What if I don't use a BMS at all?

[Bert65]

First of all, thank you for having me on your Forum, I am a 55 year old University Professor with a 32 ft sailboat. My former boat had solar and I loved it, so the now boat has to use solar as well

I am making my ship ready for a conversion to LifePo4 coming from SLA. My current set up is that I have a 74 Amp starter battery and a 74 Amp household battery. Charged by the alternator and a Victron IP22 charger. I want to change this to a setup with four 200Amp (probably Catl) cells replacing the household battery.

A solar panel (2x55watt) will charge the household battery through a Victron mppt smart charger and the alternator will be protected by using a Victon 25A Buck Boost DC / DC converter. The IP22 charger will be set to Lithium and from than on, will only charge the household battery. De starter battery will receive its own charger. The setup will be monitored by a Victron BMV 602s that is capable to shut the system off if there is under voltage to preserve the battery.

All these devices have a considerable amount of intelligence and all have LifePo4 settings. They all take care of over voltage and the monitor takes care of under voltage

I want to take my household battery out every winter and top balance it. So never charged under 0 dg Celsius

Why would I still want and need a BMS?

 

[Whinny]

My short answer................Yes

 

[Bert65]

So you think I do, but why? What will the BMS do for the battery that the other systems don't?

 

[Dzl]

So you think I do, but why? What will the BMS do for the battery that the other systems don't?

The fundamental and unique purpose + capability of a BMS is CELL LEVEL battery management and protection. It is the only device in a system that is capable of (1) seeing, plus (2) taking (protective) actions based upon cell level conditions. The most important of these conditions are low voltage and high voltage protection/disconnection. Most other functions of a BMS can be accomplished with another component, but cell level protection requires a BMS.

Cell damage happens at the cell level, so the ability to act based on cell level conditions is indispensable. Particularly if you are building with cheap raw cells that are often fairly mismatched (despite what a seller may tell you) but it also applies to decently matched cells. Consider a situation where you have cells at 3.45, 3.45, 3.45, 4.30. Without a BMS the system just sees the total voltage of the pack 14.55, which is within acceptable limits, however your 4th cell is at a dangerously high voltage. A BMS would have recognized that early on and cut charging before it got to a dangerous level.

 

[Bob B]

I think we are getting kind of tired answering this question ... do a search for "do I need a BMS".

 

[Pyke13]

What happens if you have a runaway cell? Either up or down. What’s going to stop it causing long term damage to itself

 

[Gazoo]

What happens if you have a runaway cell? Either up or down. What’s going to stop it causing long term damage to itself

A BMS...lol.

 

[Bert65]

The fundamental and unique purpose + capability of a BMS is CELL LEVEL battery management and protection. It is the only device in a system that is capable of (1) seeing, plus (2) taking (protective) actions based upon cell level conditions. The most important of these conditions are low voltage and high voltage protection/disconnection. Most other functions of a BMS can be accomplished with another component, but cell level protection requires a BMS.

Cell damage happens at the cell level, so the ability to act based on cell level conditions is indispensable. Particularly if you are building with cheap raw cells that are often fairly mismatched (despite what a seller may tell you) but it also applies to decently matched cells. Consider a situation where you have cells at 3.45, 3.45, 3.45, 4.30. Without a BMS the system just sees the total voltage of the pack 14.55, which is within acceptable limits, however your 4th cell is at a dangerously high voltage. A BMS would have recognized that early on and cut charging before it got to a dangerous level.

I get your point (although I am not sure if it is possible for a single cell to hold more than 4.2 volts) But I definitely don't intent to just smack 4 cells from Ali on sale together. They will we A-grade (Winston, Catl, Sinopoly or Calb) and they will be top-balanced. I am considering adding only active cell balancing and /or a volt-meter that gives reading for all four cells.

 

[Gazoo]

I get your point (although I am not sure if it is possible for a single cell to hold more than 4.2 volts) But I definitely don't intent to just smack 4 cells from Ali on sale together. They will we A-grade (Winston, Catl, Sinopoly or Calb) and they will be top-balanced. I am considering adding only active cell balancing and /or a volt-meter that gives reading for all four cells.

The reason a BMS is recommended is in case one of the devices in a system fails and doesn't behave properly. The BMS the last line of defense for protecting the cells. Cell grading/matching has no bearing on this.

