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Old 06-06-2019, 07:40   #31
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Re: LFP LiFePo4 BMS, Batteries, etc Links

If you delve into the world of Victron comm protocols, they actually open-source and help users with connecting to other systems, more than any other commercial vendor out there.
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Old 06-06-2019, 14:55   #32
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Re: LFP LiFePo4 BMS, Batteries, etc Links

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My take on addressing failure on prolonged cruises would be:



Understand your system, preferably design it yourself. Think through possible failure modes. For each component try to imagine what failure of this component would entail. Have a plan for all failure modes identified.

Bring spare parts and tools.



Your backup plan might be "I carry a spare (e. g. BMS) which I swap in case of failure" or "I work around the problem using the following method".



For a failed BMS e. g. an identically configured BMS could be kept on the boat. However, keep in mind that installing a backup BMS will take time and may be difficult on a boat without power (at night with high seas...?).



A sensible complement to a "cold spare" BMS would be designing the circuitry to disable and override the BMS completely. Example: assuming your BMS switches a main contactor, HVC and a LVC contactor, in normal operation the BMS will engage the coils of all three contactors, effectively connecting charge bus, load bus and battery. If the BMS fails, coil activation may not happen, leaving the boat without power and charging capacity.

Solution: add circuitry to independently power all contactor coils. How this is done depends on the actual circuitry, but in general one can achieve this behaviour by making proper use of (Schottky) diodes which form hard wired logical "OR" gates. That way normally the BMS drives the contactor coils, but with the flip of the "BMS override" switch, the coils get excited regardless what the BMS does. This gives you emergency power and takes the urgency out of the situation.



In most cases the system will be fine even without the BMS. After all it's your insurance if the charge system or the cell voltages hit protection levels. In mid SoC level operation, the BMS will 99.999 % do nothing at all, so in the "override" mode you only need to be aware to keep out of extremely high and low SoC situations but otherwise you are fine for the time being, allowing you to postpone actual repair until the time is right. You could cross an ocean in this mode, if need be, and repair the BMS afterwards.



Regarding solid state relays: essentially they are MOSFETs. Theirs typical fail state is "closed", so if you wish to plan for failure, the prudent way might be to have two of them in series, triggered by the same output. And have a monitoring on each of them, of course.



That's probably overkill, though...


This is what I did on my BMS. I have two manual switches, one to lock the main battery contractor on, and one to lock it off. Otherwise itís in auto mode and controlled by the BMS.

If you can program your charge sources appropriately, the BMS really does nothing other than hold the contactor on and monitor cells. In a healthy system, you can lock the contactor on and turn off the BMS and run fine for a very long time. Mine has done exactly nothing in the past 6 months, just as itís supposed to.

Now if your BMS design is such that it actively controls the charge sources, it might be a more vital component, and you frankly may want to rethink that approach.
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Old 07-06-2019, 00:28   #33
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Re: LFP LiFePo4 BMS, Batteries, etc Links

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If you delve into the world of Victron comm protocols, they actually open-source and help users with connecting to other systems, more than any other commercial vendor out there.
Yes, but who in this forum is able to read source code and keen on developing own controllers, in a failure situation who has a CAN BUS analyzer handy, and a logic analyzer or at least a 2 channel oscilloscope in his toolbox?

But most probably have a multimeter or a light bulb with 2 wires... Just saying, keep it simple. On land, I have this gear or can borrow it somewhere, on the water in a remote place where I barely understand the locals it may be a challenge...
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Old 12-06-2019, 05:19   #34
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Re: LFP LiFePo4 BMS, Batteries, etc Links

I was planning on making a 400AH bank using 8x200AH cells. One of the main reasons that I wanted to do this was so that I could, if there were ever a need (and as CatNewBee had suggested in one of their threads), remove half of the bank if something went wrong and still have a 12v system albeit with half of the available AH.
I was putting the order in with REC (side note: They haven't been great at answering my specific questions), and was planning on purchasing the REQ Q BMS 16s. They then told me that "this was suitable for 48V systems." This makes a lot of sense since it is called the "16S" duh! Still though, it seems insane that the unit would not be able to be wired for a 2P4S bank! Again, I feel like the person that I am dealing with over there is sort of "phoning it in" and not really answering my questions.

