MPPT chargers dying upon battery disconnect?
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Joerg
Jan 30, 2023, 1:40:50 PMJan 30
to
Attention, this is one of those rare electronics-related questions :-)
Will be rigging up a small power-failure backup system. Essentially a
12V LiFePO4 battery (with BMS in there), inverter, solar panels, MPPT
charger.
When reading up on stuff and talking to people I found something very
disconcerting. It seems that MPPT charge controllers can and often do
commit suicide when operated without a battery. It even says that in the
manuals a lot. The trigger event can be as simple as the BMS opening for
some reason and ... *PHUT* ... MPPT is gone. A friend had that happen,
twice.
They can also send out a substantial voltage spike or short their
MOSFETs during such events, the latter letting full solar panel voltage
onto the 12V power bus. Both of which can kill rather expensive gear
connected to the 12V bus.
Why is that? Are the engineers designing this stuff not very bright? I'd
never release a design with such "traits".
Does anyone know lower-end MPPT charge controllers (20-40A range) that
are properly designed in this respect? Preferably ones that are also low
noise so they don't mess with measurements on the lab bench. The Genasun
brand is low noise but AFAICT they only come for very low solar panel
voltages.
Of course, one solution is to build a massive crowbar to blow a fuse and
protect the connected gear. The MPPT charger might possibly still die.
--
Regards, Joerg
http://www.analogconsultants.com/
Will be rigging up a small power-failure backup system. Essentially a
12V LiFePO4 battery (with BMS in there), inverter, solar panels, MPPT
charger.
When reading up on stuff and talking to people I found something very
disconcerting. It seems that MPPT charge controllers can and often do
commit suicide when operated without a battery. It even says that in the
manuals a lot. The trigger event can be as simple as the BMS opening for
some reason and ... *PHUT* ... MPPT is gone. A friend had that happen,
twice.
They can also send out a substantial voltage spike or short their
MOSFETs during such events, the latter letting full solar panel voltage
onto the 12V power bus. Both of which can kill rather expensive gear
connected to the 12V bus.
Why is that? Are the engineers designing this stuff not very bright? I'd
never release a design with such "traits".
Does anyone know lower-end MPPT charge controllers (20-40A range) that
are properly designed in this respect? Preferably ones that are also low
noise so they don't mess with measurements on the lab bench. The Genasun
brand is low noise but AFAICT they only come for very low solar panel
voltages.
Of course, one solution is to build a massive crowbar to blow a fuse and
protect the connected gear. The MPPT charger might possibly still die.
--
Regards, Joerg
http://www.analogconsultants.com/
Fred Bloggs
Jan 30, 2023, 1:55:48 PMJan 30
to
You have to remove power to the MPPT charge controller before you remove the battery.
Joerg
Jan 30, 2023, 2:48:21 PMJan 30
to
Li-Ion battery can literally "remove itself" via BMS action. Any
external stuff should be resilient in that case and most MPPT chargers
seem not to be.
In medieval Europe they would have dunked the design engineer into a
moat several times for "moral cleansing" :-)
Fred Bloggs
Jan 30, 2023, 3:09:17 PMJan 30
to
Joerg
Jan 30, 2023, 3:35:54 PMJan 30
to
like this and know how it's done correctly. I just need to know which
MPPT mfg also knows this, since I don't want to roll my own again.
You say your BMS disconnects the battery? Most of the time it's supposed
to communicate with the charger and tell it to back off the current.
battery, as is customary. They generally do not have any communications
interfaces. And with a properly designed MPPT it isn't necessary.
Very occasionally a large Li-Ion battery will have Bluetooth
connectivity but that's mainly for relaying the charge status.
Check to see if your MPPT is compatible with Li having a BMS disconnect,
or that the Li is compatible with an MPPT. Send an email query to the
MPPT manufacturer customer support.
[...]
boB
Jan 30, 2023, 5:07:46 PMJan 30
to
On Mon, 30 Jan 2023 12:35:43 -0800, Joerg <ne...@analogconsultants.com>
wrote:
I have dealt with this directly. The problem is that when the MPPT
is charging hard and that load is abruptly disconnected, it is like a
tug of war with a rope where if one person lets go, the person at the
other end may fall down. In this case, the PWM is such that if the
current suddenly goes to zero, the input voltage of a synchronous buck
converter can go sky-high and breal the FETs.
