LFP power vs energy density - know your cells!

Substrate

Solar Addict
All LFP is the same right? No, and not knowing the difference can lead to disappointment, or even cell abuse.

I'll go so far as to say that @Will Prowse teardown of Miady battery demonstrates this, amongst other things reserved for comment later. I thank Will for his teardowns, so not a critique of the man, just a comment about something he may have missed.

Basically there are TWO types:
1) Energy Dense LFP (designed to actually start a vehicle). Headway, A123, Shorai, K2, Kokam, etc)
2) Power Dense LFP (not designed to start a vehicle, but designed for long power draw). Most of the prismatics and cylindricals not mentioned above.

You can fake the ability with power-dense cells to start a vehicle with added capacity, but this is not recommended.

The cells used in Will's Miady test are of the second category, power-dense. Think overgrown garden solar-walkway cells. Even if you take a bms out of the equation, a limited 0.5C charge rate is a giveaway. Does that mean they are bad? No, they are just designed for a specific application - long power draws, not high energy bursts.

A chart of the difference may help understand. In past years, when EV was the thing, using large prismatics was very popular. Brands like Sinopoly, Winston, A123 pouches diy, GBS, Headway, Kokam all had differing densities, and were best suited for different applications.

Not knowing this, meant abuse if you didn't get it right. Most of us discovered it by trial and error and repetetive flaps of the open wallet.

So here's the chart. Go to the last image for a quick peek. Guess where the Miady cells fall? Wonder where most of our inexpensive modern prismatics fall?

The chart is a bit stale, but useful for demonstration.


The gist of what I'm saying is that Will's test of the Miady cells should have been much gentler. Take it easy bro when first testing those solar walkway cells out of the gate! Especially given the price, the consumer is expected to be the one doing the initial balance by more normal cycling, where the bleeder resistors have time to operate. The hammer test on these unbalanced batteries may actually have done a little bit of damage - but I have no way to prove that. :)

In the end, extreme examples exist of why you should know where your cells fall in this curve.

1) Trying to start a motorcycle, let alone anything else with a Miady battery is a bad idea.

2) Building a large house-bank out of Headways, or in my case once Shorai starter LFP batteries, is a total waste of money. In power-dense application, you are paying for an energy capability that you are not taking advantage of. A mismatch of application. And a bloody wallet if built to any reasonable house-bank capacity!

Moral: if you are using Miady's, please give TIME for those weedy little bleeder resistors a chance to work on those non-balanced /random-pulls from the assembly line and tack-welded, lower your cv to normal values, and don't hammer them right out of the gate. There is a reason a Miady is not a Battleborn and not priced like one up front. Don't test it like one right out of the box. A little TLC with solar walkway lights inside goes a long way.:)

And you are still not advised to start a vehicle with a Battleborn either. Match the power vs energy density to your application, and one won't be disappointed, or suspect of getting "B" grade cells.

They may in fact be "A" grade cells intended for their power application based upon their power-density. But as usual, keep an eye out for sharks trying to re-classify garbage from being initially designed for being energy-dense to suddenly being power-dense after abuse.
 
Last edited:

Substrate

Solar Addict
To keep it humorous and show that I have no ill-will, let's have some quick fun.

All I'm saying is not to pair up your Miady-based race back against this one from an A123 in 2007:


Conversely, don't build a huge house-bank out of A123 just to light up your garden walkway!

Despite the fact that each might proclaim 2000 cycle life. Heh, depends on your real-world application. :)
 

Substrate

Solar Addict
Remember the "NANO" buzzword?

Nano-phosphate. Nano-bot. Nano-nutrients. Nano-meatloaf.

Obvious translation is very small. To change your energy vs power-density, one of the easiest ways was to grind your LFP active material much finer.

This means lithium ions have more nooks and crannies they can intercalate into and out of, and in extreme cases, less heat. But of course, just like doping your active materials with rare-earth minerals, or coming up with the proprietary secret-sauce electrolyte additives to achieve a specific goal, this costs more.

