limiting low light voltage output from solar panels

Thread Starter

Chipper

Joined Jul 19, 2018
60
Hi all, I have a 350 watt solar panel project that is connected to a MPPT. I'm using this to charge a 13v 200Ah LiFePO4 battery pack through a BMS. The BMS will enable charging when it senses greater than 9v at the charge pin. I use the resulting enable signal to close a relay that connects the panels to the MPPT (and more importantly disconnects them in the event of a overcharge situation) The panels will provide well over 9v even in very low light conditions (i.e: dawn), long before they produce enough current to charge the battery. I would like to somehow keep the panel voltage output low until the panels can produce enough current to effectively charge the cells. I think about a Zener diode and perhaps a resistor. The BMS apparently draws very little current when detecting charge power being available. In short I'd like to prevent the current draw of the 30 amp automotive type relay until the panels are ready to charge.
 
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Ian0

Joined Aug 7, 2020
759
How about wiring the relay coil to the solar panels? A 30A automotive relay might put enough load on the panels to keep the voltage down, it will also conveniently switch on at about 9V, and won't load the battery if it is switched on too soon.
 
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ronsimpson

Joined Oct 7, 2019
1,024
Part of the problem is the panel output good voltage with no load.
Using a relay like below the power can be sent to two places. Panel is connected to the center pin. When the coil is engaged the power goes to the MPPT controller. When the coil is not engaged the power goes to a load. (car light bulb, tail light or head light) This load will keep the voltage down.
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Next idea; keep the MPPT controller connected all the time. When the voltage is good cut the (car light load) out. When the voltage is too low connect the new load.
 

Thread Starter

Chipper

Joined Jul 19, 2018
60
Part of the problem is the panel output good voltage with no load.
Using a relay like below the power can be sent to two places. Panel is connected to the center pin. When the coil is engaged the power goes to the MPPT controller. When the coil is not engaged the power goes to a load. (car light bulb, tail light or head light) This load will keep the voltage down.
View attachment 220633
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Next idea; keep the MPPT controller connected all the time. When the voltage is good cut the (car light load) out. When the voltage is too low connect the new load.
I will think about this solution. Might just do the job. My main concern was to be able to prevent an overcharge situation
 

ronsimpson

Joined Oct 7, 2019
1,024
Maybe I do not understand.
BMS Battery Management System.
MPPT is a Max Power Point Thing. "Thing" wrong word. lol

Why not Panel--MPPT--BMS--Battery? The MPPT is to get the most power out of the panel. The battery monument is to charge the battery right and to cut it off when full.
 

Thread Starter

Chipper

Joined Jul 19, 2018
60
Maybe I do not understand.
BMS Battery Management System.
MPPT is a Max Power Point Thing. "Thing" wrong word. lol

Why not Panel--MPPT--BMS--Battery? The MPPT is to get the most power out of the panel. The battery monument is to charge the battery right and to cut it off when full.
My BMS; Orion jr BMS is really good at tracking and controlling the LiFePO4 battery pack, but requires methods to switch off and on charging and discharge. I've built all of these control circuits and redundencies with things I've learned here on All about Circuits. My MPPT (Tracking) :^) is a RedArc (australian mfd) and it manages power from panels as well as truck alternator. I'm probably way over-thinking all of this and should have just bought some company's packaged system for my truck camper, but I've had fun putting it all together and earned some things in the process. The system also charges from shore power. Let's just say it got complicated. I'll play with these suggestions and see what works. Right now I'm just switching the panels off and on manually.
 

Daniel Sala

Joined May 28, 2015
65
Hi,

Dreadful idea for voltage good + charge current good ANDing into the BMS power good sensing pin with an R2R quad op amp, a voltage reference and some resistors, lacking some of your numbers like charge current:

_20201027_091610.JPG

Done this very quickly, so caveat emptor. Also, didn't check if summer ('AND gate' op amp) resistors 10ks in parallel are seen as 5k to the 4k7 resistor. Obviously, you'd need to re-calculate most things to suit whatever sense resistor were used depending on permissible voltage drop across it and BMS power good trigger voltage level, etc.

Op amps as comparators: not those with ESD protection diodes at inputs (could short-circuit inputs together and weaken or plain fry the part), ideally not with large Vos, that's about it. LMC6464 is good general purpose quad OA that can be used as a comparator.

Looks like it could work fron my basic understanding of circuits. If not, it's just a nice showcase of assorted op amp uses...
 

bobwareham

Joined Feb 10, 2013
5
Hi all, I have a 350 watt solar panel project that is connected to a MPPT. I'm using this to charge a 13v 200Ah LiFePO4 battery pack through a BMS. The BMS will enable charging when it senses greater than 9v at the charge pin. I use the resulting enable signal to close a relay that connects the panels to the MPPT (and more importantly disconnects them in the event of a overcharge situation) The panels will provide well over 9v even in very low light conditions (i.e: dawn), long before they produce enough current to charge the battery. I would like to somehow keep the panel voltage output low until the panels can produce enough current to effectively charge the cells. I think about a Zener diode and perhaps a resistor. The BMS apparently draws very little current when detecting charge power being available. In short I'd like to prevent the current draw of the 30 amp automotive type relay until the panels are ready to charge.
Sound like your MPPT charge is dum it should do all that on its own?
 