They suggested that I use the REC ACTIVE but of course that is made for only 4 cells.
Does anyone know if there is another way around this that could use a REC device (I really like what I do know about them) and still be able to do a 2P4S bank?
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Old 12-06-2019, 06:27   #35
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Re: LFP LiFePo4 BMS, Batteries, etc Links

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I was planning on making a 400AH bank using 8x200AH cells. One of the main reasons that I wanted to do this was so that I could, if there were ever a need (and as CatNewBee had suggested in one of their threads), remove half of the bank if something went wrong and still have a 12v system albeit with half of the available AH.
I was putting the order in with REC (side note: They haven't been great at answering my specific questions), and was planning on purchasing the REQ Q BMS 16s. They then told me that "this was suitable for 48V systems." This makes a lot of sense since it is called the "16S" duh! Still though, it seems insane that the unit would not be able to be wired for a 2P4S bank! Again, I feel like the person that I am dealing with over there is sort of "phoning it in" and not really answering my questions.

They suggested that I use the REC ACTIVE but of course that is made for only 4 cells.
Does anyone know if there is another way around this that could use a REC device (I really like what I do know about them) and still be able to do a 2P4S bank?

I m running a 4p4s bank my bms sees the parallel cells as a single cell because that is what they are wired up as . YMMV . ( I am not running the REC bms mine is custom built for my needs)
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Old 12-06-2019, 06:56   #36
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Re: LFP LiFePo4 BMS, Batteries, etc Links

This may be a stupid question but is it possible to do an XP4S, where x>1 setup with a BMS that doesn't use cell boards on the terminals but can also still monitor each individual cell?
Instead of having the BMS see each paralleled string, I'd much rather it be able to gather data from each cell.
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Old 12-06-2019, 07:02   #37
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Re: LFP LiFePo4 BMS, Batteries, etc Links

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This may be a stupid question but is it possible to do an XP4S, where x>1 setup with a BMS that doesn't use cell boards on the terminals but can also still monitor each individual cell?
Instead of having the BMS see each paralleled string, I'd much rather it be able to gather data from each cell.
cells in parallel will function as a single cell .
If your specific desire is to monitor each individual cell then I would suggest the following
1p4s x2p ( build 2 separate 12v banks then parallel them to give you the desired ah capacity .)
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Old 12-06-2019, 07:12   #38
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Re: LFP LiFePo4 BMS, Batteries, etc Links

Has anyone put together a table or chart for the different LFP systems?

Price, warranty, availability, ease of use

I had thought a few months back that the Trojan Trillium would be the answer to all but they are dragging their feet on production and still not really updating in regards to warranty (someone said 4/8 years but not confirmed)

I am ready to buy 15 to 20kwh of lithium but I want to pay under a kilobuck a kwh and I want a full package with at least a 3 year warranty by a relatively large stable company.
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Old 12-06-2019, 07:51   #39
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Re: LFP LiFePo4 BMS, Batteries, etc Links

buying a drop-in, or proprietary system with the BMS already wired in

is very different from buying bare cells and putting in your own protective gear, even if just trying to find a good BMS

really should be separate threads

Even a set of given questions discissing about one member's needs may be worth starting a new thread to keep some focus.
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Old 12-06-2019, 08:00   #40
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Re: LFP LiFePo4 BMS, Batteries, etc Links

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This may be a stupid question but is it possible to do an XP4S, where x>1 setup with a BMS that doesn't use cell boards on the terminals but can also still monitor each individual cell?

Instead of having the BMS see each paralleled string, I'd much rather it be able to gather data from each cell.
It is physically impossible to individually monitor cells wired in parallel.

