mppt and bms deeper questions.

[Dacflyer]

I hope my questions make sense.
My SCC is set up for 48v system, this will be feeding into a BMS into my batteries.
does my SCC need to be slightly higher or an exact voltage so that the BMS can properly charge the batteries ?
I'm using Epever in USER mode , this will feed into JK BMS to a 14s ( 48v )
What other info should i provide to get a proper answer?

 

[chrisski]

I’m getting ready to turn on my first litjium in a couple of days, made from 8 cells.

I look at the BMS as a last resort. My SCC will be set to a voltage short of what the BMS will cut off. In fact, my Daly BMS only shuts off for over voltage undervoltage, and same with currents, which is the same thing my Overkill BMS for the larger battery will do. So the BMS only does on / off. So my parameters in the BMS will be over the voltage the SCC will provide to prevent it from shutting down, and for current in the BMS, that is set to over what I expect the system to need, but the wiring still capable of handling.

14s seems like an odd number of cells. Most 48 volt battery packs are 16s.

 

[OffGridInTheCity]

I hope my questions make sense.
My SCC is set up for 48v system, this will be feeding into a BMS into my batteries.
does my SCC need to be slightly higher or an exact voltage so that the BMS can properly charge the batteries ?

The BMS has nothing to do with charging - its just a pass-thru. A BMS is a monitor and if it senses something out of spec it's supposed to block current flow - but its all or nothing and is not a 'charger'.

I'm using Epever in USER mode , this will feed into JK BMS to a 14s ( 48v )

Assuming that when you say 14s - you mean lithium ion cells. The max for lithium-ion is 4.2v/cell, so 14 * 4.2v = 58.8v is the absolute max you can charge. If you charge over this, it can lead to fire. **If you have LifePo4 cells then you want 16s for a 48v nominal (e.g. regular voltage range) battery - and the voltages below do not apply - its a much flatter curve and requires a different answer

Back to 14s lithium-ion ....
Typically you don't try to charge to 100% (58.8v) as this will *greatly* shorten the lifespan AND it could be dangerous because one of the 14s could be at 4.15v and another at 4.25v and still add up to 58.8v. Also, there's just very little power between 4.2v and 4.15v.

Personally - I'd stay at 4.1v/cell (or less) but certainly no more than 4.15v/cell max. 14 * 4.1v = 57.4v. So use 51.4v for your absorb/float settings. In my home powerwall, I use 4.0v/cell as my max.

FYI - here's a famous web page / chart showing max voltage vs lifespan - https://batteryuniversity.com/article/bu-808-how-to-prolong-lithium-based-batteries

 

[OffGridInTheCity]

14s seems like an odd number of cells. Most 48 volt battery packs are 16s.

14s is typically lithium-ion (18650 cells are common / higher voltage range than LifePo4) vs 16s when using LifePo4 cells.

 

[Steve_S]

There is a point missed. The SCC may read 48.000 V at its output to battery terminals BUT likely to be less at the battery itself. With ANY Lithium (which is not brute force tech like FLA) volts & millivolts count. The also applies to the Inverter for cutoffs etc. For example, if you must cutoff for low voltage at 40.000 (2.500Vpc for LFP) not 39 or 41.

And the GOTCHA ! there always is. Voltage Readings between the SCC, BMS, Inverter are not constant between charge / discharge. Its a nasty monkey to work out but I found the easiest way to beat it, is to take the charge & discharge voltage readings, and splitting the difference and compensating with the settings. The actual settings had to be compensated for on my Samlex Inverter, but the Midnite Solar SCC has a simple compensation setting just for this. I do not believe the EPEver SCC's have this, so likely have to correct using the settings.

The BMS has only the task of guarding the battery cells & pack as a whole. If anything goes out of set params, it's job is to stop whatever activity. Some have balancing functions but that is not "charge" as such, that is just transferring volts between cells or burning off hi voltages.

Hope that helps, Good Luck.

 

[Dacflyer]

The BMS has nothing to do with charging - its just a pass-thru. A BMS is a monitor and if it senses something out of spec it's supposed to block current flow - but its all or nothing and is not a 'charger'.


Assuming that when you say 14s - you mean lithium ion cells. The max for lithium-ion is 4.2v/cell, so 14 * 4.2v = 58.8v is the absolute max you can charge. If you charge over this, it can lead to fire. **If you have LifePo4 cells then you want 16s for a 48v nominal (e.g. regular voltage range) battery - and the voltages below do not apply - its a much flatter curve and requires a different answer

Back to 14s lithium-ion ....
Typically you don't try to charge to 100% (58.8v) as this will *greatly* shorten the lifespan AND it could be dangerous because one of the 14s could be at 4.15v and another at 4.25v and still add up to 58.8v. Also, there's just very little power between 4.2v and 4.15v.

Personally - I'd stay at 4.1v/cell (or less) but certainly no more than 4.15v/cell max. 14 * 4.1v = 57.4v. So use 51.4v for your absorb/float settings. In my home powerwall, I use 4.0v/cell as my max.

FYI - here's a famous web page / chart showing max voltage vs lifespan - https://batteryuniversity.com/article/bu-808-how-to-prolong-lithium-based-batteries
View attachment 63947

I was planning like 4.0v max volts per cell and maybe 3.0 vpc minimum.
does this sound ok, ?

 

[OffGridInTheCity]

I was planning like 4.0v max volts per cell and maybe 3.0 vpc minimum.
does this sound ok, ?

4.0v * 14 = 56v max - really good for long life
3.0v * 14 = 42v min - is safe but..... I'd go with 3.4v or 3.5v minimum.

Explanation...
If you look at a standard discharge curve of lithium-ion, you'll find that the 'knee'

is around 3.5v. It can vary 'a little' (3.4v) but 3.4v to 3.5v is a good standard.

What this means is that when you go below 3.5v, there is little power left in the cell and the cells in series start varying 'much more' in their individual voltages as you drop rapidly thru 3.3, 3.2, 3.1, 3.0v. So if you go down to 3.0v/cell - you could easily have some cells at 2.8v and some at 3.3v all adding up to 3.0v *14 = 42v. This doesn't help long life.

ME - I use 3.5v as my bottom. 3.5v * 14 = 49v. If you have some measurement equipment you can see this for yourself.
- Discharge down to 3.0v * 14 = 42v and use a voltmeter to measure the individual cells.
- Use an iCharger X8 (with USB to computer software) or something similar OR just track discharge-power + voltage on a graph and you can find your specific 'knee'

Back to that Battery University page ... https://batteryuniversity.com/article/bu-808-how-to-prolong-lithium-based-batteries
we have this famous graph

which shows that if you limit both max (charge) and min (discharge) voltage ranges you 'might' get dramatically longer life. In fact, my whole powerwall was designed around this and I'm shooting for the 75-25% (light blue) line.... Hope it turns out to be true

 

[Dacflyer]

ok,, so if according to the chart, i could go with 4.1v max and 3.5v min.. that is about optimal...
I'll have a little diminished capacity, but much longer lifespan ?
So, these figures are what i should put into the charge controller?
there are a few other setting i'll have to figure out. but at least i have the voltages to use now.