In my old designs, I tested for this same thing except with lead acid
batteries and the circuit breaker to the battery from the MPPT
controller.
Communications from the BMS to MPPT, if it is even there, is most
likely not fast enough to stop this but might be in some cases.
Was this a Chinese product ?
boB
wrote:
is charging hard and that load is abruptly disconnected, it is like a
tug of war with a rope where if one person lets go, the person at the
other end may fall down. In this case, the PWM is such that if the
current suddenly goes to zero, the input voltage of a synchronous buck
converter can go sky-high and breal the FETs.
In my old designs, I tested for this same thing except with lead acid
batteries and the circuit breaker to the battery from the MPPT
controller.
Communications from the BMS to MPPT, if it is even there, is most
likely not fast enough to stop this but might be in some cases.
Was this a Chinese product ?
boB
Lasse Langwadt Christensen
Jan 30, 2023, 6:55:08 PMJan 30
to
no longer a battery on output, where does the energy come from?
there is of course some energy in the inductor but is that really enough
to rise the voltage with reasonable output and input caps?
Joerg
Jan 30, 2023, 7:04:20 PMJan 30
to
would prevent this. So far all the converters I designed react
immediately and do not let the output overshoot even with a suddenly
disconnected load. It can be done within one cycle so you don't need
much in terms of capacitance. One of them was an MPPT design. That one
you could even use as a stand alone power supply, sans battery, provided
the panels delivered enough juice.
It's the same in other situations. I designed an actuator circuit where
then the client encountered the unforseen situation where the load would
accidentally come off. It's taken care of by a FET that immediately
takes over and bleeds off the energy in the inductor for that one cycle.
IOW it briefly goes linear. If something shoots up sky-high that is a
design flaw in my opinion.
> In my old designs, I tested for this same thing except with lead acid
> batteries and the circuit breaker to the battery from the MPPT
> controller.
>
corrosion or loosening at one of the contacts. I once sat there in a
borrowed old Chevy truck. Wouldn't start after loading my stuff and
others around me became impatient. Popped the hood, wiggled some cables
and the positive terminal on the battery came right off.
> Communications from the BMS to MPPT, if it is even there, is most
> likely not fast enough to stop this but might be in some cases.
>
then the alert doesn't come and ... whaddabam.
> Was this a Chinese product ?
>
units some time in February to have a look.
Anyhow, I'll include a big fat crowbar on this system because I don't
trust design engineers in that industry much anymore. There's too much
money connected to the 12V bus.
boB
Jan 30, 2023, 10:10:44 PMJan 30
to
On Mon, 30 Jan 2023 16:04:10 -0800, Joerg <ne...@analogconsultants.com>
Well, there are also capacitors on the battery side but they should
hold less energy than the PV input capacitors.
Yes, the input voltage 'shouldn't' go high but the battery side
certainly does go high. I think you are right about the input voltage
not going high, but having fixed this many years ago, I can't remember
the failure mechanism exactly. But it can go at least to the PV's Voc
voltage, but much above that, (just a few volts) the PV array should
clamp that.
What was the nominal PV input voltage of this controller ? Was it set
up as a 24V array or 48V array or maybe higher voltage ? That can
also make a difference. What kind was it ? I am familiar with a lot
of the commercial ones.
It might be also that the battery voltage flying up is shoving current
the other way, too fast and even with the indutor in there can over
current the FETs. The input capacitors could make a good load for
that backwards current and hurt things as well.
Is the controller designed to work backwards reliably, too ?
What I find is that the PV input caps, with much higher PV voltage
than battery voltage, is a great source of blow-up energy for the
controller but that is usually from HV to LV battery direction.
Ultimately it is the high current from the battery and shorted FETs
that can cause the PCB to catch fire if things go very wrong.
One thing to do is to catch the Vbattery side going too high *FAST*
and immediately shut off the converter(s)
boB
hold less energy than the PV input capacitors.
