Alternators - leave them alone and forget about them

[Jeff F]

There are many scenarios where the cause of the BMS disconnect affects all the series drop-ins, and all disconnect either together or one after another.

I think series wiring is largely out of scope in the context of the discussion. Let's assume 12v.

They are complex, and become your hobby to manage. AGM with current charging and regulation tech are an install and forget for many years prospect.

I don't disagree, but for me and I'm sure many others management is not onerous.

The end-of-life scenario is potentially challenging. Failure cases that seem remote now become more prevalent as the system ages. Many of us routinely deal with issues and challenges created by shortsight a generation ago when our boats were built. But OTOH lithium should outlast AGM, so using say a 10 year window LI can and should provide trouble free service vs AGM.

 

[Jeff F]

Jeff See PM
Regards

Saw that, and appreciate comments. Maybe share here? We're all learning...

 

[Jeff F]

I'm never more than 50 miles from shore or a day or two from civilization.

I should add that my wife stays home, and I mostly traveling solo. So I am able to assume risk and deal with minor inconvenience that many would find intolerable.

Again, part of the use case.

 

[Jeff F]

My batteries are individually cabled. I have to believe that there is a way to recognize when current stops flowing on a battery.

Actually of course there is a way. Each individual battery gets its own disconnect switch, and you periodically switch between them at the end of a charge to make sure they're all on.

You're not concerned with gradual failure, just identifying terminal cases, given say 4 batteries.

 

[Jeff F]

Just one more thought - does anyone know of a meta-BMS device?

Let's say I want to use 2 or more stupid batteries, each with a 200a BMS, wired in parallel.

Why couldn't one build a basic device between that bank and the alternator? Basically it has to monitor current and voltages across the circuit, and shut down the alternator if anything goes out of range. You could have limits set against known individual battery limits. Could provide full battery-level monitoring. Connect each battery to the device.

There are many ways to fully solve for this without having to go to cell-level monitoring.

Maybe this is better suited to a new thread, but you can see how I got to here.

TT, this is where the ATC/ATD gets implemented.

 

[Jeff F]

But they where also very difficult to detect as they were dying a slow, camouflaged death, causing the charger to output slightly more, vented corrosive gas in the bilge and when finally viewed with an infrared camera showed internal hot spots.

Yep. I was motivated to go LI in part because I had inadequate ventilation in my battery compartment.

LA batteries are stupid and dangerous. We've just learned to live with them.

 

[twistedtree]

Just one more thought - does anyone know of a meta-BMS device?

Let's say I want to use 2 or more stupid batteries, each with a 200a BMS, wired in parallel.

Why couldn't one build a basic device between that bank and the alternator? Basically it has to monitor current and voltages across the circuit, and shut down the alternator if anything goes out of range. You could have limits set against known individual battery limits. Could provide full battery-level monitoring. Connect each battery to the device.

There are many ways to fully solve for this without having to go to cell-level monitoring.

Maybe this is better suited to a new thread, but you can see how I got to here.

TT, this is where the ATC/ATD gets implemented.

You could do that, but you really need to know what each cell is doing, not just the total pack voltage. Here's and example why, using over voltage as an example.


LFP cells are fully charged at 3.60V per cell, which is 14.4V when you string 4 of them together in a 12V battery. But let's take an exaggerated example to show the problem. Say the cells are out of balance such that 3 of them are at 3.5V, and the fourth is at 3.6V. That's 14.1V, so your chargers will keep charging striving for 14.4V. That will drive the high cell into an over voltage condition and the BMS will disconnect the battery. The same thing can happen on discharge if you have a low cell.


This is why you can only predict a BMS disconnect if you have visibility at the cell level, and only the BMS has that visibility.


But to your idea, I have read of some people who have cut open their drop-ins and hooked up a separate BMS to monitor and report cell info and associated alarming.

 

[twistedtree]

You could do that, but you really need to know what each cell is doing, not just the total pack voltage. Here's and example why, using over voltage as an example.


LFP cells are fully charged at 3.60V per cell, which is 14.4V when you string 4 of them together in a 12V battery. But let's take an exaggerated example to show the problem. Say the cells are out of balance such that 3 of them are at 3.5V, and the fourth is at 3.6V. That's 14.1V, so your chargers will keep charging striving for 14.4V. That will drive the high cell into an over voltage condition and the BMS will disconnect the battery. The same thing can happen on discharge if you have a low cell.


