Annti:
be careful with bigger batteries. I tried to install one bigger size in my E32 750iL under the rear seat, only 15 mm higher, but I could not completely fit the electric rear seat. Had to change again to the smaller one.
Varta states for the E38 750
Länge [mm] 353 length
Breite [mm] 175 width
Höhe [mm] 190 height
Also note where the plus and minus poles are, there are different versions, and also the hose for the drain of gases and acid.
Here are some details
http://www.varta-automotive.de/index...plication.html
If you do not use the car so often, maybe you make it like I do with my Highline. I bought a cheap solar battery charger. That does not recharge a drained battery but is maintains it. Just connect one with each battery and place it on the dash. But of course you need some sunlight for that
A blocking diode prevents a reverse flow of energy.
http://www.batterystuff.com/solar-chargers/