lifePo4 State of charge

[cutter3322]

Hello, I currently have 3 lifePo4 batteries connected in parallel and I would like to know the state of charge of each. What device do you suggest to me? Thank you

[team karst]

I also have 3 in parallel. I’m not aware of a way to measure SOC individually. I use a Victron to measure total SOC.

[cutter3322]

Example BMV-702. https://www.victronenergy.com/battery-monitors/bmv-702
?!?

[wholybee]

If they are in parallel and in good health, the SOC should be the (very close to) same for all of them.

Do they have a BMS you can connect to to check cell voltages etc? Usually the BMS will give an estimate of SOC.

[T1 Terry]

Originally Posted by wholybee

If they are in parallel and in good health, the SOC should be the (very close to) same for all of them.

Do they have a BMS you can connect to to check cell voltages etc? Usually the BMS will give an estimate of SOC.

Voltage across multiple batteries in parallel will be the same, but that has very little to do with state of charge. The internal resistance of an LFP cell is 0.05v from static or rested voltage .... in other words, the voltage must be 0.05v higher than the rested voltage for any current to flow in and 0.05v lower than the rested voltage to flow out. This means between charging and discharging there will be a minimum of 0.1v per cell. Put 4 cells in series and this adds up to 0.4v per 12v battery. The difference between a rested LFP cell at 100% SOC and 0% SOC is 0.4v .... a 4 cell in series fully balanced 100% SOC battery and a fully balanced 0% SOC battery is therefore 1.6v. Bring that 0.4v into the equation 1.6 / 0.4 = 25% .... and that relies on the 4 cells of each battery being fully balanced as far as state of charge goes, not just voltage ..... the chances are slim the average SOC over multiple batteries in parallel reflects that actual SOC of each battery in the parallel string.
The voltage across any 4 cell battery does not indicate the SOC in all 4 cells is the same either, but the voltage of each cell will be within 0.05v accurate when it comes to judging the state of charge and balance across the battery .... when it's at rest, no load and no charging and any cell balancer inactive ....
At least with the cells in parallel to build the capacity, each cell in that parallel string will be within 0.05v of each other so close to all being at the same state of charge.
The only true voltage indication of state of charge is when the cell is above 3.45v rested means it's at 100% SOC, and 3v or less rested it is at 0% SOC. t

To get any idea of the SOC between 100% and 0% can only be calculated, and this is really only an estimate, all be it a very close one, by counting the Ah out against the Ah in.
You could do Coulomb counting, but then you need to know a lot more about the losses between charging and discharging at different current rates, cable and connection losses .... way too hard to get true accuracy, so Ah counting is close enough.
To find out the SOC of each battery in a parallel string, you need an accurate Ah counter, like a Victron 712BMV, connected to each battery in the parallel string. Fully charge each battery by holding them at 14v for as long as it takes till any current stops flowing in, then the 100% SOC will be accurate enough for this purpose, but this proceedure will need to be repeated every mth or so to reset the BMV to an accurate 100% SOC.

If you really do go down this path, please record the current draw from each battery when different loads are applied to the parallel string and if there is a difference in the state of charge recorded for each battery after a long or heavy discharge load .... I'm sure it will be an eye opener for many who post and read here .....

T1 Terry

[Davidhoy]

Originally Posted by cutter3322

Hello, I currently have 3 lifePo4 batteries connected in parallel and I would like to know the state of charge of each. What device do you suggest to me? Thank you

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

[s/v Jedi]

The installation is flawed. Every lifepo4 battery must have it’s own battery monitor (it’s own shunt).

This is one of the reasons to build bigger batteries, using bigger cells, yourself.

Same problem exists in cell level when multiple cells are put in parallel.

The only way to get the paralleled batteries at the same SOC is to give them a full charge at maximum allowed voltage.

[SV__Grace]

This is probably not helpful for your specific situation but an integrated system, while expensive, is also the safest and easiest to manage and monitor.


We refit our 1985 boat in 2017 with all Mastervolt components and we have the ability to monitor, modify, and manage pretty much anything. Behind our electrical panels you will see as many ethernet cables as electrical wires.


