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

[john61ct]

Do not waste time trying to get two different SoC meters to match up at more than one SoC point, ideally near your defined stop-discharge point

which should be nowhere near zero SoC.

None is ever exact, a guesstimate within 4-6% is the best to hope for with today's technology.

Choose to believe the one you think is closer to accurate, or (my choice) the one that causes you to stop (slow down) discharging sooner,

lower average DoD% will greatly extend battery lifetime.

At low current rates, overall LVC or at least alarms should be like 3.2Vpc. At high C-rates maybe 3.1V, I would not go lower than that at rest, treat 3.0V as drop-dead cutoff, try to get recharged up past 3.2V asap.

Sounds like you've already read this:

https://marinehowto.com/programming-a-battery-monitor/

[john61ct]

If you wanted to be OCD about it, buy a Link Pro to be the tie breaker.

[JmanC]

Thanks John noted about SoC gauges.

These are the cell voltages according to bms monitor on my 12v dropin when charging to 14.5V set on battery charger.
3.71 3.70 3.64 3.50
I held 14.5V on battery charger for hours until no current flow into battery. But still the cells are this far apart.
The battery is pretty new with only a few dozen cycles.

I guess some of the internal balancers are not working?

[mikedefieslife]

Surely your BMS internal or external shouldn't allow a cell to go passed 3.65

[CatNewBee]

Not true, 3.65V is the value where the cell is considered full, the cut off is usually much higher, between 3.750V and 4.00V.
Cells get damaged if going higher than this, 4.000V is the initialization voltage recommended by the manufacturers for new cells.

Of course you can configure a cut off when the first cell hits 3.650V, but then the other cells will not be full and you will not get a 100% SOC on the battery, therefore drop-ins usually allow charging of a single cell way above this to allow balancing and charge of the remaing cells to 3.65V.

Originally Posted by mikedefieslife

Surely your BMS internal or external shouldn't allow a cell to go passed 3.65

[mikedefieslife]

Originally Posted by CatNewBee

Not true, 3.65V is the value where the cell is considered full, the cut off is usually much higher, between 3.750V and 4.00V.
Cells get damaged if going higher than this, 4.000V is the initialization voltage recommended by the manufacturers for new cells.

Of course you can configure a cut off when the first cell hits 3.650V, but then the other cells will not be full and you will not get a 100% SOC on the battery, therefore drop-ins usually allow charging of a single cell way above this to allow balancing and charge of the remaing cells to 3.65V.

I’ve not seen that before.

Allowing cells to go higher than 3.65 is contrary to everything I’ve read everywhere.

My cell manufacture lists a cut off voltage of 3.65. My BMS has this specced too.

I appreciate the cells in question may be different but I’ve really never heard of anyone going higher than 3.65 when top balancing and 13.8-14.2 during everyday charging

[CatNewBee]

Originally Posted by mikedefieslife

I’ve not seen that before.

Allowing cells to go higher than 3.65 is contrary to everything I’ve read everywhere.

My cell manufacture lists a cut off voltage of 3.65. My BMS has this specced too.

I appreciate the cells in question may be different but I’ve really never heard of anyone going higher than 3.65 when top balancing and 13.8-14.2 during everyday charging

This is the manufacturer specification.

3.65V cut off is a very conservative setting, that mean your charge bus will be disconnected when the first cell reaches 100% SOC, operating your battery at SOC around 95% or worse. Your charging sources will be brute force disconnected and may get some damage before they reach the absorbtion voltage and gracefully stop charging, except you configure them even lower and miss even more capacity plus accept slowly drifting unbalanced cells.

If you want 100% balanced SOC at 4 cells 3.65V or 14.6V absorption, the cut off must be set to 3.750V to allow weaker cells to catch up. This is how most drop in battery BMS are configured btw.

Look at the specs and make your calculations. The BMS is a safety devise and not meant to control charging regulary. It engages only if things go really wrong and voltages go up above the safety margin (below 2.800V or above 4.000V for this cells)

[nebster]

Be naturally skeptical of manufacturer recommendations. They may have no clue what they are talking about, or they may simply parrot what the guy before them wrote down, or they may have different incentives.

