Thanks Main Sail. That's interesting indeed. How does the energy density compare with conventional batteries?
Ken
Ken,
That is a tough one to answer because when compared to lead there are far too many ways to evaluate that equation. LiFePO4 is nowhere near as energy dense as LiCoO2 but leaps & bounds beyond lead. Often when comparing Li to Li we use Wh to Kg or Wh to Liter but with lead this does not translate well because the "C" rate changes the capacity rather drastically where with Li it does not.
To equal the
usable cruising capacity of our 400Ah LiFePO4 battery, which is 320Ah's (100% SOC to 80% DOD), I would need a 900Ah lead acid bank cycled (50% SOC to 85% SOC). However, that is only part of the story. To equal the
real world cycle life, using lead acid, I would need to be shallow cycling that lead acid battery by about 10% or a depth of discharge to just 90% SOC which would mean a lead acid bank of approx 3200 Ah's...
USABLE CAPACITY RANGE:
Approx 80% of an LFP banks capacity is
fully usable. With lead acid you often have just 30-35%
usable capacity (50% SOC to 80-85% SOC) due to
charge acceptance current limiting. With LFP current limiting or acceptance taper is very, very short in duration, even at relatively low charging voltages of 13.8V - 14.0V.
CHARGING SPEED:
Very, very short current taper even with large current sources. Charge to nearly full before even attaining absorption voltage. This of course is entirely dependent on your charging voltage and your current source. We charge at 120A steady (160A alt set up to run at 120A continuously) and our current taper lasts only 30-35 minutes. If I bumped the voltage to 14.2V, which I won't do but could, I could charge to 100% SOC with about 4-5 minutes of current taper. Compare that to HOURS and HOURS of current limited charging using a 120A charge source on 400Ah lead acid batteries. With a small charge source, like solar or wind, we will hit 99.5%+ SOC before any current limiting occurs. Our bank literally has to be chock full before our solar array can even get to 13.8V.... These batteries can take immense current, and charge extremely fast, but really tend to do extremely well with .3C to .5C in charge current.
WEIGHT/SPACE:
Less than half the weight of lead Ah to Ah and almost always more compact. Our 400Ah LFP bank weighs 182 pounds less than a 400Ah lead acid AGM high performance bank. However, to equal the
usable capacity of a 400Ah LFP bank one would would need approx 900Ah's of lead acid. This makes the 400Ah LFP bank approx 400 - 700 pounds lighter (depends upon lead batteries chosen) than the equivalent usable capacity in lead acid.. This is another reason why it is tough to put an energy density figure on Li when comparing to lead acid.
LEAD IS HEAVY:
The typical lead acid bank consists of 65-70% of the weight being comprised of what I prefer to call "
dead lead" or the excess lead you carry around but that you can not actually use, for optimal cycle life. If you have a usable capacity of just 30-35% of the bank,
when out cruising, this means that you are carrying around 65-70% of that weight in
unusable dead lead capacity. Our 400Ah LFP bank weighs 130 pounds & a full 80% of it's capacity is readily usable. This means just 20% of it we are choosing not to use or you simply don't want to use it for
best longevity. As a result, we carry around a meager 26 pounds of
unusable LFP battery on our boat.
If we want to equal the
usable capacity of this 400Ah LFP bank in lead, we would need 8 GC2 6V golf cart batteries or approx 900Ah's. If you wanted a high performance AGM system you'd be looking at approx 700 pounds of TPPL AGM battery. 35% of 900Ah is a usable capacity of 315Ah's. 80% of the 400Ah LFP bank is a usable capacity or 320 Ah's. The 900Ah lead bank weighs 520 pounds for GC2's or 702 pounds for TPPL AGM.. If you use just 35% of the GC2 bank then you are hauling around 338 pounds of "dead lead" or 338 pounds of unusable capacity adn 456 pounds of unusable capacity for the TPPL AGM bank. Twenty six pounds of unused LFP or 338-456 pounds of "
dead lead".. Energy density/usability points to ponder...
STEADY VOLTAGE:
Another un-measurable benefit not often included in
energy density discussions is that LFP banks have a very strong & flat charge & discharge curve with a very steep & fast rise or drop at either end. These ends are called the "knee's". LFP's will maintain voltages well above that of any fully charged lead acid bank, and remain very close to their 3.3VPC / 13.2V nominal voltage level. They will hold extremely steady voltages, with little change, all the way to 80% DOD. They will maintain this very tight voltage range even under
normal house loads. Espar heaters, refrigeration, watermakers, windlass and winch motors etc. etc. will all perform better. Your equipment likes higher voltages and dislikes voltage sag. Even bilge pumps will pump more water. Voltage sag that can drop out electronics during bow thruster or windlass use is almost entirely eliminated.
CHARGING EFFICIENCY:
Charge efficiency is also referred to as the Coulombic efficiency. These batteries are as near 100% efficient as I have ever seen on my test bench. Take 200Ah's out and put 200 Ah's back in and you hit the voltage and net accepted current at almost the exact same Ah's out to Ah's in. Until LFP I had never witnessed anything like this, even with the best AGM's. Lead acid ranges from 70% to as high as 90% +/- efficient but you still need to put back in 10-30% more than you took out, and this is with healthy lead acid batteries. The last 5% can be less than 50% efficient meaning your solar or other energy source is being 50% wasted when pushing towards 100% SOC, as you need to. As lead acid batteries sulfate the charge efficiency or Coulombic efficiency gets even worse.
NO NEED TO RECHARGE TO 100% SOC:
We know the Achilles heel of lead is sulfation and in order to fight off sulfation we need to charge them to 100% SOC as often as possible. This proves very difficult for many boaters and cruisers unless your boat resides at a dock after each sail or sits on a mooring with an adequate solar system. LFP batteries
do not need to get back to 100% SOC. This is a major win for LFP. When we come back from a weekend on the water, and our battery is at 30% SOC, I really don't care. I shut down the boat, and the solar, and go home. LFP batteries actually prefer to sit at 50-60% SOC rather than at 90-100%.. Nothing to do with energy density but everything to do with not needing to waste energy or worry about ruing the batteries because you failed to get back to 100% SOC.
SULFATION DAMAGE FROM PSOC USE, WHAT'S THAT?
Sulfation is by far and away the cancer and #1 killer of lead acid batteries. LFP batteries do not sulfate, no cancer, so there is no need or worry about constantly getting back to 100% SOC before you leave your boat. Lead acid batteries deal the most poorly in the
actual way we use our boats no matter how energy dense they are or are not, when compared to LFP.
Energy density is really a non-issue if we look at all the other benefits. Course LFP is far from a perfect "
drop in replacement" but how you use it can mean major differences between lead and LFP
