Lithium Battery.... bettery fuel economy ?

[fastegg]

The guys selling lithium car batteries are claiming they provide better fuel economy.
I thought you could have 2 batteries, one standard lead starting battery, and a small lithium running battery. 4WD shops sell a battery isolator, a device that has both the starting battery and an aux battery conected.... When you start the car, it starts with the starting battery, when that is fully charged, it then switches to the aux battery and charges it. Now my theory is if you had this isolator in your car but the aux battery is a small motorbike size lithium battery that when fully charged, its voltage is in the high 13's (volts) so there is absolutely no load on the altinator... unlike having a 60lb lead battery always being trickle charged. There's a few guys in Australia selling lithium batteries and their claiming 5% better fuel economy. The reason i thought of having 2 batteries, one being a $50 lithium is that a full size lithium is $500-$700.
Does this theory/story have any validity ?

[Frank Lee]

This is currently (a pun! ) being discussed in another thread.

[fastegg]

Could you direct me to this thread ?...

[Frank Lee]

Civic EPES: Alternator Delete Project (with additional lithium battery...)

It is easy to see what's currently being discussed by hitting the "recent posts" button at the top. It's also easy to find all related threads using the "search" function.

There's another thread(s) regarding using lightweight batteries too.

[oil pan 4]

Why not ditch the lead acid battery all together?

Lightweight Batteries

[fastegg]

Ive just been reading about this guy that did his own tests with firstly just unpluging the alternator, then secondly taking the alt out all together.
He got about 5% unpluging the alt then 10% taking the alt out and running a shorter fan belt for the water pump. Then charging the batt up each day with a battery charger. So, if you used a lithium batt good for 2000 cycles ? and a weight saving of 15kg....
10% better FE and more power at the wheels.....
maybe using a 14 volt battery to avoid things playing up when volts drop...