Hi there,
Keep in mind that every time the battery is totally disconnected from the ECM it will lose all the settings, so your van won't run right until you've driven it for 15-20 miles, because it will start off with the default settings. Same with the radio station settings. If you can wire the battery brain to bypass the ECM it should be ok to use.
Usually that sort of battery drain is due to the "keep alive" wire to the radio. Some radios draw as much as 70 ma when the vehicle is off.

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Richard Wooldridge
'01 ML320
'82 300D 4.3L V6/T700R4 conversion
'82 380SL, '86 560SL engine/trans. installed
'79 450SL, digital servo update
'75 280C