Allow me to share my solution to:
self dischargesulphation during even slight discharged states (read short trips, especially infrequent)limited life span with close to full starting performance of 2-5 years if treated well, months if notacid fumes eating car chassis and other partsexcessive weight, 2.8kg for 4S2P shown belowenvironmental hazard of lead
The DIY Headway 38120HP 4S2P pack will easily start any of my 4 and 6 cylinder engines, and quite likely the 8 litre diesel in our boat which I already started (only just) with a similar pack with only 4 cells and half the capacity. These should see me to my grave given thousands of full discharge cycles before 20% degradation in capacity and no full discharge cycles being applied to them.
That's wired into my 360 with the original rusty tray removed. And this is in another car:
Downsides:
cannot run them flat, flat = deadshould not run them over 14.8should not run them unbalancedbalance circuits for alternator and starting currents not easy to come by - alternative arrangements neededlow capacity comes with small size/light weight despite high starting ability
In summary, I plan to run these in my cars using an isolator switch on the firewall to keep them safe and a balance circuit that is not in series with the pack and the car such that it can balance the cells but not prevent over charging or over discharging - that's up to the regulator in your alternator and your isolator switch or frequent use. Having said that, the 16AH pack above has spent two periods of 1 month unattended in my 360 on the side of the road and started like a CHAMP with no sign of being drained after each period, so the switch may be paranoid, but I have a lot of cars so none of them are daily used, weekly to fortnightly for the frequent ones.