Wanted to share what is the current the market on lightweight lithium batteries for our cars. For years I've been using a Deka ETX14 AGM battery, which is an 11lb 12Ah battery, rated at 220CCA, and has never had a problem cranking and starting the motor in my 3.2 or GT3. Great battery and little attention or worry ever needed if on a battery maintainer when the car sits. In looking for a lighter replacement in lithium, I recently got this Houny YTX20L-BS, which is a true 20 Ah LiFePO4 battery with 480CCA, and internal BMS to protect from over discharge and over charge, and weight just under 6 lbs. Cost: $105 on Amazon. I specifically searched for something in the 20 Ah capacity range, as it gives more reserve than the Deka, of which for a lead battery, actual usable Ah is half of its rated capacity if you want to be able to start your motor. In other words on a 12 Ah AGM battery, if it drops below half capacity or 6 Ah, it may not be able to crank the starter, vs this lithium will still start the motor at 85% discharge. So this 6lb 20Ah battery would equal to a 40 Ah 30lb lead battery
In the 3.2 it cranks over the motor so much more rapidly and easily, and engine starts more immediately, you don't even need to compare with another battery to tell the difference. To verify its capacity, I used a lithium balance charger to bring the battery to full charge at 14.4v, then did a discharge down to 8.0v, which measured at 21.7 Ah. I brought the battery back up to about 10.5v, and started measuring the charge to bring it back up to 14.4v. Measured total capacity shows 20.7 Ah, meeting and exceeding its rated capacity.
When comparing batteries, don't be fooled by listings using a PbEq equivalent for Ah capacities, which means true capacity is 1/3 of that. Battery weight is a good indicator. For example if the battery weights 3 lbs, the max capacity for LiFePO4 is 8-10 Ah, and usually 7-8.