LiFeYPo4 questions

[tasman]

Thanks, that answers my question very well. be using a 200Ah bank in a converted bus, with 460/730w solar array, there's 460w 12v panels and a 250w 24v panel which can be switched from the start batteries to house, through an MPPT. Doing it as a learning tool for using lifepo4 systems and then will change over our 2kw house off grid system to lifepo4. We currently have a mix of AGM and gel, neither are satisfactory and gel seems very over rated.

Have started off with a 800cca lifepo4 start battery and a 38Ah lifepo4 for fridge/freezer and accessories in our Sahara. Found the start batteries didn't have the capacity to start and run accessories when the engine is off, so have both batteries in the battery holder and could fit another couple in as well.

The next obstacle is to find a decent amp solar controller with adjustable low and high voltage cut off, so far haven't found one, they all seem to high and to low. Then have to find a 2500-5000 inverter with the same ability. Our current inverter works well and is wired into the buses 240v system, can switch it fro inverter to grid when we have power on the road which saves lots of messing round.

Buying a ready made 200Ah lifepo4 battery from china with included BMS and will learn from that installation, then hope to builds my own house bank, with an adjustable BMS.

[PNjunction]

A series connected set of 4 LifePo4 cells (4S) at a "nominal" 3.2v rating is what you make a standard nominal 12v battery from.

As a set, assuming they are balanced to begin with, any charger that can bring the SOC to 14.0 to 14.6v will work. Since there is very little capacity from 90% SOC onwards, there is little need to go as high as 14.6v, and enhanced cycle life will result from lower voltages, although absorb will take just a smidgen longer. A happy medium would be to charge until reaching 14.2v / 3.55v per cell and let the absorb current drop to C/20 or thereabouts and shut off the charger (or solar charge controller). If you can't stop the charge immediately, it won't kill them outright, but don't let it stay that way forever.

At the low end, one shouldn't go past the 80% DOD mark, and for a 4S pack, that would be about 12.85v / 3.2v per cell. This is a "rested" voltage measurement of at least an hour or more.

At fractional charge / discharge rates <1C, you may not encounter severe out of balance issues, (again assuming they came balanced in the first place) however one should always monitor cell voltages individually on a regular basis in case action is necessary to balance the pack. Depending on who you talk to, what application you are doing, or how much quality monitoring is in place, (like EV, RC or other heavy duty motive power applications), then a dedicated BMS may be a wise choice. However one must weigh the consequences of a failed BMS taking out a perfectly good battery. BMS / No BMS is an endless internet thread.

As always, consult the manufacturer's own recommendations, but be sure to note that "hard limit" damaging voltages do not mean you should visit or get close to them regularly. Unless they indicate otherwise, I'd recommend staying within an 80% to 10% DOD ratio, ie 12.85v at the discharged end (3.2v cell), and 14.2v (3.55v) at the charged end.

Note that once you remove the charging source after a full charge, the cells will eventually settle down to about 13.3v or so on their own and stay that way for up to a year with no load. When dealing with Lifepo4 voltages one wants a voltmeter that has very good accuracy. We're talking about a Fluke 87V accuracy (not just the amount of digits) as being a minimum for anyone serious about lifepo4 care.

Float is not necessary nor needed on a lifepo4, but if you did have some sort of parasitic load that would drain the battery down dangerously, then you could "float" it at about 13.8 to maybe 14.0v at the most. Ideally, find and kill the parasitic load.

[tasman]

Just getting into lifepo4 batteries and don't have the tech nohow to really understand it yet, iv'e been looking at makng up a 200ah pack using lifepo4 3.2v cells and am a little confused.

Are these 3.2 cell suitable for a 12v pack and if soi what would be the low voltage level for proper use. In this thread it says 3.2-3.1, but the cells I'm looking at are only 3.2v. So how do you work out the best low voltage for them, or should I be looking at cells with a higher voltage. Thanks.

[PNjunction]

The storage for lifepo4 would be about 3.1v per cell, measured with an accurate voltmeter.

I think we are over-engineering the actual storage voltage. I still haven't seen any comparative charts for cycle-life vs variable storage voltages, just that it is a best-engineering practice to store them somewhat discharged.

It dawned on me that the reason the powersports lifepo4 seem to be stored at only a shallow 80% SOC may be for operational readiness. That way when a rider takes the battery home and just replaces his old one, there will be enough power to start the vehicle. The manufacturer isn't counting on the consumer to charge the battery first with his own charger.

Had it been stored at the more commonly recommended 40% SOC, and sat around for 2 to 3 years, there may not be enough power to start the vehicle, and also drag the cells down too far on the very first start. Heh, I see this a lot with parking-lot swapouts at auto parts stores with lead too. Guy buys a quality brand, but old stock, starts vehicle, and does nothing but 5 minute coffee runs and ruins the battery by short cycling at low soc's. - of course swearing off brand-x batteries the rest of his life.

