Porsche Boxster Owner's Manual page 271

Charge state
A well charged battery will not only prevent starting
problems but will also last longer.
In order to avoid unintended battery discharge
Switch off unnecessary electrical loads in city
traffic, on short trips or in a line or traffic.
Always remove the ignition key from the
ignition switch when leaving the car.
Avoid frequent operation of the Porsche
Communication Management system when
the engine is not running.
Battery care
Ensure that battery is securely mounted.
Keep terminals and connections clean
and properly tightened. Corrosion can be
prevented by coating the terminals and
connections with petroleum jelly or silicone
spray.
Ensure that vent caps are securely tightened
to prevent spillage.
Checking the electrolyte uid level
Generally, the electrolyte level must be checked
more often in summer than in winter, and more
often when driving long distances.
When adding water, use only clean containers.
In no case may alcohol (e.g. window cleaner
residues) be permitted to enter the battery.
Unscrew and open the filler vent caps of each
cell.
With the car on a level surface, the fluid level should
meet the indicator mark in each cell.
If necessary, top up with distilled water.
Do not use acid. Only fill up to the mark,
otherwise the electrolyte will overflow when
the battery is being charged and cause
damage.
Battery charging
Automotive batteries loose their efficiency when
not in use. The charge available in your battery
can be measured with a battery hydrometer. We
recommend that the battery voltage be tested
by your Porsche dealer who has the appropriate
equipment.
If the car is not driven for prolonged periods, the
battery must be charged at least every 6 weeks.
A discharged battery allows rapid formation of
sulfates, leading to permature deterioration of the
plates.
Danger!
Hydrogen gas generated by the battery
could cause an explosion, resulting in serious
personal injury or death.
Charge battery in a well ventilated area.
Never charge a frozen battery. It may explode
because of gas trapped in the ice. Allow a
frozen battery to thaw out first.
If you get electrolyte, which is an acid, in your
eyes or on your skin, immediately rinse with
cold water for several minutes and call a
doctor.
Practical Tips, Emergency Service
269