Float of LFP

[wholybee]

Originally Posted by barcoMeCasa

Isn't that going to wrong way based on the data?

Assuming hitting the 13.8v up the knee a bit as a reliable data point for SOC (seems to be) if the smartshunt SOC isn't reset it gets lower every day. It thinks less energy is going back in and/or more energy is going out than there really is.
Will need to have a look but istm even if your generic figures are close to the mark the victron still gets it wrong and low set to 100% & peukert at 1.
Bit unsure about Peukert, discharge setting it above 1 will likely make things worse but google/google scholar didn't come up with much useful info regarding peukert when charging. Setting efficiency to less than 100% will make the error worse as well afaics.
Testing required to confirm this.
Haven't found a way to tune this out so if the daily incremental in the victron SOC can be manually tuned out reliably a bit then that would be a nice thing

I do know that manufactures recommend 99% and 1.03, and it is very close for me. I think the Victron with those numbers does a better job than my BMS does. It is really close, even after a few weeks of not resetting at 100% it's maybe 5% off when I finally do charge to 100%
I'm not sure what is causing your error, but certainly you don't have greater than 100% efficiency. And if the install is correct, the Victron quite accurate.

[barcoMeCasa]

Originally Posted by wholybee

I do know that manufactures recommend 99% and 1.03, and it is very close for me. I think the Victron with those numbers does a better job than my BMS does. It is really close, even after a few weeks of not resetting at 100% it's maybe 5% off when I finally do charge to 100%
I'm not sure what is causing your error, but certainly you don't have greater than 100% efficiency. And if the install is correct, the Victron quite accurate.

Tnx.

Turns out my known to be lacking understanding of Peukerts is even less than thought

Never really dug deep into where the current draw helps and where it doesn't

Smartgauge site has a xls calc down near the bottom, SmartGauge Electronics - Peukert's Equation - what it means to you

So looks like if I average about 10A on a 300Ah batt a peukert of 1.05 will actually help the batt last a bit longer >


I know from logging that the suggested smartshunt values of 99% & 1.05 had the soc drift down each day but will have another hit at it, tweaking each day to see if it's possible for better accuracy on this bank.

[s/v Jedi]

Originally Posted by barcoMeCasa

Tnx.

Turns out my known to be lacking understanding of Peukerts is even less than thought

Never really dug deep into where the current draw helps and where it doesn't

Smartgauge site has a xls calc down near the bottom, SmartGauge Electronics - Peukert's Equation - what it means to you

So looks like if I average about 10A on a 300Ah batt a peukert of 1.05 will actually help the batt last a bit longer >


I know from logging that the suggested smartshunt values of 99% & 1.05 had the soc drift down each day but will have another hit at it, tweaking each day to see if it's possible for better accuracy on this bank.

Now that senses are back and logic is winning, maybe I can explain this better:

Once the settings are precise for reasonable indication, all that is needed is an accurate reference and a reset of the monitor based on that reference. All accuracy in the world is based on a reference.

So for lifepo4 the reference of choice is a fully charged battery and we determine this by experimentation with charge voltage for our specific battery and charging conditions. We check that we get into the upper knee with the voltage of our choice.

So let’s say we determine that for us this voltage is 13.8% so we set this as our absorption voltage and plotting the graph we see that this brings us to 98.5% SOC.

This will be our reference. By choosing this voltage, we have decided to charge up to 98.5% and thus we never use the upper 1.5% (or 1%, or 0.5%, whatever voltage and absorption time you choose)

Now weuse this reference to automatically calibrate our BMV or SmartShunt every time we do this full charge. For some this happens daily, for others now and then and some need to run an engine with alternator or a generator with AC charger to complete a full charge, but periodically we all do this.

So with absorption voltage set at 13.8, in the BMV settings, we set “charged voltage” at 13.6. This means that when the voltage goes higher than 13.6V, the monitor will reset to 100% WHEN CHARGING STOPS. As soon as current flows out of the battery instead of in, the monitor is set to 100% charged.

