Are my BMS settings good for more lifetime ?

[crozen]

Hi guys,

I bought an 100Ah LiFePo4 4x3.2v cells.
My system is 12v.

I have a Daly BMS. According to these charts (https://www.solacity.com/how-to-keep-lifepo4-lithium-ion-batteries-happy/), i want to use 20%-80% range of the total capacity for more lifetime.
Are my settings correct ?

Thanks.

 

[A.Justice]

3.2v and 12.8v are very high low voltage cutoffs. With voltage sag, you could have a LVD condition with more than 60-70% of the battery left.

I would go for 11-11.6v and 2.65-2.9v per cell, that cuts off the bottom 10-20%.

 

[crozen]

3.2v and 12.8v are very high low voltage cutoffs. With voltage sag, you could have a LVD condition with more than 60-70% of the battery left.

I would go for 11-11.6v and 2.65-2.9v per cell, that cuts off the bottom 10-20%.

But 11.6v is about only 7% of the bottom according to this chart ?

 

[A.Justice]

Measuring capacity based on voltage is very difficult with LiFePo4 cells (search for LiFePo4 voltage chart on here). The reason I would set it lower is because of voltage sag. If the battery was 12 volts, and you turn on a load, it could easily dip the voltage to 11.5 or lower. You would trigger the LVD before you got to 20-30%.

I would suggest using a shunt meter and a relay. It would measure actual power use, and cut the system off when a certain watt-hour specification is met, rather than trying to cut it off by guessing the voltage.

 

[crozen]

Measuring capacity based on voltage is very difficult with LiFePo4 cells (search for LiFePo4 voltage chart on here). The reason I would set it lower is because of voltage sag. If the battery was 12 volts, and you turn on a load, it could easily dip the voltage to 11.5 or lower. You would trigger the LVD before you got to 20-30%.

I would suggest using a shunt meter and a relay. It would measure actual power use, and cut the system off when a certain watt-hour specification is met, rather than trying to cut it off by guessing the voltage.

Is it the same while charging ?
The voltage is increased on charging but when you cut if off, the voltage drops. So it doesn't mean that you have charged to 80% of the capacity.

 

[A.Justice]

Is it the same while charging ?
The voltage is increased on charging but when you cut if off, the voltage drops. So it doesn't mean that you have charged to 80% of the capacity.

That is correct. The only real way to know with any certainty how much power is left is with a shunt / hall sensor. The BMS should really just be there to prevent damage to the cells, rather than controlling the charge and discharge cycles. I Separate low voltage disconnects and high voltage disconnects should be used to control your loads.

I have my HVD at 14.3v and LVD at 10.6v, but I only charge to 13.85v and I have a LVD module and relay that cuts off at around 11v.

 

[crozen]

That is correct. The only real way to know with any certainty how much power is left is with a shunt / hall sensor. The BMS should really just be there to prevent damage to the cells, rather than controlling the charge and discharge cycles. I Separate low voltage disconnects and high voltage disconnects should be used to control your loads.

I have my HVD at 14.3v and LVD at 10.6v, but I only charge to 13.85v and I have a LVD module and relay that cuts off at around 11v.

Do you monitor the charging and shut it off manually when it reaches your specifications ?
I tought that i could use a BMS to stop the charging ?
But as you said, it is only to prevent damage.

 

[snoobler]

+1 to @A.Justice BMS purpose - The BMS is to prevent catastrophe. Keep it's settings closer to cell limits. Running to 3.65V or 2.5V/cell isn't a big deal, and it won't measurably impact cycle life if only done occasionally.

Adjust your chargers and loads so that they cut off when you want them to.

One of the biggest and easiest things you can do to extend cycle life is to limit charge voltage and current. Simply by charging to 3.45V/cell, that is less stressful to the cells and should improve life. The tradeoff is that charging will take notably longer and require a longer absorption phase since the charge is happening at a lower current.

 

[crozen]

Thank you guys. ?

 

[crozen]

Does the BMS really can balance cells ?
My first cell hit 3.55v and trig the protection while the last one is 3.35v on charging. (this is my first charging)
But at idle, they are almost the same voltage.

