Even though the solar panels usually taper the current in the evening you still need a charge controller. Could be a relay (mechanical or SSR), a PWM or an MPPT. For example, let’s assume you have a 24v system and you are using 24v panels. Usually, you do not gain much by using an MPPT vs PWM if you use 12v panels with a 12v battery or 24v panels with a 24v battery if you connect them in parallel. But even in that case usually the 24v panels have a 44v VOC and a 35v VMP (very close to the 29.2 absorption voltage). The important thing is that the voltage is over absorption, so you need something to control the charge. If you need an on/off MPPT charge controller, check the Smart Controller from Victron (the ones with the on/off jumper) or a Midnite Classic (with aux2 input). If you have a Raspberry PI you can use the ones that have Modbus. Either by LAN (ethernet) or via RS485 like Midnite Classic or the MorningStar MPPT 60. If you know a little bit of electronics, you can use most MPPTs and turn them off (turn down the voltage, effectively turning them off) by simulating a temperature increase of the battery. Using the Battery Temperature Sensor (simulating it). Most controllers use a 10k thermocouple. Just put a 10k resistor and decrease the resistance when you want to reduce the voltage by putting a 2.5k resistor in parallel (using a relay).
Does anyone make a simple MPPT with on/off?
- Thread starter johntaves
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You would have to come up with a VERY contrived example to show PWM is more efficient - and even then it would be only at one temperature, one battery state, one irradiance etc. 99.9% of the time MPPT gets more power out of your PV.
It is how most BMSes work. They prevent damage to the cells. They, in general, do not terminate charge at an appropriate voltage. They are a last ditch effort at preventing damage/fires.
All BMSes sold now have settings for LFP or li-ion which disable all the lead acid features (temperature compensation, float voltage.)
That's the info I was looking for, thanks!
I have no problem doing the modbus, ethernet, rs485 route, but I'm not thrilled with that because sending an "OFF!!!!" signal via a complicated communication protocol is lousy.
The thermocouple is also doable, but Ugh!
I'd be happier with a simple open circuit == off.
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OK, my bad. I will accept the correct belief system.
BMS's must have large amp mosfets to be able to shut off the current, because loads and MPPTs should not have simple on/off switches that can be trivially controlled by a very low power relay.
Chargers should control the charge because a BMS has a big mosfet to snap off the current, because the charge controller knows nothing about the individual cells. They should also be able to hold a voltage and taper the current because doing on/off and letting the sun taper the current is much too complicated. The battery would get a headache with that juice flowing +, then -, then + again at a frequency similar to what my BEV battery experiences every time I drive it. It's bad enough that we have clouds, and day/night, doing this cycling. Plus it is ideal that these controllers that know nothing about the SOC determine when to stop because it is most intelligent to stop at 100% all the time. It would be a waste of capacity, not to mention all that intelligence, to charge to say 90% every day and do that tapering stuff only once a week or so. This is why they call them "intelligent" controllers, obviously.
It is imperative that we always buy an MPPT because a lot more circuits, with more software, and a higher price, is always better than less, less, and less. Well, 99.9% of the time, which is good enough for all of us.
Also it is important that we ignore heat sinks. Whenever we are trying to eek out the most efficient system, we should not question electronic components that produce heat. After all, heat does not affect circuit longevity or efficiency.
Hopefully this will keep the correct belief system police from knocking at my door and locking me up. I appreciate all the help from those trying to get me to comprehend the current regime.
I apologize for being so thick.
BMS's must have large amp mosfets to be able to shut off the current, because loads and MPPTs should not have simple on/off switches that can be trivially controlled by a very low power relay.
Chargers should control the charge because a BMS has a big mosfet to snap off the current, because the charge controller knows nothing about the individual cells. They should also be able to hold a voltage and taper the current because doing on/off and letting the sun taper the current is much too complicated. The battery would get a headache with that juice flowing +, then -, then + again at a frequency similar to what my BEV battery experiences every time I drive it. It's bad enough that we have clouds, and day/night, doing this cycling. Plus it is ideal that these controllers that know nothing about the SOC determine when to stop because it is most intelligent to stop at 100% all the time. It would be a waste of capacity, not to mention all that intelligence, to charge to say 90% every day and do that tapering stuff only once a week or so. This is why they call them "intelligent" controllers, obviously.
