Lithium a better performance per $ than lead?

11 Posts
Nov 23, 2009 07:55 pm
Lithium a better performance per $ than lead?

Let met start by saying that I built my own off-grid 400 watt solar PV system with 6 of 100AH @ 12V lead acid batteries.  My new project is with an electric moped that uses LiFePO4 cells.  I put together a table that shows their relative merits.  Keep in mind these are the chemistry that isn't susceptible to thermal runaway like LiPo or Lion.  Is it time to make the switch for PV installations?

(Sorry for the layout, I can't post a proper table without using HTML tags)

*Lead Acid
Volts:           12
AH @ Standard C: 92
Cost:            $200.00
Lbs:            63
Operating Temp:    -40ºC to 60ºC
Standard C Rate: 0.05
Cycles @ Standard C: 3100
Power Density:   55.2
DoD:            30%
Usable Whr:    331.2
$ / Whr:     $0.60
Lbs / Whr:    0.190217391

Volts:            12.8
AH @ Standard C: 40
Cost:            $232.00
Lbs:        13.2
Operating Temp:   -45ºC to 85ºC
Standard C Rate: 3
Cycles @ Standard C: 3000
Power Density:    1536
DoD:            80%
Usable Whr:    409.6
$ / Whr:    $0.57
Lbs / Whr:    0.032226563
11 Posts
Nov 24, 2009 12:21 pm
Re: Lithium a better performance per $ than lead?

Sorry about the sloppy looking original post.  I had the epiphany, but I didn't have much time to explain myself.  The main point to take away is this:

If you buy a 100AH lead acid battery and want it to last, you'll only use 30AH (30% Dod).  That will get you around 3000 charge-discharge cycles.

However, with a 100AH LiFePO4 (lithium iron phosphate) battery you can use 80AH (80% DoD) of its capacity and still be left with 3000 cycles.  In case you ran it to 70% DoD you would get 5000 cycles.

So, even though the lithium batteries are more expensive for the same rated AH, you actually get a lot more usable capacity with LiFePO4.

Now, add the fact that lead acid batteries can only reach their rated AH capacity if you discharge them at no more than a 1/20 C rate and you watch your real world capacity diminish further.  LiFePO4 generally are rated between 2.5C and 3C, giving them much greater overhead and efficiency.  Take this example:

A 12V, 100AH lead acid battery has an ideal usable capacity (30% DoD) of 360 watt hours, at a discharge rate of 60 watts (1/20C).

a 12.8V, 100AH lithium iron phosphate battery has an ideal usable capacity (80% DoD) of 1,024 watt hours, at a discharge rate of 3,840 watts (3C).

After taking into account the DoD of these two chemistries, lead acid comes in at around $0.60 per watt-hour, and LiFePO4 comes in at around $0.57 per watt-hour.

Other advantages of LiFePO4:
-Operating temperature of up to 85ºC, rather than lead's 60ºC
-Truly a sealed, maintenance-free, non-toxic, and non-spillable paste
-No worry about plate thickness or corrosion

The only thing we would need are solar charge controllers and inverters designed for lithium iron phosphate.  That is, a charger with a goal of 3.7V per cell (3.2V nominal), and an inverter with a low voltage disconnect of 2.7V per cell.
« Last Edit: Nov 24, 2009 05:04 pm by Alan Dennis »
11 Posts
Nov 24, 2009 05:28 pm
Re: Lithium a better performance per $ than lead?

Where are my numbers coming from?

For my solar project I bought six "100AH" (Rated 92AH @ 1/20C) 12V deep-cycle AGM lead acid batteries from the local Batteries Plus.  These were model #WKDC12-100P.  They cost me $200 each.  I then made my own 2/0 gauge interconnect cables from parts purchased at a local Marine Store and Home Depot.  These were additional costs, but I'm not including these here.

