The BMS Was Supposed to Protect my Batteries?

[High Current]

Maybe I missed it but I do not see anywhere in this spec a mention of the voltage the battery input can take when the BMS is tripped. If your trying to design a system taking this into account, you would need to know this number. Without this number, you cant design a system. You can speculate that its some common breakdown voltage of junctions of a device such as a fet but a zillion variables could make that a bad speculation.

If it were my boat, I'd assume the failure could also occur while the battery switch was in the ON position. I would therefore want to make sure I used a protection device that could limit the surge to 16-ish volts for the sake of my GPS, VHF, etc... It's a safe bet that any 12V BMS has MOSFETs that are good enough to protect against at least 20V.

 

[dlochner]

Yes.. The point earlier is that a solar panel might have an open circuit (no load ) voltage of 100 volts but it will not put out more current than its short circuit spec which is usually close to the maximum power point current. A solar panel looks like a constant current source over much of its operating voltage range and when connected to something like a battery that is below the open circuit voltage, the panel just injects near is short circuit current into the battery and this generally will only result in an initial voltage rise of the panel current times the internal impedance of the battery. Lets even go to an 800 watt panel (it was on an RV) with an open circuit voltage of 100 volts and a short circuit current of 10 amps. 4 milliohm battery impedance results in the initial battery voltage increase of a small 40 mv. Not damaging.

But now the charge controller is damaged so that 10 amps just keeps getting put into the battery with no regulation so the battery voltage rises because its getting charged. Pretty common knowledge that you can damage a battery putting an unregulated solar panel on it.

But as high current pointed out, after some time of unregulated charging the BMS will trip and disconnect the battery from the panel. Now the panel has no load and the voltage jumps up to the open circuit spec of the the solar panel.

Here is a lithium battery spec from Victon 7. Technical data

Maybe I missed it but I do not see anywhere in this spec a mention of the voltage the battery input can take when the BMS is tripped. If your trying to design a system taking this into account, you would need to know this number. Without this number, you cant design a system. You can speculate that its some common breakdown voltage of junctions of a device such as a fet but a zillion variables could make that a bad speculation.

Victron didnt give a number for max battery input voltage when the BMS is tripped.. If this is important, why isnt it specified. If you damage a charge controller so its no longer regulating and leave it connected to a battery, your likely to damage that battery for a wide range of panel open circuit voltages.

If you are referring to the video posted by Rod, you missed a key point in his discussion. The BMS did not trip, the MMPT controller failed allowing the panel's full voltage of about 100v to flow to the battery. The MOFSETS on the BMS were rated at 60v, the high voltage caused the MFSETS to fail which then allowed the full 100v to pass to the battery cells, overheating the battery.

One of the points he was making is we often don't know what the MOFSET ratings are and many are only at 60v. When designing the solar array the voltage should be kept below the MOFSET's rated voltage, so in the case of a MMPT failure the BMS can do its job and not burn up.

 

[Maine Sail]

Lets use an example of a 400 watt solar panel that is somehow 100 volts open circuit. Maximum current out of this 400 watt panel with a 100 volts open circuit might be about 5 amps (maximum power point voltage of 80 volts and 5 amps) The short circuit amp of this panel might also be about 5 amps.

When you apply that solar panel directly to a battery,. it can only put 5 amps into the battery. No more than this. What happens to the battery voltage if you put in 5 amps. Maybe it goes up .1 volts, maybe .3 volts.

Remember that the solar panel 100 volts is open circuit with no load or zero current. This is way different than a 100 volt power supply that can attempt to put out 50 amps at 100 volts.

In our case above, when the 400 watt solar panel example was put directly on the battery, the battery voltage might only increase by a few hundred millivolts.

This is how solar panels work.. They are basically a current source with an open circuit voltage limit.

FYI, google what the internal impedance of a 100 amp hour LFP battery is. I got 2 to 4 milliohm.

At 4 milliohm, that 400 watt panel with 5 amp short circuit current only raised the battery voltage by .02 volts. That will not cause a problem.

However, a 400 watt solar panel connected to a battery with no regulation (ie, the failed MPPT controller looked like a short between in and out) will cause the battery to over heat and damage itself if its left on for long enough. Most likley the battery cooked because there was no regulation on the charge long after the battery was full. The mystery is what is the BMS supposed to do if the battery is being overcharged because regulation was damaged.

