LifepO4 battery recommended max charge current in a series-parallel connections

[fabieville]

I have (4) 24V 100AH Redodo Lifepo4 battery that i connect in series-parallel connections to give me a total of 48V @ 200Ah to power my 48v system that I have. Each of these battery contains a BMS that the manufacturer supports a recommended max charge current of 50Ah.

What i want to know is due to the fact that its (4) separate BMS in my battery bank if I decide to charge the pack with 200Ah would it be automatically shared between all 4 BMS so basically each BMS would be drawing about 50amp? So in that case I would be able to pump that high 200Ah tru the bank and don't have to worry that one or some of the BMS might be drawing more than 50amp?

Or due to the fact that my bank is rated 48v @ 200ah should I limit the charging current to 50% of my battery bank which would be 100ah the max charge current that should be going to the battery bank?

 

[BentleyJ]

I have (4) 24V 100AH Redodo Lifepo4 battery that i connect in series-parallel connections to give me a total of 48V @ 200Ah to power my 48v system that I have. Each of these battery contains a BMS that the manufacturer supports a recommended max charge current of 50Ah.

Current is measured in Amps. Ah is Amps x Time. So lets use the proper terminology.
When 2 x 24V batteries are connected in Series the Voltage doubles to 48V and the Ah rating of the resultant 2S battery pack stays the same, 100Ah because current is flowing through both batteries at the same time, it has no were else to go. Thus both BMS "see" the same current.
In this case the maximum charging current would be 50A or 0.5C for that single 2S series string.

What i want to know is due to the fact that its (4) separate BMS in my battery bank if I decide to charge the pack with 200Ah would it be automatically shared between all 4 BMS so basically each BMS would be drawing about 50amp?

That is incorrect. Per the above explanation a 2S2P battery consists of only 2 series strings. 200A would be divided between by 2 so each series string is getting 100A thus all 4 BMS have 100A flowing through them. Keep in mind its not just the BMS that has current limits, the cells can be damaged or have life reduction due to excessive current. Also,depending on how the battery is wired together it may not be sharing current evenly between cells or strings. That is a different discussion. Lastly, 4 batteries can be connected as either 2P2S or 2S2P configuration. Not trying to confuse things but needs to be mentioned here.

So in that case I would be able to pump that high 200Ah tru the bank and don't have to worry that one or some of the BMS might be drawing more than 50amp?

Incorrect, see above.

Or due to the fact that my bank is rated 48v @ 200ah should I limit the charging current to 50% of my battery bank which would be 100ah the max charge current that should be going to the battery bank?

Yes, 100A (not Ah) would be 0.5C. That is considered the max, Do you drive your car at the max RPM all the time?
A more conservative charge rate for LiFePO4 is 0.2 to 0.3C.

 

[fabieville]

Thanks for the info. Much appreciated, so I will limit my charging current to 100A max. My battery bank cut off voltage is 51.2v so therefore my max array should not pass 5120watt or maybe limit the array slightly lower to compensate for when i have a surge in array power production so that it doesn't pass 100A going into the battery?

 

[DIYrich]

100A is 0.5c. I would limit to 40A or 0.2c if you gave enough time to charge. 0.5 is max rate.

 

[BentleyJ]

so I will limit my charging current to 100A max.

You would be better off at 50A which is 0.25C. Is there some reason you need to charge the batteries very fast?

My battery bank cut off voltage is 51.2v so therefore my max array should not pass 5120watt or maybe limit the array slightly lower to compensate for when i have a surge in array power production so that it doesn't pass 100A going into the battery?

Not sure the above makes sense, or I'm not reading it correctly. When you say battery bank Cut-Off voltage are you referring to when the inverter stops producing power due to its internal settings or when the BMS cuts out and disconnects the load? Or perhaps you are referring to one of the settings on the Charge Controller. In any case, 51.2V is essentially the midpoint of the operating range of a 48V LFP battery, its not a desirable control point for either High or Low cut off.

Disclaimer: The following is not intended to be condescending or offensive in any way. The forum is here to help other DIY'ers. I would encourage you to ask more questions and give us more specific details about the other components in your system and how they are connected together. It seems like you may not have a complete understanding of how your system works as a whole and how to set the User Menus and control parameters.