It is imperative that we always buy an MPPT because a lot more circuits, with more software, and a higher price, is always better than less, less, and less. Well, 99.9% of the time, which is good enough for all of us.
Also it is important that we ignore heat sinks. Whenever we are trying to eek out the most efficient system, we should not question electronic components that produce heat. After all, heat does not affect circuit longevity or efficiency.
Hopefully this will keep the correct belief system police from knocking at my door and locking me up. I appreciate all the help from those trying to get me to comprehend the current regime.
I apologize for being so thick.
Don’t be so hard on yourself, BMS’ with comms to chargers/inverters have been the best practice for lithium for over a decade.
It’s only if you want a cheaper solution that you use the compromised method you describe.
Where i live I can get less than five year old 5kw Sunny Boy inverters, including 5kw of PV and all the racking and cabling for less than $1k. It makes the choice simple.
The problem with simply switching off the charger is you miss the opportunity to taper current when nearly full, at low or high temperatures, or when coming out of deep discharge. What the e-bike BMS gang are yet to find out is that doing these things in a residential system will make the batteries last that much longer that the extra expense of a proper BMS is more than covered.
It’s only if you want a cheaper solution that you use the compromised method you describe.
Where i live I can get less than five year old 5kw Sunny Boy inverters, including 5kw of PV and all the racking and cabling for less than $1k. It makes the choice simple.
The problem with simply switching off the charger is you miss the opportunity to taper current when nearly full, at low or high temperatures, or when coming out of deep discharge. What the e-bike BMS gang are yet to find out is that doing these things in a residential system will make the batteries last that much longer that the extra expense of a proper BMS is more than covered.
Right, because it is impossible to turn charging back on after the cells have recovered to a lower V when the amps are naturally lower from the Sun. See above, where I accept the stated religion.
Do any of these MPPT charge controllers do this besides not charging when near freezing and below? Many will state they have temperature compensation, which was useful with lead acid, but does that mean they do something useful for Li at deep discharge?
After a quick check I found this thread which makes it clear you have to really pay attention when you buy these "intelligent" "controllers". But, again, these MPPTs are not hacks from the Lead Acid world. One can say they are very very intelligent. It would be stupid to put that logic into the BMS that must know the cell voltages and temps. No, it is far more intelligent to put that logic outside the one thing that actually must know it is Li. That would be a compromise, and sadly cheaper.
sunshine_eggo
Happy Breffast!
Didn't see it on the first page. Sorry if it's a duplicate. Many Victron MPPT have an external control circuit. Circuit closed = MPPT on. Circuit open = MPPT off. I use this for my Batrium to cut off the MPPT in the case of over-voltage.
I thought that was the case but failed to find the info. I searched Victron again and found this hairball.
"All larger units feature the Victron standard remote on/off terminals. All models that don’t feature anonboard Remote on/off terminal can be remotely controlled by using the VE.Direct non invertingremote on/off cable – ASS030550310. Note that this prohibits using the VE.Direct port for anythingelse."
But note that if the VE.Direct non invertingremote on/off cable is pulled out, then the MPPT charges. In other words, the default is to charge.
The minimum with the proper on/off remote is over $500!
If this isn't adding more complexity to do something simple, I don't know what is. Clearly this is "intelligent" and not some hack because this MPPT descended from lead acid.
I thought you wanted simplicity! Let me know your algorithm for controlling a charger in that manner.
What is your plan for limiting charge current in Summer mornings when the cells are coming out of deep discharge?
Your problem is you think you have discovered something new, when BMS’ such as REC have had lithium specific algorithms for over a decade that ensure maximum cell life and performance.
If you can’t afford it that’s fine, you’ll have to do your best like all those using the e-bike BMS’ , just don’t kid yourself you have come up with a simpler or better system.
I wrote it above. When attempting to get to 100%, stop charging when any cell gets to xY. Turn charging back on when all cells are below yV. I set mine to be 3.5 and 3.4V respectively.
