost no voltage is disaster for the capacity.
• Disconnect the battery if you're not using the car and not charging with a trickle charger.

How to charge the battery?

• A good battery is charged to 14,4 volts. A low battery voltage is 10,4 volts. Do not go lower than this as it might damage the battery.
• Charging can best be done disconnected from the car, as the ecu's don't like a charging voltage over 14 volts.
• If you charge the battery in the car, keep the voltage low like 14,4 volts.
• Charging current should be like 6 amps. Allow time to recharge. Short charging cycles do not help for capacity.
• Use a trickle charger. This charges like 900mA constantly or a better charger that only tops up till 14,4 volts and then reduces current.

Rejuvenating a bad battery?

there are several stories about this.

• One is about pulse charging. Use 16v short pulses on a 30Hz frequency to try to solve sulfur sticking on the lead plates. Some say it works, some say it is a waste of time. I've tried this for two weeks with no obvious results.
• One solution is using an old skool charger that effectively has 50Hz half rectified DC at 15volts. Charge current can initially go up to 10 Amps so it acts like a boost load. (take the battery out of the car or at least disconnect it) It works like pulse charging as stated above. The battery should not get hot. Keep an eye on this.
• Some say over charge the battery as much as you can and then drain it to 10,5v. Repeat many times. It is like training. I've tried this as well with narrow pulse charges as high as 19v, and it looks like charging waves to seem to make the battery happy but total capacity is still minimal and not gaining much.
• Another approach is rinse the battery with distilled water and replace acid. That only works if it is an open battery. Most are not. So I can't test.

Bottom line:

• A battery has a lifecycle of about 6 years or so, maybe more in daily use.
• If you drain the battery a few times until it is completely flat you reduce the lifetime to 1 year or even less.
• Do not over-charge the battery, like charging it at 16 volts or higher !!! The battery will not gain anything by doing this. You will hear bubbles in the battery and H2 gas will flow from the battery breather holes. H2 plus O2 and a spark will ignite like explode and you don't want that happening with your lead-acid battery close to you.  
   

Regards, Theo
http://928gts.jenniskens.eu