lifePo4 State of charge

[T1 Terry]

Originally Posted by s/v Jedi

You are saying that a bigger capacity cell is build up from the smaller capacity cells that are paralleled inside the housing? No, that is not the case. You can see how prismatic cells are manufactured in plenty videos on Youtube.

It doesn’t matter how many words, how many years… it is inherently better to have only cells in series, which is why all high end systems have exactly that.

You clearly understand how the plates in a Prismatic LFP and LYP cell are joined to the terminal block, you know these connections can not be accessed, yet you still hold the same line of belief that your are right because others do the same thing ....... Not much I can say is there .......

T1 Terry

[s/v Jedi]

Originally Posted by T1 Terry

You clearly understand how the plates in a Prismatic LFP and LYP cell are joined to the terminal block, you know these connections can not be accessed, yet you still hold the same line of belief that your are right because others do the same thing ....... Not much I can say is there .......

T1 Terry

So it’s plates. Plates are inside cells and are very different from cells connected in parallel, for starters because they are in the same electrolyte and thus at equal charge level, while cells connected in parallel can be at different charge levels. It doesn’t matter how you twist it, this is how it is.

Before lithium we got the big gel cells from Sonnenschein. You can still get these, 2V cells up to 3,000Ah (http://www.sonnenschein.org/PDF%20fi...0technical.pdf) before that the flooded cells like we used to have. That is how high end systems were built and today with lifepo4 you can get cells easily up to 1,000Ah but they even exist up to 10,000Ah. If you need 12V you put 4 of those in series and you’re done.

For redundancy you create two batteries. You can parallel those, as long as following the guidelines I listed above: each it’s own BMS (good for redundancy as well) and each it’s own battery monitor to determine SOC.

The only reason for connecting cells in parallel is budget restrictions. Cheap cells only come in small size. BMS’s and battery monitors cost money. But don’t try to make it appear as a better solution from a technical standpoint.

[T1 Terry]

Originally Posted by s/v Jedi

So it’s plates. Plates are inside cells and are very different from cells connected in parallel, for starters because they are in the same electrolyte and thus at equal charge level, while cells connected in parallel can be at different charge levels. It doesn’t matter how you twist it, this is how it is.

Before lithium we got the big gel cells from Sonnenschein. You can still get these, 2V cells up to 3,000Ah (http://www.sonnenschein.org/PDF%20fi...0technical.pdf) before that the flooded cells like we used to have. That is how high end systems were built and today with lifepo4 you can get cells easily up to 1,000Ah but they even exist up to 10,000Ah. If you need 12V you put 4 of those in series and you’re done.

For redundancy you create two batteries. You can parallel those, as long as following the guidelines I listed above: each it’s own BMS (good for redundancy as well) and each it’s own battery monitor to determine SOC.

The only reason for connecting cells in parallel is budget restrictions. Cheap cells only come in small size. BMS’s and battery monitors cost money. But don’t try to make it appear as a better solution from a technical standpoint.

Comparing what worked with different chemistry cells in the past with LFP or LYP cells just doesn't work, the whole interactions inside the cells is completely different.

Sorry, you lost me about the shared electrolyte ..... it's not like there is a concentration change in the lithium ion content between different levels of SOC, we aren't talking about some measure of specific gravity for the LFP electrolyte like there is for lead acid electrolyte .....

My experience is what I'm sharing here, 10yrs plus and a lot of systems that are still operating without problems. The problem systems have all been where the battery was built using single large capacity cells where multiple smaller capacity cells in parallel built to provide the same capacity do not suffer the same issues. 3 x 100Ah cells in parallel compared to single 300Ah cells in series, all Winston LYP cells so not a product quality issue, just the effect of different tolerance points on the mass production acceptability scale produces different capacities and different internal resistances.

