LFP battery chatter ( AKA LiFePo4, Lithium Iron Phosphate )

[reed cundiff]

PNJunction

Always enjoy your writings and appreciate your information.

Notice that Solar Panel Forum is getting friendlier to LFP and Northern AZ is getting chllier.

We generally get down 30 to 40% DOD (of 3.4 V per cell float) by morning and are at float in mid-afternoon. We were obtaining 950 W today at noon which is fairly good rate for the flat panels on the 5th wheel at this time of year. Six inches of snow night before last and not much charging yesterday.

Reed and Elaine

[PNjunction]

There has been a lot of discussion on the Aussie forums about hand balancing and setting the individual cells at 3.4 V. This probably is all that is needed for a single battery of 4 cells. Apparently proper upper and lower balancing takes quite a while.

Yep it all depends on the method you use to achieve that. What is often overlooked is that the ONLY thing you are "balancing" is the exposure to voltage that each cell receives, and not necessarily an equality of capacity.

For example, I have two batteries - a 20ah GBS 12v, and a 40ah GBS 12v setup. In the interest of science, I removed one cell from the 40ah battery, and replaced it with a cell from the 20ah battery. With my hobby charger, I was easily able to "top balance" this frankenstein setup. Of course, that 40ah battery is now limited to the smallest in the bunch - effectively it is only a 20ah battery in operation, limited to that small cell married into the battery on a goof. But boy did my charger do a GREAT job of top balancing that freak setup. So all I have accomplished is limit the exposure of voltage to each cell, yet all the while I have wasted the capacity overall.

There are two ways to look at lifepo4 in operation:

Use it like a lead-acid, and base your discharge from 100% SOC down to some comfortable level of discharge.

OR, ideally, START from a 50% DOD as the norm, and *expand* from there.

Why?

The following is total propeller-head material - so beware.

With lifepo4, we want to control unwanted parasitic reactions. Higher voltages averaged over time aggravate lithium-plating. Low voltages over time aggravate the battery's desire to eat away at the copper electrode, usually accompanied by gassing during the process leading to swelling. Applying a full-current charge at this point is a secondary point of damage.

So ideally one wants to charge / discharge their battery to 50% DOD, put it in a cool environment, and never use it.

So again, the further away from the high and low voltage points in operation the better. But is that practical for your application? Would you be comfortable running from say 20-70% DOD all the time, leaving you with little autonomy? Or would it be better to run from 100 - 50% DOD? If battery life is your sole concern, then the former is desirable, but may paint you into a corner when it stays dark longer than you expect. Still, that is the ideal however impractical that may be. It all depends on your application, and monitoring ability.

The major point here is that if one spends a lot of time balancing unnecessarily at high voltages, you are just exposing and aggravating the high-voltage lithium plating effect, desite the warm fuzzy feeling about achieving a near-perfect *voltage* balance. In a sub-c application like ours, balancing on each cycle is like brushing your teeth 100 times a day. Great for your smile, but no so much for your tooth enamel. (bad analogy, as it is more like the more you brush/balance, the more plaque/plating you develop)

I think as engineers we tend to over-think the process. Ideally, balance the battery with whatever method you feel comfortable. Then choose how you intend to run the battery - as a lead-acid from fully charged down to some arbitrary dod cutoff, or as an expansion from the middle 50% DOD point, or as a "bottom balanced" battery better suited to EV drag racing. Battery life overall will depend on your application. Whether it makes practical or financial sense is up to you.

That's the problem with the freedom that lifepo4 presents. We are all looking for the one and only ideal solution when in fact there are many depending on your needs. It sure is nice having that choice though!

[PNjunction]

Once balanced, you could merely disable that feature, and run conservatively utilizing only the HVD and LVD.

I guess I should be a little more clear on that. The HVD and LVD are there as safety backups, and are not being actively used to control the charge / discharge process - that is left to your controller and monitoring to handle under normal circumstances. LVD and HVD are separate functions for times when things go awry. So while your controller has a lower and upper voltage boundary, these are backups for extreme circumstances.

So with proper monitoring and engineering the capacity you truly need, you shouldn't be hitting these lvd and hvd events, but they are there just in case.