This is why you can only predict a BMS disconnect if you have visibility at the cell level, and only the BMS has that visibility.


But to your idea, I have read of some people who have cut open their drop-ins and hooked up a separate BMS to monitor and report cell info and associated alarming.

And YES, this is exactly where ATC and ATD come from. The BMS sees that one cell is fully charged at 3.6V, and signals that charging needs to stop. The prevents what would otherwise be a certain disconnect, followed by an alternator surge, etc. The ATC signal can be easily adapted to turn off either an internally or externally regulated alternator, shut off other chargers, solar chargers, etc.

 

[Jeff F]

You could do that, but you really need to know what each cell is doing, not just the total pack voltage. Here's and example why, using over voltage as an example.


LFP cells are fully charged at 3.60V per cell, which is 14.4V when you string 4 of them together in a 12V battery. But let's take an exaggerated example to show the problem. Say the cells are out of balance such that 3 of them are at 3.5V, and the fourth is at 3.6V. That's 14.1V, so your chargers will keep charging striving for 14.4V. That will drive the high cell into an over voltage condition and the BMS will disconnect the battery. The same thing can happen on discharge if you have a low cell.


This is why you can only predict a BMS disconnect if you have visibility at the cell level, and only the BMS has that visibility.


But to your idea, I have read of some people who have cut open their drop-ins and hooked up a separate BMS to monitor and report cell info and associated alarming.

I'm really not concerned with an unexpected BMS shutdown. I recognize it as a signal to swap out the battery. I keep a spare on the rack.

I fully understand the failure model for the battery. Explain again why I should care.

Edit to add: a battery failure wouldn't necessarily call for a call to stop charging, as B.S. pointed out earlier.

 

[Jeff F]

Step back for a moment. Think of each battery as a cell. Now, how can you manage that? Easily I think.

Edit to add: an uncommunicative cell with a BMS.

 

[twistedtree]

Step back for a moment. Think of each battery as a cell. Now, how can you manage that? Easily I think.

So if one of your batteries does a disconnect, you toss it?

 

[BruceK]

If one cell is toast, unless you can replace them individually, isn`t the battery toast, like other batt types?

 

[SteveK]

Actually of course there is a way. Each individual battery gets its own disconnect switch, and you periodically switch between them at the end of a charge to make sure they're all on.

You're not concerned with gradual failure, just identifying terminal cases, given say 4 batteries.

I suppose a shunt on each negative would display a constant flow from each, or no flow of current.

 

[Jeff F]

So if one of your batteries does a disconnect, you toss it?

Yup. At your leisure, given sufficient in parallel.

 

[Jeff F]

This obviously allows a higher expected BMS/battery failure rate that comes with garbage batteries.

 

[Barking Sands]

Step back for a moment. Think of each battery as a cell. Now, how can you manage that? Easily I think.

Edit to add: an uncommunicative cell with a BMS.

Thats how I am forming my opinion as well. Each 12v battery as a modular, line replaceable unit in a parallel string. Depending on your AH requirements you could size them accordingly to get the optimum number of units. I would say probably 4. I am sure you could make an argument for anything between 3 and 8. But I think 4 covers some wide ranging scenarios. Lets say you have moderate requirements and would only require 200 amps out of the entire pack. Enough to run a decent Inverter for a limited time. If I decide on four 100AH batteries in parallel with 100 amp BMS's and you lose 1 battery you can continue with the mission as planned but just be down to 3/4 capacity until the replacement can be installed. You just have to equalize the new battery to the pack before hook up. Keeping a small charger on board would be a good idea too so you could accomplish that. Changing a failed unit out would be 2 terminal screws, a hold down and some fiddling around getting the voltage where you want it. But your mission would be uninterrupted. Or at least the interruption would be short and at a time of your choosing.

Of course you can scale that up by purchasing four, six or however many batteries in 200 AH or even 300.400 AH flavor to suit needs of total AH capacity and available amp delivery. You can do the same for 24 volt 100Ah or 200AH units too.

 

[Barking Sands]

So if one of your batteries does a disconnect, you toss it?

Or make a warranty claim to replace the spare you just put it (in a perfect world of course )

 

[Jeff F]

Right.

Now extend that to include a meta-BMS.

If you've got a dozen 100a all under a meta-BMS you can reasonably contemplate a 500a max pack rating.

The mbms keeps a close eye. If one pops, alarm. If two pop shut things down with ATC/ATD. Maybe detune your max slightly. You can set the mbms threshold wherever you want.