If multiple cells are in parallel, some with a higher capacity that others, some with a different internal resistance as others, it balances out, this is the advantage of multiple cells in parallel.
These differences willresults in the voltage balance between any two cells in series varying to a high degree and this alters between charging and discharging. All active cell balancing equipment rely on cell voltage and this is what the balance from one cell to the next is, moving capacity from the high voltage cell to the low voltage cell. A cell that drops it's voltage lower than another cell in the series string while discharging, will constantly be topped up by the active BMS, even if the SOC was the same before the active balancer got involved, yet the same cell will have a higher voltage when charging causing the BMS to try and move capacity back out of the cell to the other cells. This can even show up as an issue between either charging or discharging and rest voltage. The cause is simply a different internal resistance, nothing wrong with the cell but that is not the way it looks when monitoring the cell voltages.

The objective is to build a hassle free storage battery and this is the better way to do this from our experience.

T1 Terry

[ccannan]

"with lifepo4 the SOC can be very different for the same voltage."

How can that be true? I know the curve is very flat but surely a precise voltage measurement must equal a specific SOC.

Also, I thought the mantra was parallel first, then series. Jedi, you seem to be saying that isn't the best method.

I've got my charging regime such that I never charge higher than 13.9v, and the battery has never been lower than 13.2v.

[ccannan]

Every lifepo4 battery must have it’s own battery monitor (it’s own shunt).

When you say monitor are you saying a coulomb counter or monitor that looks at the voltage of each cell?

[goboatingnow]

Prismatics don’t really have a common electrolyte

They are effectively in parallel internally

I agree “ battery management systems “ aren’t needed but a battery monitoring system is.

Lithium “ needs “ HVC “ and “LVC “ systems because either of these are detrimental to lithium’s.

After that everything else is a nice to have.

Cell balance is only of relevance if you are trying to extract every last piece of storage.

In practice on fractional C charge applications and low C discharge Li tech usually converges rather then diverges. So after an initial manual cell balance, this aspect can be ignored or performed manually occasionally

soc using voltage is merely an estimation as is columb counting ( which is essentially the same as Ah counting ). At least li has a Peukert close to 1 so the estimation is more accurate then lead acid

In a system where the battery is constantly under some load voltage based Soc can be highly misleading.

Reasonable Soc can be derived from amp counting reset by full charge triggers.

[ccannan]

Please explain:

Reasonable Soc can be derived from amp counting reset by full charge triggers.

[s/v Jedi]

Originally Posted by ccannan

"with lifepo4 the SOC can be very different for the same voltage."

How can that be true? I know the curve is very flat but surely a precise voltage measurement must equal a specific SOC.

Also, I thought the mantra was parallel first, then series. Jedi, you seem to be saying that isn't the best method.

I've got my charging regime such that I never charge higher than 13.9v, and the battery has never been lower than 13.2v.

I am talking about lifepo4 cells connected in parallel. They will always have a different internal resistance and most probably also different resistance on connections made to their terminals. This, by laws of nature, means that one cell is charged more than another.
Next, when charging stops (sun goes down) let’s assume the cells are in rest. The voltage drops and the highest cell starts discharging into the lowest cell. In theory. In practice it turns out that this balancing is minute and will not keep up with the unequal charging. Also, there is no BMS or balancer that can fix this, you need to take the pack apart to do that.

If you create two batteries of four cells in series instead of one battery that has a 2p4s configuration, then every individual cell is monitored and can be balanced without taking anything apart. This is the technically superior setup. It also costs more so it’s not the most economical setup. But it provides a much higher redundancy as well as no-break availability. From a technical standpoint the advantages are impossible to ignore.

[s/v Jedi]

Originally Posted by ccannan

Every lifepo4 battery must have it’s own battery monitor (it’s own shunt).

When you say monitor are you saying a coulomb counter or monitor that looks at the voltage of each cell?

Both, but there’s a trick that I use. First thing is to buy a cell monitor which is between $10-$30 on Amazon, Ebay etc. I see them for $7 on AliExpress. Next you wire a harness to every battery that you can connect this cell monitor to. Now you can manually check a battery without having to measure each cell with a multimeter. I’m afraid that even though I have this setup, I still use my high end multimeter because it is much more accurate.

