The P38 is very sensitive to low voltage. A simple low voltage can make the BECM (Body Electronic Control Module, main computer located under the front RH seat) throw up all kinds of random faults such as gearbox warnings.
To prevent against this all seasoned P38 owners will tell you to fit the biggest battery you can. The Hankook MF31-1000 is the current (no pun intended) battery of choice and is available via the link below the picture.
Better pictures to come.
There are plenty of complaints about the batteries in P38s going flat overnight, particularly if a smaller battery has been fitted. One common cause for this can be signals being picked up by the security system keeping the BECM constantly active. The frequency used by the remote signal from your key is close to that used by modern Wi-Fi. If you are parked in a strong Wi-Fi area the security system thinks it is being swamped by 'code grabbing' equipment and so wakes the BECM and immobilises the vehicle.
There is a filter in the system located near the RH C-pillar that Land Rover updated twice, once mid way through production and then again after production had ceased so only available over the parts counter. The latest version is supposed to be effective but is rather expensive. But enthusiasts from one of the forums (linked at the bottom of this page) have created their own for a lot less which I have fitted.
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