If your charger successfully completes the charge, then this works perfectly and 100% now means 100% of your usable capacity because you never charge it further than this.

Now the problem some experience: when the voltage goes above 13.6 but you stop charging before completing it. Now the monitor will reset to 100% but you aborted the charge.

This can happen when you kill the engine or generator, i.e. on purpose. But this can also happen accidentally: when you charge with solar and you barely make it to full charge with the daily cycles, then after you reach 13.6V a large load can switch on and consume more than the solar can deliver, meaning the rest comes from the battery and the monitor resets to 100% as it sees the current reverse.

So to combat this problem, you can raise the “charged voltage” to be closer to the 13.8V absorption voltage. This reduces the time span that this problem can happen. But it can also lead to missed resets because voltage measurements can differ between charger and BMV.

To find a good solution, I did a lot of experiments with Victron protocols called DVCC and SmartNetwork. DVCC is done by a GX device like the Cerbo. SmartNetwork is done by the “Smart” Bluetooth versions of Victron gear. Both methods communicate voltage, current and temperature as measured by the BMV to other devices like MPPT controllers.

So now the MPPT controller gets it’s voltage from the BMV. This means they both use the same measurements and thus this allows us to bring the “charged voltage “ very, very close to the absorption voltage. I went up to 30mV and it still worked. This is 0.03V so about 10 times less than the regular recommendation from Victron (which doesn’t take DVCC in account).

SmartNetwork works just as precise. Make sure all the devices are in Bluetooth range of each other and setup the network. When done, the MPPT will show that it is linked to the BMV and receiving voltage, current and temperature from it.

So to recap, if you have a Cerbo GX and have setup DVCC or you have smart versions of the devices and setup SmartNetwork, then you can set your “charged voltage” very close to absorption voltage. If absorption voltage is set at 13.8V then try a “charged voltage” of 13.75V and see if it reliably resets to 100% when the charger switches to float.

Hope this is helpful

[T1 Terry]

Originally Posted by sailorboy1

Alright, lots of people writing like what they write are proven facts. But no source is given for a study that supports it. Let's remember that just because it is on the internet or you read it somewhere doesn't make it true unless there is test/study documentation that backs it up. Just being able to retype what you read doesn't cut it.

Unfortunately, the Junsi graphs and tests carried out over the last 12 yrs on various house batteries, was on my computer and on the hard drive storage at the workshop when it burnt to the ground.
There is another source of operating records contributed to a private forum with 125 members who have recorded all their build and 3 yrly service data over that same period and to date .... but it is a private forum with strict conditions the information recorded there is for use of the forum members only, not for public distribution, so, even if I wanted to write a 12 yr study paper on using LFP and LYP cells to build strong long lasting off grid battery systems, I can't use that information.

The fact I've moved on from LFP/LYP and even LTO lithium chemistries to sodium ion, due to the bad press any battery with lithium in it's name has received and the mass of misinformation and completely wrong information regarding LFP and LYP chemistry batteries, I'm going to put that 12 yrs hands on experience to rest so I can stop this it sure hasn't been easy and I've just had enough to be honest ... the last couple of PM's from the admin on these lithium subjects has just pushed me past my limits ..... it seems even free valuable information has to have sugar sprinkled on it so the sensitive aren't offended
You win I surrender

T1 Terry

[sailingharry]

Originally Posted by s/v Jedi

Now the problem some experience: when the voltage goes above 13.6 but you stop charging before completing it. Now the monitor will reset to 100% but you aborted the charge.

Jedi, I fully understand your discussion. My question is real-world impact. I'm not a Cerbo user, and my eventual LFP implementation will be someplace between "no BMS crap" and "The Jedi nirvana." I'm interested in being "good" but perfect doesn't bother me much.
If I set up as you had initially done, and reset the battery monitor to 100% when it is really some value less, how much less is that? If it's 5%, I really couldn't care. If it's 10%, I probably don't care. If it's 15%, that's serious. Really, all a battery monitor does is tell you about where you are, and at least in my current AGM world I am happy with the readouts as long as I hit 100% at least once a week. Sure, it's wrong -- but it doesn't impact me.
So, the question -- how big an issue was your "problem" before you made it 10 times smaller?