I turn the loads off if any one cell gets below 3.1V. I could be wrong but I don't think that is a deep discharge. Also, correct me if I am wrong, doesn't the Sun gradually increase the amps every morning?
Thank you for telling me what my problem is. To be honest though, I did invent the Sun and how it rises and falls every day.
OK, thanks, you've done a very good job of understanding what I have implemented, and explained how it is a poor system, and how the other systems work better.
Depending on your panel orientation, for sure it is possible to emulate the job of a BMS using the suns natural movements.
I can’t imagine why anyone would intentionally make their system so complex to implement
Please tell me where in the world there is a place that is never cloudy in the morning?
One of the major factors in reduced lifespan of LiFePO4 is fast charge from a deep discharge. You can rely on no cloudy mornings if you like - i’ll stick with a proven system. Good luck ?
I can’t imagine why anyone would intentionally make their system so complex to implement
Please tell me where in the world there is a place that is never cloudy in the morning?
One of the major factors in reduced lifespan of LiFePO4 is fast charge from a deep discharge. You can rely on no cloudy mornings if you like - i’ll stick with a proven system. Good luck ?
DIYrich
Solar Wizard
I'm late to the party. It is my understanding that the MPPT optimizes power production from the panels. Charge Controller handles the charging of the battery. BMS protects the battery from over/under voltage and does cell balancing.
Your question seems to be if there is a "simple" device that can control the MPPT to replace the Charge Controller. Then you talk about having excess production off grid, and don't need an MPPT to optimize production. Do you have an MPPT or not? Or are you building a new system?
I think you need an MPPT that can produce the right stable voltage to charge the battery to eliminate the Charge Controller. I thought MPPT passes the highest wattage, and varries the voltage to find the highest watts.
Assuming you have rapid shutdown of the panels, it seems to me that you can use a raspberry pi to turn off and on production via the rapid shutdown signal.
If you have excess production, then I would stick with the charge controller. If you are building a system and want to use an MPPT without a charge controller, rapid shutdown with a PI might work.
Your question seems to be if there is a "simple" device that can control the MPPT to replace the Charge Controller. Then you talk about having excess production off grid, and don't need an MPPT to optimize production. Do you have an MPPT or not? Or are you building a new system?
I think you need an MPPT that can produce the right stable voltage to charge the battery to eliminate the Charge Controller. I thought MPPT passes the highest wattage, and varries the voltage to find the highest watts.
Assuming you have rapid shutdown of the panels, it seems to me that you can use a raspberry pi to turn off and on production via the rapid shutdown signal.
If you have excess production, then I would stick with the charge controller. If you are building a system and want to use an MPPT without a charge controller, rapid shutdown with a PI might work.
pollenface
Solar Addict
The first thing that comes to my mind is a relay on the PV wires.
Australia.
A BMS needs either mosfets OR contactors if you are concerned about mosfets failing. IMHO, one of those two is a MUST.
A BMS can control charging devices, such as an MPPT controller or (usually more importantly) an alternator regulator. But the BMS is NOT controlling the charge parameters, just turning them on/off. The MPPT is still it's own intelligent device, and should still determine when full charge is, and stop charging accordingly on its own. The MPPT still goes through the bulk and absorption phases. And the BMS is still the emergency last line of defense. IMO, a relay is still needed, because there are examples of MPPT controllers failing as a short circuit to the solar panels, and no "stop charging" command is able to stop them. Or, an owner/installer programs the MPPT incorrectly. And the relay/fets also protects from over discharge, which control of the MPPT offers no protection from.
Plenty of people use PWM controllers, and a few use basic on/off controllers. They work. The MPPT gets more power from the panels, that's all.
Ohm's law proves your statement incorrect, and that you fundamentally misunderstand the article and MPPT controllers.
V=IR
Voltage from a panel will remain constant when open circuit, regardless of irradiance. In that case R=infinity, so V will be the maximum voltage the panels can produce. When you apply a load, ohms law applies. If the load is constant (for example if the panel is connected *directly* to a battery) R is not changing and you cannot reduce current without voltage also falling. That is electronics 101 and basic physics.