When i bought them, all had 3.05v

 

[snoobler]

Does the BMS really can balance cells ?
My first cell hit 3.55v and trig the protection while the last one is 3.35v on charging. (this is my first charging)
But at idle, they are almost the same voltage.

Top balancing synchronizes the cells to true 100% charge. Unbalanced cells with even a very small difference in their state of charge can have wildly different voltage responses near full charge.

BMS balancing tends to be at very low current. 24 hours of balancing might knock off 1Ah if it ran all the time, and it won't.

A properly configured BMS can eventually top balance a pack, but it may take weeks/months, and it will frustrate you along the way.

 

[crozen]

Top balancing synchronizes the cells to true 100% charge. Unbalanced cells with even a very small difference in their state of charge can have wildly different voltage responses near full charge.

BMS balancing tends to be at very low current. 24 hours of balancing might knock off 1Ah if it ran all the time, and it won't.

A properly configured BMS can eventually top balance a pack, but it may take weeks/months, and it will frustrate you along the way.

So, what is the solution ?
Let it go charge/discharge normal usage, and overtime it will be balanced or there is a faster way ?
I don't have a 3.65v charger and charge them fully one by one...

 

[snoobler]

Top Balancing "How to"

For current best practices on top balancing cells, please watch the video and download the PDF at the following page: Top Balancing Cells Using a Low Cost Benchtop Power Supply If you are interested in a more general tutorial of what balancing is about and *Why* to balance, check this out...

diysolarforum.com

 

[A.Justice]

Do you monitor the charging and shut it off manually when it reaches your specifications ?
I tought that i could use a BMS to stop the charging ?
But as you said, it is only to prevent damage.

I use something like these and a relay to shut off my inverter. I got them a long time ago and they are different from the ones I have, but look similar.

Amazon.com

As far as top balancing goes, it's pretty much necessary to do. I would suggest getting a bench power supply, they can be used for all kinds of stuff, and can charge whatever battery configuration (3.6v single cells, or a whole pack) you have. I have two of these (link below), but there are other brands available that are just as good. I would go with 10 amps though, 5 is painful to charge large cells with. I put off buying one untill I absolutely needed it, but wish I had got one sooner. They are incredibly useful tools, for everything from diagnostics and troubleshooting, to testing systems out before you plug 1000's of potential amps worth of power into untested components.

Amazon.com

I've "balanced" cells before I had a power supply by putting them all in parallel for MORE THAN A MONTH, it worked well enough for smaller (25ah) cells. Luckily I didn't need to use them with any urgency, but it can be done.

 

[crozen]

Thnx guys ?

 

[Just John]

First, you need to top balance that pack.

I have been experimenting a lot with that magic 80% DOD the manufacturers recommend. Notice the manufacturers don't define it beyond that, no charge to, or discharge to volts. They do define a 100% "standard" charge and 0% discharge (for 280AH Eve cells, charge to 3.65v and stop when current in drops below 14 amps, then discharge to 2.5v as full normal cycle).

Seriously, above 3.45v is barely 2 amp hours capacity. Below 3.0v is about 12 amp hours, and the battery rises 5 degrees C below 3.0v. So, I am getting about 95% of capacity (i.e. measured amp hour storage) by charging to 3.4v per cell and stopping when current drops to 1 amp in, then discharge to 3.0v.

The drawback is the long slow drop in current to 3.4v. With a standard charge, the 1 hour wait after charge, the cell will drop from about 3.55v to 3.45v. The minute you draw any power, it drops to 3.375v and really drops pretty steady after. So you actually want the higher voltage to drive the current, and it is really difficult to judge when full without waiting for the tail current. I have noticed very consistently, that the claim of 98% efficiency, is not true. More like 99.9% efficient. Seriously, I have measured it a LOT, if you emptied a cell of 270 amp hours, it takes 270.1 amp hours to fill it up again.

Silly as it sounds, the easiest and best way is measure amp hours in and out. You can use voltage, but that takes a while to settle after charging. Both in and out you get substantial "bounce". With solar, it could be hard to detect a tail current on a cloudy day.