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/
MPPT chargers dying upon battery disconnect?
Started by ●January 30, 2023
Reply by ●January 30, 20232023-01-30
On Monday, January 30, 2023 at 1:40:50 PM UTC-5, Joerg wrote:> 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.I've seen that as a warning, in the user's manual, even for those little solar battery maintainers to keep a seldom used car battery topped off. You have to remove power to the MPPT charge controller before you remove the battery.> > -- > Regards, Joerg > > http://www.analogconsultants.com/
Reply by ●January 30, 20232023-01-30
On 1/30/23 10:55 AM, Fred Bloggs wrote:> On Monday, January 30, 2023 at 1:40:50 PM UTC-5, Joerg wrote: >> 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. > > I've seen that as a warning, in the user's manual, even for those little solar battery maintainers to keep a seldom used car battery topped off. > > You have to remove power to the MPPT charge controller before you remove the battery. >Yeah I know, and that really doesn't make sense in such a design. Any 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" :-) -- Regards, Joerg http://www.analogconsultants.com/
Reply by ●January 30, 20232023-01-30
On Monday, January 30, 2023 at 2:48:21 PM UTC-5, Joerg wrote:> On 1/30/23 10:55 AM, Fred Bloggs wrote: > > On Monday, January 30, 2023 at 1:40:50 PM UTC-5, Joerg wrote: > >> 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. > > > > I've seen that as a warning, in the user's manual, even for those little solar battery maintainers to keep a seldom used car battery topped off. > > > > You have to remove power to the MPPT charge controller before you remove the battery. > > > Yeah I know, and that really doesn't make sense in such a design. Any > 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.There are a bunch of different switching topologies. My guess there is a bunch of circulating magnetic energy that causes failure when the load is removed abruptly. 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. 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.> > In medieval Europe they would have dunked the design engineer into a > moat several times for "moral cleansing" :-) > -- > Regards, Joerg > > http://www.analogconsultants.com/
Reply by ●January 30, 20232023-01-30
On 1/30/23 12:09 PM, Fred Bloggs wrote:> On Monday, January 30, 2023 at 2:48:21 PM UTC-5, Joerg wrote: >> On 1/30/23 10:55 AM, Fred Bloggs wrote: >>> On Monday, January 30, 2023 at 1:40:50 PM UTC-5, Joerg wrote: >>>> 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. >>> >>> I've seen that as a warning, in the user's manual, even for those little solar battery maintainers to keep a seldom used car battery topped off. >>> >>> You have to remove power to the MPPT charge controller before you remove the battery. >>> >> Yeah I know, and that really doesn't make sense in such a design. Any >> 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. > > There are a bunch of different switching topologies. My guess there is a bunch of circulating magnetic energy that causes failure when the load is removed abruptly.That would be a serious case of engineering blunder. I've designed stuff 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. That is not realistic when you are using an off-the-shelf Li-Ion 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. Everytime I asked the answer was no :-( [...] -- Regards, Joerg http://www.analogconsultants.com/
Reply by ●January 30, 20232023-01-30
On Mon, 30 Jan 2023 12:35:43 -0800, Joerg <news@analogconsultants.com> wrote:>On 1/30/23 12:09 PM, Fred Bloggs wrote: >> On Monday, January 30, 2023 at 2:48:21 PM UTC-5, Joerg wrote: >>> On 1/30/23 10:55 AM, Fred Bloggs wrote: >>>> On Monday, January 30, 2023 at 1:40:50 PM UTC-5, Joerg wrote: >>>>> 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. >>>> >>>> I've seen that as a warning, in the user's manual, even for those little solar battery maintainers to keep a seldom used car battery topped off. >>>> >>>> You have to remove power to the MPPT charge controller before you remove the battery. >>>> >>> Yeah I know, and that really doesn't make sense in such a design. Any >>> 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. >> >> There are a bunch of different switching topologies. My guess there is a bunch of circulating magnetic energy that causes failure when the load is removed abruptly. > > >That would be a serious case of engineering blunder. I've designed stuff >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. > > >That is not realistic when you are using an off-the-shelf Li-Ion >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. > > >Everytime I asked the answer was no :-( > >[...]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
Reply by ●January 30, 20232023-01-30
mandag den 30. januar 2023 kl. 23.07.46 UTC+1 skrev boB:> On Mon, 30 Jan 2023 12:35:43 -0800, Joerg <ne...@analogconsultants.com> > wrote: > >On 1/30/23 12:09 PM, Fred Bloggs wrote: > >> On Monday, January 30, 2023 at 2:48:21 PM UTC-5, Joerg wrote: > >>> On 1/30/23 10:55 AM, Fred Bloggs wrote: > >>>> On Monday, January 30, 2023 at 1:40:50 PM UTC-5, Joerg wrote: > >>>>> 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. > >>>> > >>>> I've seen that as a warning, in the user's manual, even for those little solar battery maintainers to keep a seldom used car battery topped off. > >>>> > >>>> You have to remove power to the MPPT charge controller before you remove the battery. > >>>> > >>> Yeah I know, and that really doesn't make sense in such a design. Any > >>> 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. > >> > >> There are a bunch of different switching topologies. My guess there is a bunch of circulating magnetic energy that causes failure when the load is removed abruptly. > > > > > >That would be a serious case of engineering blunder. I've designed stuff > >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. > > > > > >That is not realistic when you are using an off-the-shelf Li-Ion > >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. > > > > > >Everytime I asked the answer was no :-( > > > >[...] > 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.a sync buck can pump energy from the output to the input but if there is 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?