Next step is the battery monitor. Use a Victron BMV712 smart monitor. The trick comes with wiring the shunt. Follow manufacturer instructions for measuring midpoint voltage. This requires one extra wire from the shunt to the battery, connecting it to the positive terminal of cell2 for a 4S, cell 4 for an 8S etc. setup. Now you can see the voltage difference between the lowest half of the cells and the highest. If balance is slipping, you will notice that by a higher differential on the monitor and you can set an alarm as well.

Apart from periodic monitoring with the cell monitor, you can connect it to a battery that the monitor shows an increased differential for that midpoint voltage.

As with everything around each battery, keep a logbook for every routine inspection (date, time, SOC, temperature and voltage of each cell) and start adding normal observed midpoint range during cycles. This will show trends or out of order situations immediately.

[ccannan]

Originally Posted by s/v Jedi

I am talking about lifepo4 cells connected in parallel. They will always have a different internal resistance and most probably also different resistance on connections made to their terminals. This, by laws of nature, means that one cell is charged more than another.
Next, when charging stops (sun goes down) let’s assume the cells are in rest. The voltage drops and the highest cell starts discharging into the lowest cell. In theory. In practice it turns out that this balancing is minute and will not keep up with the unequal charging. Also, there is no BMS or balancer that can fix this, you need to take the pack apart to do that.

If you create two batteries of four cells in series instead of one battery that has a 2p4s configuration, then every individual cell is monitored and can be balanced without taking anything apart. This is the technically superior setup. It also costs more so it’s not the most economical setup. But it provides a much higher redundancy as well as no-break availability. From a technical standpoint the advantages are impossible to ignore.

Thanks for your explanation. Could you also address the other question I had about how you define a monitor, as merely a method of checking voltage of each cell or a coulomb or Ah counter.

[s/v Jedi]

Originally Posted by ccannan

Please explain:

Reasonable Soc can be derived from amp counting reset by full charge triggers.

Not sure who wrote that, but here is my take on the Victron Smart BMV: it is perfect for lifepo4 but only when configured correctly. Study the manual especially where it comes to “charged voltage” and their recommended settings. The SOC reading must revert to 100% every time your charger, solar controller etc. finish charge (switch to float, i.e. go to a lower voltage). When you don’t do that, SOC value errors accumulate over time making the reading not accurate enough.

This means that when you set your charger to not charge the battery to the full 100% (to increase lifespan) that the monitor shows 100% at your chosen maximum, even when that is only 95% SOC.

When you can live with that, it performs perfectly. It also has something very useful: a programmable relay. You can program the relay to switch on at 30% SOC and off at 70% SOC and use that as an on/off signal for solar controllers etc. The Victron controllers have a compatible input. You can enable that for use as a “storage mode”, i.e. when you don’t use the boat.

[s/v Jedi]

Originally Posted by goboatingnow

Prismatics don’t really have a common electrolyte

They are effectively in parallel internally

I agree “ battery management systems “ aren’t needed but a battery monitoring system is.

Lithium “ needs “ HVC “ and “LVC “ systems because either of these are detrimental to lithium’s.

After that everything else is a nice to have.

Cell balance is only of relevance if you are trying to extract every last piece of storage.

In practice on fractional C charge applications and low C discharge Li tech usually converges rather then diverges. So after an initial manual cell balance, this aspect can be ignored or performed manually occasionally

soc using voltage is merely an estimation as is columb counting ( which is essentially the same as Ah counting ). At least li has a Peukert close to 1 so the estimation is more accurate then lead acid

In a system where the battery is constantly under some load voltage based Soc can be highly misleading.

Reasonable Soc can be derived from amp counting reset by full charge triggers.

No, they are not, you are confusing prismatic cells with pouch cells. Here is a video showing the manufacture of prismatic cells: https://youtu.be/peeR_ZiXCc8
You will see that after the plates are stacked and tabs welded, the stacks are inserted into the housing and then the electrolyte is added. There are no compartments and there is only one single safety overpressure valve.