Lead Acid AGM
$200 / (92AH * 12V * 30%) = $0.604 per watt-hour
(92AH * 12V * 30%) / 63 Lbs = 5.26 watt-hours per pound


For my electric moped, an XM-3500li that I'm adding extra batteries to, I purchased four 40AH 3.2V ThunderSky cells from Evolve Electrics.  They were $58 each, 5 day shipping included.  The interconnects came with the batteries, and they were actually already wired up in series, saving me extra time and cost.

Prismatic LiFePO4
$58 / (40AH * 3.2V * 80%) = $0.566 per watt-hour
(40AH * 3.2V * 80%) / 3.3 Lbs = 31.03 watt-hours per pound


The operating temperatures come from the manufacturers' data sheets.
46 Posts
Nov 24, 2009 06:33 pm
Re: Lithium a better performance per $ than lead?

Alan, I think we are close. LiFePo4 (lfp) are great, but they do have their shortfalls as well. They work great hot, but not as well as lead acid in cold temperatures. A lead acid cell, kept properly charged will live for years and years. Lithium batteries of any type begin to degrade the moment they are manufactured. LFP also have some of the worst c/rate of the lithiums. c rate is more of a determing factor for longevity on lfp than DoD. Their c rate may be equal to or worse than that of lead acid.
If your packs were big enough this may be acceptable for a solar system. For a scooter, just the reduction in weight would surly be worth it. I have a 36v, 12ah battery bank on my ride, and it weighs around 65lbs. Sucks.
Just like solar, the problem with lithium isn't it's particular ups or down, it's the up front cost.
A company by the name of Axion Power is supposedly developing a "light" lead acid. It will be interesting to see if it happens, but i'm not holding my breath... -m
1 Posts
Sep 10, 2011 12:00 pm
Re: Lithium a better performance per $ than lead?

I looked at this same idea. I started with a different premise because my interest was in Electric Vehicles, and since most people don't drive more than an hour at a time to commute, I looked at the comparable 1 hour ratings of the Lead Acid and Lithium choices, and for them LiFePO4 Makes so much more sense, and IS Cheaper to buy - on the first cycle, as I calculated it out.

I am so convinced that they are better than every packaging format of Lead-Acid - I have begun to clarify my thoughts, using specific comparisons of exact battery models - and put them in whitepapers on my website. they can be found at -> Select Whitepapers in the menu, and then read those that I have posted. There are two so far, in pdf format. the first is a nice single page letter-size, double-sided fit, and the second is four pages, two sheets letter size, each double sided.

Easy reading, and should be of some use in making choices - even in the Renewable Energy Field.

However - with all that said, I have never seen any specs on what a LiFePO4 (Lithium Iron Phosphate) Cell can do in a 20 hour load test similar to how Lead Acid Batteries are done. THAT would be interesting - to see if they can deliver any appreciable larger capacity in the slow drain approach - as is used in Solar Energy and Wind Energy Storage off-grid.

Another However - I also see Renewable Energy/Solar Energy Batteries (Lead Acid) Marketed with a 100 hour rating (0.01C) - and here is where I see that counting: You have no wind, or no appreciable sun for four days, how much energy can you use over that 4 days from your batteries without starting up your backup generator? These type of batteries don't usually look like car batteries, are usually 6V, 4V, or even single cell at 2V types, and very heavy.

So - while I know that for an EV (Electric Vehicle) the LiFePO4 is already a winner on price in terms of direct usable energy on the first cycle, I have no way to know if that is true for Renewable Energy Storage in slow draw applications, where how much they weigh only counts during carrying them into place, and does not affect the daily energy use as it would in a moving vehicle.

However - I do believe the life of the Lead Acid Battery has to be considered, so the usual deep cycle capacity of 300 - 400 cycles alone, could be the primary difference between them and a Lithium Iron Phosphate Cell Set that has a 1500 Cycle Capacity at the same 100% DOD (Depth of Discharge).

Here is the thing - it is hard to find fully supported data for Lead Acid Batteries that cover more than the 20 hour and 5 hour ratings for them. To get a full range down to 1 hour is a long and slow hunt to find spec sheets. I know of just a few mfg'rs that provide such detailed data, and in the LiFePO4 Side - similarly - for the Renewable energy use, most just list the ratings at 1 hour - excellent in the EV world, but not quite as critical for the usual off-grid home, where you want to run for a day or so without input energy, hence the reason for the storage in the first place.