[reed cundiff]

Our system came with a fairly good BMS designed for EVs. We were running the air conditioner (Dometic) and it was drawing around 1800 W which would be about C/5 rate (though I think we were getting about 1 kW input from solar). I watched the BMS monitor as it busily demonstrated balancing the 4 cells in each of the 4 batteries in succession. Have done the same when running the micro-wave. Well, yes indeed there are some very slow days while boondocking.

There has been a lot of discussion on the Aussie forums about hand balancing and setting the individual cells at 3.4 V. This probably is all that is needed for a single battery of 4 cells. Apparently proper upper and lower balancing takes quite a while.

Hopefully, there will be BMS's designed for the much lower C rates of solar systems, particularly the smaller systems we have on RVs.
Reed and Elaine

[Living Large]

Note that a bms doesn't always mean you *have* to have balancing activated. Once balanced, you could merely disable that feature, and run conservatively utilizing only the HVD and LVD. As a "Sub-C" application like housebank solar setups, once *quality* cells are balanced initially they stay that way unless there are outside conditions (high-resistance interconnects etc) forcing an unbalance.

The miniBMS I was looking at appeared to have individual boards at each battery, for shunting, and then the simple series loop to allow for triggering LVD or HVD via a control board. I assume that setup would not allow for the disabling you mention, unless there is a jumper or switch at each board.

I came across "Housepower BMS" at some point. If I am not mistaken, that is the one that has modules that can handle 4 batteries. Possibly in the past, they had one that could handle 12 (what I would want), but it was discontinued for a reason I can't recall. Maybe I am thinking of a different brand. You run wires from this module to the batteries, instead of having individual boards located at each cell.

I guess with either design, the function you mention could be easily implemented, but it isn't something I would have looked for. Excellent point to consider - thanks! Beats the heck out of disconnecting wires. Based on what I have read and seen here, I wouldn't need shunting under normal circumstances. I could turn it on every X months for a period of time, and monitor the cells to confirm progress.

[PNjunction]

Personally, if I was going the BMS route I'd probably choose the HousePower BMS from CleanPowerAuto LLC. Many options out there.

Note that a bms doesn't always mean you *have* to have balancing activated. Once balanced, you could merely disable that feature, and run conservatively utilizing only the HVD and LVD. As a "Sub-C" application like housebank solar setups, once *quality* cells are balanced initially they stay that way unless there are outside conditions (high-resistance interconnects etc) forcing an unbalance.

I think you are going into this with eyes wide open, so that is a very good sign.

[Living Large]

Also, when paralleling is necessary to get the capacity you need, with LFP you are probably best off with a ladder interconnection while with FLA a ladder connection should be avoided.

Yeah, I did pick up on that. I seem to recall one reason is you would need a rats nest for a BMS for your parallel strings (a shunt for every single cell vs. one for each voltage building block). I'm not sure there was another reason stated.

[Living Large]

Yeah, a cell IS a battery, but I think you get the point. Ideally, simplify and get the least amount of cells in parallel necessary to do the job.

Yep. I am interested in what will happen when I ask the seller of 180Ah cells about a BMS. I got a quote on shipping, and the rep offered to discuss BMS. I need to take him up on that. There will be 3 cells in parallel for each 540 Ah "battery", and hopefully a standard BMS will shunt that. It probably will not be a problem. He also mentioned his company has a prepackaged solution that he believes would be better. I just need to jump in the water. Thanks for your comments.

[inetdog]

Ideally, simplify and get the least amount of cells in parallel necessary to do the job.

Also, when paralleling is necessary to get the capacity you need, with LFP you are probably best off with a ladder interconnection while with FLA a ladder connection should be avoided.

[PNjunction]

so I'd ask again, is there less of a concern with LFP? If I could parallel three 200 Ah CALB batteries (whereas no more than two is advised with FLA), I'd be happier than a pig in slop.

With LFP, because we are dealing with individual cells, and not batteries per se (that is until we interconnect many of them together), you parallel them to get your capacity first, and then series connect them to get your voltage.

With FLA, one is usually dealing with entire batteries, and not individual cells. That is, they are already pre-manufactured of several cells in a series connected string internally, and you can't get access to the individual cells. There are those however, who can buy 2v fla batteries, and construct anything they like.