Unless you have a mechanism for "un-paralleling" isolating them from each other while not in use, to perform periodic checks.

The good new is, parallel cells are by definition going to be at the same voltage at any given time, so no worries about the weaker ones hitting the top or bottom before or after the others in that group.

With pairs, each carrying half the load, a dramatically bad cell will quickly show up in your monitoring the BMS, but yes a catastrophic failure (rare) could also take its partner.

Or as newhaul suggests, create 4S modules, each protected by its own BMS and then parallel those.
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Old 12-06-2019, 08:01   #41
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Re: LFP LiFePo4 BMS, Batteries, etc Links

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Originally Posted by strongmonty View Post
This may be a stupid question but is it possible to do an XP4S, where x>1 setup with a BMS that doesn't use cell boards on the terminals but can also still monitor each individual cell?
Instead of having the BMS see each paralleled string, I'd much rather it be able to gather data from each cell.
As Newhaul wrote, paralleled is fine and the way to go. What caught my eye was your "each parallel string" which implied that cells within strings might be in series and that is not good unless the BMS is complex enough to ensure a balanced charge is going to each cells individually.

There's a good story in this thread:
https://robotics.stackexchange.com/q...-the-same-time

The cells may start virtually the same, but the absolute slightest manufacturing difference in internal resistance will lead to this type of loss of a cell.

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Old 12-06-2019, 08:08   #42
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Re: LFP LiFePo4 BMS, Batteries, etc Links

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I am ready to buy 15 to 20kwh of lithium but I want to pay under a kilobuck a kwh and I want a full package with at least a 3 year warranty by a relatively large stable company.
Where in the world are you located?

If in the US, Victron, Mastervolt and Lithionics are the only systems providers that aren't one-man shows afaik.

To meet that price target, you may need to sacrifice the warranty idea, seek out a trusted US-based supplier of quality cells bought bare, and then pay someone to assemble with the BMS.
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Old 12-06-2019, 08:17   #43
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Re: LFP LiFePo4 BMS, Batteries, etc Links

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I was putting the order in with REC (side note: They haven't been great at answering my specific questions),
. . .
it seems insane that the unit would not be able to be wired for a 2P4S bank! Again, I feel like the person that I am dealing with over there is sort of "phoning it in" and not really answering my questions.
You really need to know exactly what you want, know what you're doing when dealing with sales or customer service.

The people that really know what's going on, their time is worth hundreds of dollars an hour giving advice / consulting, margins are too slim to just give away free help on systems design.

Maine Sail actively repels requests for even paid consulting of that type, just too frustrating and of course imagine the liabilities in the tort-happy US.

Australia's been very lucky with very well trusted one-man operations, one main guy in particular, but he does not just give away his knowledge to the public for free anymore.
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Old 12-06-2019, 08:24   #44
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Re: LFP LiFePo4 BMS, Batteries, etc Links

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What caught my eye was your "each parallel string" which implied that cells within strings might be in series [/IMG]
Yes, by definition units in **series** comprise "a string".

Best to use "a group" for cells in parallel.

I use "module" for sub-bank units that will function outside the bank as an independent unit, not necessarily at the whole-bank voltage. Say a 48V bank, where 24V modules can be swapped out for running the electric dinghy, and 12V modules used to feed portable powerpaks for camping or day trips.
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Old 12-06-2019, 09:09   #45
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Re: LFP LiFePo4 BMS, Batteries, etc Links

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Yes, by definition units in **series** comprise "a string".

Best to use "a group" for cells in parallel.

I use "module" for sub-bank units that will function outside the bank as an independent unit, not necessarily at the whole-bank voltage. Say a 48V bank, where 24V modules can be swapped out for running the electric dinghy, and 12V modules used to feed portable powerpaks for camping or day trips.
did you miss how it is properly stated xpxs
As to the rest of your statement .
you mean you will when and if you ever actually own Lfp batteries.
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