It's not recommended to charge more than 3.65 volts. An overcharged cell builds up gasses internally and causes the cell to bloat. They are your cells and if you want to take the risk of going without a BMS then that's your decision.

 

[MisterSandals]

55 year old University Professor

LifePo4

Suspecting that you're not a chemistry professor.

although I am not sure if it is possible for a single cell to hold more than 4.2 volts

The key word is "hold". I have an air compressor tank that will "hold" 250psi. If i try to put 300psi in there, it probably won't "hold" 300psi so it has a safety valve. A BMS is a LOT like this safety valve that monitors CELLS and not just overall battery voltage as your BMV 602 does (though i am not familiar with it in particular).

And as Dzl pointed out with his cell voltage example, that is REALLY how the cell voltages diverge at the extremes of the charge voltages (below ~2.8v and above ~3.5v).

You can act as your own BMS if you have a way to watch individual cells closely while charging and discharging. Having a a conservative charge profile and conservative discharge habits can greatly reduce how much attention this requires. And the key word there was "reduce" as it will only reduce your chances of damaging cells.

 

[Dzl]

I get your point (although I am not sure if it is possible for a single cell to hold more than 4.2 volts)

I'm not sure either, but that detail is unimportant here, the point is that without cell level protection, a single cell can get to a damagingly high or low voltage while the pack level voltage doesn't indicate (or see) any problem and therefore can't protect against it. A functional active cell balancer lessens the likelihood of this happening to some degree (as do matched cells, as do conservative pack level voltage limits), but it is not a protection layer and doesn't provide any of the safety that a BMS does.

If you understand the distinction and understand the risks, it is your prerogative to elect to use or not use a BMS. My two pence is that its a relatively cheap protection layer that provides (1) cell level protection that no other component offers (2) a second 'layer' of protection to protect against equipment failure or misconfiguration (3) automated protection to protect against user error and human forgetfulness/distraction. While I think a BMS makes sense in 99% of the cases here on this forum, I won't say its the only way.

Like a seat belt, you don't actually need one if nothing goes wrong, and you can take other steps to lessen the likelihood of something going wrong, but like a seatbelt if something does go wrong its value is immediately apparent. Unlike a seatbelt, we are (mostly) not talking about human life here, we are talking about monetary loss and damage to the system, and a lesser risk to human safety, so to some degree it comes down to how much you value your system and your $$ and whatever structure or vessel your system is being installed into.

But I definitely don't intent to just smack 4 cells from Ali on sale together. They will we A-grade (Winston, Catl, Sinopoly or Calb) and they will be top-balanced. I am considering adding only active cell balancing and /or a volt-meter that gives reading for all four cells.

While verifiably matched grade A cells, that are properly balanced, should charge and discharge more evenly than unmatched grey market cells, and can and have been used without a BMS, personally, I would be more inclined to use a BMS in this situation, as the cost per cell is significantly higher, and relatively speaking, the cost of adding the protection a BMS overs is proportionally cheaper, even if its somewhat less mandatory.

As a side note, where would you source Grade A CATL cells? The others you mention have distributors and legitimate retail sales channels, I have not seen the same with CATL but my knowledge is quite limited here.

 

[Bert65]

@Dzl : thanks for taking the time for a serious response (and to all others as well). My point was (and still is a bit) that there is three sources of energy for the LifePo4 battery, all of them backed with considerable intelligence and a battery monitor.
I remember that, for larger systems, we used 2volt Gel cells and I do remember they did not use a BMS. But of course a Lithium battery is something else, even in LifoPo4 form. I am looking into the https://www.lithiumbatterypcb.com/ or Overkill BMS right now. If only it is for a total ease of mind, definitely worth a few pennies.

Catl has an importer in Europe, Quantron AG, so far only for batteries. They also started building a factory in Germany.

 

[codfish]

@Dzl : thanks for taking the time for a serious response (and to all others as well). My point was (and still is a bit) that there is three sources of energy for the LifePo4 battery, all of them backed with considerable intelligence and a battery monitor.
I remember that, for larger systems, we used 2volt Gel cells and I do remember they did not use a BMS. But of course a Lithium battery is something else, even in LifoPo4 form. I am looking into the https://www.lithiumbatterypcb.com/ or Overkill BMS right now. If only it is for a total ease of mind, definitely worth a few pennies.

