LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

[goboatingnow]

Originally Posted by diugo

That is totally incorrect. Peukert applies to all battery chemistries. It applies perfectly to ZiC, NiFe, NiCd, NiMH, other Li-ion chemistries, and yes---even to vaulted LFPs. LiFePO4 is very good, but still far from perfect.

Peukert's law, presented by the German scientist W. Peukert in 1897, expresses the capacity of a lead–acid battery in terms of the rate at which it is discharged. As the rate increases, the battery's available capacity decreases.


Of course the law , which simply models the discharge curve can be used on Li. But part of the problem is we don't know the correlation between the LaW and actual Li in the field especially high C environments. The data is only being assembled. We don't even have a standard way of specifying Li capacity.

Peukerts is only one o f the issues.


Dave

[charlie p]

Dave,

I assume you were referring to my post. There is staying away from the knees and there is staying away from the knees. The knees are not discrete points, rather they are regions where dV/dSOC is large enough to be reliably measured. It depends a lot on where along the knees you want to put the charge and discharge cutoffs.

On the charge side, you have to get at least to the start of the knee, otherwise there is no way to know where you are. The lowest reliably detectable voltage rise will clearly be before 100% SOC. Similar situation at the low end. When the voltage starts to drop, you will be at low but not zero SOC.

As you suggest, there is evidence that narrowing the voltage range slightly is good for the batteries. Lets assume that is true. What we have then is simply reduced capacity in return for longer life. That means it depends entirely on the numbers. How much reduction for how much longer life. 30% reduction for 10% longer life would be a bad deal. 10% reduction for double the life would be a big win. I don't have these numbers. If you do, that would be quite interesting and I'm sure of interest to all LFP users.

Now, back to the cumulative amphour meter problem. I think it works fine in your scenario, it's just a question of how one defines 0% and 100% SOC. Lets say you have a 100 Ah rated battery but choose to stop charging at 95% and stop discharging at 5%. These numbers can be whatever one thinks is best, as long as they are close enough to the knees to be reliably detectable. Effectively we now have a 90 Ah battery. So tell the meter it's a 90 Ah battery, set the counter reset points accordingly and let it run. The 5 Ah above the top and below the bottom don't exist. The meter will reliably measure the %SOC of the EFFECTIVE capacity. I think I've got this right but let me know if I've overlooked something.

As far as discharging to zero being "of course, madness", remember that it will actually be the 5% or whatever zero, not physical zero. I agree that it might be awkward depending on ones setup, but it would not be done often. My understanding is that it is not harmful to LFP batteries as long as you don't go TOO low. On the same topic, is there definitive data on how cycle count affects capacity on these batteries? It's a big problem with cumulative meters on LA batts because you don't find out the capacity has dropped until the LVC alarm goes off when the meter says 30%.

Charlie

[goboatingnow]

Originally Posted by charlie p

Dave,

I assume you were referring to my post. There is staying away from the knees and there is staying away from the knees. The knees are not discrete points, rather they are regions where dV/dSOC is large enough to be reliably measured. It depends a lot on where along the knees you want to put the charge and discharge cutoffs.

On the charge side, you have to get at least to the start of the knee, otherwise there is no way to know where you are. The lowest reliably detectable voltage rise will clearly be before 100% SOC. Similar situation at the low end. When the voltage starts to drop, you will be at low but not zero SOC.

As you suggest, there is evidence that narrowing the voltage range slightly is good for the batteries. Lets assume that is true. What we have then is simply reduced capacity in return for longer life. That means it depends entirely on the numbers. How much reduction for how much longer life. 30% reduction for 10% longer life would be a bad deal. 10% reduction for double the life would be a big win. I don't have these numbers. If you do, that would be quite interesting and I'm sure of interest to all LFP users.

Now, back to the cumulative amphour meter problem. I think it works fine in your scenario, it's just a question of how one defines 0% and 100% SOC. Lets say you have a 100 Ah rated battery but choose to stop charging at 95% and stop discharging at 5%. These numbers can be whatever one thinks is best, as long as they are close enough to the knees to be reliably detectable. Effectively we now have a 90 Ah battery. So tell the meter it's a 90 Ah battery, set the counter reset points accordingly and let it run. The 5 Ah above the top and below the bottom don't exist. The meter will reliably measure the %SOC of the EFFECTIVE capacity. I think I've got this right but let me know if I've overlooked something.

As far as discharging to zero being "of course, madness", remember that it will actually be the 5% or whatever zero, not physical zero. I agree that it might be awkward depending on ones setup, but it would not be done often. My understanding is that it is not harmful to LFP batteries as long as you don't go TOO low. On the same topic, is there definitive data on how cycle count affects capacity on these batteries? It's a big problem with cumulative meters on LA batts because you don't find out the capacity has dropped until the LVC alarm goes off when the meter says 30%.

Charlie

Yes I agree , in general. The problem being what one determined is an acceptable knee voltage and how much in practice that's value varies. We can't design amp counters that require us to characterise the cells first.