[T1 Terry]

Originally Posted by s/v Jedi

Now that senses are back and logic is winning, maybe I can explain this better:

Once the settings are precise for reasonable indication, all that is needed is an accurate reference and a reset of the monitor based on that reference. All accuracy in the world is based on a reference.

So for lifepo4 the reference of choice is a fully charged battery and we determine this by experimentation with charge voltage for our specific battery and charging conditions. We check that we get into the upper knee with the voltage of our choice.

So let’s say we determine that for us this voltage is 13.8% so we set this as our absorption voltage and plotting the graph we see that this brings us to 98.5% SOC.

This will be our reference. By choosing this voltage, we have decided to charge up to 98.5% and thus we never use the upper 1.5% (or 1%, or 0.5%, whatever voltage and absorption time you choose)

Now weuse this reference to automatically calibrate our BMV or SmartShunt every time we do this full charge. For some this happens daily, for others now and then and some need to run an engine with alternator or a generator with AC charger to complete a full charge, but periodically we all do this.

So with absorption voltage set at 13.8, in the BMV settings, we set “charged voltage” at 13.6. This means that when the voltage goes higher than 13.6V, the monitor will reset to 100% WHEN CHARGING STOPS. As soon as current flows out of the battery instead of in, the monitor is set to 100% charged.

If your charger successfully completes the charge, then this works perfectly and 100% now means 100% of your usable capacity because you never charge it further than this.

Now the problem some experience: when the voltage goes above 13.6 but you stop charging before completing it. Now the monitor will reset to 100% but you aborted the charge.

This can happen when you kill the engine or generator, i.e. on purpose. But this can also happen accidentally: when you charge with solar and you barely make it to full charge with the daily cycles, then after you reach 13.6V a large load can switch on and consume more than the solar can deliver, meaning the rest comes from the battery and the monitor resets to 100% as it sees the current reverse.

So to combat this problem, you can raise the “charged voltage” to be closer to the 13.8V absorption voltage. This reduces the time span that this problem can happen. But it can also lead to missed resets because voltage measurements can differ between charger and BMV.

To find a good solution, I did a lot of experiments with Victron protocols called DVCC and SmartNetwork. DVCC is done by a GX device like the Cerbo. SmartNetwork is done by the “Smart” Bluetooth versions of Victron gear. Both methods communicate voltage, current and temperature as measured by the BMV to other devices like MPPT controllers.

So now the MPPT controller gets it’s voltage from the BMV. This means they both use the same measurements and thus this allows us to bring the “charged voltage “ very, very close to the absorption voltage. I went up to 30mV and it still worked. This is 0.03V so about 10 times less than the regular recommendation from Victron (which doesn’t take DVCC in account).

SmartNetwork works just as precise. Make sure all the devices are in Bluetooth range of each other and setup the network. When done, the MPPT will show that it is linked to the BMV and receiving voltage, current and temperature from it.

So to recap, if you have a Cerbo GX and have setup DVCC or you have smart versions of the devices and setup SmartNetwork, then you can set your “charged voltage” very close to absorption voltage. If absorption voltage is set at 13.8V then try a “charged voltage” of 13.75V and see if it reliably resets to 100% when the charger switches to float.

Hope this is helpful

The system I've found that has worked well for the last 12 yrs on a lot of off grid systems, is using cell voltage as the over riding control for charging and discharging.
If the quality of the cells is good, then the advertised capacity can be used as the base line for determining where the 20% capacity warning alarm should be sounded, but the 100% charged needs to be a reliable value as well.

The battery capacity can be tested using the cell voltages to determine all the capacity has been extracted, set your own continuous peak load current (amps) for the 2 hr test and watch for any cell going under the 3v mark while under load.
This will tell you two thinks, is the battery delivery rate sufficient for the peak demands you need, if the voltage drops below the 3v in any cell while under load in less than the 2 hrs, either that cell has a problem, or if all the cells are hovering around the 3v mark, either the battery capacity isn't enough to supply demand, or the battery delivery isn't enough to supply demand. How close you get to the 2 hr mark will tell which one is the problem.