Jettman

Joined Apr 5, 2019
3
Solar panels are current sources shorted by what you could call a body diode. This body diode is what limits the voltage at each cell. In an open circuit condition, you are seeing the total voltage of all those diodes in series. In low light condition where the currents are tiny, the voltage is being limited by the shunt resistance of the cells. As the illumination increases, the current goes up and you see the rise in voltage.

Using voltage to decide when to connect the panels to the load will give a false indication that the panel is able to supply any useful current. By placing a shunt resistor across the panel and measuring the shunt voltage you can determine the current and therefore the level of illumination.

A simple circuit for doing the switchover is placing a relay across the solar panel as mentioned earlier and have the normally closed contact attached to a shunt resistor and the normally open contact connected to the load. As illumination and panel current increases, the voltage across the shunt will rise until the relay energizes. This will remove the shunt and attach the panels to the load. The relay will not de-energize until the panel voltage falls below what is needed to keep the relay on.
 

Hugh Riddle

Joined Jun 12, 2020
72
Hi all, I have a 350 watt solar panel project that is connected to a MPPT. I'm using this to charge a 13v 200Ah LiFePO4 battery pack through a BMS. The BMS will enable charging when it senses greater than 9v at the charge pin. I use the resulting enable signal to close a relay that connects the panels to the MPPT (and more importantly disconnects them in the event of a overcharge situation) The panels will provide well over 9v even in very low light conditions (i.e: dawn), long before they produce enough current to charge the battery. I would like to somehow keep the panel voltage output low until the panels can produce enough current to effectively charge the cells. I think about a Zener diode and perhaps a resistor. The BMS apparently draws very little current when detecting charge power being available. In short I'd like to prevent the current draw of the 30 amp automotive type relay until the panels are ready to charge.
Please could you sketch us a block diagram of your system incl the BMS, MPPT and relay.

I might have got the wrong picture but why do you want to keep the panel voltage down in twilight during which little panel power is available and neither overcharging or over-fast charging will occur? In stronger light, battery current must be continuously monitored and be respectively cut-off or limited (a battery protection task) overriding, when necessary, the MPPT action. Will your control elements provide all these actions?

BTW, I'm working on a (much lower-powered) product that involves exactly the same basic considerations and controls charging using only the MPPT's 'variable DC-DC transformer' action. Perhaps some high-power MPPT's work that way.
 
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Thread Starter

Chipper

Joined Jul 19, 2018
60
Please could you sketch us a block diagram of your system incl the BMS, MPPT and relay.

I might have got the wrong picture but why do you want to keep the panel voltage down in twilight during which little panel power is available and neither overcharging or over-fast charging will occur? In stronger light, battery current must be continuously monitored and be respectively cut-off or limited (a battery protection task) overriding, when necessary, the MPPT action. Will your control elements provide all these actions?

BTW, I'm working on a (much lower-powered) product that involves exactly the same basic considerations and controls charging using only the MPPT's 'variable DC-DC transformer' action. Perhaps some high-power MPPT's work that way.
Thanks for the taking the time to all that answered my post! I will draw up a block diagram as suggested and post here. My MPPT will absolutely control all that needs to be controlled with the panels as well as the combined alternator charging operations. I merely have added a pair of relays to provide a backup way to prevent overcharge to battery cells. The BMS turns off a signal relay that sends power to two automotive relays (one for panels, one for alternator) when the specified voltage is reached. My thinking here is I wanted to avoid the maintenance charging the the RedArc device provides and allow the battery to cycle to a lower voltage before entering a charging cycle once again...all controlled by BMS settings. Right now I’m manually switching between shore power charging and off-grid charging
 

Thread Starter

Chipper

Joined Jul 19, 2018
60
One more thing: I am using power supplied by the various charging sources applied to a BMS input to control the BMS output to the signal relay that is in turn controlling the auto relays
 

Hugh Riddle

Joined Jun 12, 2020
72
Thanks for the taking the time to all that answered my post! I will draw up a block diagram as suggested and post here. My MPPT will absolutely control all that needs to be controlled with the panels as well as the combined alternator charging operations. I merely have added a pair of relays to provide a backup way to prevent overcharge to battery cells. The BMS turns off a signal relay that sends power to two automotive relays (one for panels, one for alternator) when the specified voltage is reached. My thinking here is I wanted to avoid the maintenance charging the the RedArc device provides and allow the battery to cycle to a lower voltage before entering a charging cycle once again...all controlled by BMS settings. Right now I’m manually switching between shore power charging and off-grid charging
A thought crossed my mind: I was surprised your PV panel showed 9V pre-dawn. Could that be caused by the BMS-MPPT 'leaking' (milliamps of 'reverse' current from the battery to the PV panel? On my (tinier) scale, I've had to introduce a diode to block the c.1mA emerging from the power inputs of DC-DC converters or low drop-out (LDO) linear IC regulators, which was preventing my using the panel to sense twilight. Connecting a 12V car relay's coil across the solar panel at pre-dawn should throw 'light' on that.

It also occurred to me that your relay coil could overheat as 12V PV panel voltages can reach more than 20V (above the continuous automotive spec) particularly in winter (high panel volts at low temperatures) when the BMS-MPPT limits or cuts-off charging current. Adding a zener diode could worsen the worst-case situation.

Why don't you trust the BMS-MPPT to prevent overcharging? I believe Li cells are easily damaged by overcharging and float/maintenance charging is discouraged but had the impression that suitable modules could handle those aspects.
 
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