Because of this, an LFP consumer in a power-dense application might not want to totally put the blinders on solely comparing lifecycles. Perhaps question why a power-dense LFP which is easier to manufacture, might cost nearly as much as an energy-dense LFP which requires more care and tolerances is!

Recently I've seen marketing bullet-points for some LFP calling themselves "Automotive grade".

So what does that mean? Where does this battery fall on the power vs energy density scale? Or is it just something marketing made up?

You know what I mean. Maybe I am a disgruntled propeller-head after all. :)
 

HRTKD

Boondocker
Aging propeller head here.
propeller.gif


I looked at the amps required to start my on-board generator (~200+) and what my BMS could produce continuously (240 amps from two 120 amp BMS). I decided to use a small lead acid battery to start the generator instead of the LiFePO4. The generator's spec calls for a higher CCA than my BMS is rated for. Could the LiFePO4/BMS do it? Yes. Another forum member has tested it with his similar system. I'm just not comfortable with it.

I would really like to ditch the lead acid battery, but there's more involved than just connecting the LiFePO4 to the generator.
 

Substrate

Solar Addict
Yeah, don't do that often with your existing LFP.

But if you want to ditch the lead-acid starter, just use cells designed for that application. Typically somewhere in the spec will be "nanophosphate" or other wording.

You might be able to go off-the-shelf with a larger motorcycle LFP. Brands like Antigravity, Shorai, EarthX. You'll freak out on how small they are in normal capacity to just start something. Matching CCA and other stuff like that is best handled by those manufacturer sites.

A fake specification for them is Pb/Eq - which is a very simplistic way for the layman consumer to try and pick the right LFP capacity based upon the original oem lead-acid recommendation. If you have the space, most suggest to give yourself some headroom and +1 it, or pick the next size up.

Yeah, ditch the lead and go LFP. Just be prepared to pay for that capability. Shorai's are kind of cool because they have a built-in bms charge balance connector if you want to go that far and not simply use the 2-terminal connections. Other manufacturers have other features, like a built-in bms with basically higher lvd levels, simply because letting an LFP reach what we would consider a normal cutoff, means that if you don't have a charger around, you can't start anyway! Kinda' solves that catch-22 out in the field.

So different application, with some different concerns. Look into a beefy motorcycle LFP starter and you might be surprised and ditch the lead.

SHORAI warning - do NOT short those built-in balancing port pins by trying to fat-finger your multimeter probes when taking a measurement. Simply do not try to measure with a multimeter from that port! Bring them out on a dedicated cable! The cap covering those balance pins is on a hinge. The temptation is to flick it open with your fingernail kind of skewed. Don't do that either - pull it STRAIGHT out a little bit. Otherwise, you'll bend and short the furthermost balance pin. Been there. :)
 
Last edited:

curiouscarbon

Solar Addict
i might have missed my coffee, but perhaps power and energy were swapped in the OP?

energy dense = more kilowatt Hour per mass/volume
power dense = more peak watt discharge per mass/volume

shamelessly plugging this spreadsheet

some of the far right columns help keep track of C ratings, surge duration, initial internal resistance, and cycle life at stated C rate. like you said 2000 cycles with one cell is not the same as 2000 cycles with another cell.

 

Substrate

Solar Addict
Thank you! I did have too much coffee! Just swap the energy / power around.

This was a global forum test, and you are the first one to pass! :) <running and ducking>

I felt something was wrong, so thanks for the correction.
 

Steve_S

Offgrid Cabineer, N.E. Ontario, Canada
Many manufacturers of LFP Cells also have a "Burst Rate" for High Discharge Demands. With many Standard ESS grade cells, momentary 5C Burst Rates can be handled but NOT suggested, as that is for rare use cases.
There are different grades of LFP which should be fairly obvious, and it is covered at Battery University & elsewhere.
There are some which are targeted for High Discharge Rates but they are not cheap. Generally for such a requirement like "Starter" batteries, LTO is suggested because they can handle hard fast deep discharges.