My 9 Cents worth!


PS Check out for more EV information, and if you want to catch my various thoughts on the fly - join the google groups list there!

11 Posts
Sep 12, 2011 01:35 pm
Re: Lithium a better performance per $ than lead?

Lithium makes more sense now than ever.
Lead specs are on the left / Lithium specs are on the right

Type: Universal brand AGM / Thundersky brand LiFeYPO4
Voltage: 12.8V / 3.2V
Rated Ah: 200Ah / 200Ah
Rated Capacity: 2,560 Wh / 640 Wh  (Ha, just wait!)
Weight: 132 Lbs / 16 Lbs
Volume: 1,666 cubic inches / 307 cubic inches
Toxicity: Lead and acid / inert
Fire Danger: can vent hydrogen / none
Orientation: Upright only / Upright or on its side

Hour rating: 20 hours  /  1 hour
Recommended Depth of Discharge: 30% / 80%
Cycle Life: 550 cycles / 2,500 cycles
Shelf Life: 6 years / 10 years
Self discharge: 3% per month / 3% per month
Peukert's Constant: 1.20 / 1.08
Usable watt-hours (at rated discharge): 768Wh / 512Wh
Price: $375 / $250

Cell capacity at various loads.  Peukert's Constant, as well as the hour rating is the key here.
C/100 =275.9 Ah / 251.8 Ah
C/20=200.0 Ah / 232.3 Ah
C/5=151.57 Ah / 216.8 Ah
C/2=126.2 Ah / 207.1 Ah
1C =109.9 Ah / 200.0 Ah
2C=95.6 Ah / 193.2 Ah
3C=88.2 Ah / 189.3 Ah
Lithium has a much flatter capacity curve than lead.

Dollars per watt-hour (lower is better).
Lead on left / Lithium on right
C/100: $0.353 / $0.388
C/20: $0.488 / $0.420
C/5: $0.645 / $0.450
1C: $0.885 / $0.488
Even at C/20, Lithium still wins on a $/Wh basis.

Let's consider a battery pack that we want to get 10KWh (Usable) at a C/20 rate:
Again, lead is on the left / Lithium is on the right
Cell count: 13 / 16
Usable Capacity (Actual capacity under load and to DoD) : 9.98KWh / 9.52KWh
Rated Capacity (The actual marking on the cells):  33.28KWh / 10.24KWh
Price: $4,875 / $4,000 - Lithium wins
Weight: 1,716 lbs / 256 lbs - Lithium wins (by a lot)
Volume:  21,658 cu in / 4,912 cu in - Lithium wins (by a lot)

Lithium wins on price, weight, volume, cycle life, shelf life, layout flexibility, and safety.  Convinced?  Any additional information I should provide?
By the way, my lithium scooter is still going strong.
« Last Edit: Sep 12, 2011 01:47 pm by Alan Dennis »
11 Posts
Sep 12, 2011 01:51 pm
Re: Lithium a better performance per $ than lead?

I made a couple additions to my last post regarding safety.  LiFeYPO4 is the safest form of lithium.  You may be familiar with LiPoly, which are commonly used in laptops and RC airplanes.  They have twice the energy density, but are prone to exploding and catching fire if over charged or punctured.  If you use LiPoly in an off-grid array, expect your house to burn down shortly thereafter.  That being said, LiFeYPO4 is safer than lead acid (especially non-sealed cells).
11 Posts
Sep 12, 2011 02:29 pm
Re: Lithium a better performance per $ than lead?

Alan, I think we are close. LiFePo4 (lfp) are great, but they do have their shortfalls as well. They work great hot, but not as well as lead acid in cold temperatures.

I realized I never replied to this.  Lead does not operate below freezing.  Lithium can operate at diminished power down to -20C (or rather, whatever the electrolyte variant is rated to, but -20C is typical).

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