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

Fabian · June 2023

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?

Dave Angelini · June 2023

I would limit it until you know exactly how the BMS will play together. Your issue is one that can "KILL" your expensive battery. It is also one of the reasons that a quality battery (and you pay even more for) will be designed for a large inverter load like an outback radian or Schneider XW. They will spell out the minimum number of batteries to safely handle these currents. You would think it will divide among the 4. You need to know it will. It could get down to how the 4 are wired also.

Too fast of charge and too fast of discharge can instantly destroy LFP and their electronics. Not like the failure modes of Lead Acid that are much slower and give some warning.

Horsefly · June 2023

Fabian said:

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?

First off: I may be looking at the wrong one, but when I looked for the Redodo 24V 100Ah battery the one I found says it is rated for 100A continuous charge and 100A continuous discharge. You should double check that. For now, let's assume you are right that the limit on your battery is 50A.

If you have two batteries in series, whatever current is going through one is going through the other. So if you put 50A in that series pair, they both are seeing 50A. So in a perfect world, you're two strings of two should be OK with 100A. Unfortunately, the world is not perfect. Here's some things to be aware of:

One of the main things the BMS is supposed to do is limit the current. For a battery that has a 50A charging limit, the BMS will probably actually have a higher cutoff limit, like maybe even 100A. The way it does this is to actually disconnect the battery from the source of the charge. If you are putting your batteries in 2S2P (two batteries in series, connected to another series string of two) this can have some unforeseen impacts to the battery in series with the one that cutoff (sudden disconnect from the system), and to the batteries in the parallel string (some of the extra current will suddenly go through the other string).
The manufacturer seems to suggest 2P2S (two batteries in parallel, connected in series with another two batteries in parallel). This can help limit some of the problem in #1.
Generally, the LiFePO4 chemistry can handle 1C charge or discharge. For a 100Ah battery, that would be 100A. The 50A limit from the manufacturer is a common conservative limit. You still should try and observe the limit that the manufacturer has specified, but I'm saying if you go over it by a bit for a short while, it won't cause any problem.
I've never heard of this battery company, and I would suspect they are one of the many fairly cheaply manufactured batteries. Doesn't mean they are not good enough to use, but one of the main places they can cut corners is the BMS. A cheap BMS can fail to do the protection of the cells that it should, which can definitely cause a battery to turn into a worthless doorstop.

Bottom line: I would hook your batteries up as 2P2S, rather than 2S2P. That is, parallel two batteries, and parallel the other two batteries, and then hook these two parallel sets in series. Then limit your charging to 100A, and do what you can to limit your discharging to 100A, and you should be fine. If the BMS in each battery is any good, it should protect from over-current, but the over-current cutoff can cause some other issues. Best to avoid over-current if you can.

Fabian · June 2023

I rewire it to 2P2S as u suggested. The manufacturer recommended i disassemble the pack and do a balancing every 6months.

Suppose I decide to get (2) 24V Lifepo4 charger and connect them.to each side of the series connection do u think it would charge each of them to 28.8v so that all batteries would be reading the same voltage whilst still connected in the parallel-series connection? This charging would take place once every couple months just to bring the SOC and voltage reading close as possible and i think i would let this occurs only when the outback mppt has reach 54.6v or near a full charge so basically when the charge controller reaches 54.6v and begins to float charge then the 2 chargers would commence charging of each side of the battery pack of so that there won't be any voltage difference between them or the most would be 0.01v at the end of the charge cycle. I would only do this when i notice a voltage difference between the 2 sides later on in the future as i don't expect to see it anytime soon because i have been using the batteries over 3 months now and the 2 sides are still showing the same voltage reading.

I would go with either a 1 or 2 Amp 28.8v Lifepo4 charger

Can this method work?

BB. · June 2023

BMS, I think, do not "regulate" current into/out of the batteries... They are just an on/emergency off switch (as I understand). They are the emergency off switch to prevent battery/cell damage.

Paralleling batteries (and strings of batteries) can share current--But with low resistance wiring (and cells), it is difficult to ensure 50/50 "perfect" current sharing.

If the cells/battery voltages between the two strings are evenly matched, you have a better chance of current sharing between strings.

This site shows how to parallel multiple battery strings:

http://www.smartgauge.co.uk/batt_con.html

And you can use an AC+DC Current Clamp DMM to measure the current sharing:

https://www.amazon.com/UNI-T-Digital-Handheld-Resistance-Capacitance/dp/B0188WD1NE (low cost meter)
https://www.amazon.com/Auto-Ranging-Resistance-Klein-Tools-CL800/dp/B019CY4FB4 (mid cost meter)

There are AC only clamp meters--Those do not work for for DC current measurements.

-Bill

Fabian · June 2023

I currently set my Outback MPPT charge controller to bulk charge to 55.2v with 20mins absorption time and also float charge at 55.2V Is this setting ok?
Will this setting bring me close to 100% SOC and will the BMS able to balance the cells or do I need to increase the absorption time/Bulk Voltage?
Do I need to set a lower float voltage?

Horsefly · June 2023

55.2V is a good conservative setting for both bulk and absorption. I would probably set the absorption time to something more like 40-60 minutes, and set the tail current (which I think Outback calls "Charge Return" or something like that) to 0.02C, or 4A in your case.

I would set the float voltage to something lower, like in the area of 53.5V or so. Although there is some debate, most people using LiFePO4 believe that it shortens the life of the battery if you hold it at full charge. By cutting back to 53.5V or so, the battery will pretty quickly drop down to below 100% SoC, but hold pretty high while you have sun. LiFePO4 doesn't need float like lead acid does, but rather allows you to use the solar production for loads and not depleting the battery until you don't have solar.

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

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