Edit to add: if that were to occur it might call for pack balancing rather than battery replacement.

Further edit: a smart mbms could handle this.

Wow, I'm being allowed edits for a long time.

Let's extend the commodity approach. What could you do with 100 ah 50a BMS?

 

[Jeff F]

Or make a warranty claim to replace the spare you just put it (in a perfect world of course )

Or sell off as a second.

 

[Jeff F]

I should be filing a patent .

I like where this has gone.

 

[twistedtree]

Right.

Now extend that to include a meta-BMS.

If you've got a dozen 100a all under a meta-BMS you can reasonably contemplate a 500a max pack rating.

The mbms keeps a close eye. If one pops, alarm. If two pop shut things down with ATC/ATD. Maybe detune your max slightly. You can set the mbms threshold wherever you want.

Edit to add: if that were to occur it might call for pack balancing rather than battery replacement.

Further edit: a smart mbms could handle this.

Wow, I'm being allowed edits for a long time.

Let's extend the commodity approach. What could you do with 100 ah 50a BMS?

I think it all comes down to the reasons why a battery's BMS disconnects, how you diagnose it, and what's involved to bring it back on line or replace it.


One battery disconnecting in a bank doesn't mean the battery is bad. It might, but it might not. It could just have a cell that's out of balance and hasn't had enough time on the balancer to correct it. The people I mentioned who were cutting open drop-ins and adding their own BMS was largely motivated by just this. Apparently some of the drop-ins can't really keep up with balancing.


The disconnect could also be due to over current in one battery. The current in parallel batteries will never be exactly the same, and might vary quite a bit. This means that one battery will always be the first to trip. It doesn't mean the battery is bad. Also note that in this situation you are likely to get a cascading shutdown since when one battery disconnects from an overload, it leaves the others with a larger load and will likely cause another to trip, etc.



There are also temp out of range, and low voltage shutdowns to consider.


Then you need to know that the battery disconnected. With most drop-ins you can't even tell that it disconnected without removing it from the bank and testing it. Let's say you have per-battery disconnect switches so you can periodically test the batteries one at a time. And if it has disconnected, you have no way of knowing why. You could measure voltage and perhaps get insight from that, but it's not a certainty.



You mentioned some sort of bluetooth connected app in some batteries as a means of visibility into each battery. That would help a lot and let you do a periodic check to find a battery that has disconnected, and presumably find out why it disconnected.


Now let's look at recovery and repair. If the fault is believed to be transient, then you are ready to put it back in service. Otherwise you need to replace the battery. After that, you need to reintroduce the battery back into the bank, but they will be at different SOCs. So you need to synchronize them, probably by bring the bank to full charge, and separately bringing the replacement battery to full charge. Then you can through the switch and merge the battery back into the bank.


I'm not saying this an unsolvable problems. Not at all. Just pointing out all that's involved, just in case less experienced people see it as a simple solution.


I have to say that it all seems a lot more complicated than just building with a fully functional BMS in the first place. Are drop-ins really that much less expensive? So much effort goes into working around the limitations of drop-ins, rather than more directly accepting and working with LFP battery characteristics. Drop-ins seem easier, but always seem to turn into one complex work-around after another.

 

[Jeff F]

Quick answer: I'm proposing that someone get to work and build a better MBMS.

I'll read more carefully later.

 

[Jeff F]

You buy $200 batteries, try them out, and first time they trip you sell them for $50.

You can do this seasonally. If you want to figure out why they tripped you can. But others will be interested. Get them to report findings back to you.

 

[DDW]

....
Apparently some of the drop-ins can't really keep up with balancing.

....
Also note that in this situation you are likely to get a cascading shutdown since when one battery disconnects from an overload, it leaves the others with a larger load and will likely cause another to trip, etc.



There are also temp out of range, and low voltage shutdowns to consider.


Then you need to know that the battery disconnected. With most drop-ins you can't even tell that it disconnected without removing it from the bank and testing it.

Rod Collins tested a number of cheap drop ins and found the balancers to be completely inadequate in some.

Another problem with a drop in disconnect is, depending on their reset strategy, you may never know it is happening, and may not be able to duplicate it on test. If in the example one is going overcurrent and disconnecting, but the others pick up the slack, then the disconnected one automatically reconnects after 5 seconds (or whatever their strategy is) you may not know for months.

I'm sure Jeff will say, "why do I care?". But I would.