You cant think of a solar panel as a fixed voltage source since its not even close to that.

The problem is not when the batt bank is at low SoC but when it approaches full. The voltage hockey sticks above HVD, the BMS shuts down but the voltage continues to rise until the BMS is fried and then it's game over..

 

[walt]

Im curious as to why if this over voltage that fries a BMS is important, why cant I find a spec for this voltage...

FYI, here is a power mosfet with a drain to source breakdown of 400 volts. https://www.digikey.com/en/products...jRqIKQp0kiwx9lThpgJp9kfsmm8letyUaAoaYEALw_wcB

Somehow this 60 volt keeps getting brought up but its a complete guess. I easily found a cheap part with a 400 volt breakdown. I spent a little time trying to find an actual spec for a LFP battery but could not. Maybe someone can find something.

I did watch the video. The circuit was burned up and you cant tell what failed.

And a solar panel acts like a current source. Not a voltage source. Even with an open circuit NO LOAD voltage of say 100 volts, attach it directly to a battery and the battery voltage rise will be completely due to the current of the solar panel. Even for the 800 watt panel example, that might be less than 100 mv. This will not damage the battery or the BMS initially.

It seems we agree that when the MPPT controller initially failed, this was similar to just connecting a solar panel directly to a battery with unregulated charging which is what likely generated the heat that damaged the battery.

Whatever FET (totally being guessed about) would not have failed initially because the battery voltage rise may have only been .1 volt. But at some point, if the BMS tripped, the load would have gone to zero and the open circuit voltage of the panel would now appear on the input. If whatever is in the input is sensitive to the open circuit voltage when tripped, it could have failed. Remember I found a FET that could withstand 400 volts. When whatever that circuit is failed, did it fail as an open or a short, we dont know or even if it did fail.

You could speculate further that the BMS should have prevented the battery from heating up like it did from being connected to an unregulated solar panel so it must have failed as a short so the solar panel kept dumping current into the battery.

Lots of speculation. And simply a MPPT controller failing so it does not regulate can damage all sorts of batteries.

I think there is still the perception that a solar panel acts like a voltage source. It does not. A solar panel is a voltage limited current source which behaves differently. Look a a curve and you will see about constant current over most of its operating voltage range, then the current tapers off as you get near the open circuit voltage. You get maximum power when you adjust to the load so the panel operates at its maximum power point (which is what an MPPT controller does).

I dont like to argue with folks who make such a valuable contribution here.. so hopefully can sort of gracefully bow out. Just mainly wanted to make that point about how a solar panel works..

 

[dlochner]

FYI, here is a power mosfet with a drain to source breakdown of 400 volts. https://www.digikey.com/en/products...jRqIKQp0kiwx9lThpgJp9kfsmm8letyUaAoaYEALw_wcB

Apparently the BMS manufacturers don't want to use a 400v MOFSET for whatever reason and prefer to use lower rated ones, probably because they are cheaper.

 

[walt]

Probably.. I really have no idea since the BMS tripped state over voltage spec seems to not be one that is published. Im sure the manufacture knows this. And if the BMS got zapped from over voltage while tripped, does it "typically" fail as an open or a short. It would be interesting if someone found a battery with BMS that had this specified.

I worked in consumer electronics and something like this would somewhat be determined by returns. If you are getting too many returns, the design spec would get increased until the failure or return rate was acceptable low. Its a balance of cost to manufacture vs cost of returns. Example of this was a part that got damaged by lightning. We had an internal spec but of course it wasn't published since it would mean nothing to a consumer plus you may not want your competitor to know something like this.

 

[FastOlson]

Reading all the replies in this excellent thread, I idly wonder how many of us are reinforced in our opinion to continue to wait to move (onward, upward, sideways, whatever...) away from our perfectly functioning lead/acid traditional DC storage systems? It's not like I am against "change" per se, but do sense that we are still not close enough to a "drop in" DC storage solution. Yet. It will likely occur, but maybe not tomorrow or next week...