If you look at Winston cells for example, they offer different Grades / Families relative to purpose, then look at the cell cost differences. Best to be seated when doing so.

There are now a couple of ESS Systems (Similar to Tesla PowerWalls) which also include a set of Super-Capacitors which can discharge at very high rates to serve big Surges & Demands on a power system without impacting the actual cells. Current reviews & tests are proving this out as an efficient solution for several problems. By this fall Retail Commercial versions will be available but word is trickling out in the Tradies and looks like they will only cost a slight bit more due to the Super-Caps. LG, Generac, Kholer, GoodWe and a few others have gone deep into this area along with many others.
 

Substrate

Solar Addict
For sure. There are many different grades of performance from solar-walkway LFP to A123 at the other end. :)

So what may be happening in some circumstances is not manufacturers trying to pawn of grade-b cells, but in fact grade-a cells that are in another performance bracket, and matching a lower-current bms to that spec.

But yeah, keep your head on a swivel for the cheapskate outfits!
 

kaspre

New Member
ESS Systems (Similar to Tesla PowerWalls) which also include a set of Super-Capacitors which can discharge at very high rates to serve big Surges & Demands on a power system without impacting the actual cells. Current reviews & tests are proving this out as an efficient solution for several problems. By this fall Retail Commercial versions will be available but word is trickling out in the Tradies and looks like they will only cost a slight bit more due to the Super-Caps. LG, Generac, Kholer, GoodWe and a few others have gone deep into this area along with many others.

Sounds like a great idea. Is anyone DIY this yet?
 
Last edited:

carbon60

Solar Enthusiast
I was musing over beers about using capacitors to start ICEs we have lying around for various purposes. Added advantage would be the rather exciting charge and start sequence, which will require added sound effects for sure! :cool:
 

Seven30

New Member
What about a small set of LTO cells? Maybe a relatively light connection to main bank to act as current limit resistor. My RV has a 900amp surge which is far too high for my main bank hence retention of the double 8D monsters but a small LTO bank could handle that surge easily.
 

Substrate

Solar Addict
Anything is possible. Due to their high self-discharge, supercaps are most seen in the regenerative-braking EV/Hybrid world. Mobile stereo shootouts and so forth.

LTO? I suppose, but I have no experience with them. It all comes down to a matter of budget, practicality, and DIY one-off skills.
 

carbon60

Solar Enthusiast
Who cares about self-discharge in a starter system? I would envision "Push to charge, release to start." with an awesome charging whine–before you release and the big diesel rumbles to life. :cool: (Which only happens rarely, since you have so much PV and a giant LFP bank, of course…)
 

Seven30

New Member
Just keep in mind starting a BIG diesel in winter can be challenging!
.
I guess this is why all the complex battery isolation systems exist.
 

kaspre

New Member
All true, but @Steve_S was referring to home battery systems with super caps. I assume to handle load surges like HVAC kicking on, or a water pump? In that case, I suppose that the rate of super capacitor self-discharge would need to be considered as part of the system design.
 
Last edited:

Seven30

New Member
Oh right. I have a question on that though. If a supercap supplies starting load for AC doesnt it also present a similar drain in the main batteries?
Is some form of current limiting required?
 

Steve_S

Offgrid Cabineer, N.E. Ontario, Canada
All true, but @Steve_S was referring to home battery systems with super caps. I assume to handle load surges like HVAC kicking on, or a water pump? In that case, I suppose that the rate of super capacitor self-discharge would need to be considered as part of the system design.
Yes, my response was in the Residential ESS context, which is probably the easiest to implement using supercaps to supplement. They store & can discharge a high Crate burst easy enough and they can take their time charging as well so it doesn't "load" the battery bank hard. Or in situations like a small commercial operation or big hobby, a "woodshop for example", we all know tools like that have a Surge and varying loads as they run and the harder they work. The Caps could take the surge hit off the battery system making them a lot happier.
 
Top