 

[Barking Sands]

Now let's look at recovery and repair. If the fault is believed to be transient, then you are ready to put it back in service. Otherwise you need to replace the battery. After that, you need to reintroduce the battery back into the bank, but they will be at different SOCs. So you need to synchronize them, probably by bring the bank to full charge, and separately bringing the replacement battery to full charge. Then you can through the switch and merge the battery back into the bank.


I'm not saying this an unsolvable problems. Not at all. Just pointing out all that's involved, just in case less experienced people see it as a simple solution.


I have to say that it all seems a lot more complicated than just building with a fully functional BMS in the first place. Are drop-ins really that much less expensive? So much effort goes into working around the limitations of drop-ins, rather than more directly accepting and working with LFP battery characteristics. Drop-ins seem easier, but always seem to turn into one complex work-around after another.

Can you run through the recovery and repair of a single cell in a purpose built pack with single integrated BMS? Maybe also what a cell failure looks like in practice? Also, can you point to an example of a battery system that you are likely thinking of to contrast? Maybe even a picture. I am sure you have made a post before. Maybe just point me in the right direction so I can take a gander. Much appreciated.

I get the feeling we are still adding ALL the details in a pro/con list still. I think that list is incomplete as of yet. I also get the feeling there will be more than one way to do these things in a perfectly acceptable manner. But IMo we still need to add details to the discussion.

 

[Barking Sands]

Rod Collins tested a number of cheap drop ins and found the balancers to be completely inadequate in some.

Another problem with a drop in disconnect is, depending on their reset strategy, you may never know it is happening, and may not be able to duplicate it on test. If in the example one is going overcurrent and disconnecting, but the others pick up the slack, then the disconnected one automatically reconnects after 5 seconds (or whatever their strategy is) you may not know for months.

I'm sure Jeff will say, "why do I care?". But I would.

Rod was describing the cheapest of the cheap direct from China from several years ago. It might actually be hard to find a battery like that now unless you go searching for it...lol.
But...I am not sure what everyone has in their own mind for the "cheap" drop ins. Some might be seeking out that $199 Lifepo4 "deal".
I am hoping most are thinking of the more typical $300-400 range 12 volt 100AH battery from Amazon such as Ampere Time or Chins for the low end.
You can also get their "smart" batteries for about a 20% premium.
Any of these drop ins for a boat house bank IMO should have an on/off switch and bluetooth monitoring at the cell level along with warning. Here is the battery I just purchased for my Golf Cart:

https://www.epochbatteries.com/?rfsn=7026032.670492

But they also have:
12 volt 100AH
24 volt 100AH
36 volt 100AH
48 volt 100AH.

All have:
Bluetooth that can see all cells
many warnings
self heating for cold weather charging
IP67 rated waterproof case with mounting bushings
many protections (high/low temp, voltage, amperage etc)
11 year warranty from a large company (engineered in the US)
Data sheets for all testing and specs
2 CAN ports
On/Off switch on each unit
A remote dash mounted monitor that sounds all the warnings noted in bluetooth. This is connected to a CAN BUS plug
many certifications such as UL, CE. FCC and SAE
removable top, seems to be semi serviceable

Three videos from Will Prowse, Lithium Solar and some other guy that have rave reviews about the internal build quality.

And this battery cost $599 currently. That seems impressive. But I will test it when I get it in my hands and see how that translates into the discussion we have been having. I asked them about the CAN plug output that the external monitor plugs into and whether or not it could interface with Victron stuff. He said they are looking into that but that they would very much like to make it communicate with Victron stuff.
He also stated they plan to come out with 270Ah and 540AH versions.

 

[Jeff F]

For the purpose of this excercise I'm going to use the cheapest available option. Let's assume 100ah 50a BMS.

What's our example pack specs? Capacity and max amps.

Unfortunately I operate pretty much exclusively from my mobile so no pretty pics from me. Happy to delegate that out.

Edit to add: I'd suggest 300a for pack. At 12v it's hard to exceed.

 

[Jeff F]

I'm particularly interested in nailing down function and features of MBMS.

 

[Jeff F]

Once we have specs nailed down the floor is open. Lots of folks here have built LI packs. There's lots of practical as well as theoretical knowledge on tap here.

Thanks again to all.

 

[rslifkin]

To me, the insight into BMS behavior, etc. is not just about what happens after a disconnect, but knowing you're at risk for one before it happens. Knowing that could prevent an unexpected loss of house power due to the overload cascade TT mentioned if you're under heavy load when part of the bank disconnects, causing an overload on the other BMSes.