 

[dLj]

Reading all the replies in this excellent thread, I idly wonder how many of us are reinforced in our opinion to continue to wait to move (onward, upward, sideways, whatever...) away from our perfectly functioning lead/acid traditional DC storage systems? It's not like I am against "change" per se, but do sense that we are still not close enough to a "drop in" DC storage solution. Yet. It will likely occur, but maybe not tomorrow or next week...

I think it all depends upon how you lean. Are you more inclined to stay with lead acid or more inclined to move into LiFePo chemistry.

If you are inclined to stay put - then this may reinforce that inclination as it all seems to be too much.
If you are inclined to move over - it can help you move over as it adds into the "knowledge base" of what to look out for...

It's kind of like the old expression: If you believe you can't, or, if you believe you can: you are right.

dj

 

[walt]

If it were my boat, I'd assume the failure could also occur while the battery switch was in the ON position. I would therefore want to make sure I used a protection device that could limit the surge to 16-ish volts for the sake of my GPS, VHF, etc... It's a safe bet that any 12V BMS has MOSFETs that are good enough to protect against at least 20V.

I think are are devices like you mentioned, whatever you put in should be able to handle the short circuit current of the solar panel and this could be significant power.

When I googled "how much voltage can 12 volt lfp battery handle with the BMS in a tripped state" you get numbers from AI ranging from 20 to 60 volts.

Remember that a working MPPT controller would never try and apply above say 15 volts. This whole discussion relies on the MPPT controller failing in a mode that connects input to output. Is that at all common, I somewhat dont think so.

The only MPPT controller Ive had fail was when I accidently shorted the solar panel with the sun shining and the controller was operating. But I dont think it failed as a short.. but am not sure of the details. Im thinking it just damaged the input circuit so the controller stopped doing anything.

Its attractive to have a panel voltage (with MPPT) significantly higher than the battery because this greatly reduces the size of wire from the panel to the controller. For every doubling of voltage, the power loss in the wire is 1/4th. And in any case, if you have a solar controller like this fail so there is no longer any regulation, you may fry the battery regardless. My trailer solar setup has a 24 volt panel with a 12V battery (lead acid) but you could even have this cause some issue with BMS and this rare failure mode if the BMS only handled 20 volts. On the other hand, if the BMS could handle 60 volts, it (maybe) would have protected the battery while my lead acid battery is likely to get damaged regardless.

 

[colemj]

Here is a lithium battery spec from Victon 7. Technical data

Victron batteries require external BMS's. There is no internal BMS.

Mark

 

[colemj]

Im curious as to why if this over voltage that fries a BMS is important, why cant I find a spec for this voltage...

I think that was one of the points Mainesail was making. Nobody publishes or knows this spec unless one opens it up like Mainesail did.

Somehow this 60 volt keeps getting brought up but its a complete guess.

No, Mainesail looked at it, and looked up the spec sheet. There was no guessing.

It seems we agree that when the MPPT controller initially failed, this was similar to just connecting a solar panel directly to a battery with unregulated charging which is what likely generated the heat that damaged the battery.

An internal BMS sits between the charge controller and the battery (else it wouldn't work at all). Its input is isolated from its output. Its input can experience different voltages than its output. Particularly transient ones.

Mark

 

[colemj]

If it were my boat, I'd assume the failure could also occur while the battery switch was in the ON position. I would therefore want to make sure I used a protection device that could limit the surge to 16-ish volts for the sake of my GPS, VHF, etc... It's a safe bet that any 12V BMS has MOSFETs that are good enough to protect against at least 20V.

I've installed MOV surge devices across the main battery bus bars, as well as the main DC distribution panel (and AC panel, but that isn't relevant here). Electronics are behind isolated DC-DC power supplies, that hav a TVS diode across their inputs. Theoretically, these will clamp at 16-17V.

Granted, this wasn't done with this failure example of a BMS in mind at all, but it does seem a bit useful there.

Mark

 

[colemj]

Reading all the replies in this excellent thread, I idly wonder how many of us are reinforced in our opinion to continue to wait to move (onward, upward, sideways, whatever...) away from our perfectly functioning lead/acid traditional DC storage systems? It's not like I am against "change" per se, but do sense that we are still not close enough to a "drop in" DC storage solution. Yet. It will likely occur, but maybe not tomorrow or next week...