Reply by ●January 30, 20232023-01-30
On 1/30/23 2:07 PM, boB wrote:> On Mon, 30 Jan 2023 12:35:43 -0800, Joerg <news@analogconsultants.com> > wrote: > >> On 1/30/23 12:09 PM, Fred Bloggs wrote: >>> On Monday, January 30, 2023 at 2:48:21 PM UTC-5, Joerg wrote: >>>> On 1/30/23 10:55 AM, Fred Bloggs wrote: >>>>> On Monday, January 30, 2023 at 1:40:50 PM UTC-5, Joerg wrote: >>>>>> 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. >>>>> >>>>> I've seen that as a warning, in the user's manual, even for those little solar battery maintainers to keep a seldom used car battery topped off. >>>>> >>>>> You have to remove power to the MPPT charge controller before you remove the battery. >>>>> >>>> Yeah I know, and that really doesn't make sense in such a design. Any >>>> 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. >>> >>> There are a bunch of different switching topologies. My guess there is a bunch of circulating magnetic energy that causes failure when the load is removed abruptly. >> >> >> That would be a serious case of engineering blunder. I've designed stuff >> 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. >> >> >> That is not realistic when you are using an off-the-shelf Li-Ion >> 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. >> >> >> Everytime I asked the answer was no :-( >> >> [...] > > 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. >There is supposed to be at least some capacitance on the output that 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. >I'd even test with lead acid because there could always be a gradual 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. >True. Plus it shall not be relied upon anyhow. Comms can fail quietly, then the alert doesn't come and ... whaddabam.> Was this a Chinese product ? >I was told it was Californian. I'll probably receive one of the failed 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. -- Regards, Joerg http://www.analogconsultants.com/
Reply by ●January 30, 20232023-01-30
On Mon, 30 Jan 2023 16:04:10 -0800, Joerg <news@analogconsultants.com> wrote:>On 1/30/23 2:07 PM, boB wrote: >> On Mon, 30 Jan 2023 12:35:43 -0800, Joerg <news@analogconsultants.com> >> wrote: >> >>> On 1/30/23 12:09 PM, Fred Bloggs wrote: >>>> On Monday, January 30, 2023 at 2:48:21 PM UTC-5, Joerg wrote: >>>>> On 1/30/23 10:55 AM, Fred Bloggs wrote: >>>>>> On Monday, January 30, 2023 at 1:40:50 PM UTC-5, Joerg wrote: >>>>>>> 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. >>>>>> >>>>>> I've seen that as a warning, in the user's manual, even for those little solar battery maintainers to keep a seldom used car battery topped off. >>>>>> >>>>>> You have to remove power to the MPPT charge controller before you remove the battery. >>>>>> >>>>> Yeah I know, and that really doesn't make sense in such a design. Any >>>>> 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. >>>> >>>> There are a bunch of different switching topologies. My guess there is a bunch of circulating magnetic energy that causes failure when the load is removed abruptly. >>> >>> >>> That would be a serious case of engineering blunder. I've designed stuff >>> 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. >>> >>> >>> That is not realistic when you are using an off-the-shelf Li-Ion >>> 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. >>> >>> >>> Everytime I asked the answer was no :-( >>> >>> [...] >> >> 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. >> > >There is supposed to be at least some capacitance on the output that >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. >> > >I'd even test with lead acid because there could always be a gradual >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. >> > >True. Plus it shall not be relied upon anyhow. Comms can fail quietly, >then the alert doesn't come and ... whaddabam. > > >> Was this a Chinese product ? >> > >I was told it was Californian. I'll probably receive one of the failed >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.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
Reply by ●January 31, 20232023-01-31
On a sunny day (Mon, 30 Jan 2023 10:55:43 -0800 (PST)) it happened Fred Bloggs <bloggs.fredbloggs.fred@gmail.com> wrote in <01c089c0-1b8f-4f5d-9596-faa9982e5a4fn@googlegroups.com>:>On Monday, January 30, 2023 at 1:40:50 PM UTC-5, Joerg wrote: >> 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. > >I've seen that as a warning, in the user's manual, even for those little solar battery maintainers to keep a seldom used car >battery topped off. > >You have to remove power to the MPPT charge controller before you remove the battery.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?
