running 2 batteries Options

[yellowchinaman]

I've seen people having an extra car battery in the boot for the extra ICE they have.
I am thinking of installing a small extra battery in the arm rest to boost the power of my existing racing battery.
Question is, what is the proper way or doubling a battery and is there a problem with mixing different battery types and size? For instance, can I add a small 12v motorcycle battery with the 12V car battery?

[yellowchinaman]

Thanks guys.
You see have a small battery mod which Im dependant on using a Caterham Banner battery. (The smallest 12V with enough amp for the GT4)
Only problem is that it never holds very well and when the car is not in use it lives off a trickle charger.
As a track car it's ok. But as a normal road car it's not ideal.
After say 5 days the battery doesn't produce enough to start up.
Its not a simple case of changing for a better battery because there is no other battery smaller or that size that produces enough power and for me to keep it in that location.
I know the logical thing to do is to relocate it to the boot and I can have what ever battery I want.
But I use my boot space a lot and don't want to compromise it any more hence why I was thinking of doubling up the battery in the arm rest.