Catl has an importer in Europe, Quantron AG, so far only for batteries. They also started building a factory in Germany.

Folks I agree with all of the above posts that a BMS is essential to protect the health of a battery, but ...

I do think in marine applications there is a serious tradeoff between protecting the health of the battery and the health of the boat and crew. There are numerous situations where any boat owner would gladly sacrifice a battery if it means saving the boat. If there is even 1 Ah left and it helps me get the anchor up while dragging onto a lee shore, or get out one last MAYDAY call, then please let me use it. I really can't have the BMS deciding that the health of 1 cell dropping below 2.5 V is more important. So yes, monitor it (perhaps at cell level) and feed it with high quality components (Victron), actively balance it, but ask yourself what your risk tolerance is to the various modes of failure and which failures leave you in more dangerous situations.

One alternative is using a low current BMS to control a contactor's coil voltage and adding a readily accessible switch to bypass the BMS. But I must be able to bypass it without getting out wrenches and diving into the battery compartment.

 

[MisterSandals]

One alternative is using a low current BMS to control a contactor's coil voltage and adding a readily accessible switch to bypass the BMS.

It is possible to run the charge current thru the BMS and loads (or even select loads) directly from the battery.

 

[codfish]

It is possible to run the charge current thru the BMS and loads (or even select loads) directly from the battery.

Agreed. But I wanted to make the point that people need to seriously consider failure modes and their consequences. Just simply saying "use a BMS", may not be an appropriate risk tradeoff without further qualification or easily invoked work arounds.

 

[Bert65]

It is possible to run the charge current thru the BMS and loads (or even select loads) directly from the battery.

That sounds interesting! So I could have my anchor winch rum directly from the battery? I draws an Initial 140 amps. Just by bypassing the BMS? Be course that is one other reason why I was considering this; the limits a BMS would put on both in and output of the battery. The anchor winch is now connected to the starter battery and only works with running motor. That means that in a safety situation where there is engine failure, I can not lower my anchor and am therefore considering connecting it to my household battery. I know the other option is tot buy a 150amp BMS, but those are bulky and don't come cheap

 

[MisterSandals]

That sounds interesting! So I could have my anchor winch rum directly from the battery? I draws an Initial 140 amps. Just by bypassing the BMS?

Yes. And as they say, “you become the BMS”. You will be responsible for watching the battery, at the cell level if possible.

EDIT: I should add, from my experience with being a BMS, there are quite a few tools that help. There are battery monitors with various alarms (low battery voltage, cell disparity, low cell voltage) that have saved my bacon a couple times. Things can go awry quickly so anything that can help you do your job as a BMS is good. And these monitors, like the BattGo are $30-$50.

 

[Terrapin]

That sounds interesting! So I could have my anchor winch rum directly from the battery? I draws an Initial 140 amps. Just by bypassing the BMS? Be course that is one other reason why I was considering this; the limits a BMS would put on both in and output of the battery. The anchor winch is now connected to the starter battery and only works with running motor. That means that in a safety situation where there is engine failure, I can not lower my anchor and am therefore considering connecting it to my household battery. I know the other option is tot buy a 150amp BMS, but those are bulky and don't come cheap

This is my plan for running my microwave that may require 180A to the inverter by bypassing the 120A max of the BMS. But as stated, you now own monitoring for low voltage. But also info available regarding the capacity of the battery on smart BMSs like the overkill will not reflect watthours drawn off the battery around them. It's a trade off on monitoring, but you can always use a separate shunt monitor or hall effect current monitor to combine all watts out from the battery for a capacity monitor.

 

[JeepDaddy]

You are free to do whatever you wish. You asked for advice and got it from several knowledgeable sources.

Buy the proper equipment and get proper results. Do it on the cheap and your mileage will vary.

I really can't have the BMS deciding that the health of 1 cell dropping below 2.5 V is more important.

Point taken, but there really isn't much power left under 2.5v. It isn't going to pull an anchor for sure. A manual bypass relay would be fine in those situations.

 

[Forbisher]

Point taken, but there really isn't much power left under 2.5v. It isn't going to pull an anchor for sure. A manual bypass relay would be fine in those situations.

There really is not much power under 3v and almost nothing under 2.5v in a LFP battery.