Goran what voltage is your lithium battery and did you change or modify voltage regulator in alternator to provide higher voltage and allow batteries to fully charge. 12.8V(4x3.2V) lithium battery require 15V charge voltage, and if it 's higher voltage battery using 3.6V or 3.7V cells then would need even more charge voltage.

Could you use a lipo or is the balance charging an issue.. ? I'm sure you use lipo in your RC jet ?

Shopping list to make your own:

3.2V 20Ah cells, the listing is for 3 but I would suggest 8 for 12.8V, 40Ah, about £200 for 8 http://www.aliexpress.com/item/UPS-H...276823966.html

Charging balancing circuit, 1 or 2 pieces depending how you wire it up, £11.21 each, probably best to use 2 to allow charge currents above 10A http://www.aliexpress.com/item/4-cel...251999850.html

Diodes to allow discharge not going through balancing board, as the balancing board only allows 60A discharge. So While discharging current will go through diodes and through the balancing boards when charging. 2 of them for total of 800A continuous and 5000A peak discharge current, abut £11 for pair, 1 would probably be enough but they are cheap enough to use 2 just to be sure.

Plus casing, temperature switch and fan for cooling, so about £250 total for 40Ah battery, not much difference from buying fully assembled, just satisfaction of the job you done yourself + learning experience.

Lipo are normally 3.6 or 3.7V so it's hard to get close to 12V its either 10.8-11.1V if using 3 cells in series or 14.4-14.8V if using 4. LiFePO4 are 3.2V so using 4 you get a lot closer with 12.8V compared to 12.6V on regular lead acid battery.

I use 7.4 v liposuction at home they are 1800mh but they are very small the side of a stick of rock ..

Is there an advantage to using any other forms of battery other than of course lightness ?

Its LiFePo4 so nominal voltage I guess 3.2Vx4=12.8V. It usually sits at around 13.2V and that's probably 50% to 60% charge. I dont know what cells they used, big prismatic or lots of little Panasonic cells. If Panasonic cells they kind of limit my low cost replacement option if/when they need replacing.
Its a good question about the alternator and it worried me slightly. Voltphreaks website says it should put out a minimum of 14V which would theoretically charge each cell to 3.5V per cell, which should be fine. http://www.voltphreaks.com/technologydir.php
In the video my alternator is outputting only 13.6V which would be too low.
However I kept an eye on the alternator, after a short drive it seems to build up the output to 14V or 14.1V so its just about ok. At some point I would like to clean up all the connectors between battery and alternator. At some point when I get round to it I would like to change or modify the voltage regulator in the alternator.
So far the system seems to be working ok, sometimes after a longer drive the battery holds as much as 13.4V so its getting a fairly good charge, and seems to be able to last 7 days without driving and hasn't triggered the Low Voltage cutoff yet.

You have a great idea for putting together your own battery much cheaper than the ready made ones. I'm not as confident with electronics :) but I would like to give it a go in the future.

You can use lipos, as Ainars says they have a different chemistry and higher voltage so alternator may supply too low a charging voltage? You still need a balance/maintenance circuit similar to the one that comes with the LiFePo4 starter batteries currently available.
The advantage of lipo is power density, they can output much more current for the same capacity as LiFePo4. One of the downsides seems to be volatility (some people say), they seem to go into thermal runaway more easily. But this may be mostly due to the fact that they are used in more demanding conditions drawing high current on the very limit of the battery (like in RC jets).
Also I don't know how lipo compare in longevity, can LiFePo4 last for more cycles? I dont know.
I don't think they are cheaper either, if you look at Hobbyking for some of the cheapest ones, I think a 10Ah 3 cell is close to £100, the option Ainars posted for 20Ah LiFePo4 cells is a much better price and double the capacity.

LiFePo are considerably safer in use than LiPo (which isn't actually a battery chemistry, but a construction) and straight LiPos are not really suited to automotive use as I've said before - far too much risk of a thermal runaway, just from aging as much as any in use abuse.

They're only good for a certain number of cycles as well....

I have a 3 cell LiPo I used for my RC radio and it's beginning to swell after intermittent use over a period of years - it still balance charges fine so I will continue to use it, but as soon as it swells good and proper it'll be dunked in salt water and binned

Have to agree, I would be scared of using and leaving Lipo batteries in a car.
Modern ones are quite good, for example I have some 3 or 4 year old lipos of only 850mah, used in rc jets drawing 20A fairly regularly and they have not puffed and capacity still seems to be good. But there are too many cases of fires especially when charging.
Even for the LiFePo4 that's in my car now I bought some very high temperature insulation which I'm going to install soon for peace of mind. Its Pyrogel XT flexible sheets (I wanted XTF but could not get hold of it).

I forgot to add, the recent 787 fires were LiFePo4 weren't they? That worries me a bit.