We have a "boat computer" with "Master Adjust" software and can plug it in to our electrical system to modify digital switches, view the functioning and health of our system, identify and clear alarms, etc. In reference to your question, this allows us to view the SoC, temperature, etc. of each of our lithium batteries.



We have two 12" B&G chartplotters (outside in cockpit/helm and inside), both have CZone that allows us to monitor battery charging and use as well as a bunch of digital switches. The Mastervolt network integrates seamlessly with NMEA200 which allows us to view and control anything and everything electronic in the system.



A few months ago we upgraded from AGMs to Lithium and the transition was seamless as we already had almost all the components necessary for the Lithium setup (Mastervolt Lithium batteries, of course).


I'm not an expert in other systems on other boats, but I love the system on our boat and in this arena you truly "get what you pay for," but it's not for everyone.

[ccannan]

Isn't it a safe bet that in the short term (<=1 year) that 3 LiFePO4 batteries connected in parallel that were balanced at the start of use, are most probably very close in SOC (assuming no defects)?

For S/V Jedi, what constitutes a battery? 2 or more cells connected in parallel?

[s/v Jedi]

Originally Posted by ccannan

Isn't it a safe bet that in the short term (<=1 year) that 3 LiFePO4 batteries connected in parallel that were balanced at the start of use, are most probably very close in SOC (assuming no defects)?

For S/V Jedi, what constitutes a battery? 2 or more cells connected in parallel?

No, traditionally a battery is cells connected in series. If you want a bigger capacity battery, then bigger cells were used. Even though companies like Tesla, Battleborn etc. started using many small cells in parallel, the big cells in series is still superior.

If 12V lithium batteries like the OP has are connected in series to create a 24V, 36V or 48V bank, then a single battery monitor can be used.

[ccannan]

S/V Jedi, how do you square a previous post:

I don’t think a BMS is that important for a house bank at all. Only when one needs to get every bit of capacity out of the bank it becomes critical. Example: my cells have a minimum discharge voltage of 2.5V and a maximum charge voltage of 4.0V. For 8s this comes down to 20V and 32V. If I first balance the cells and then cycle between 25V and 28V, how much out of balance need cells to go to get close to those min/max values? It will probably never happen, but a regular look at a cell monitor will make sure.

with your these more recent posts?

I ask because, I have just purchased, installed, and started to use 8, 300Ah, LiFeYPO4 cells purchased through Julia Yu. I didn't have all the necessary equipment to monitor everything, but I did check the voltages upon receipt and they were all equal. I wired them 2P4S to create a 600 Ah combined house and start battery.

Tell me how can they get out of balance.

[T1 Terry]

Originally Posted by s/v Jedi

The installation is flawed. Every lifepo4 battery must have it’s own battery monitor (it’s own shunt).

This is one of the reasons to build bigger batteries, using bigger cells, yourself.

Same problem exists in cell level when multiple cells are put in parallel.

The only way to get the paralleled batteries at the same SOC is to give them a full charge at maximum allowed voltage.

Ummmm...... a higher capacity cell compared to a lower capacity cell means the connections are inside the case where they can not be accessed with the higher capacity cell, and out side where they can be cleaned and recoated with an anti oxidising compound every 3 yrs or so using multiple smaller capacity cells in parallel ...... Using this knowledge, isn't it more of a problem using higher capacity cells than multiple smaller capacity cells in parallel?

There are so many other reasons why it is better to use multiple smaller cells in parallel than just the access to terminals, like:
1) The cost of replacement if a cell fails

2) Redundancy because a single cell can be bypassed when away from any services resulting in a smaller capacity battery that still functions allowing for a slightly modified energy use pattern until a replacement can be sourced

3) the configuration of the battery can be changed to suit the available space

There are many more reasons, like cell balancing and averaging of resistance and capacity over a number of cells either end of the mass manufacturing specs ....

Never a good idea to build a battery from single high capacity cells where the same capacity could have been achieved using multiple smaller capacity cells .... 10yrs plus experience and many system out in the wild, both ones we installed and ones installed by others that we now service, have verified what works and what just gives the user headaches .......

T1 Terry

[T1 Terry]

Originally Posted by ccannan

Isn't it a safe bet that in the short term (<=1 year) that 3 LiFePO4 batteries connected in parallel that were balanced at the start of use, are most probably very close in SOC (assuming no defects)?