I'm not going to go bananas over the actual storage voltage. If I don't have the time to accurately get down to 50% I'll just make sure to at least get out of the top 10% of my capacity for running things for a little while. If that's the difference between getting only 2850 cycles vs 3000, at my sizes I'm not going to be too upset.

[Sunking]

Some Li chemistry batteries, not sure if it also applies to the iron phosphate family, even deteriorate faster when kept at full charge than when kept at 50-75% charge.

Pretty close and it is called Storage Voltage. For LiPo it is 3.85 vpc. Just about all the smart chargers for Lithium today have a storage setting. It will either charge or discharge to the right voltage. I keep mine in a warmer part of my refrigerator in a zip lock bag with silica gel packets.

[PNjunction]

For storage recommendations, I guess longer than a few days or a week maybe, then 50% DOD seems to be about the norm. However, I have seen them arrive anywhere from 20% to 60% DOD depending on the manufacturer and application.

For example, the Shorai Lifepo4 powersports batteries are typically kept in "storage" at about 20% DOD. Their own charger has a function to either bring the battery up or down to this level before storing the bike - very handy. Presumably, one will be riding it in a week to a few months.

My Braille lifepo4 battery, which was over 2 years old, (26650 A123's inside) was sitting pretty out of the box at 13.3v! That's pretty high for long term storage, although I'm sure that the company didn't think this would end up being a shelf-queen and would sell quickly. That may mean that I'm going to suffer reduced cycle life by it sitting around at a high SOC for so long. We'll see.

These high SOC's during storage may be indicative of powersports lifepo4's being able to sit around like that, whereas low-rate prismatics designed for capacity rather than power, may be the ones that really need to be stored much lower. The Shorai's are prismatics, but apparently are tweaked.

I'm going KISS and just keeping them at 50% DOD for any long term storage that will be longer than a week. Still, I can't find any qualitative comparisons other than being at the extremes.

[inetdog]

I'm glad you brought that up. Make sure that you are not confusing a hard-damage low voltage limit (if you stay there long) with a real-world 80% DOD limit, otherwise you'll be sacrificing cycle life akin to lead-acid and losing the benefits of Lifepo4 if you go below 80% DOD on a regular basis.

For Lifepo4, That would about 3.2v per cell, or 12.8v for a nominal 12v pack for 80% DOD. Ie, if you see it drop to 3.1v, stop and recharge, even though the specs may say that 2.5v / cell is the low-voltage limit. Just because you can go there, doesn't mean you should, otherwise you'll pay the price with cycle life akin to lead-acid. Twice or more basically (at current prices) when you have to replace your lifepo4's.

It is somewhat akin to not letting your lead-acids drop to 10.75v, although no experienced person would ever want to take their Pb batteries that low, unless they were unconcerned about cycle life.

Definitely different from Lead Acid in one way, in that leaving the battery at a low charge level for awhile is not going to cause damage similar to the sulfation problem with lead acid.

So you really do need to calculate from the life cycle curve of your particular battery type whether you get more total energy out of the battery over its lifespan by only doing mini-cycles or by dropping them lower each day or waiting to recharge them.

Some Li chemistry batteries, not sure if it also applies to the iron phosphate family, even deteriorate faster when kept at full charge than when kept at 50-75% charge.
For those cells, cutting off the charging early and doing fractional cycles, say ranging between 50% and 75% SOC might give a longer working life than cycling between 75% and 100%. Do you have hard facts on that to share?

[PNjunction]

Just do not run the voltage down below the point of no return during the testing.

I'm glad you brought that up. Make sure that you are not confusing a hard-damage low voltage limit (if you stay there long) with a real-world 80% DOD limit, otherwise you'll be sacrificing cycle life akin to lead-acid and losing the benefits of Lifepo4 if you go below 80% DOD on a regular basis.

For Lifepo4, That would about 3.2v per cell, or 12.8v for a nominal 12v pack for 80% DOD. Ie, if you see it drop to 3.1v, stop and recharge, even though the specs may say that 2.5v / cell is the low-voltage limit. Just because you can go there, doesn't mean you should, otherwise you'll pay the price with cycle life akin to lead-acid. Twice or more basically (at current prices) when you have to replace your lifepo4's.

It is somewhat akin to not letting your lead-acids drop to 10.75v, although no experienced person would ever want to take their Pb batteries that low, unless they were unconcerned about cycle life.

