LiFeYPo4 questions

[PNjunction]

3.1v per cell sounds reasonable, although for me I prefer not to go below 3.15v (12.6v for a nominal 12v battery). The reason being is that once you start going below 3.2v (12.8v for 12v batt), you are entering the discharge knee, and while shallow excursions into it aren't necessarily bad, the recharge needs to be slow until the battery reaches 12.8v whereupon you can apply a normal charge current. Then again, with solar, you may not be applying much more over 0.1C anyway.

I shudder when I see guys reading the spec sheets and thinking that going as low as 2.5v is ok as an LVD limit. Maybe once or twice, but not as a practical limit! Kind of like thinking that 10.75v for lead acid is ok to take your battery down to all the time.

I think you'll be just fine.

Re the BMW - just like a Harley, it too is indescribable until you ride one with the crisp snickety-snack of the precision gearing, throttle feel etc. A different world, but just as invogorating! Hint - we can put lifepo4 under the seats of these as well, with Antigravity, Shorai, EarthX and the like for SLI duty...

[Sunking]

3.1v per cell sounds reasonable, although for me I prefer not to go below 3.15v (12.6v for a nominal 12v battery).

Are we talking LiPo battery without BMS? If so I would not dare go below 3.2 volts without BMS. Eventually a cell is going to become unbalanced and low voltage. When that happens and it locks itself out, you are in big trouble. So for a 4S LiPo I would never dare to go below 12.8.

[reed cundiff]

We do have a BMS and I check the monitor several times each day (primarily when we wake up). We have never gone below 52 V for the suite or 3.25 V per cell. We were still at 52.2 V for the 16 cell suite when we dropped to 55% DOD. It is full overcast with slight rain on coast of Oregon just north of Rockaway Beach and we were charging at around 700 W. Fully charged by 11 am.
Reed and Elaine.

[Sunking]

We were still at 52.2 V for the 16 cell suite when we dropped to 55% DOD.

Reed I must be missing something and if so I apologize. I assume we are talking about LiPo chemistry. 52.2 volts on a 16S LiPo pack is 3.27 volts per cell. On a LiPo that correlates to a DOD of more than 100% or less than 0% SOC. 55% SOC is about 3.85 volts or 61.6 volts on a 16 S pack. I run a 16S pack on my racing golf cart.

So what am I missing here?

[inetdog]

Reed I must be missing something and if so I apologize. I assume we are talking about LiPo chemistry. 52.2 volts on a 16S LiPo pack is 3.27 volts per cell. On a LiPo that correlates to a DOD of more than 100% or less than 0% SOC. 55% SOC is about 3.85 volts or 61.6 volts on a 16 S pack. I run a 16S pack on my racing golf cart.

So what am I missing here?

What you are missing is that the cell voltages are different for different kinds of LiPo (or other package type) Lithium chemistry. LiFePO₄ (also abbreviated LFP) has voltages which are several tenths of a volt different from LiCo and other higher energy density chemistries, and with a different V versus SOC curve shape. It is important to know exactly which Li chemistry is being discussed.

I suspect that your racing golf cart is using a less stable but higher energy density chemistry.

[PNjunction]

Are we talking LiPo battery without BMS? If so I would not dare go below 3.2 volts without BMS. Eventually a cell is going to become unbalanced and low voltage. When that happens and it locks itself out, you are in big trouble. So for a 4S LiPo I would never dare to go below 12.8.

Actually, you are exactly right, even though we are talking large prismatic lifepo4's typically in a 4S configuration (usually CALB, GBS, Winston, Sinopoly, Hi-Power etc) that have no deep discharge lockout or other internal protection!

12.8v (3.2v per cell) is a rested voltage, so under heavy heavy load, I could see someone taking them a bit down to maybe 12.5v (3.125 per cell), which usually comes up a bit to rest towards the 12.8v (3.2v cell) mark. But this should NOT be considered as your low-voltage daily-use target, but your max limitation! Despite the spec-sheets from the manufacturers that show lower voltages, it is not a good idea to do so. Would you routinely take a lead-acid down to 10.75v ? No way, even though you see specs for them like that.

Many are excited to see that they can actually obtain more than about 80% of the rated capacity, and love to dive into the discharge knee to get just a little bit more. Bad idea, even though you can, because even if you don't take them to the very bottom, once you are INTO the discharge knee, you need to take it easy coming out of it, about 0.1C or much less before applying a normal charge current. That warning goes unheeded usually. It is best to stay away from the knee completely.

