car/motorbike second battery

[pereczes]

Had this thread earlier about 12V car voltage stabiliser. This is complementary to it.
I found out the magic number 6 for my second battery. I think 6 is ideal to combine 3.7 V Li-ion batteries in series.
-> I understand that these batteries are alive at 3V. 6*3 = 18, this is a good voltage for a step down converter to 12V
-> for charging I need 4.2, 6*4,2 = 25,2 which is a good voltage for step up from the alternators voltage.

please tell me your opinion. Anybody has any experience in building such batteries?

was thinking about such cells, (of course they are not 5000mah, but max 2000mah)

http://www.ebay.com/itm/10-X-For-UltraFire-18650-5000mAh-3-7V-Rechargeable-Li-ion-Battery-for-Flashlight-/111323267883?pt=US_Rechargeable_Batteries&hash=item19eb62432b