What you are missing and the article is discussing is that there is a power curve, where if you adjust the load resistance, both voltage and current will change. As you adjust R, the *Watts* out of the panel changes, even if irradiance does not. Now imaging constantly changing R such that you are always getting the most *power* from the panels. This is the case where voltage doesn't change much, where you are constantly changing R to get maximum power from the panels. This is also what an MPPT controller does (maximum power point tracking), it constantly changes R such that the panels are always producing the most power possible. And THAT is why an MPPT controller produces more power than either a PWM or a direct connection, even given the loss from heat.
If you put 2 100 watt panels next to each other, one connected to an MPPT and battery, and the other directly to the battery, this is easy to observe. You could then measure the panel voltages, and the panel connected to the MPPT would be probably around 18V or so. The panel connected to the battery would be close to the battery voltage, maybe 13V. And more power would be flowing into the battery connected to the MPPT.
Not if you design a different type of BMS. Read my earlier comment on what I built. Again, I do not send any current through my BMS. My BMS and Electrodacus and maybe battrium are different.
If you read my very early posts in this thread you would discover I want to compare the MPPT with a simple straight connection.
Read what you quoted and then read your response. The two are unrelated.
I get what an MPPT does with respect to matching the panel's MPP. That is not the topic or concept you quoted.
I apologize for using sarcasm in the post that you have replied to. I should have been more blunt as follows.
You have failed to comprehend what I am doing and how I am doing it. Unfortunately this makes your comments irrelevant and worse misleading for other readers.
Thank you, but frankly, this seems like sarcasm.
I have plugged in a Victron MPPT 100/50 ($300+) and it produces minimal, if any, additional power compared to the DSSR20, which is effectively a straight connection from the battery to the panels. The MPPT picks 30V as the power point, and of course without the MPPT the voltage is the battery's at 27V. The difference between those two voltages does not amount to squat with respect to power. There are fewer amps from the panel with the MPPT, but at a higher voltage. The resulting amps to the battery are virtually identical.
I monitored the two when a cloud rolled by and couldn't tell the difference. Of course there might be irradiances that the MPPT does make an interesting difference. Regardless, if space and long cables are not an issue, I don't see how an MPPT can compete in terms of watts/$ compared to another panel.
The DSSR20 was $50CDN each, so about $75USD for the equivalent capacity to the $300 Victron.
Note of course, that this requires reasonably matched panels to the battery. A 60 cell panel matches nicely to a 24V LiFePo4 battery. A 72cell panel will not achieve more power than a 60 without the MPPT. But still an MPPT wouldn't justify its cost compared to another panel.
I monitored the two when a cloud rolled by and couldn't tell the difference. Of course there might be irradiances that the MPPT does make an interesting difference. Regardless, if space and long cables are not an issue, I don't see how an MPPT can compete in terms of watts/$ compared to another panel.
The DSSR20 was $50CDN each, so about $75USD for the equivalent capacity to the $300 Victron.
Note of course, that this requires reasonably matched panels to the battery. A 60 cell panel matches nicely to a 24V LiFePo4 battery. A 72cell panel will not achieve more power than a 60 without the MPPT. But still an MPPT wouldn't justify its cost compared to another panel.
Yes. That is expected. If the solar panel Vmp is very close to the battery charge voltage, there is no difference between a cheap PWM (or on/off relay with Lifepo4) vs MPPT CC. Disregarding the amps (because the Vmp is close to battery voltage) the difference between 30 and 27 is 90%. That is usually the efficiency of an MPPT at low charge current (usually the max efficiency is around 94 to 98% at around 80-90% the CC capacity). And at high currents you have to take into consideration voltage drop. With voltage drop in the equation, possibly the Vmp vs the charge voltage are much closer. That is why there is virtually no difference.
PD: If charging lifepo4 (with a charge voltage per cell close to 3.65), a relay or an on/off charger is okay. But when charging other types of batteries (or charging lifepo4 at lower cell voltages) it is best to use a PWM or MPPT if you want to do absorption (constant voltage) charging.
400bird
Solar Wizard
For everyone else playing along at home, it doesn't matter where the current flows, the BMS should have an independent way to stop current in or out of the battery. Either internal mosfets, a contactor, a shunt trip breaker, something. Relying on only one layer of safety (BMS turning off components) is not recommended or the safe way to do it.