Yes, the input voltage 'shouldn't' go high but the battery side
certainly does go high. I think you are right about the input voltage
not going high, but having fixed this many years ago, I can't remember
the failure mechanism exactly. But it can go at least to the PV's Voc
voltage, but much above that, (just a few volts) the PV array should
clamp that.
What was the nominal PV input voltage of this controller ? Was it set
up as a 24V array or 48V array or maybe higher voltage ? That can
also make a difference. What kind was it ? I am familiar with a lot
of the commercial ones.
It might be also that the battery voltage flying up is shoving current
the other way, too fast and even with the indutor in there can over
current the FETs. The input capacitors could make a good load for
that backwards current and hurt things as well.
Is the controller designed to work backwards reliably, too ?
What I find is that the PV input caps, with much higher PV voltage
than battery voltage, is a great source of blow-up energy for the
controller but that is usually from HV to LV battery direction.
Ultimately it is the high current from the battery and shorted FETs
that can cause the PCB to catch fire if things go very wrong.
One thing to do is to catch the Vbattery side going too high *FAST*
and immediately shut off the converter(s)
boB
Jan Panteltje
Jan 31, 2023, 12:50:53 AMJan 31
to
On a sunny day (Mon, 30 Jan 2023 10:55:43 -0800 (PST)) it happened Fred Bloggs
<bloggs.fred...@gmail.com> wrote in
<01c089c0-1b8f-4f5d...@googlegroups.com>:
Yes mine has that warning in red printed on it.
When I first tested it I ignored it of course.
Seems to be still working though.. mmm maybe I should check it again... :-)
Mainly load protection I think?
<bloggs.fred...@gmail.com> wrote in
<01c089c0-1b8f-4f5d...@googlegroups.com>:
When I first tested it I ignored it of course.
Seems to be still working though.. mmm maybe I should check it again... :-)
Mainly load protection I think?
Joerg
Jan 31, 2023, 4:52:01 PMJan 31
to
not, but then at least my electronics loads do. With MPPT chargers it
seems it's not worth to spend much money and rather treat them like
disposables.
> Yes, the input voltage 'shouldn't' go high but the battery side
> certainly does go high. I think you are right about the input voltage
> not going high, but having fixed this many years ago, I can't remember
> the failure mechanism exactly. But it can go at least to the PV's Voc
> voltage, but much above that, (just a few volts) the PV array should
> clamp that.
>
75V, two residential panels in series. I can also connect them in
parallel but that requires much beefier cables and extra diodes in case
one is shaded (they already have bypass diodes).
> What was the nominal PV input voltage of this controller ? Was it set
> up as a 24V array or 48V array or maybe higher voltage ? That can
> also make a difference. What kind was it ? I am familiar with a lot
> of the commercial ones.
>
> It might be also that the battery voltage flying up is shoving current
> the other way, too fast and even with the indutor in there can over
> current the FETs. The input capacitors could make a good load for
> that backwards current and hurt things as well.
>
the PV side are large enough?
> Is the controller designed to work backwards reliably, too ?
>
regard bullet-proof. You could not destroy it by randomly disconnecting
things. The same should go for commercial ones but obviously they can't
get it done.
> What I find is that the PV input caps, with much higher PV voltage
> than battery voltage, is a great source of blow-up energy for the
> controller but that is usually from HV to LV battery direction.
>
> Ultimately it is the high current from the battery and shorted FETs
> that can cause the PCB to catch fire if things go very wrong.
>
> One thing to do is to catch the Vbattery side going too high *FAST*
> and immediately shut off the converter(s)
>
An SCR the size of a gold ball that is capable to reliably blow a 50A
car fuse if needed. If I put that sans fuse on the MPPT output it would
present a dead short to the MPPT, like an empty battery. If it doesn't
survive that then the MPPT is real junk.
Jeff Liebermann
Jan 31, 2023, 5:42:16 PMJan 31
to
On Mon, 30 Jan 2023 10:40:39 -0800, Joerg <ne...@analogconsultants.com>
wrote:
The usual result is a warranty claim, which most MPPT charge
controller manufacturers were honor. That's being generous because
the documentation and usually a big label on the controller will warn
the user that charging without a (battery) load is a really bad idea.
