Auxiliary battery
This page shows how to install auxiliary battery in W220.
Battery
For auxiliary battery it is better to not use starter battery (e.g. most automotive batteries), but a deep-cycle battery that is designed to be regularly deeply discharged using most of its capacity.
Crown 27DC115 (EUR 200) is battery that perfectly fits in the left side of W220 trunk.
Specification:
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Installation
Battery fits tight in the left side of the trunk. For better fit wrap it in solid rubber sheets. To put back the electronics bracket, it has to be cut below the navigation device level. Audio gateway which was below the navigation unit can be attached to the navigation unit with cable ties as shown in the picture. If CD Changer is used, the space for it has to be found somewhere else. Since the lower of the electronics bracket is cut off, the bracket now is attached only with 2 screws instead of 3, however, the installation is pretty tight anyway.
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Charging
We want the auxiliary battery to be isolated from the main circuit when the car is off (to not drain the auxiliary battery and vice versa), but to connect it to the main circuit when the car is running so that alternator would charge both the starter battery and the auxiliary battery. To do this automatically the starter battery's positive terminal and auxiliary battery's positive terminals must be connected together (using wires at least 3mm in thickness) by high current (200A) relay (EUR 10). The negative terminal of auxiliary battery can be permanently attached to the chassis. The relay should be activated by positive 12-volt wire from alternator's D+ terminal (warning lamp wire). This will make sure that auxiliary battery is connected to the main circuit only when the alternator is already producing the output. The optimal place to install the relay is to put it under the rear seat and attach it to the electronics bracket. The wire from engine bay can be passed to right footwell through the right fuse box and then to the rear seat consulting rear view camera wiring guide.
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Original Auxiliary Battery
A second battery fitted on the right-hand side of the trunk above the existing battery is an optional extra in Europe. It is clearly shown in WIS/ASRA under AN54.10-P-00021 and requires a new battery holder, circuit breaker and new modified trunk paneling for the right-hand side.