 

[fabieville]

You would be better off at 50A which is 0.25C. Is there some reason you need to charge the batteries very fast?

Not sure the above makes sense, or I'm not reading it correctly. When you say battery bank Cut-Off voltage are you referring to when the inverter stops producing power due to its internal settings or when the BMS cuts out and disconnects the load? Or perhaps you are referring to one of the settings on the Charge Controller. In any case, 51.2V is essentially the midpoint of the operating range of a 48V LFP battery, its not a desirable control point for either High or Low cut off.

Disclaimer: The following is not intended to be condescending or offensive in any way. The forum is here to help other DIY'ers. I would encourage you to ask more questions and give us more specific details about the other components in your system and how they are connected together. It seems like you may not have a complete understanding of how your system works as a whole and how to set the User Menus and control parameters.

my setting of 51.2v is my battery deep discharge protection setting. I have a battery monitor that cuts the inverter output whenever the battery reaches 51.2v. So my transfer switch takes the house loads and connects it to the grid once the battery reaches 51.2v and connects it back to the inverter once the battery charges up back to 52v

 

[Horsefly]

@fabieville - I didn't notice you posted the same question here. I saw you post over at the NAWS forum, and answered you there. Note that you'll get more and better answers here anyway, and the answers you've gotten already are good.

As I pointed out on the other forum, when I looked for your battery on line, the one I found said it had a 100A charge/discharge limit, not 50A. That would be more common, as a 100A LiFePO4 battery is generally spec'd to support up to 100A charge / discharge. I think it would be good to try and limit it to 50A for long life, but I don't think the hard limit is that low.

A cutoff of 51.2V is a bit higher than you need to go. A cutoff of 49.6V would be fine, but there is nothing wrong with using 51.2V is you want.

 

[BentleyJ]

my setting of 51.2v is my battery deep discharge protection setting. I have a battery monitor that cuts the inverter output whenever the battery reaches 51.2v. So my transfer switch takes the house loads and connects it to the grid once the battery reaches 51.2v and connects it back to the inverter once the battery charges up back to 52v

As Horsefly noted its your choice to run your system as you wish. Per your statement you are wasting much of your battery capacity using 51.2V on the low side cut out. I would recommend 50V.
Are you recharging your batteries up to at least 54.4V?
These values are still very conservative and will not overly stress the batteries but will unlock more usable energy from them.

 

[fabieville]

my max charge is 54.6v how long should i set my outback mppt to stay in absorb mode? I currently have it set to 30mins

 

[BentleyJ]

30 min is too short. As an example lets say the 200Ah battery is discharged down to 20% State of Charge. It would require 80% of its 200Ah capacity to reach full charge again. That is 160Ah. So if charging is limited to 50A then in the first 2 hours you get about 100Ah but as the battery charges its internal voltage pushes back and the current falls off. The next 60Ah will take 1.5 to 2 hours with the constantly diminishing current.
These are rough values, I would set the time to 4 hours for absorb. EDIT: Does the Outback MPPT have a setting to automatically exit Absorb and enter Float at a specific voltage? If so that is the best way to handle charging. Set up the controller properly and let it do its job.

If you are using the inverter to power loads then the MPPT should be set to 3 stage charging and go into Float mode after the 4 hour absorption charge (or per the above voltage setting) so any excess PV production is used by the inverter. 54.4V is a good setting for float.

 

[fabieville]

4 batteries can be connected as either 2P2S or 2S2P configuration. Not trying to confuse things but needs to be mentioned here.

its best to connect my (4) 24v 100ah batteries in 2P2S instead of 2S2P ?

 

[googdot]

its best to connect my (4) 24v 100ah batteries in 2P2S instead of 2S2P ?

There is some good info on that in this document in the Resources section:

Cell Configurations for 12V 24V and 48V LiFePo4 Batteries

This short deck shows 1P and 2P cell configurations for 12V, 24V & 48V LiFePO4 batteries. To get the deck, click on the orange button at the top of this page. Note: Reviews, Comments and corrections are welcome Document Revision History...

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