Clarification on LiFePO4 Charging Voltage and Current

[kentdavidge]

This thread advises us to use 14.2V as absorption voltage. Questions:

(1) Is this the voltage I should set on my MPPT charge controller?

(2) That thread also say "the charge rate will be reduced at high SOC. THIS IS NOT A GOOD IDEA IF YOU ARE NOT ABLE TO BALANCE FOR VERY LONG."---so should I actually set a lower value on my MPPT (like 14.2V)?

(3) whatever the voltage set on my MPPT, can I configure my JK BMS to limit the charging current? Because I heard somewhere that LiFePO4 like being charged at 30% of its rated capacity for longevity.

 

[Aridom82]

I am confused. The Jk bms usually has the option of limiting the charging current and some of them can do active balancing too.
But how is your system organized?

 

[mikefitz]

(1) Is this the voltage I should set on my MPPT charge controller?

Yes, set absorbtion to 14.2 and float to 13.5 or 13.4.

(2) That thread also say "the charge rate will be reduced at high SOC.

The battery will take a reducing current as it nears a full charge. There is no need to change the absorbtion voltage for a well balanced battery. If very poor balance is found, a reduced absorbtion voltage will give more balance time.

can I configure my JK BMS to limit the charging current?

You can configure the BMS charge current protection level. Once reached, the BMS will disable the charge path. This feature is for protection, not charge control. The charge current should be determined by the chargers. The battery or cell supplier will specify a recomended charge current. A lower current charge is less stressful.
A charger will have a maximum specified charge current and may have provision for user setting.

 

[kentdavidge]

I am confused. The Jk bms usually has the option of limiting the charging current and some of them can do active balancing too.
But how is your system organized?

My system will consist of two 12V packs connected in parallel, one with 4 cells of 102Ah each, the other with 4 cells of 206Ah each. I have two 8S20P JK BMSes (one for each pack) with active balacing of 2A and 200A of max current. The packs will be connected to busbars, which in turn will be connected to a single MPPT controller of 100A, and to an 1500W inverter (my loads won't draw more than about 50A from the batteries).

 

[littleharbor2]

The last few 200 amp JK BMS's I installed came Factory preset to 30 amps maximum charge protection. The first time I discovered it the BMS kept shutting down and throwing the PV open circuit voltage to the inverter which did not like that at all.So when you commission JK BMS's after setting your cell count, cell type and amp hours go to the charge current setting and set it where you want. 30 amps is terribly low.

 

[kentdavidge]

The last few 200 amp JK BMS's I installed came Factory preset to 30 amps maximum charge protection. The first time I discovered it the BMS kept shutting down and throwing the PV open circuit voltage to the inverter which did not like that at all.So when you commission JK BMS's after setting your cell count, cell type and amp hours go to the charge current setting and set it where you want. 30 amps is terribly low.

That's interesting. In my previous post I explain my setup. So for example, if the BMS in the smaller pack cuts current to that pack, the other pack would still keep receiving current, right?

 

[littleharbor2]

That's interesting. In my previous post I explain my setup. So for example, if the BMS in the smaller pack cuts off current to that pack, the other pack would still keep receiving current, right?

I don't see why it would, provided the settings are correct but you really don't want your BMS shutting down except in an extraordinary situation.

 

[kentdavidge]

I don't see why it would

why it would...keep receiving current? but then my system won't work well. The remanining pack must keep receiving charge if it has a low SoC.

 

[littleharbor2]

why it would...keep receiving current? but then my system won't work well. The remanining pack must keep receiving charge if it has a low SoC.

Sorry I guess I could have clarified that a little better I don't know why it would affect the second BMS

 

[littleharbor2]

why it would...keep receiving current? but then my system won't work well. The remanining pack must keep receiving charge if it has a low SoC.

Sorry for the double pose here accidentally hit post when I didn't mean to