Small LFP battery with lots of panels?

I have a utility 24v I use to do various things like run the well. Right now it has a large lead acid bank with about 150amp of charge controllers (victron) hooked to it. I don't cycle the batteries, they are just there to cover for clouds that might be rolling through. I don't use it unless there is lots of sun.

The batteries are getting weak and I want to replace them with some cheap LFPs from Amazon. I would like to start with a single 24v 100a battery and add more as money comes in. My concern is the 150amps of SCCs hooked up. I can set them all to lower charge currrent but then when I need to run stuff when the sun is shinning my output is now lower.

My question is, does the typical BMS regulate the charge current to max or do I just need to get more batteries at the same time?

sunsurfer said:

I want to replace them with some cheap LFPs from Amazon. I would like to start with a single 24v 100a battery and add more as money comes in. My concern is the 150amps of SCCs hooked up.

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Most of these batteries will be limited to 100A, although you could get two, or buy a 200Ah pack, with 200A charge/discharge limit.
Beware some packs will be different charge and discharge ie 50A charge and 100A discharge.

OffGridForGood said:

Most of these batteries will be limited to 100A, although you could get two, or buy a 200Ah pack, with 200A charge/discharge limit.
Beware some packs will be different charge and discharge ie 50A charge and 100A discharge.

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I'm fine with a 50a charge on a 100a batt. I just don't know if the BMS will regulate the current to 50a or just shut off or let the cells cook. They don't tell you that much info, even if they knew. Lucky to get a PDF with bad engrish

sunsurfer said:

I'm fine with a 50a charge on a 100a batt. I just don't know if the BMS will regulate the current to 50a or just shut off or let the cells cook.

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The BMS can be programmed to accept a charge/discharge current limit.
If the charging current exceeds the set limit, the BMS will just cut-off.
The BMS never actively regulates the current going into the battery. That is the job of your SCC.
Your MPPT controller should automatically regulate the charging current once batteries start getting full.

The BMS will just protect your battery in case of an abnormality.

You paid alot for those Victron SCCs, make good use of them:
If you have "smart" BMSs with some kind of communication, you should be able to hook up communication to the SCCs. This will allow the BMS to throttle down charging current, when a certain SOC is reached, for example.
Look up DVCC and VenusOS. You might need a Raspberry Pi and additional Canbus adapters, but it's doable.
Andy from Off-Grid-Garage has various videos about this setup.

sunsurfer said:

I'm fine with a 50a charge on a 100a batt. I just don't know if the BMS will regulate the current to 50a or just shut off or let the cells cook. They don't tell you that much info, even if they knew. Lucky to get a PDF with bad engrish

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I'm assuming those are SmartSolar units, sounds like this is a manual process where you turn on the system on clear sunny days, can you just enable/disable the chargers via victronconnect? The cells aren't going to cook assuming your charging voltages are reasonable. Once the battery is fully charged, the amps from the SCC will go to the load, assuming the load isn't so big that it needs to draw from the battery as well.

For 24V 100Ah I can recommend the djlb....I'm pretty sure it has over 2.9kWh capacity (versus 2.56kWh nameplate) at least for the early charges. Two of those and you should be set. I don't give mine more than 50A charge rate.

If the batteries are pretty much always full. There won't be much current flowing.
Current flows from the higher voltage source to the lower voltage source.
Amount of current flowing is based on amount of voltage difference. And any system limitations. (Physical or chosen)

The remaining question is if the BMS on the cheap amazon batteries can handle a large charge potential on their own? I could not find any info if the 24v 100a cheap ones on ebay, if they can regulate the charge current on their own or if the BMS will shut off do to excess current available. I doubt the BMS is programmable on them and I do not want to swap BMS.

I'm using 5 Victron 100/30 SCCs. They are top notch for top end charge termination accuracy but that is voltage based and I worry about the low impedance of the LFP cells and the high amps available for them. The SCCs have no idea of the amp/hr capacity of the battery. I need the SCCs to stay full charge current so I can use the power to run the well and other things like cutting firewood, etc A/C this summer.

My lead acid batteries are getting to end of life. Eventually I will buy 4-8 cheap LFP batts to replace them and then I can also cycle over night, but I would like to start with a single battery if possible in the 24v 100a price range. Still not sure if I can or not. Price is pretty amazing, much cheaper then what I paid for my SOK bank.

sunsurfer said:

if they can regulate the charge current on their own or if the BMS will shut off do to excess current available.

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No regulation.
Just off and on.

timselectric said:

No regulation.
Just off and on.

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So it would be better to save up and buy a sized bank. 4 100a batts is what I would think. Thanks.

sunsurfer said:

So it would be better to save up and buy a sized bank. 4 100a batts is what I would think. Thanks.

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Or just limit the charging from the SCC's.
Maybe turn some off, until you need to use the power.
This only matters if charging after discharging.
Once the battery is full, it won't draw anything.
No matter how much charging is available.

sunsurfer said:

So it would be better to save up and buy a sized bank. 4 100a batts is what I would think. Thanks.

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What is the load on the batteries when the clouds roll by? You may have 150A of charging available but are you using all of it?

Brucey said:

What is the load on the batteries when the clouds roll by? You may have 150A of charging available but are you using all of it?

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Irrigating in the summer and running A/C will see about 2500w (includes inverter overhead). Sometimes I add other loads onto that but that is typical.

Long term goal is to add more batt capacity for running extra A/C overnight but the few minutes load between clouds is the first goal. Last summer, the batteries where struggling with more then 5 minutes between clouds.

I have a second 12v system with 20KWH of LFP so this system is not life or death