Lots of examples and horror stories online:
<https://www.google.com/search?q=solar+do+not+operate+with+battery+disconnected>
I attempted to get a list of parts replaced, but failed. I'm fairly
sure all the MOSFETs blew, but there might have been some other parts
involved.
I've seen open batteries cause failures 3 times so far. All three in
campers running whatever could be thrown together from Amazon sourced
parts. I've learned from these and two more installs that it pays to
buy everything from a single manufacturer. My favorite this week in
Renogy:
<https://www.renogy.com>
Unfortunately, I couldn't find a schematic of the charger. On my last
order, I attempted to demand a schematic or I would order from someone
else. They complied, by sending me a hand scribbled block diagram.
When I asked for an autopsy on the failed unit(s), they mentioned the
disconnected battery problem, but not the BMS (battery management
system).
The problem seems to revolve around the battery disconnect switch,
breaker or fuse. It's common advised that a high current disconnect
switch be installed in series with the battery to allow a guaranteed
disconnect in case of fire. The switch works nicely if the owner
remembers to turn off the charger or cover the solar panels BEFORE
disconnecting the battery. There are some other ways to create the
problem. I have various schemes for preventing the problem, but both
the owner and manufacturer will void the warranty if I become too
creative.
My best so far is a big contactor relay that disconnects battery,
solar panels, vehicle alternator and shore power if any manner of
fault is detected. So far, on one home system, it seems to work
except the contacts will sometimes arc weld closed. Arc welding one
pair of contacts effectively locks the contacts in the closed
position, makes the idea self-defeating.
Your crowbar solution might work but I suspect that the charger
MOSFETs will blow before the fuse opens.
Good luck.
--
Jeff Liebermann je...@cruzio.com
PO Box 272 http://www.LearnByDestroying.com
Ben Lomond CA 95005-0272
Skype: JeffLiebermann AE6KS 831-336-2558
wrote:
controller manufacturers were honor. That's being generous because
the documentation and usually a big label on the controller will warn
the user that charging without a (battery) load is a really bad idea.
Lots of examples and horror stories online:
<https://www.google.com/search?q=solar+do+not+operate+with+battery+disconnected>
I attempted to get a list of parts replaced, but failed. I'm fairly
sure all the MOSFETs blew, but there might have been some other parts
involved.
I've seen open batteries cause failures 3 times so far. All three in
campers running whatever could be thrown together from Amazon sourced
parts. I've learned from these and two more installs that it pays to
buy everything from a single manufacturer. My favorite this week in
Renogy:
<https://www.renogy.com>
Unfortunately, I couldn't find a schematic of the charger. On my last
order, I attempted to demand a schematic or I would order from someone
else. They complied, by sending me a hand scribbled block diagram.
When I asked for an autopsy on the failed unit(s), they mentioned the
disconnected battery problem, but not the BMS (battery management
system).
The problem seems to revolve around the battery disconnect switch,
breaker or fuse. It's common advised that a high current disconnect
switch be installed in series with the battery to allow a guaranteed
disconnect in case of fire. The switch works nicely if the owner
remembers to turn off the charger or cover the solar panels BEFORE
disconnecting the battery. There are some other ways to create the
problem. I have various schemes for preventing the problem, but both
the owner and manufacturer will void the warranty if I become too
creative.
My best so far is a big contactor relay that disconnects battery,
solar panels, vehicle alternator and shore power if any manner of
fault is detected. So far, on one home system, it seems to work
except the contacts will sometimes arc weld closed. Arc welding one
pair of contacts effectively locks the contacts in the closed
position, makes the idea self-defeating.
Your crowbar solution might work but I suspect that the charger
MOSFETs will blow before the fuse opens.
Good luck.
--
Jeff Liebermann je...@cruzio.com
PO Box 272 http://www.LearnByDestroying.com
Ben Lomond CA 95005-0272
Skype: JeffLiebermann AE6KS 831-336-2558
Joerg
Jan 31, 2023, 6:14:49 PMJan 31
to
> Lots of examples and horror stories online:
> <https://www.google.com/search?q=solar+do+not+operate+with+battery+disconnected>
> I attempted to get a list of parts replaced, but failed. I'm fairly
> sure all the MOSFETs blew, but there might have been some other parts
> involved.