I often wondered if my initial foray into Shorai and Antigravity lifepo4 batteries for powersports would pay off, and it DID when I looked at the charge algorithm of the Optimate TM-291 lifepo4 charger! I try to emulate that with my own larger setups.

So if lurkers want to know what a commercial charger designed specifically for lifepo4 does, and not rely on internet forum testimony, take a look at what the Optimate TM-291 does!

If it encounters a lifepo4 battery (12v nominal of course) resting under 12.8v, it goes into a "save" mode, and applies very very low current until the battery terminals reach 12.8v, whereupon it proceeds with it's normal 5A charge. Tecmate / Optimate are not some fly by night outfit just winging it and trying to apply their own lead-based charger tech to lifepo4. They actually know what they are doing.

So the commercial charger outfit totally agrees with those who have 4-5 years or more of charging prismatic lifepo4 experience under their belt and their settings more or less agree with each other.

Yes, balance is an issue, but it is totally overblown in our application to make sales based on fear, unless one is using trash/used componentry.

[Sunking]

Actually, you are exactly right,

Thanks I thought I was right, just checking to see if I was on the same page. I use LiPo for two things: RC toys, and another big toy of a golf cart. My POV is really for Electric Vehicles. It is one thing to ruin a $50 LiPo battery without a BMS and a completely different thing than ruining a $10,000 EV pack. Slow charging an EV pack is not going to fly. The goal is 15 to 20 minutes at most using 5C. With that said I use balanced chargers for both RC and Golf Cart. No BMS to speak of because both RC models and Golf Cart have LVC circuits to prevent from going to far down.

[PNjunction]

OH, don't worry about slow-charging - that is only because my solar setup won't reach the 1C that is usually the comfortable limit for the large prismatics, with short 3C bursts, but in our storage application, we'll never reach or need that! Most recommend that for daily use, don't exceed 0.5C charge/discharge. That is WAAY over the top for a large house-bank, so no worries.

It is something I would keep in mind for my smaller portable stuff, where I can charge in 30 minutes if I had to. That is one good thing about lifepo4 house banks - unless you are already maxed out at 1C, you have some room for improvements in the array size if you need a quicker charge. Let's see, an 800ah bank could take about 400A charge current and not blink an eye.

I do keep the Optimate around as a subsitutue for an adjustable bench supply for the time I totally miscalculate the power draw and take it to the bottom. The Optimate will automate that recovery process for me safely, provided I get to it right away to limit the degradation.

[Mike90250]

Li gurus -
I've got a dump trailer, with hydraulic pump and electric brakes. It has a circular RV connector with a battery recharge contact in it. And a cheap-o dead, deep cycle battery. And I assume the next battery I put in, will fade away too. Only needs about 5 minutes of battery to run the brakes if the trailer disconnects. Rest of the time, it sits in the field, battery bleeding away. Would this be a good application for Li ? Brand / model suggestions ?

[Sunking]

Li gurus -
I've got a dump trailer, with hydraulic pump and electric brakes. It has a circular RV connector with a battery recharge contact in it. And a cheap-o dead, deep cycle battery. And I assume the next battery I put in, will fade away too. Only needs about 5 minutes of battery to run the brakes if the trailer disconnects. Rest of the time, it sits in the field, battery bleeding away. Would this be a good application for Li ? Brand / model suggestions ?

Yeah I think so Mike. What kind of discharge current do you need and for how long?

First thought is use a RC LiPo 4S pack. They come as large as 10 AH with up to 50C discharge current. On the Litium Iron Phosphate for E-Bikes, lawn mowers, and Ev's as many amp hours as you want and can afford. Catch is most of them are limited to 3 to 5C discharge rates.



For my racing golf cart I use Calb 100 AH cells. Pricey though at $120 per cell.

[PNjunction]

Mike - as much as I like lifepo4, I'm not sure it would make sense in your dump trailer application that sees infrequent use. Personally, I'd use an AGM, and make sure that there are no parasitic loads during storage.

However, if you were to incorporate lifepo4, the performance would be phenomenal during the project. And that can be a problem! The deep discharge knee is a tempting mistress that beckons you to go past 80% DOD, since performance compared to lead is mind-boggling. Unless you had the discipline to obey your own monitoring or not set your LVD too low, you'd be asking yourself "heck, I'll dump a couple more loads!", and now that's where the trouble starts.