Still, the usual recommendation when dealing with pre-constructed FLA *batteries* is not to parallel many of them together.

With LFP, you may have no choice because you are dealing with constructing individual cells into a battery of your own design.

Yeah, a cell IS a battery, but I think you get the point. Ideally, simplify and get the least amount of cells in parallel necessary to do the job.

[Living Large]

The lower the RI of any battery chemistry the better.

NiCd has the lowest Ri of any battery chemistry and very flat discharge curves and thus why military and NASA use them in rockets, missiles, torpedoes, radios or other device that need enormous amounts of current. Down side to NiCd is energy density of around 20 to 30 wh/Kg

Lithium, well most lithium batteries are second lowest Ri. Lithium has two huge advantages. 1. Energy density from 90 to 300 wh/Kg, highest of all batteries. 2 The Ri starts low at 100% SOC, and decreases as charge goes down to about 30% Soc then starts to rise again. That is a huge advantage and the only battery chemistry that does that.

NiMh is third and like NiCd has low energy density.

Pb ranks 4th and has some unwanted characteristics. Most annoying is they start fairly low Ri at 100% SOC and increases as the battery discharges. Couple that fact with a fairly steep discharge curve cause some headaches. Example if you have a constant power load device you may start with say 10 amps of current at full charge with .2 volts sag, then when you get to say 50% DOD the current increases to 11 amps and with higher resistance voltage sag increases two fold to 6%. Literally a snow ball rolling down hill effect.

Good summary - thanks again.

[Sunking]

If I interpreted this right, the low Ri is a plus both initially and during use. Thanks.

The lower the RI of any battery chemistry the better.

NiCd has the lowest Ri of any battery chemistry and very flat discharge curves and thus why military and NASA use them in rockets, missiles, torpedoes, radios or other device that need enormous amounts of current. Down side to NiCd is energy density of around 20 to 30 wh/Kg

Lithium, well most lithium batteries are second lowest Ri. Lithium has two huge advantages. 1. Energy density from 90 to 300 wh/Kg, highest of all batteries. 2 The Ri starts low at 100% SOC, and decreases as charge goes down to about 30% Soc then starts to rise again. That is a huge advantage and the only battery chemistry that does that.

NiMh is third and like NiCd has low energy density.

Pb ranks 4th and has some unwanted characteristics. Most annoying is they start fairly low Ri at 100% SOC and increases as the battery discharges. Couple that fact with a fairly steep discharge curve cause some headaches. Example if you have a constant power load device you may start with say 10 amps of current at full charge with .2 volts sag, then when you get to say 50% DOD the current increases to 11 amps and with higher resistance voltage sag increases two fold to 6%. Literally a snow ball rolling down hill effect.

[inetdog]

If I interpreted this right, the low Ri is a plus both initially and during use. Thanks.

Yes. It also helps if the Ri is relatively uniform across the batteries.

[Living Large]

The current moving between two batteries with a different SOC is directly proportional to the voltage difference and inversely proportional to 2 x Ri, the internal resistance of each battery.
So even though the LiFePO₄ may have a low Ri, the lower voltage difference helps keep the parallel batteries from sharing damaging current when first connected.

If I interpreted this right, the low Ri is a plus both initially and during use. Thanks.

[inetdog]

1.

Lithium Iron Phosphate , I thought the normal abbreviation was LiPo.

Lithium Iron Phosphate is most unambiguously abbreviated LiFePo4, shorter either LiFe (rare) or LFP (as in this thread title.)

LiPo stands for Lithium Polymer and refers more to the cell packaging than to the basic cell chemistry.

Lithium Phosphorus (which would just be LiP, not LiPo) is not a valid cell chemistry.

2. One reason that paralleling LiFePO₄ cells directly (ladder or not) is less problematic than doing same with FLA is the flatness of their voltage versus SOC curve in the useful region. The current moving between two batteries with a different SOC is directly proportional to the voltage difference and inversely proportional to 2 x Ri, the internal resistance of each battery.
So even though the LiFePO₄ may have a low Ri, the lower voltage difference helps keep the parallel batteries from sharing damaging current when first connected.