Noobie Building LifePo4 Battery - BMS question and more

Hi guys, I just ordered 4 280Ah battery 3.2V cells. I plan on building a 12V van solar system with it.

I'm confused a bit about the BMS, there are many options out there. I don't need low temp shutoff function as I'll be buying a victron energy mppt that has one.

The Daly one seems like a good cheap option, however what's the difference between the separate and common port? Is the discharge rate of the most energy hungry combo of devices I'll be using enough to go off to calculate the size?

Is there anything else I should look into buying for cheap on Aliexpress for the battery or the system while I wait for the cells to come?

loverofpeace said:

Is the discharge rate of the most energy hungry combo of devices I'll be using enough to go off to calculate the size?

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Most folks put the BMS between the SCC and battery. Therefore, the size (ah handling) of the BMS should be based on the max charge current, plus a bunch.

The separate port models usually have a ridiculously low charge current ability. The separate port model in theory lets you control charging and discharging with the parameters of the BMS. The BMS is just a switch on the negative lead - when the parameters are exceeded, the switch opens. Think of it as a last line of battery defense, shut everything down switch.

There is some balancing functionality too but that is often at the extremes of the charge range and minuscule (millivolt range which is dwarfed by charging currents).

MisterSandals said:

Most folks put the BMS between the SCC and battery. Therefore, the size (ah handling) of the BMS should be based on the max charge current, plus a bunch.

The separate port models usually have a ridiculously low charge current ability. The separate port model in theory lets you control charging and discharging with the parameters of the BMS. The BMS is just a switch on the negative lead - when the parameters are exceeded, the switch opens. Think of it as a last line of battery defense, shut everything down switch.

There is some balancing functionality too but that is often at the extremes of the charge range and minuscule (millivolt range which is dwarfed by charging currents).

Click to expand...

Thanks a bunch for the clear explanation. So with 2 100w panels with max charge current of 5.56a each would a 20a one be sufficient or would you go bigger than that?

Also the 4S for BMS's refers to how many cells the battery has correct? So in my case this one would be sufficient or do I have this wrong?

loverofpeace said:

Also the 4S for BMS's refers to how many cells the battery has correct? So in my case this one would be sufficient or do I have this wrong?

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4S in the context of a BMS basically means "12V," more specifically it can handle 4 cells in series. So yes, for your application a 4S BMS is perfect.

Thanks a bunch for the clear explanation. So with 2 100w panels with max charge current of 5.56a each would a 20a one be sufficient or would you go bigger than that?

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You want to size your BMS for the max current that might ever be passing through it. Like MisterSandals mentioned, if you buy separate port, it will have separate ratings for charge and discharge, will just have one combined rating.

One thing to keep in mind, many people here recommend you try to keep your continuous current below ~60-80% of the BMS's rating for cheaper BMS' like the Daly. Up to you whether you want to be cautious like this or not, but since the price difference isn't that much, its probably worth oversizing the BMS in my eyes.

Im building something similar sounds like have 4 cells of the 280ah batteries will be connected as a 4S. Making this as a portable very universal charge station and to power my fridge when camping. I've yet decided on my SCC but I do have the Chargery BMS which I like having the external display that I have mounted in a pelican micro case to keep any moisture out.

tim0shel said:

Im building something similar sounds like have 4 cells of the 280ah batteries will be connected as a 4S. Making this as a portable very universal charge station and to power my fridge when camping. I've yet decided on my SCC but I do have the Chargery BMS which I like having the external display that I have mounted in a pelican micro case to keep any moisture out.

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Nice. Is yours 100A? I just looked at the chargery one and the display is pretty nice.

Dzl said:

4S in the context of a BMS basically means "12V," more specifically it can handle 4 cells in series. So yes, for your application a 4S BMS is perfect.



You want to size your BMS for the max current that might ever be passing through it. Like MisterSandals mentioned, if you buy separate port, it will have separate ratings for charge and discharge, will just have one combined rating.

One thing to keep in mind, many people here recommend you try to keep your continuous current below ~60-80% of the BMS's rating for cheaper BMS' like the Daly. Up to you whether you want to be cautious like this or not, but since the price difference isn't that much, its probably worth oversizing the BMS in my eyes.

Click to expand...

Thanks I will. If I wasn't to go the ultra cheapo way like Daly is there one you'd recommend?

loverofpeace said:

Thanks I will. If I wasn't to go the ultra cheapo way like Daly is there one you'd recommend?

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Nothin wrong with Daly in terms of a cheap basic BMS.

But the chargery BMS and the battery hookup bms are worth looking into. I really like the Chargery, it was my top choice for a while, now I'm planning to use the SBMS0.

Dzl said:

Nothin wrong with Daly in terms of a cheap basic BMS.

But the chargery BMS and the battery hookup bms are worth looking into. I really like the Chargery, it was my top choice for a while, now I'm planning to use the SBMS0.

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Thanks, I ended up buying the 120a 4S with the low temp cutoff that will recommends on his site. I figure it's better have good margin like you guys mentioned and not cheap out on it. Thanks for your help

loverofpeace said:

Nice. Is yours 100A? I just looked at the chargery one and the display is pretty nice.

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I have the BMS8T - 300 but the BMS you can run whatever size Relay that you think you will need. So I have a 300 amp relay which will be the main load and will have smaller one for the charge side.

I looked into the Chargery, and was leaning that way until I spent more time researching all the options.
In the end, I settled on a Bluetooth version with a low temp cutoff its cheap and I will be able to look at it almost constantly until I get comfortable or bored then I can ignore it until I have a concern about charge level. When I reach this point, I don't want to have to worry about what to do with the display.
I went with the 120A version due to my engineering background. You always want to have margin between what your use is and your capacity. This keeps you out of trouble when extreme situations arise.
At high temperatures, electronics lose efficiency and batteries lose capacity. This is worst case scenario especially if you are trying to cool off by using an inductive load like a fan or the compressor on your fridge. These loads have high initial spikes of current that are not necessarily on the spec sheet. This inrush can overload your system or blow fuses if you do not have enough margin.