For infrequent use, after say a yearly dumping project, you could very easily just walk away from it half-discharged and not charge it again until just before using it again next year. No parasitic loads of course, and you do NOT float these things. Of course if you just tossed in an agm, then you'd have to make sure you had that fully charged prior to leaving.

Cycle life would be outstanding, but with your infrequent use, you may not realize any financial gain from the extended cycles over lead. I'd think carefully before you leap. In my mind, the ONLY reason for you to go with lifepo4 in that application is if you demanded full performance all the way down to 80% DOD.

Sunking - you obviously benefit from the weight savings, and motive-power performance - and knowing that you treat your batteries right, you would gain financially over lead in the long run. Ordinarily I think you'd do fine with the standard CALB CA series (aka "grey case"), but if you want to start hammering them, then the latest "FI" series might be up your alley.

For the rest of us - standard lifepo4 prismatics like GBS, non-specialized CALB's, Winstons, Sinopoly, Hi-Power and the like, will lead a pampered life since we are running them at relatively low-voltages and currents, and should easily exceed double the standard cycle ratings, *as long as we stay out of the steep knees. *

[PNjunction]

Voltages are thrown around forums like so much candy, but not often are they measured with any sense of trust in the metering equipment itself.

This is what I'm talking about - this measurement with Fluke metering I trust. (that's not me, and I do NOT have all that bms wiring on my bank). Still the most impressive demo! And for me, he took 400ah bank waaaaay too far!!! I hope for the bank's sake that when he finally decided to recharge it, he did so with 0.01C or less current until the terminal voltage reached 12.8v before applying a normal charge current.

https://www.youtube.com/watch?v=oJ8AfJsOhsM

On the other hand, here is a fellow trying to check / set voltages on expensive lifepo4's, with a 2-dollar meter. Yeah, I wouldn't trust this as far as I could throw it. The mind boggles if there is no faith in your metering:

https://www.youtube.com/watch?v=9FSEZE_9OLg

I guess what I'm trying to say is that people doing the DIY thing should really invest in metering that they can trust before testing the waters with lifepo4. A bottom-of-the-line Fluke 114 I would trust with no problem whatsoever. My 115 in the 6v dc autorange is about as accurate as my 87V, so you don't need to spend a lot to get a level of trust for mere low voltage dc measurements for lifepo4 work.

[Sunking]

Sunking - you obviously benefit from the weight savings, and motive-power performance - and knowing that you treat your batteries right, you would gain financially over lead in the long run.

I chose lithium because of the performance boost. Economically AGM would have saved 50% and last just about as long. The most critical spec in RC and EV is weight and discharge performance. Those Calb CA100's cost $120 each and I run a 60 volt battery pack nominal (19 cells) in the golf cart. Total weight of batteries is 143 pounds. Before that were just 4 12 volt 150 AH AGM's for a total weight of 360 pounds. So there is some significant weight and volume differences going on.

[PNjunction]

The weight savings is significant, about half to 1/3 that of lead depending. I've got a feeling that in your hands, you will handily beat the typical life of an agm, but it might take about 6-8 years to find out. You benefit from your motive power application, whereas I can easily hand-carry a 60ah battery in one-hand for portable use.

Quick note for lurkers who see that demo of the 400ah bank, and wonder how to wire say an 800ah bank out of those 400ah cells, which at this time seem to be the largest individual cell capacities I've seen.

Would you merely duplicate the existing 400ah setup, and parallel them together like you would with two 12v lead-acid batteries? NO!

You build your CELL CAPACITY first, and THEN wire those cells in series. In other words, each "cell" for an 800ah bank of Winstons would consist of two of the 400ah cells wired in parallel. You would then series-connect these paralelled cell groups for a nominal 12v. (or 24v with 8 sets of paralleled groups instead of 4)

[Sunking]

PN personally IMHO if one needs more capacity than 400 AH at a given voltage is considering the wrong voltage to begin with. Show me an EV with a 600 volt 400 AH pack weighing around 1350 to 1400 pounds, 5C Charge/Discharge rate, recharge from 10 to 90% in 15 to 20 minutes, and I will show a you luxury 6 passenger Electric Vehicle with all the Bells and Whistles that can cruise 100 mph for 900 miles, and accelerate 0 to 60 in 4 seconds.

Point I am making here is at 400 AH a battery watt hour capacity at:

12 volt = 4.8 Kwh
24 volt = 9.6 Kwh
48 volt = 19.2 Kwh
96 volt = 38.4 Kwh
192 volt = 76.8 Kwh
.........
600 volt = 240 Kwh

Low voltage is extremely limited and just flat out suks.