Test: Alternator vs. no alternator = 10% MPG gain @ 70 km/h

[Ladogaboy]

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.

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.

[brucepick]

Aha. So a big enough Li battery could possibly enable you to run without the alt for some considerable time/distance, if you can charge it up from AC when not driving.

This site: LithiumMoto
offers a 11.5 AH 12V* battery for $300. Unless there are much more economically priced Li batteries available, it looks like you'd need $900 for ~34 AH of 12V, and more $$ for more AH.

*I think a Li cell pack isn't really 12V, right? This raises more questions on cell voltage and charging. Is all that already covered somewhere else on this forum?

Oh, I got it. It's on this page: http://ecomodder.com/forum/showthrea...ies-18547.html

[Ladogaboy]

And that is one of the points that I was getting at: I don't think the batteries from LithiumMoto are going to be anywhere near as "good" for this application as the batteries that you could build for yourself. While the LithiumMoto 11.5 Ah battery still has more energy available overall, it is not sufficient to power a vehicle sans alternator for very long, and, in terms of weight savings, I don't think it is that much better of a value than Odyssey's 15lb AGM battery with (if I remember correctly) ~ 10 Ah.

The big problem with building one's own battery sufficient to run a car without an alternator is cost because, at the cheapest I've found, it would cost about $700 to build a 100 Ah, 12V battery that weighs ~ 30 lbs. Price drops significantly with lower Ah, but as others have stated, those batteries might only be sufficient for short commutes.

[brucepick]

Thanks, Ladogaboy. Now of course I have more questions for you...

How do you calc AH for batteries in series? 4 x 3.3V cells in series = 13.2V. If each cell holds 2.5 AH, does the pack have 10 AH capacity at 13.2V? or 2.5 AH at 13.2V?? Or???

Then assuming you want more AH, you could put 16 cells in the pack, series-parallel, to make the same 13.2V. That would give you 4x the AH from the first scenario, right? True that you could merely double or triple the # of cells.

Looks like you're aiming your calcs towards a target of 100AH. Reason? I suspect you have a good one. Can you say what is usually needed to run the car's basics - fuel pump, ignition, computer? I did some calcs for headlights, down below.

I got into this because EOC with headlights on runs down the (lead-acid) battery kinda quickly so I started thinking about a separate deep cycle battery to power the lights. But if Li cells can hold enough AH they might be able to power everything for a day, and then get recharged overnight.

I calc'd 2 headlight bulbs @ 55W each = 110W. I rounded that up to 150 to give some headroom to run some other stuff too.
150 W at 12V = 12.5 Amps
Run for 3 hours = 37.5 AH

[deathtrain]

Series = voltage
parallel- Amps

if you have (2) 3.3vdc @ 2.5Ah cells in series. you would have 6.6vdc @ 2.5Ah
if you have (2) 3.3vdc @ 2.5Ah cells in parallel. you would have 3.3vdc @ 5Ah

[Ladogaboy]

Well, there's a lot more to it, and I only really feel comfortable explaining the basics.

Li cells tend to be 3.4-3.7 volts. I've seen the numbers for max, min, and nominal voltages vary.

If you run them in series, the voltage increases, but the amp hours stay the same. If you run them in series, the voltage stays the same, but the amp hours increase.

The amp hour numbers can be very deceptive because they can vary based on discharge rate, and FLA/AGM batteries do not seem to have a consistent standard for explaining what their claimed amp hour ratings are based on. Essentially, the faster you drain the battery, the less overall power you'll get out of it.

Since most of us are trying to determine the actual *work* we will be getting out of the battery, I prefer to look at the W/KW hours. To determine the Wh in a battery, just multiply the voltage (e.g., 12V) by the amp hours (e.g., 100 Ah). Caveat: See above regarding the varying standards for Ah ratings. Now you have the watt or kilowatt hours of energy in the battery. Of course, you don't want to exceed the depth of discharge of the battery, so you will want to figure out what percentage of that energy is actually available (i.e., < 50% DoD for FLA/AGM and < 80% DoD for Li).

Also, this is kind of working backward. You'll need those calculations, but the first thing you'll want is how many Wh it is actually taking you to run your car. For instance, if you are spending 100 Wh/mile (just pulling that out of a hat), a battery that provides you with 1 Ah of useable electricity is only going to get you 10 miles before you start to risk seriously damaging the battery or having to stop driving. So the first step really should be determining how much electricity it is taking you to run your car.

Also, on a side note: I only chose 100 Ah because a) it was the size battery I was considering when I was thinking about building an EV (on the shelf right now) and b) it seemed like a nice, round number to do math with. :-P

[brucepick]

Thanks, Ladogaboy.

These questions aren't exclusively for Ladogaboy...

I did some more reading on LIPO's.
I found this in the .pdf on A123's ANR26650M1-B cell:
Recommended Standard Charge Method
- - 1C to 3.6V CCCV, 45 min
Recommended Fast Charge Method to 80% SOC
- - 4C to 3.6V CC, 12 min

I dug around till I found that CCCV = "Constant Current Constant Voltage" (I think).
But what are "1C" and "4C"?

And, I don't seem to find the latest ANR26650M1-B's being sold anywhere.
[EDIT] I found M1-A's at HobbyKing.com, $17.99, with price dropping to 11.99 as you watch (!!). ANR26650M1-B's [/EndEdit]
I did find ANR26650M1-A's offered at $9.99 - see voltmanbatteries.com. I haven't yet chased down the differences between the A's and B's.

Another question that has refused to yield an answer: What amount of Amps draw am I likely to find, just running the car's engine with it's management systems? Any idea on that?

[brucepick]

Originally Posted by brucepick

... But what are "1C" and "4C"? ...

For anyone reading this and still wondering, I found the answer by searching "lipo C rating explained".

Here it is:
Say you have a 10 AH cell. It can provide 10 amps for an hour, or 1 amp for 10 hours, or 2 amps for 5 hrs, etc.

Say the 10 AH cell is rated at 1C. That means you can charge it (or discharge it) at 10 amps. If it's rated 2C, you can charge/discharge it at 2x that rate, or 20 amps. If rated 5C you can charge/discharge it at 5x that rate, or 50 amps. Exceed that rating and you'll overheat it or damage it some other way.

There's usually a second, higher rate also posted. That's a higher charge/discharge rate that can only be maintained briefly. The spec should state how long it can discharge safely at the higher rate.

[Ladogaboy]

Yes, C means Coulomb. It's a measurement of rate of charge.

Coulomb - Wikipedia, the free encyclopedia

I'm heading out, so I can't really reply to all of your questions... But, I believe that (if I remember correctly) MetroMPG already hit on the best way to assess the draw on your battery. Measure your battery's charge. Disconnect the battery. Drive for a given distance/time. Remeasure your battery's charge. From that, you should be able to derive how much electricity your car is drawing.

[toc]

What if the exhaust pressure was used to spin some sort of fin in the exhaust stream which in turn spins the alternator to produce electricity ?