So as you say we pick a knee voltage at just reset on that. To date I've no data on the variation of such voltage , and in some cases where a high C bank is being subjected to low C recharge my understanding is the knee comes on quickly. ( it does in my small cells )

I suppose all you can say , is the designed charge cutoff point is the reset point. Irrespective of actual SOC.

Li s are severely damaged by over discharge , so no one reccomends going anything below about 20% ( If detectable )

As I said Amp counters are fine for mom and pop idiot gauges. , I certainly would not use them to control anything.

Dave

[mcarling]

Originally Posted by charlie p

I assume you were referring to my post. There is staying away from the knees and there is staying away from the knees. The knees are not discrete points, rather they are regions where dV/dSOC is large enough to be reliably measured. It depends a lot on where along the knees you want to put the charge and discharge cutoffs.

Originally Posted by goboatingnow

Yes I agree , in general. The problem being what one determined is an acceptable knee voltage and how much in practice that's value varies. We can't design amp counters that require us to characterise the cells first.

So as you say we pick a knee voltage at just reset on that.

I don't think Charlie was suggesting that we choose a knee voltage. I think he was suggesting that we pick a threshold for dV/dSOC (the derivative of the voltage with respect to SOC i.e. the rate of change of the voltage as charging occurs i.e. the slope of the graph of Voltage against cumulative current). If so, I agree. I would use dV/dSOC rather than voltage as a threshold to identify the knees (just as we do when we look at charging graphs).

[daddle]

At-rest volts is the accurate way to assess state of charge as a condition. That is what all the technical papers say. And that is what people are saying here even if they don't realize it. The problem is that the at rest condition is at least a few minutes for a rough reading, and according to some manufacturers up to 24 hours for accuracy. Hardly convenient. And, yes, the measurement in the middle region is significantly less accurate due to the flatness of the voltage/SoC function there. Then to determine the state of charge as Amp-hours one must perform a controlled calibrated charge-discharge to construct a table or function.

A huge amount of research has been done on complex ways to measure SoC while in use. The complex methods can be accurate, but they are unwieldy in practice.

A bit of good news that the inaccuracies in SoC measurements tend to favor being safe in the vicinity of overcharge and undercharge. During discharge the readings will be lower than actual SoC, while during charge the readings higher. This amounts to reducing battery Amp-hour storage.

As for long life, both in calendar time and in charge-discharge cycles the research indicates that charging as little as possible is best. This is the opposite of what cruisers generally want. Seems it would be best that while the boat is relatively idle, in a marina, at anchor perhaps, or unattended, that the charge cutoff be set closer to 50%. Significantly undercharged batteries last significantly longer that fully charged batteries. That is why the new batteries arrive near 50% charge. Likewise it is best to only charge enough to meet the demand until the next charge. This is probably uncomfortable to the cruiser mindset where planning for battery energy is problematic. It would suit the aviation practice well as most aircraft are only fueled enough to safely reach the next fuel stop.

[T1 Terry]

We know 3.4v is fully charged, so 3.5v would indicate all cells in parallel string were fully charged/would accept no further charging, look at it what every way you like, they are as full as you are going to get. At this point 100% can be determined. the problem comes in for those that don't want to return to 100%, errors will creep in and he 100% figure can only be confirmed by a return to all cells being above 3.5v. The longest test I have carried out using the Plasmatronics PL20, a shunt and solid state relay control for the solar charging is 3 mths, at the end of this period, the 720 ah discharged match with 0% SOC and fully recharged, all cells above 3.5v matched with 99%. I could have dragged the cells down to 2.8v per cell and let zero be reset there but I chose not to at that time. To me, that says the 20% SOC would be a safe target for essential load shut down, but I would not use the 100% figure as the point to stop charging, that must be a cell voltage thing.

The Peukert’s factor seems to be misunderstood by a lot of people. It's Wh out v Wh in that determines the Peukert’s factor, not Ah out v Ah in. As we don't use Wh meters the Ah meter is close enough for what we are looking for, it's a guide, not a hard and fast set point, anyone who calculates capacity to 1% is asking for a failure. We don't need the engineers 100% safety margin to cover the arse, but 20% would be a sensible approach. If you know in a 12 hr period your system will use 30% of the battery capacity you best make sure the SOC meter reads better than 50% SOC before you leave the boat for a 12 hr period. That's what the SOC meter is useful for, a guide, battery voltage won't do it.

T1 Terry

[nimblemotors]

Originally Posted by goboatingnow

..
So this leaves is in the tricky position in real of possibly never getting a really full or empty reset. Amp counters are notoriously error prone where such conditions occur and in practice you have no way of cross checking them.
Dave

Huh? you see a "really full" when the voltage climbs, and "really empty" when it drops. You can detect the end points when the amp counting gets off.
i.e. you allow charge back in 10amps, but it goes "really full" at 8amps. There has been enough drift so you must reset. You now have 100% FULL, and can back off 10% from there. Same at the bottom. If you get too much reset, you must become more conservative on the counting.
Software has memory and can adapt, one of the great wonders of the 21st century.