The evidence is very strong for the benefits of conservative voltages with this chemistry.

[mikedefieslife]

CNB isn't the whole point of top balancing so that all the cells are very close within a few mV of each other at the top end. Hence when 1 cell is at 3.65, the others are close to it too. Otherwise why bother top balancing at all. You might as well just run unbalanced cells.

[john61ct]

Yes those cell level voltages are very far apart and indicate the BMS is faulty.

Use another BMS or dedicated balancing gear to do the job, many options there.

The factors to look at are:

"start balance" voltage should be well below **your preferred** Absorb voltage setpoint, and that really should be adjustable IMO. Some balancers will balance at any voltage, and need IMO to be controlled to only do so as SoC is nearing the top at the end of the charge cycle

"balance current" should be high enough that your cells - in their current condition **and** as they wear over the next few years - get to fully balanced before the point **where you prefer** to end the charge cycle. No such thing as too high, but note that most give a high spec, actual current may depends on the voltage delta between cells.

______
I consider 3.65Vpc to be a very high target / stop-charge / Absorb voltage, especially for normal cycling usage, and most especially if needs to artificially be held for a long time just to get balancing finished.

And yes such poor design of balancing circuitry is the norm for most cheap BMS, and even many that cost more than say $100.

If you do convert your balancing process to another BMS you could get a "non-protective" version with a better design

and keep your existing BMS - after thorough testing - for its protective functions - just keep your normal cycling charge voltage setpoint lower than its start-balance setpoint, in effect disabling its balance function.

Or use dedicated balancing gear, only connected when needed.

Some even adapt RC hobby chargers for that purpose.

Just do not let multiple balancing devices work at the same time on the same set of cells.

[john61ct]

Another option is manual re-balancing, but that is only convenient with a bank designed for it, and the right gear so you can do so anytime as needed.

And IMO only suitable for a bank that only requires very occasional re-balancing, perhaps only while the cells are all still in a very high %SoH.

[CatNewBee]

That is true, but if you follow the recommendations of john and the others with shallow charging and staying at 50% your balancers will fail over time to their jobs properly, one cell will climb to 3.65V while the other is far away, eventually the bms will then engage at 3.65V if set so and you will experience a capacity loss and disconnect events while charging even at lower voltage settings for Absorption. A BMS set to disconnect let's say at 3.75V instead will allow a longer charge within safe margins and help the balancers do their work and finally fix the difference. It will also ensure you have a higher capacity available after charging without early disconnects what would make the problem worse over time or wreck the controller.

Originally Posted by mikedefieslife

CNB isn't the whole point of top balancing so that all the cells are very close within a few mV of each other at the top end. Hence when 1 cell is at 3.65, the others are close to it too. Otherwise why bother top balancing at all. You might as well just run unbalanced cells.

[nebster]

Meh, my cells are gently charged and have never been rebalanced, and all of them (96 independent ones) are still tracking very close to their initial state. That's after 1.5 years and hundreds of cycles.

With stable cells, bringing a battery high regularly just to rebalance may be the worse choice.

[ReneJK]

Originally Posted by gscriba

Hi John,
Yes, I've done my research. Saw all that. Thanks. Looking for more hands on practical experience from those here who have set up and are using their LiFePO4 and BMS system.

I am using the Simarine Pico with 600Ah Lifepo4 GBS cells

[Maine Sail]

Originally Posted by nebster

Meh, my cells are gently charged and have never been rebalanced, and all of them (96 independent ones) are still tracking very close to their initial state. That's after 1.5 years and hundreds of cycles.

With stable cells, bringing a battery high regularly just to rebalance may be the worse choice.

My Winston / Thundersky cells are now over 10 1/2 years old and still remain in balance. They also are still exceeding their capacity.

I've physically disabled the balancing portion of the BMS as that was the only glitch I ever had with the pack. (it is still active for protection)

If you start with well matched cells, and do a good initial balancing, charging at 13.8V to 14.2V is quite reasonable, and at least for our boats pack, has meant 10.5 year old cells with nearly 1300 cycles and still they're exceeding capacity...