Mppt charge controller current path question

[Roland Saul]

A question or two about current path through a 20 amp mppt controller....

Given an mppt controller that provides separate Battery and Load terminals, does this by itself indicate that the two circuits are totally separated? If I apply a load to the load terminals, will that load ever be shared between batteries and array voltage or will it draw only from the batteries? Seems difficult to me to properly control any shared current draw between two sources of varying voltage, but I'm just trying to get my head around it. How does the controller isolate the load so that it just draws from the battery, and is there a layman's explanation as to how it does this, if it does?

[sensij]

The load terminals will have the same voltage as the battery terminals; they are connected. There is a relay between them that will open if the battery voltage drops too low, offering protection for your battery from a load that would discharge it past the point of no return, and possibly protecting your load from damage that could occur by operating a too low of a voltage (drawing more current to achieve the same power). In general, the load terminals will only support light loads... to avoid nuisance tripping of the relay, a heavy load like an inverter is probably better off hooked up to the battery directly.

In a true MPPT controller, the solar array might have a common negative (or more rarely, a common positive) with the other terminals, or it might be isolated and floating. You can check the documentation, but a meter will tell you too. The array voltage will almost definitely not be the same as the battery (or load) voltage, and will not affect what is happening on the load terminal. Easy example... when the array voltage is zero at night, the load terminal will still have the same voltage as the battery.

Some cheaper MPPT controllers offer MPPT in bulk only, but absorb and float are PWM. In that case, you would find that the array voltage measures the same as the battery voltage during the absorb and float portions of the charge cycle. However, once the sun sets and the array voltage drops, the array terminals are either opened or reverse current flow is blocked by diodes, so that the battery voltage is unaffected by the zero array voltage.

[Mike90250]

.....
Some cheaper MPPT controllers offer MPPT in bulk only, but absorb and float are PWM. In that case, you would find that the array voltage measures the same as the battery voltage during the absorb and float portions of the charge cycle. However, once the sun sets and the array voltage drops, the array terminals are either opened or reverse current flow is blocked by diodes, so that the battery voltage is unaffected by the zero array voltage.

AFIK, there are no MPPT controllers that use MPPT for absorb. My Midnight Classic will, as light is fading, display MPPT FLOAT for a couple minutes, then drops out of FLOAT and just displays MPPT as the sun goes away. It's just trying to hang on and charge with what it can. But I'm pretty sure absorb and float are just PWM as the controller stops tracking and logs a low voltage.

[Roland Saul]

Ah, most excellent info, thanks . I was unsure whether my inverter could connect directly onto the battery posts or whether I had to connect it strictly at the load terminals of the controller..

Further to that question then, if I apply a typical load, say, a 10 amp 12 volt spray pump, directly to the battery, what regulation happens when I turn it on while the panels are producing say 8 amps, which is their peak?? Does the current coming from the batt terminals on the controller just go to point of least resistance, and regulate itself? If the current from the panels drops while the battery is under load, like a cloud obscuring the sun, does the path of least resistance self-correct and then flow more from the battery to the load?

[SunEagle]

Ah, most excellent info, thanks . I was unsure whether my inverter could connect directly onto the battery posts or whether I had to connect it strictly at the load terminals of the controller..

Further to that question then, if I apply a typical load, say, a 10 amp 12 volt spray pump, directly to the battery, what regulation happens when I turn it on while the panels are producing say 8 amps, which is their peak?? Does the current coming from the batt terminals on the controller just go to point of least resistance, and regulate itself? If the current from the panels drops while the battery is under load, like a cloud obscuring the sun, does the path of least resistance self-correct and then flow more from the battery to the load?

Pretty much like you have described. The panels will provide what the batteries need to charge first. What ever is extra will go to a load.

The load will draw from the batteries and if available from panels depending on what it needs.

Most people that live off grid try to schedule their big loads during the best charging time of the day so that they use everything from the panels and don't discharge the batteries as quickly.

[thastinger]

AFIK, there are no MPPT controllers that use MPPT for absorb. My Midnight Classic will, as light is fading, display MPPT FLOAT for a couple minutes, then drops out of FLOAT and just displays MPPT as the sun goes away. It's just trying to hang on and charge with what it can. But I'm pretty sure absorb and float are just PWM as the controller stops tracking and logs a low voltage.

As a MN CC guy myself, help me with your terminology. Is "bulk" and "absorb" the same thing to you or 2 different cycles and settings? My absorb cycle is 4 hrs long, during that period the CC still tracks as clouds go by etc. or it appears to do so to me. I don't notice the CC behaving differently between bulk and absorb. To be honest, I've never really observed it once it reaches float.

Here is my question. After 4 hrs at 59.2V in the absorb cycle, when it goes into float the voltage instantly drops to the set 52.1V....where did the 7V go? Happens to fast to be discharged as heat and the loads haven't changed. That's a lot of power that is just instantly gone.

