Quote:
Originally Posted by brucepick
Yes you can save a bunch of weight by substituting a lithium-based pack for the old lead acid battery. Weight savings is mostly helpful in acceleration contests and kind of helpful in city driving.
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The issue with Li batteries isn't the weight savings, it's the energy density and the safe depth of discharge (DoD) range. Most flooded lead acid (FLA) and absorbed glass mat (AGM) car batteries only have ~ 30 Ah of energy (finding information on this can actually be very, very difficult). Deep-cycle FLA/AGM might have double or triple that. Maybe even 100 Ah, let's say. 12V * 100 Ah = 1,200 Wh or 1.2 KWh. At a 50% maximum safe DoD, that's .6 KWh. And what does an average deep-cycle weigh? 80 or 90 pounds?
Now, a 100 Ah Li cell weighs ~ 7 pounds (and it takes four to make a 12v battery), so 28 lbs total (you'll want to add a few pounds for casing, terminals, etc.). So, for 1/3 of the weight of a deep cycle, you get the same energy storage, right? But wait, Li batteries can safely be run to a greater DoD (~ 70-80%) without affecting their "memory" or energy storage capabilities. So instead of .6 KWh, you have .96 KWh of available electricity. And again, that's at a 1/3 (easily) the weight. Now, if you wanted to increase the number of cells (or Ah of the cells) in the Li battery to match the FLA/AGM weight, you'd end up with 3.6 Ah of total energy, and 2.88 Ah of usable energy. That's almost 5x the energy for the same amount of weight used.
Now, switching gears a bit. What I've been wondering is why not kill several birds with one stone. Many aircraft have combined the alternator and the starter in one piece, and I'm wondering why auto manufacturers moved away from that. It seems somewhat redundant to me to have both an electrical starting motor (which is only activated when starting the engine and is nothing but dead weight after) AND a belt-driven alternator that causes additional parasitic loss. There has already been talk about converting standard internal combustion cars into hybrids by installing an electrical start/stop motor. Installed properly, couldn't that motor also serve as an alternator during deceleration, braking, etc.? It would probably be very difficult to pull off, but it seems feasible to me.