For S/V Jedi, what constitutes a battery? 2 or more cells connected in parallel?

Unless it is a top of the line, integrated BMS system that actually has direct communication inside each battery, then, no. After rescuing many system owners who were on the verge of giving up after spending many $$ following poor advice, even after a few mths they will not even be close to the same SOC, unless meticulous care was taken on how the system was linked and set up, but there is still no guarantee that the batteries in parallel will be the same SOC, unless they return to fully charged voltage and held there for long enough to balance the cells every few days. Not practical with a system in full time use.

The shortest was 1 mth, a lot at 6 mths had one battery fail due to exceeding cell voltage limits and simply replacing with a new battery resulted in similar symptoms appearing after only a few weeks ...... Very few have exceeded 2 yrs without a system failure, yet to see one in full time use reach 3 yrs.

T1 Terry

[s/v Jedi]

Originally Posted by ccannan

S/V Jedi, how do you square a previous post:

I don’t think a BMS is that important for a house bank at all. Only when one needs to get every bit of capacity out of the bank it becomes critical. Example: my cells have a minimum discharge voltage of 2.5V and a maximum charge voltage of 4.0V. For 8s this comes down to 20V and 32V. If I first balance the cells and then cycle between 25V and 28V, how much out of balance need cells to go to get close to those min/max values? It will probably never happen, but a regular look at a cell monitor will make sure.

with your these more recent posts?

I ask because, I have just purchased, installed, and started to use 8, 300Ah, LiFeYPO4 cells purchased through Julia Yu. I didn't have all the necessary equipment to monitor everything, but I did check the voltages upon receipt and they were all equal. I wired them 2P4S to create a 600 Ah combined house and start battery.

Tell me how can they get out of balance.

I’m still not using a BMS, but I’m monitoring my cells. I can report that after a full season of use, without BMS, balancers etc. the maximum cell deviation is only 8mV.

Yes, these are Winston cells but no they are not in parallel, they are all in series, meaning that the charge current is exactly the same for each cell, even if their internal resistance is a little different.

When you put cells in parallel, the current divides over the parallel cells according to resistance. As it’s pretty impossible to get that exactly equal, the current will be different for each cell and balance is lost. This is the fundamental difference and why series connection of larger cells is superior.

The cells in parallel do not balance themselves because they will only go to equal voltage and with lifepo4 the SOC can be very different for the same voltage.

So in your case, using 300Ah cells… you have Winston cells from Julia which are top quality and probably sequential serial numbers like mine, which will minimize the problem I described above. Still, your setup isn’t optimal and you would be better off by making two 300Ah batteries instead of one 600Ah battery. Each with it’s own battery monitor (and its own BMS if you use any)

Alternatively you can put all 8 in series but that requires a 24V inverter/charger as well as DC-DC converters to run the boat. Still, that is what I did.

Jedi came with two 12V 900Ah batteries. Each battery had 6 cells of 900Ah each. So this was 1,800Ah at 12V. I now have 400Ah at 24V which is less than half but I can do twice as much as before.

[s/v Jedi]

Originally Posted by T1 Terry

Ummmm...... a higher capacity cell compared to a lower capacity cell means the connections are inside the case where they can not be accessed with the higher capacity cell, and out side where they can be cleaned and recoated with an anti oxidising compound every 3 yrs or so using multiple smaller capacity cells in parallel ...... Using this knowledge, isn't it more of a problem using higher capacity cells than multiple smaller capacity cells in parallel?

There are so many other reasons why it is better to use multiple smaller cells in parallel than just the access to terminals, like:
1) The cost of replacement if a cell fails

2) Redundancy because a single cell can be bypassed when away from any services resulting in a smaller capacity battery that still functions allowing for a slightly modified energy use pattern until a replacement can be sourced

3) the configuration of the battery can be changed to suit the available space

There are many more reasons, like cell balancing and averaging of resistance and capacity over a number of cells either end of the mass manufacturing specs ....

Never a good idea to build a battery from single high capacity cells where the same capacity could have been achieved using multiple smaller capacity cells .... 10yrs plus experience and many system out in the wild, both ones we installed and ones installed by others that we now service, have verified what works and what just gives the user headaches .......

T1 Terry

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.