Now I am sure you will reason that the SOC between the plates in the stack are not all equal and somehow better in smaller cells and that wiring or busbars between cells connected in parallel are better than the welded stacks of plates in prismatic cells but of course you would be wrong again: a single prismatic cell will always outperform two cells of half the capacity connected in parallel. The welded plates inside a cell are always better than the bolts, nuts, busbars and cables you use to connect individual cells in parallel.

[T1 Terry]

Originally Posted by ccannan

"with lifepo4 the SOC can be very different for the same voltage."

How can that be true? I know the curve is very flat but surely a precise voltage measurement must equal a specific SOC.

Also, I thought the mantra was parallel first, then series. Jedi, you seem to be saying that isn't the best method.

I've got my charging regime such that I never charge higher than 13.9v, and the battery has never been lower than 13.2v.

I tried to explain about the difference in voltage between charging and discharging in a previous post, this isn't the most difference you will see but is the minimum. Check the voltage curve on a lithium cell/battery and see how much 0.05v changes the state of charge at cell level on the graph, that is the difference between it discharging and at rest, 0.10v is the difference between discharging and charging.
As an example, look of the left hand graph, this is for a 200Ah battery so 0.5CA is a 100 amp load, but the graph is roughly the same as you go up or down in cell capacity, as long as you keep in mind that the 0.5CA means 50% of the advertised capacity as the load.
That top 0.5CA line has the 0.05v drop included, now add 0.1v to represent the difference between charging rather than discharging, the line is now at 3.4v ...... can you see why voltage along is no indicator of state of charge?

As far as the stopping at 13.9v, depends where it is measured and if still charging or rested voltage ..... To get an accurate sense of just what voltage the end of charge needs to be, watch the current still going in at 13.9v, then try 14v, if the current climbs and stays up there for a while, the charging voltage was too low, if it is only a very short burst of current and then the 14v is held steady with roughly the same current flow as it was at 13.9v, then the battery was actually fully charged.
I set the end of bulk charging to 14v, absorption charging to 13.9v for 30 mins, then float at between 13.7v and 13.8v, 0.1v differential before charging resumes, sometime built in but actually adjustable with the Victron range of chargers.

T1 Terry

[T1 Terry]

Originally Posted by s/v Jedi

No, they are not, you are confusing prismatic cells with pouch cells. Here is a video showing the manufacture of prismatic cells: https://youtu.be/peeR_ZiXCc8
You will see that after the plates are stacked and tabs welded, the stacks are inserted into the housing and then the electrolyte is added. There are no compartments and there is only one single safety overpressure valve.

Now I am sure you will reason that the SOC between the plates in the stack are not all equal and somehow better in smaller cells and that wiring or busbars between cells connected in parallel are better than the welded stacks of plates in prismatic cells but of course you would be wrong again: a single prismatic cell will always outperform two cells of half the capacity connected in parallel. The welded plates inside a cell are always better than the bolts, nuts, busbars and cables you use to connect individual cells in parallel.

Ummm..... clearly you have never seen inside a Winston prismatic cell. In you video, they are making very small capacity cells, but at the 5:30 min mark, you can see these connected in parallel/series to build their batteries ... what you say shouldn't be done.

If you want to see a photo of just what the inside of a quality Winston LYP cell looks like, let me know and I'll post a photo of one with the top off so you can see just how the plates are connected to the terminal plate ....

T1 Terry

[dreuge]

Originally Posted by Davidhoy

What type of batteries do you have? DIY or integrated? What BMS are you using, and does it have external communications?

The reason for my questions is that I'm in a very similar position, and just yesterday created a prototype device that connects via Bluetooth to my three (soon to be six) JDB/Overkill Solar BMS boards, collects the data, and outputs NMEA 2000 battery status messages which can be displayed by any MFD device on my network. Not sure if it would work with your batteries though. Eventually I'll probably open source the project, and the hardware is available in kit form online too.

-David

This is very interesting. I also use the JBD-SP04S020 BMS. I just purchased a new one with Bluetooth and RS485 communications. I have been using the standard iOS app for Bluetooth communications and I am developing the RS485 to rasp pi for remote monitoring. As I'm learning along the way, I am looking forward to your open source project.