Don't lose perspective. The issue being discussed is very rare requiring three simultaneous things to occur, that aren't even possible on most boats. So rare that Mainesail's warning caught most people by surprise because they had never heard of a case before. I think it also caught Mainesail by surprise.

IMO, LFP is way past the development and economic stage of replacing LA batteries. It is always difficult to explain the advantages of LFP to people who are using LA, but every single person who eventually converts has a "holy crap" moment and wonders why they didn't do it much earlier.

Mark

 

[Davidasailor26]

Reading all the replies in this excellent thread, I idly wonder how many of us are reinforced in our opinion to continue to wait to move (onward, upward, sideways, whatever...) away from our perfectly functioning lead/acid traditional DC storage systems? It's not like I am against "change" per se, but do sense that we are still not close enough to a "drop in" DC storage solution. Yet. It will likely occur, but maybe not tomorrow or next week...

I think this is more a risk from the MPPT and high voltage array more than the battery chemistry. If an MPPT on a 100V array charging lead acid failed it would still dump 100V into the lead acid bank. That would also lead to a pretty spectacular event. At least with lithium an MPPT failure on a 50 volt array would be safely caught by the BMS. With lead the 50V would still dump into the 12V battery and probably destroy it.

 

[Davidasailor26]

@Maine Sail I’m curious about the 4S 60V BMS consideration. If using an external BMS for the full bank then I understand it could be subject to the full array voltage. But if these are drop-in LiFePO4 with integrated BMS’s, then wouldn’t each BMS in the series only be exposed to 1/4th of the total array voltage, more likely staying under their threshold?

 

[FastOlson]

Considering reply 34, among other good ones, my lack of -or need for- solar generation is a part of our "More Basic" power system. If adding solar I would consider a Full Monte new system. While I understand the pitfalls of our lead-acid battery system, it functions very well and meets all of our needs. That's kind of the bottom line for all these DC sources, when you get right down to it.

I have also stopped regularly attending meetings at Ned Ludd's house; those folks became a bit too extreme.......

 

[Stu Jackson]

David, it's been awhile since I played around with electricity, but I do seem to recall that in the battery systems being discussed, the VOLTAGE is evenly distributed while the CURRENT gets split.

with integrated BMS’s, then wouldn’t each BMS in the series only be exposed to 1/4th of the total array voltage, more likely staying under their threshold?

 

[Davidasailor26]

David, it's been awhile since I played around with electricity, but I do seem to recall that in the battery systems being discussed, the VOLTAGE is evenly distributed while the CURRENT gets split.

I think that’s true of circuits with components in parallel - in those cases the voltage is equal across each component but they each carry 1/4th of the current. But with components in series, like a 4S battery bank adding to 48V, each component carries the full current and 1/4 of the voltage.

 

[Stu Jackson]

Except that's NOT what Maine Sail describes in the first few sentences of his YouTube.
He says "...over 100 volts into a 12V LP battery..."

But with components in series, like a 4S battery bank adding to 48V, each component carries the full current and 1/4 of the voltage.

 

[Maine Sail]

@Maine Sail I’m curious about the 4S 60V BMS consideration. If using an external BMS for the full bank then I understand it could be subject to the full array voltage. But if these are drop-in LiFePO4 with integrated BMS’s, then wouldn’t each BMS in the series only be exposed to 1/4th of the total array voltage, more likely staying under their threshold?

When the charge FETs cut off, even with four in parallel, you still have 85V AFE chips that control the typically 60V FETS. Batts in parallel are all at the same voltage. Some batts use 80V FET's but I've yet to see a BMS, with n-channel FET's, that exceed 100V. Most budget priced batts use 85V AFE's & 60V FET's. Even if you found a batt that had high voltage FET's you'd need to find high voltage AFE's too and drop-ins just don't use high voltage AFE's.

These days, thousands of boats, especially cats, are running PV arrays of well over 100V and some pushing 150 - 200+ V. These 150V-200+V boats really need to be using fully isolated MPPT's. Isolated MPPT's are insanely expensive, and boat owners insanely cheap, so in most cases just don't let your array voltage exceed 60V. or use a device like the sterling..