Now you know for definite what the genuine battery capacity is, you can set the battery ah meter (coulomb counter) so the % SOC is an actual figure you can rely on to be repeatable.

The next step is to determine the battery reaches 100% SOC on a regular basis so the meter resets its 100% SOC point, this point drifts and can really lead the system operator into trouble ..... if the reset happens before 100% SOC is actually reached, the available capacity is less so the whole SOC % is no longer accurate and the end of available capacity can come as a rather unpleasant surprise.
If the 100% reset is never reached, the meter will continually under estimate how much capacity you actually have resulting in alarms or non essential equipment being powered down when the battery is actually fine ... then the system is no longer trusted and all sorts of problems are imagined in a system that is actually working as it should. Sadly, this can lead to systems being damaged because the system operator starts to second guess parts of the equipment have failed and they start to either bypass them, or replace them for no good reason.

By using a good battery monitor, the 100% SOC can be reset automatically when all the required parameters are met ... and not before ....

By using a cell voltage over ride control system, a once a mth 100% reset can be also automatically be achieved while also top balancing the battery.

By using the "equalise" function in the charging system, set the equalise to occur every 30 days and to run for 10 mins after the equalise voltage has been reached.
Set the equalise voltage to the total of the number of cells in series x 3.5v. Sert the auto charge cut to 3.6v in any cell and the reset time to 10 mins. This will allow the active cell balancer to move excess capacity from the high voltage cell/s to the low voltage cell/s, then the charging starts again until either a cell reaches 3.6v, or the charge controller reaches the voltage that equals 3.5v x the number of cells in series ... eg 3.5 x 4 cell for a 12v battery = 14v, for an 8 cell 24v battery, 28v and so on ....
Now the controller will hold that voltage for 10 mins to allow the cells to saturate charge and the cell control monitor will stop any cell going over 3.6v.

At the end of this "equalisation" cycle, the battery should be close to 100% SOC, each cycle each mth will get the last bit completed.

Now, if the battery monitor is set to the equalisation voltage, the charge set to 10 amps or less and the time to 1 min, the 100% SOC will be close enough to accurate to be reliable for the sake of monitoring how well the battery stored energy is coping with the loads applied and if the charging is keeping up with demand ....

Now you can get on with more enjoyable things than battery watching

T1 Terry

[s/v Jedi]

Originally Posted by sailingharry

Jedi, I fully understand your discussion. My question is real-world impact. I'm not a Cerbo user, and my eventual LFP implementation will be someplace between "no BMS crap" and "The Jedi nirvana." I'm interested in being "good" but perfect doesn't bother me much.
If I set up as you had initially done, and reset the battery monitor to 100% when it is really some value less, how much less is that? If it's 5%, I really couldn't care. If it's 10%, I probably don't care. If it's 15%, that's serious. Really, all a battery monitor does is tell you about where you are, and at least in my current AGM world I am happy with the readouts as long as I hit 100% at least once a week. Sure, it's wrong -- but it doesn't impact me.
So, the question -- how big an issue was your "problem" before you made it 10 times smaller?

You only start resetting the battery monitor after observing how far you charge the battery. So if you start with an absorption voltage of 13.8V and an absorption time of 30 minutes and you find that this puts you back up to 98% SOC or more, only then do you configure the BMV to reset to 100% when the charge current stops after voltage did reach 13.75V

So you only give up 1-2% of capacity in exchange for not getting HVC disconnects by the BMS etc.

You don’t need a Cerbo for this… if you have a Smart BMV and Smart MPPT then you can enable the Bluetooth smart network to get the same accuracy. If you don’t have these smart devices then you can still try the 13.75V and simply check if the BMV resets to 100% when the charge switches to float. If it doesn’t then lower the “charged voltage” a bit more, like 13.7V and try again.


By the way, I am not at all a fan of BMS systems that can brew your coffee while charging batteries etc. I want it to be minimal function of protecting the battery.