>
will have punctured as well.
> I've seen open batteries cause failures 3 times so far. All three in
> campers running whatever could be thrown together from Amazon sourced
> parts. I've learned from these and two more installs that it pays to
> buy everything from a single manufacturer. My favorite this week in
> Renogy:
> <https://www.renogy.com>
>
> Unfortunately, I couldn't find a schematic of the charger. On my last
> order, I attempted to demand a schematic or I would order from someone
> else. They complied, by sending me a hand scribbled block diagram.
> When I asked for an autopsy on the failed unit(s), they mentioned the
> disconnected battery problem, but not the BMS (battery management
> system).
>
> The problem seems to revolve around the battery disconnect switch,
> breaker or fuse. It's common advised that a high current disconnect
> switch be installed in series with the battery to allow a guaranteed
> disconnect in case of fire. The switch works nicely if the owner
> remembers to turn off the charger or cover the solar panels BEFORE
> disconnecting the battery. There are some other ways to create the
> problem. I have various schemes for preventing the problem, but both
> the owner and manufacturer will void the warranty if I become too
> creative.
>
understand is why the majority of MPPT designers can't get this done
properly. When I designed an MPPT charger it wouldn't even have occurred
to me that such a single-fault scenario should cause an issue, and it
didn't. My client tested the heck out of it because they said that
installers make all kinds of mistakes, especially if they are the homeowner.
Out in the wild one has to assume the worst. For example people
parallel-charging from the unregulated 12V output of an old gas
generator, revving it a bit via the carburetor linkage to "goose the
charge top-off". Then, after a while, the BMS comes ...
> My best so far is a big contactor relay that disconnects battery,
> solar panels, vehicle alternator and shore power if any manner of
> fault is detected. So far, on one home system, it seems to work
> except the contacts will sometimes arc weld closed. Arc welding one
> pair of contacts effectively locks the contacts in the closed
> position, makes the idea self-defeating.
>
> Your crowbar solution might work but I suspect that the charger
> MOSFETs will blow before the fuse opens.
>
about 16V because of the SCR? It essentially puts a dead short across
the MPPT output. If it can't even take that the design engineers should
go wash dishes or whatever. I wouldn't call them engineers.
Lasse Langwadt Christensen
Jan 31, 2023, 6:39:30 PMJan 31
to
and able to handle a dead/shorted/missing battery
boB
Feb 1, 2023, 12:52:15 AMFeb 1
to
On Tue, 31 Jan 2023 13:51:52 -0800, Joerg <ne...@analogconsultants.com>
What I meant was that if it is a bi-directional (synchronous) buck,
then it is possible for the PV input voltage to be raised above Voc
from the battery. The PV array, being a series of diodes, will clamp
the voltage at approximately Voc, or what would be Voc, even at night.
This assumes something is not right of course.
>
>> What was the nominal PV input voltage of this controller ? Was it set
>> up as a 24V array or 48V array or maybe higher voltage ? That can
>> also make a difference. What kind was it ? I am familiar with a lot
>> of the commercial ones.
>>
>
>I don't know yet. My friend has jury duty out of town and is gone.
>
>
>> It might be also that the battery voltage flying up is shoving current
>> the other way, too fast and even with the indutor in there can over
>> current the FETs. The input capacitors could make a good load for
>> that backwards current and hurt things as well.
>>
>
>I don't quite follow. How can there be a problem if the capacitors on
>the PV side are large enough?
>
When the battery breaker trips, the energy in the battery side caps
might not be enough to charge the PV input caps very much because of E
= 1/2 CV^2 depending on the capacitance in the controlle
>
>> Is the controller designed to work backwards reliably, too ?
>>
>
>Very unlikely. The one I designed wasn't either but it was in this
>regard bullet-proof. You could not destroy it by randomly disconnecting
>things. The same should go for commercial ones but obviously they can't
>get it done.
Non-synchronous is good in that respect. Just a bit less efficient
because of the diode drop on the low side of the 1/2 bridge.
>
>
>> What I find is that the PV input caps, with much higher PV voltage
>> than battery voltage, is a great source of blow-up energy for the
>> controller but that is usually from HV to LV battery direction.