[Mike90250]

BULK is the time/process to get low batteries up to the ABSORB voltage. This will vary with loads, depth of charge from the previous night and if the wife wants to dry her hair with the blow dryer. The Controller will run MPPT mode to get all possible power into the batteries.
When the specified voltage has been reached, say 59.2V, the controller starts the ABSORB Timer, for however long you have it set for your batteries (mine is 90 minutes, yours may be 2 hours or even 4 hours, what ever the time is needed to get your batteries specific gravity to "full". LFP batteries only need 1 minute) 4 hours is an unreasonably long time, since you may need 3 hours to get the BULK phase completed, and never complete a full charge in 1 day without generator. But generally the Bulk & Absorb voltage, is the SAME. Absorb is a timed stage, or when the battery slows down accepting amps (End Amps setting).
But, while in absorb, the battery does not need as much amps as it did in Bulk, and the way controllers reduce amps, is to lower the volts (pressure in the water hose) and that will slow down the amps. When the advantage of MPPT is no longer needed to supply amps, the controller slows down and uses PWM to regulate the charging. When the timer signals the end of Absorb, things switch to the FLOAT voltage setting, and then the controller needs only to supply a small amount of amps, and that is nearly always done with PWM.

[thastinger]

The 4 hr absorb time was recommended by Trojan's tech line. I never found any time setting recommendations in their lit so I called to ask. Been running like that for a couple years now on the same set of T605 batteries, seems ok but I have nothing to compare things to.

[Sunking]

The 4 hr absorb time was recommended by Trojan's tech line. I never found any time setting recommendations in their lit so I called to ask. Been running like that for a couple years now on the same set of T605 batteries, seems ok but I have nothing to compare things to.

For RE applications Trojan has a whole different set of voltages and terms.

Today Trojan uses the term "Daily Charge" for batteries that are cycled daily which is just a common sense term. It means to use a Constant Current mode which is your BULK setting until the battery reaches 2.47 volts per cell and current tapers off to 5%. then Float @ 2.2 volts per cell.

[thastinger]

It means to use a Constant Current mode which is your BULK setting until the battery reaches 2.47 volts per cell and current tapers off to 5%. then Float @ 2.2 volts per cell.

Yep, that's exactly where I'm at on my settings. EQ is 62.1
My Cotek inverter has a 60V high limit so I have to shut it off to do the last phase of an EQ cycle. It flashes a yellow "input level" light upon daily absorb but has been working fine for the 2 years now. I only EQ the bank when I'm there (vacation place which is unattended 25ish out of 30 days per month).

So you're saying I should adjust my absorb time based off of charge current? How do you ever tell that if there are small loads running on the system? My system is designed to provide 2.5Kwh but 26 of 30 days it only sees .5Kwh to run the beer fridge.

[Sunking]

So you're saying I should adjust my absorb time based off of charge current?

I don't think you can. For solar applications you want the pump as much charge as you can in a day. Set Bulk = Absorb = Float to 2.46 vpc or 59 to 60 volts on a 48 volt system. What that does is forces the controller to operate as Constant Current > Constant Voltage. It will stay in CC mode until the voltage reaches 59 or 60 volts, then switches to CV mode and you will see the current taper off

How do you ever tell that if there are small loads running on the system?

Same way you always do. The charger will always b eon with the sun shinning. If the load is less than what the panels are capable of supplying the power comes from the panels. Same as always. For example if the batteries have fully charged and current has tapered, if a load comes on you will see the amps rise on the controller indicating the power is from the panels.

The issue is solar is a very soft source unlike a hard source of the grid. There are not enough sun hours in a day to properly cycle through all 3 stages fully. Trojan, Rolls and all battery manufactures are getting hammered with warranty claims for RE users. The failures are sulfated battery from chronic under charging. So Trojan Rolls and other manufactures are upping the voltage running on the corrosion side of the fence vs sulfate side.

Absorb stage is not a timed event. It is a CV mode until current tapers to 3% of C. Unless you use a very small percentage of your batteries power, you will never get there on a solar system.

[thastinger]

I don't think you can. For solar applications you want the pump as much charge as you can in a day. Set Bulk = Absorb = Float to 2.46 vpc or 59 to 60 volts on a 48 volt system. What that does is forces the controller to operate as Constant Current > Constant Voltage. It will stay in CC mode until the voltage reaches 59 or 60 volts, then switches to CV mode and you will see the current taper off


Same way you always do. The charger will always b eon with the sun shinning. If the load is less than what the panels are capable of supplying the power comes from the panels. Same as always. For example if the batteries have fully charged and current has tapered, if a load comes on you will see the amps rise on the controller indicating the power is from the panels.

The issue is solar is a very soft source unlike a hard source of the grid. There are not enough sun hours in a day to properly cycle through all 3 stages fully. Trojan, Rolls and all battery manufactures are getting hammered with warranty claims for RE users. The failures are sulfated battery from chronic under charging. So Trojan Rolls and other manufactures are upping the voltage running on the corrosion side of the fence vs sulfate side.

Absorb stage is not a timed event. It is a CV mode until current tapers to 3% of C. Unless you use a very small percentage of your batteries power, you will never get there on a solar system.

My system is designed to produce 2.5Kwh/day. Since it is at our vacation place, most days it only runs my converted chest freezer that acts as a beer fridge(.5Kwh). Should I be doing something different with my charge voltages since 26 out of 30 days a month I spend 4 hrs at close to corrosion voltage when I really don't need to? My system runs at 25% capacity over 80% of the time, would you cut back the absord voltage and time a bit in that situation?