Very new beginner--BMS vs Charge Controller. Commonalities, Differences, Overlapping capabilities? Would like help please.

[SIsolar]

I am pretty much a neophyte here so please forgive me if I am asking the most rudimentary questions of if this has been asked before.

Background:

I have built a little 12v DC only battery box with a 15 ah SLA battery complete with charge controllers. I copied a YouTube video. It was a good introduction.

I am building a much more advanced battery generator based on a 20 ah LiFePo4 battery (this might get an upgrade--didn't want to go too expensive at first) and has a built in inverter. This is built on a rolling tool case. I am about 85% done, BUT

I am already dreaming up a large ammo box build complete with a home-built LiFePo4 battery, meaning constructed from individual cells. All the info I see has an attached BMS which makes sense, but much of the functionality of the BMS seems to overlap with that of a charge controller (low voltage cutoff, etc.). I know that I would need a charge controller in this system somehow, but the builds I have seen don't have them. I assume that the charge controller is some type of outside unit.

My basic question is how do I connect a charge controller to a BMS? Is it as simple as plugging in the Positive and Negative leads from a BMS into the charge controller? Do I plug in leads from the battery unit directly, bypassing the BMS (seems wrong somehow)?

I would love any feedback I can get. And if you need any more details, I will happily provide them.

SIsolar

 

[Rocketman]

The BMS - (battery monitor system) is the item that looks over the battery cells and if something is going wrong it shut down - protecting the expensive cells.

A SCC (solar charge controller) - charges the battery from the solar panels. Yes some of the settings seem redundant- but think of it this way…. You NEVER want to have the bms to have a problem- so you design everything else within the bms specs. For example, my bms shuts off at 14.8v 3.7v per cell. My SCC stops charging at 14.3v. I should never get a cell to 3.7v with that setup.

Any other type of charger (battery charger), must also carefully charge the batteries according to lithium profiles.

I also do the same thing on the discharge side, I have everything that connects to the battery has a low voltage disconnect that is above the bms. All my lights,pumps,etc run through a Victron Smart Battery Protect (BP-65). That way if I am not at my rig, my lights, etc. shut down, then my inverter (that runs my fridge). Then the BMS would shutdown. (After a really long time because there is not anything running except the bms).

 

[Rocketman]

Just realized I didn’t answer your real question…

Your cells + bms = your battery. (Just like you would buy a lithium battery from Amazon).

You should have a good lug or terminal that is your battery positive and negative. Any charging or loads should connect to the battery positive & negative.

Don’t forget fuses!

 

[SIsolar]

Rocketman,

Thanks for getting back to me. I guess I was overly verbose in my initial question. So per your answer, does this mean that the BMS and SCC both connect to the same electrical point? Or does this mean that the setup looks something like:

Solar panel > SCC > BMS > Individual Battery Cells?

This latter setup seems to make more sense as being in a series connection, either the SCC or the BMS can shut off the whole charge/discharge cycle.

Again, thanks for the reply.

And no, I certainly won't forget fuses, though I tend to prefer circuit breakers!

 

[Rocketman]

I think you have it, but Let me restate it another way.

Battery = (cells +BMS).

You will have a battery positive and negative. (Often in cheaper bms’s one wire or set of wires out of the bms will be the battery negative). I would have a small buss bar as the battery positive and negative. label it as “+” & “-“.

Attached to the battery “+” & “-“ will be: the SCC, the loads, and the inverter(another load).

The SCC will have panels attached to it, to charge the battery.

The inverter will have A/C loads, and any DC loads (like lights) will also attach to the buss bar.

All items coming off the bus bar will have fuses or circuit breakers.

Also, you may want to have a battery monitor (I like Victron Smartshunt (but there are cheaper ones out there). This is your “fuel” gauge for the battery. So you know if the battery is 90% full or 10% full.

Remember you could use a lead battery for this setup, and it would attach to the + & -. The lithium cells need the bms so you don’t accidentally kill them. That’s why lithium cells plus bms = a battery).

Also, don’t rely on the bms to save the battery - it should never have to cut-off. Your chargers should charge to a good voltage (below the bms cutoff) your inverter and loads should have low voltage cutoffs above where the bms cuts off.

The bms’s job is to monitor the cells, balance the cells, and prevent the battery from being ruined. If you design your system well, the bms should almost be redundant (except for the balancing the cells - but they should have been top-balanced before assembly)

Good luck

 

[SIsolar]

OK, got the part about the Battery=cells+BMS. Actually, that makes sense and it just sounds like the way that any lithium battery should be (do 18v tool battery packs have a BMS? Seems like they should.).

My LiFePo4 system does in fact come with a battery monitor which seems essential for any of these types of projects. Incidentally, When I power my system on, my battery meter turns on and every single LCD turns on also. This looks like everything is reading "8". I can't make any meaningful information out of this though obviously it is getting power. Any ideas here?

And the rest of the setup is like an SLA battery setup--the battery pack then connects to the SCC which itself both receives electricity from the solar panel and distributes power out the load terminals on the SCC.

So I think I have the SCC and the BMS figured out. It sounds like if I really want Bluetooth connectivity, I better get it in both the BMS and the SCC.
Any ideas about my battery monitor only reading "8"?