>>
>> Ultimately it is the high current from the battery and shorted FETs
>> that can cause the PCB to catch fire if things go very wrong.
>>
>
>That sounds like a seriously flawed design.
>
>
Any design can have this problem if the bottom FETs short AND the
relay contacts weld shorted to the battery. Double fault so that
would be a rare occurrance.
>> One thing to do is to catch the Vbattery side going too high *FAST*
>> and immediately shut off the converter(s)
>>
>
>Well, I am going to add in a crowbar anyhow to protect my electronics.
>An SCR the size of a gold ball that is capable to reliably blow a 50A
>car fuse if needed. If I put that sans fuse on the MPPT output it would
>present a dead short to the MPPT, like an empty battery. If it doesn't
>survive that then the MPPT is real junk.
Crowbar should work as long as nothing breaks before the breaker
trips.
Electronics stuff breaks sometimes. Just have to do our best so if it
does, the damage is low as possible.
boB
then it is possible for the PV input voltage to be raised above Voc
from the battery. The PV array, being a series of diodes, will clamp
the voltage at approximately Voc, or what would be Voc, even at night.
This assumes something is not right of course.
>
>> What was the nominal PV input voltage of this controller ? Was it set
>> up as a 24V array or 48V array or maybe higher voltage ? That can
>> also make a difference. What kind was it ? I am familiar with a lot
>> of the commercial ones.
>>
>
>I don't know yet. My friend has jury duty out of town and is gone.
>
>
>> It might be also that the battery voltage flying up is shoving current
>> the other way, too fast and even with the indutor in there can over
>> current the FETs. The input capacitors could make a good load for
>> that backwards current and hurt things as well.
>>
>
>I don't quite follow. How can there be a problem if the capacitors on
>the PV side are large enough?
>
might not be enough to charge the PV input caps very much because of E
= 1/2 CV^2 depending on the capacitance in the controlle
>
>> Is the controller designed to work backwards reliably, too ?
>>
>
>Very unlikely. The one I designed wasn't either but it was in this
>regard bullet-proof. You could not destroy it by randomly disconnecting
>things. The same should go for commercial ones but obviously they can't
>get it done.
because of the diode drop on the low side of the 1/2 bridge.
>
>
>> What I find is that the PV input caps, with much higher PV voltage
>> than battery voltage, is a great source of blow-up energy for the
>> controller but that is usually from HV to LV battery direction.
>>
>> Ultimately it is the high current from the battery and shorted FETs
>> that can cause the PCB to catch fire if things go very wrong.
>>
>
>That sounds like a seriously flawed design.
>
>
relay contacts weld shorted to the battery. Double fault so that
would be a rare occurrance.
>> One thing to do is to catch the Vbattery side going too high *FAST*
>> and immediately shut off the converter(s)
>>
>
>Well, I am going to add in a crowbar anyhow to protect my electronics.
>An SCR the size of a gold ball that is capable to reliably blow a 50A
>car fuse if needed. If I put that sans fuse on the MPPT output it would
>present a dead short to the MPPT, like an empty battery. If it doesn't
>survive that then the MPPT is real junk.
trips.
Electronics stuff breaks sometimes. Just have to do our best so if it
does, the damage is low as possible.
boB
boB
Feb 1, 2023, 1:02:03 AMFeb 1
to
On Tue, 31 Jan 2023 15:14:41 -0800, Joerg <ne...@analogconsultants.com>
wrote:
Don't forget the other side of power electronics blowing up.
Manufacturing itself can screw up a good design.
boB
wrote:
Manufacturing itself can screw up a good design.
boB
Joerg
Feb 9, 2023, 2:48:45 PMFeb 9
to
On 1/31/23 9:52 PM, boB wrote:
> On Tue, 31 Jan 2023 13:51:52 -0800, Joerg <ne...@analogconsultants.com>
> wrote:
>
>> On 1/30/23 7:10 PM, boB wrote:
[...]