I've had guys thinking that their 12v nominal drop-in lifepo4 pack state that 9-10v is the low voltage limit, but had to warn them that if they do that often, they are no better than they were with Pb in regards to cycle life. So if you are going to test, do not take it to the hard-limit low voltage point, but to the one you'll actually use, which should be about 3.2v at the lowest per cell.

Also forgotten is that once below 3.2v per cell, you can't just jam current into them - if you do go lower, then your best bet is to apply .01C until your cells reach 3.2v, and THEN you can apply a normal charge current. Many forget this and a double-whammy is now occurring. Cells are taken too low, and an application of full current in the sharp discharge knee hurts them further.

[inetdog]

How can an amateur like me recognize if a battery is the safer LiFePo4 (Iron) or the more risky LiCo02 (Cobalt)? Is there any way to detect it?

The best way to tell the difference is to look at the manufacturer's specification sheet for it.

If all of the original labels have been removed, you can get a good idea by looking at the fully charged voltage, the discharge voltage curve and the energy density per weight from a controlled discharge test. Just do not run the voltage down below the point of no return during the testing.

[PNjunction]

Wikipedia does a good job in the safety section of explaining and contrasting it with Lipo/cobalt:

http://en.wikipedia.org/wiki/Lithium_iron_phosphate_battery

The main point being that it does not like to give up oxygen atoms, the fuel for fire. Other points can be seen, but the price for this added safety is less power density - which just means that for the same amount of power, Lifepo4 will be larger than LiCo02.

Admittedly it can be very hard to get to this information when the signal to noise ratio on the web and in the media tends to overlook it, and instead presents the drama of cobalt instead of iron-phosphate. Two other notable attack vectors are price and politics.

[SolarEU]

LiFePo4 (Iron) VERSUS LiCo02 (Cobalt)

Unfortunately for readers who may stumble upon this, they aren't aware of the unintended FUD by associating the much much safer LiFepo4 with Lico02 Cobalt / Lipo's.

How can an amateur like me recognize if a battery is the safer LiFePo4 (Iron) or the more risky LiCo02 (Cobalt)? Is there any way to detect it?

[SunEagle]

No worries about me my friend. I charge all my LiPo's with a Icharger fine tuned for whatever pack I put in it. I just use the Astron as the source. Bu tfor a large format 12 volt AGM, I use the Astron.

I figured you would have it all in control but I wanted to let anyone else reading these threads that charging LiPo's can be exciting at the least opportune time. Having a fire proof charging container or bag is little in cost then replacing fire damaged workshop or home.

I use my Revolectrix Cellpro Multi 4 charger. I like the multiple battery selection and charging profiles already installed. I also use the bag which I place on some ceramic tiles to keep it off the floor. Can't be too safe.

[PNjunction]

...FWIW RC batteries are LiPo, not cobalt.

Nope - the common Lipo battery IS primarily Cobalt oxide. At least the cathode is. The anode is primarily a solid polymer. But the dangers of super-critical charge requirements are still there, hence the need for bags and fireplaces when charging. You still have a temperamental Li-Cobalt battery on your hands.

Unfortunately for readers who may stumble upon this, they aren't aware of the unintended FUD by associating the much much safer LiFepo4 with Lico02 Cobalt / Lipo's.

You can take a 4S nominal 12v Lifepo4 to up to about 15.2v although that is NOT recommended, but is a good recipe for non-dramatic lithium plating, which raises the internal resistance and lowers capacity. Take it to about 30v, and then unsavory reactions will start occuring. This is very unlike Cobalt oxide, which a mere few 10ths of a volt of overcharge turns into a critical fire safety concern. Take a lead-acid flooded to 30v, and you'll get plenty of spewing battery acid and hydrogen, so yep - all batteries are a safety concern when abused.

The good news is that I've started the regimen of massively extended and unnecessary float voltages on my little Lifepo4 26650's. If / when I eat my hat, you'll be the first to know!

[Sunking]

And those LiPo's will eventually over heat and flame. They don't like physical abuse or over charging. The charging bag is a real cheap investment to keep the fire from spreading. I have seen them go off in a metal ammo can. Talk about letting the smoke out.

No worries about me my friend. I charge all my LiPo's with a Icharger fine tuned for whatever pack I put in it. I just use the Astron as the source. Bu tfor a large format 12 volt AGM, I use the Astron.

[SunEagle]

My Astron has both voltage and current limit controls on the face so I can charge about any battery I desire usin ga CV with current limit which is basically what any Lithium uses. FWIW RC batteries are LiPo, not cobalt.

And those LiPo's will eventually over heat and flame. They don't like physical abuse or over charging. The charging bag is a real cheap investment to keep the fire from spreading. I have seen them go off in a metal ammo can. Talk about letting the smoke out.