I build all my own hardware and software, so I can't address what products do or don't do, in that sense I am of no help to anyone.

JackB

[diugo]

I need an accurate SoC "idiot gauge" for two reasons. Primarily, to ensure that my charger settings always keep my bank between 60-80%. I am hopeful that once the proper charge and float setpoints are methodically obtained, that the "60" figure can vary slightly according to my daily use, and still return to 80%.

Secondarily, if an extended cloudy period is forecast, I would likely want to bump the SoC to near 100% while the sun is still shining, to reduce the likelihood of running out of juice.

[diugo]

Originally Posted by T1 Terry

The Peukert’s factor seems to be misunderstood by a lot of people. It's Wh out v Wh in that determines the Peukert’s factor, not Ah out v Ah in. As we don't use Wh meters the Ah meter is close enough for what we are looking for

It's kinda silly to say that since Ah meters are used, there is no Peukert effect.

Ohm's Law says a 4-millohm battery internally wastes less than a watt at 15 amps, but a whopping 90 watts at a charge current of 150A. Thus a single 150Ah charge cycle only returns 144Ah.

It's not about one small 6Ah difference---it is the 6Ah every charge cycle that quickly adds up and renders an uncorrected Ah meter useless, even as a crude guide.

[T1 Terry]

Think you need to get some cells and move from the theory to the practical testing and try out you theory. My cells are discharged approx. 280Ah between normal solar recharges, sundown to sun up, they charge at what ever solar is available, up to 150 amp sometimes. In long periods of poor solar the batteries will drop close to zero capacity before the sun gets them back to full, I have put 720Ah back into the battery pack in a single day, it's a 720Ah battery pack, as i said in the previous post, the error after 3 mths was 1%. I put that down to errors between the shunt and the calibration of the shunt adaptor.
7.2Ah in 3 mths isn't a very big error, but it's still too big to rely on for a 100% recharge signal, trying to force an extra 7.2Ah into a full battery would destroy it.

T1 Terry

[deckofficer]

Terry,

Since this thread is approaching 1800 posts and my memory isn't that good, could you repost what cells and configuration you have? I know you were one of the first to make the change, but can't remember seeing any pictures of your housebank. I'm a visual type, I see a picture and can remember that.

Thanks.

[diugo]

Originally Posted by T1 Terry

Think you need to get some cells and move from the theory to the practical testing and try out you theory. My cells are discharged approx. 280Ah between normal solar recharges, sundown to sun up, they charge at what ever solar is available, up to 150 amp sometimes. In long periods of poor solar the batteries will drop close to zero capacity before the sun gets them back to full, I have put 720Ah back into the battery pack in a single day, it's a 720Ah battery pack, as i said in the previous post, the error after 3 mths was 1%. I put that down to errors between the shunt and the calibration of the shunt adaptor.

I have cells. From Balqon, remember? I tried the simple straight amp-hour counting thing for a week, and got a lot more than a 1% error at charging currents not even exceeding 20A. That's why I'm on such a crusade, since readers here will likely experience much higher currents.

Think you need to read the online FAQ for your PL20 controller. It clearly says the displayed SoC can exceed 100%. More to the point, the PL series will automatically reset any SoC over 100% to 100% at the beginning of every discharge cycle. So your "1% error after 3 mths" was likely just 1% during the last cycle of your supposed test.

[Lagoon4us]

Thinking some chargers will reset the fuel gauge when they kick in or top up, sure i read it in BMS guff some where......

[T1 Terry]

There are few different combinations and 2 different battery packs but I'll try and explain the ideas behind each lot of testing
This is the first pack, 360Ah @12v, made up of 90Ah cells, in total 3 boxes of them this size.
This set up is to give an example of size compared to 2 x 6V 125Ah Yuasa AGM on the left and a 105Ah Vision AGM on the right, 250Ah @12v of Yuasa AGM behind. The test was to see if connecting the cells and load across the end of the string would produce a difference in cell capacity across the pack after 3 mths of service, the answer is, no it didn't, all 16 cells had 90 Ah plus capacity at the end of the test.

[T1 Terry]

Originally Posted by diugo

I have cells. From Balqon, remember? I tried the simple straight amp-hour counting thing for a week, and got a lot more than a 1% error at charging currents not even exceeding 20A. That's why I'm on such a crusade, since readers here will likely experience much higher currents.

Think you need to read the online FAQ for your PL20 controller. It clearly says the displayed SoC can exceed 100%. More to the point, the PL series will automatically reset any SoC over 100% to 100% at the beginning of every discharge cycle. So your "1% error after 3 mths" was likely just 1% during the last cycle of your supposed test.

It will only reset the 100% at the end of an equalise charge. The reason for the over 100% reading is to allow for the creeping error that lead acid batteries introduce.
What type of Ah counter are you using?

T1 Terry