> On Tue, 31 Jan 2023 13:51:52 -0800, Joerg <ne...@analogconsultants.com>
> wrote:
>
>> On 1/30/23 7:10 PM, boB wrote:
>>> Yes, the input voltage 'shouldn't' go high but the battery side
>>> certainly does go high. I think you are right about the input voltage
>>> not going high, but having fixed this many years ago, I can't remember
>>> the failure mechanism exactly. But it can go at least to the PV's Voc
>>> voltage, but much above that, (just a few volts) the PV array should
>>> clamp that.
>>>
>>
>> It will go to Voc if the series FET shorts out. So in my case around
>> 75V, two residential panels in series. I can also connect them in
>> parallel but that requires much beefier cables and extra diodes in case
>> one is shaded (they already have bypass diodes).
>>
>
> What I meant was that if it is a bi-directional (synchronous) buck,
> then it is possible for the PV input voltage to be raised above Voc
> from the battery. The PV array, being a series of diodes, will clamp
> the voltage at approximately Voc, or what would be Voc, even at night.
> This assumes something is not right of course.
>
boost or SEPIC, for small stuff. Sometimes bucks are synchronous but
usually not intended to be used bidirectionally. What for?
>
>>
>>> What was the nominal PV input voltage of this controller ? Was it set
>>> up as a 24V array or 48V array or maybe higher voltage ? That can
>>> also make a difference. What kind was it ? I am familiar with a lot
>>> of the commercial ones.
>>>
>>
>> I don't know yet. My friend has jury duty out of town and is gone.
>>
>>
>>> It might be also that the battery voltage flying up is shoving current
>>> the other way, too fast and even with the indutor in there can over
>>> current the FETs. The input capacitors could make a good load for
>>> that backwards current and hurt things as well.
>>>
>>
>> I don't quite follow. How can there be a problem if the capacitors on
>> the PV side are large enough?
>>
>
> When the battery breaker trips, the energy in the battery side caps
> might not be enough to charge the PV input caps very much because of E
> = 1/2 CV^2 depending on the capacitance in the controlle
>
anything large into the front side. When the battery vanishes those caps
are the only energy storage there is on the output side and preciously
little of that since the manufacturers don't want to waste money.
>
>>
>>> Is the controller designed to work backwards reliably, too ?
>>>
>>
>> Very unlikely. The one I designed wasn't either but it was in this
>> regard bullet-proof. You could not destroy it by randomly disconnecting
>> things. The same should go for commercial ones but obviously they can't
>> get it done.
>
> Non-synchronous is good in that respect. Just a bit less efficient
> because of the diode drop on the low side of the 1/2 bridge.
>
>
>>
>>
>>> What I find is that the PV input caps, with much higher PV voltage
>>> than battery voltage, is a great source of blow-up energy for the
>>> controller but that is usually from HV to LV battery direction.
>>>
>>> Ultimately it is the high current from the battery and shorted FETs
>>> that can cause the PCB to catch fire if things go very wrong.
>>>
>>
>> That sounds like a seriously flawed design.
>>
>>
> Any design can have this problem if the bottom FETs short AND the
> relay contacts weld shorted to the battery. Double fault so that
> would be a rare occurrance.
>
The problem with this design flaw is that it is caused by a single fault
that might not even be a fault: The BMS opens. This could simply happen
because the battery isn't happy about the temperature. Or the amount of
peak load. Or as in one case I know of a simple screw terminal
connection had come loose. Or whatever. The result in all these cases
can be and usually is that the battery "goes away". That fact alone
triggering a MPPT charger meltdown is a serious design flaw in my book.
>
>>> One thing to do is to catch the Vbattery side going too high *FAST*
>>> and immediately shut off the converter(s)
>>>
>>
>> Well, I am going to add in a crowbar anyhow to protect my electronics.
>> An SCR the size of a gold ball that is capable to reliably blow a 50A
>> car fuse if needed. If I put that sans fuse on the MPPT output it would
>> present a dead short to the MPPT, like an empty battery. If it doesn't
>> survive that then the MPPT is real junk.
>
> Crowbar should work as long as nothing breaks before the breaker
> trips.
>
> Electronics stuff breaks sometimes. Just have to do our best so if it
> does, the damage is low as possible.
>
occurring scenarios such as a battery vanishing in the electrical sense.
Not doing so is poor engineering. Even if the engineers are younger and
may be not yet competent enough, that's what we have design reviews for.
Or should have ...
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