Can someone help with BMS, Charge controllers

Hi everyone

For a little background on this post, I have built (really, copied a YouTube build) a small SLA ammo can DC only battery box.  That got me interested in making a larger, AC capable solar generator which I have about 85% finished.  It has a LiFePo4 battery that I may upgrade in the future and an inverter.  I am still fiddling with a couple of components, but I am thinking about a third battery box, which will be based on a battery (perhaps 80 AH) that I will build from prismatic cells.  I know for this I will need a BMS.  But the functions of the BMS seem to overlap with those of a decent charge controller (low voltage shutoff, etc.) that I am not exactly certain what I will be looking for in a  BMS, charge controller or even how to hook the two up.  I have my eyes set on a BMS that has a Bluetooth connection (seems handy!) but a decent charge controller likely has this too.  To make matters more confusing, many of the builds that I see online consist of a 5.56 magazine ammo box (about twice the size of a .50 cal ammo box), with a large, prismatic cell battery, a BMS, but no charge controller.  I assume that it is charged with a charge controller used externally.

So with all that information thrown at you could anyone help me decipher what functions a BMS solves (I knows that it balances the cells, but what about other protections?), what the charge controller does, will their functions safely overlap, and how to hook these up.

I know this is a lot so thanks much in advance,

Eric

Here is the youtube link

https://www.youtube.com/watch?v=WFJlMU272wU

My understanding is that the charge controller has a different purpose. It regulates the overall charging voltage, to put less stress on the cells during charging. I don't exactly understand the details, but the optimum charging voltage profile varies with cell chemistry.

The BMS is usually internal to the battery unit, down stream of the charge controller. It deals with fine tuning the charge on individual cells with the battery.

The confusion comes because some BMS units duplicate the charge controller, and some chargers are "dumb" units that don't do any fancy regulation and just sit at the final charge voltage for the whole charging period.

My experience is all from the world of DIY ebike conversion, not solar installs. I'm not sure how transferrable the terminology is.

There are excellent facebook communities for people building their own batteries. I'm a member of one specifically for 18650 cells, but they would certainly be able to clarify this stuff for you.

Michael,

Do all lithium battery packs have a BMS?  As in do tool batteries have a BMS built into them?

Good question.

I think the answer is no, they don't ALL have them in. But they probably should.

The charge balancing of a BMS prolongs battery life considerably. As individual cells age their charging profiles change and they can end up charging at different rates from each other. In a battery pack a cell that charges just 1% slower than the other cells may, for example, end up at 4.0V when the others are at 4.2V. And on each charging cycle this can get more and more exagerated.

As some cells charge more slowly and end up at lower voltage, other cells in the series end up being at an over voltage as the "dumb" controller just applies a total voltage across the whole pack.

So a pack that is 4 18650 Li-Ion cells in series would nominally charge to :
4.2V + 4.2V + 4.2V + 4.2V = 16.8V

Without a BMS as the cells age they may charge to:
3.9V + 3.9V + 4.6V + 4.2V = 16.8V

The pack still reads the same total charge, but the over charging of one cell, and undercharging of the others, drastically shortens cell life. And if the over-voltage gets bad enough you can potentially dramatically burn out a cell (think - unquenchable lithium fire).

For a small semi-disposable pack like a power tool that may be a non-issue. For a power wall for a home, it is a bit issue.

Michael,

Wow!  Thanks.  So maybe those expensive tool batteries really should have a BMS, but probably don't--makes them a resale item!

And you largely confirmed much of what I thought I already knew--that more advanced BMS's have some overlapping qualities of a solar charge controller.

So I would like this last build to have Bluetooth connectivity so I can easily monitor it from my phone (handy!) and several BMS and several charge controllers have this built in.  I am trying to figure out how to connect these together.  Does the BMS and the Charge Controller both sort of come together at the same point on the battery pack?  Should one feed to the other?  Is this going to vary by hardware?  I can go on and on, but this is all good information to have.

Thanks in advance,

Eric

OK, I am looking at a diagram.  It looks like the Battery, Charger and Load all electrically connect to the same point.  Sound right?  I am going by the 2nd to last diagram on the let side of the screen in the link below.

https://a.co/d/e1Tx9hi

The BMS is wired to the junction between each block of parallel cells. It balances the cell voltages by doing tiny voltage adjustments during the charging process. I dont' exactly understand how the wizardry works. But it needs to be designed into the pack from the start, not be an after thought. It draws the power for itself from the main charge point.

For your smart bluetooth connection - you want that coming from the BMS so it can tell you fine detail about the health of each block of parallel cells. That way if any cells have problems you can use it to diagnose which block the failing cell is in. It will also be able to give you summary figures for the whole unit, like total voltage.

Right, I understand that a BMS is there to make sure that each cell charges and discharges properly.  I guess what I am getting at is how do they get wired in?  Are they in a series with the solar charger?  Do they connect to the same electrical point?  And I guess I am finding that it differs based on the BMS.

Thanks for all the help Michael, I am sure I will be back for more.

Eric

Solar Panel > Charge Controller > BMS > Individual cells in the battery pack

Thanks S Bengi!  That is what I was looking for.

Eric

Eric Hanson wrote:Right, I understand that a BMS is there to make sure that each cell charges and discharges properly.  I guess what I am getting at is how do they get wired in?  Are they in a series with the solar charger?  Do they connect to the same electrical point?  And I guess I am finding that it differs based on the BMS.

Thanks for all the help Michael, I am sure I will be back for more.

Eric

BMS are usually wired in series from your negative connection on your battery pack.

Then there is another set of wires that connect the BMS to each cell.