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SaylorMade · 11-08-2023, 10:27

So, in a few threads here a side-topic has arisen, about wether or not the charger should be in control of when to stop charging a LFP bank.

I think it's worth having a seperate thread about this, since it seems quite important to me, and I would like to hear the different arguments.

For the debate let us assume that the LFP bank has a BMS which will let the user set the different parameters.
We will also assume that the BMS is setup with appropriate values for OVP as per LifePo4 Cells specs, and also an OVP on pack voltage.

My own point of view is that the BMS should control when to stop charging, since only the BMS knows the voltage of each individual cells, and it is my understanding that a charger can only feel pack-voltage.

The arguments I have heard for NOT letting the BMS be in charge ("first-line-of-defence) is that BMS are not to be trusted, but this seem counter intuitive to me, since we will rely on the BMS anyway to keep the cells balanced anyway, so that pack-voltage can be any safe indicator of when max voltage has been reached.

My own setup:
12KWh 48v bank for propulsion controller by a JK BMS with a charger from the supplier.

1.2 Kwh 12v house bank with JBD BMS which is charged mainly from a "stupid" charger just putting out 14.6 volts until the BMS disconnects.

Both of these are relying on disconnect from the BMS when the first cell reaches the set value for max voltage.

I realize that the sudden cut-off from a BMS may have implications for an alternator so that should be adressed seperately.

wholybee · 11-08-2023, 10:56

Something that seems to be completely missed by those promoting using a BMS to control all the charge sources. The individual cell voltages are the same. If they are not, then there is a problem, the BMS should notify you of that so you can take care of it. With that being the case, there is no difference between using cell voltages or pack voltage to charge.

Appropriate charging can be achieved from either a BMS or a charger. That charging cycle would be to charge to a particular voltage, hold that voltage for an absorption period, and then terminate charging without entering a float stage. A BMS that just turns charging on or off, or stops charging at a programmed voltage is *not* correctly charging the battery. BMSs are available that charge correctly, but they are the most expensive BMS's available, and they would be communicating to more expensive chargers that are perfectly capable of going through the exact same charge cycle on their own.

Using the BMS to control everything, there is only a single point of failure. It isn't that it's likely, its not. But if the BMS fails, you ruin your battery. If the BMS is instead only a defense, then the BMS failing will not harm the battery. The charger needs to fail, AND the BMS needs to fail.

So, with that, and that there simply are not any advantages to the BMS controlling everything (not a longer battery life, not safer for the battery, not more capacity, not cheaper, not easier) the best way IMHO is to let chargers do their job. That is, incidentally the way the longest LFP batteries in boat service are set up. Set up by pros, in their own boats, nearly 15 years ago, and still going strong.

The WORST installation is what you are doing with your house bank. Using a BMS to abruptly cut off charging from dumb charger. You are not charging the battery correctly, which will lead to a shorter life. Plus there is the real risk that if something goes wrong the FETS will short, and your battery will be smoking and a melted pile in short order. An alternator load dump has a high probability of doing that.

Replace your dumb charger with a smart charger that has an LFP profile. And set the charger and BMS such that the BMS NEVER disconnects. The disconnect is a last line of defense, and with a BMS such as yours that can't control a charger via a CAN bus, that is mandatory. There really isn't a question or debate in your particular case.

sailorboy1 · 11-08-2023, 10:58

to me the BMS is protective device that never should ever get "tripped"

SaylorMade · 11-08-2023, 11:07

Quote:

Originally Posted by sailorboy1

to me the BMS is protective device that never should ever get "tripped"

That is an assertion, but arguments for that assertion is what I am looking for.
Again - you still rely on the BMS to balance the cells, right?
If it does not do that you are still screwed if relying on the pack voltage?

sailorboy1 · 11-08-2023, 12:06

Quote:

Originally Posted by SaylorMade

That is an assertion, but arguments for that assertion is what I am looking for.
Again - you still rely on the BMS to balance the cells, right?
If it does not do that you are still screwed if relying on the pack voltage?

I don't rely on my BMS to do anything!!!!!!!!!!!!!! Far as I am concerned in is nothing but a protection device for the batteries that should NEVER be needed.

Balancing is hocus pocus IMO

wholybee · 11-08-2023, 12:10

Quote:

Originally Posted by sailorboy1

I don't rely on my BMS to do anything!!!!!!!!!!!!!! Far as I am concerned in is nothing but a protection device for the batteries that should NEVER be needed.

Balancing is hocus pocus IMO

You may consider it hocus pocus, but I bet your BMS is balancing the cells without you knowing or worrying about it. Which is great, because there are too many other things to worry about.

SaylorMade · 11-08-2023, 12:22

Quote:

Originally Posted by sailorboy1

I don't rely on my BMS to do anything!!!!!!!!!!!!!! Far as I am concerned in is nothing but a protection device for the batteries that should NEVER be needed.

Balancing is hocus pocus IMO

OK ... so you are ok with a rogue cell being charged well above limits?
I am not sure I get this line of thought

Panope · 11-08-2023, 12:36

How do electric cars do it?

SaylorMade · 11-08-2023, 13:18

Quote:

Originally Posted by Panope

How do electric cars do it?

I don't know - do you?

sailorboy1 · 11-08-2023, 13:35

Quote:

Originally Posted by wholybee

You may consider it hocus pocus, but I bet your BMS is balancing the cells without you knowing or worrying about it. Which is great, because there are too many other things to worry about.

This sounds amuck amuck amuck

Which is witchcraft spell talk about casting one to solve nothing

I have never read a story of a boater having a battery problem because of "balance". Did thread earlier I. Year and most are doing nothing about balancing. I asked my manufacturer and they basically said there is no reason

sailorboy1 · 11-08-2023, 13:38

Quote:

Originally Posted by SaylorMade

OK ... so you are ok with a rogue cell being charged well above limits?
I am not sure I get this line of thought

Because you are looking to do some complex something about nothing

wholybee · 11-08-2023, 14:17

Quote:

Originally Posted by sailorboy1

This sounds amuck amuck amuck

Which is witchcraft spell talk about casting one to solve nothing

I have never read a story of a boater having a battery problem because of "balance". Did thread earlier I. Year and most are doing nothing about balancing. I asked my manufacturer and they basically said there is no reason

Well, basically every BMS made does at least passive balancing, so in any reasonable installation cells will never be out of balance. That is why you rarely hear about it. But there are still a few people on this board that had issues because they didn't follow manufacturers' guidelines.