Any MPPT controller with proper or approximate SOC for LiFePO4?

[Mr.Michael]

Since the MPPT controller knows when the battery is full and knows the load on the battery it seems like simple math as long as either the capacity is set correctly or figured out after a few cycles. The controller can even reset by itself to 100% after full charge is achieved. The only unknown is if you happen to have a load directly connected to the battery, like an inverter. I set the capacity on my Rich Solar MPPT to match my battery but the controller seems not to care. I would assume the Renogy MPPT behaves identical as those seem to be close cousins, same software works on both and same BT module as well.

Does anyone make a MPPT controller that keeps track of in vs out and shows an approximate SOC?

 

[fat_old_sun]

Unless a solar controller (MPPT or PWM) communicates with a battery monitor, it has no way of knowing the *net* current flow into a battery bank. Most standalone controller go to float mode after having been in the absorb stage for some specified time. What you're looking for a amp-hrs counting charger that puts back what was taken out during the previous discharge cycle. For example, if you have a 200 Ah bank, and you take out 75 Ah during the night, the next day the charger must deliver net 75 Ah to the bank and some more to account for charging inefficiency.

 

[Mr.Michael]

Doesn't all current in and out of the battery passes via the controller?
I understand the limitations about losses and an approximate SOC is fine. The Rich Solar controller I have was set using the PC software to know about my battery. I set the capacity to 60Ah, so it should be able to calculate an approximate SOC. However, I discharged 17Ah at night from a full charge. That would be roughly 28% discharge leaving 72% remaining and the controller showed "SOC = 100%". That obviously is far from usable. I am not too concerned if the SOC is off by a few points but I want a general idea at a glance of what is going on. After all, those are computers and I just want them to compute numbers for us.

 

[fat_old_sun]

Doesn't all current in and out of the battery passes via the controller?

No. Why or how would the solar charger know about the current flowing out of the battery? The controller is not installed in a manner to monitor *all* current flow. The controller knows what it delivered to the batteries. However, there may be a load siphoning off current from the batteries at the same time. The only indication that the battery is filling up is the rising voltage. This is what triggers most charge controllers to flashing a "100% charged" message. However, there may still be some residual unconverted lead sulfate in the cells. Hence the need for amp-hrs based charging.

 

[Dzl]

Doesn't all current in and out of the battery pass via the controller?

No. A SCC handles just 1 thing, solar charging. A solar charge controller doesn't even handle all charging in many cases (alternator charging, shore power charging, generator charging, etc). Its possible you are thinking of a BMS (battery management system).

I believe Victron's BMV line will accomplish what you want, will also reviewed a cheaper chinese alternative to the BMV-712 at some point, and some BMS' would accomplish this as well (see: Chargery BMS). Searching for current shunt might be a good start to your research

 

[offgriddle]

~ My inverter is connected directly to my lifepo's just because I don't want to pass load current through my lifepo's and add to the heat quotient nor do I want to power another relay open and status light.
~ The inverter's low voltage alarm goes off before the BMS's cut off threshold, which is cool because I don't want to discharge my lifepo's to minimum specs. anyway.
~ I have often wondered how my battery monitor display knows the precise SOC of my batteries; I guess it must make that determination based on voltage.

 

[FilterGuy]

Some controllers do have a load output put they tend to be very low wattage. If that is the only load hooked up to the system then yes, the controller could conceivably know the SOC. I have not seen one that does this, probably because it is rare that the load output on the controller is rarely the only load.

 

[FilterGuy]

~ I have often wondered how my battery monitor display knows the precise SOC of my batteries; I guess it must make that determination based on voltage.

A battery monitor is watching the voltage across a shunt hanging off the battery minus. A shunt is nothing more than a very low value resistor of a known value. Consequently, the monitor can use ohms law to calculate the current: Vs=I*R. ( Vs is the measured voltage across the shunt, I is the current and R is the known resistance). Note that in one direction the voltage will be measured as a positive value and in the other direction it is a negative value. This means the monitor knows if the current is going into the battery or coming out of the battery. By tracking the current in and out over time it knows the Ah SOC of the battery. Since the Monitor also has a line to the battery+, it can also calculate Watts and Watt Hours using Watts Law: P=I*Vb (P= Power in Watts, I=the calculated current and Vb = the measured battery voltage)


Edit: Added diagram

 

[offgriddle]

A battery monitor is watching the voltage across a shunt hanging off the battery minus. A shunt is nothing more than a very low value resistor of a known value. Consequently, the monitor can use ohms law to calculate the current: Vs=I*R. ( Vs is the measured voltage across the shunt, I is the current and R is the known resistance). Note that in one direction the voltage will be measured as a positive value and in the other direction it is a negative value. This means the monitor knows if the current is going into the battery or coming out of the battery. By tracking the current in and out over time it knows the Ah SOC of the battery. Since the Monitor also has a line to the battery+, it can also calculate Watts and Watt Hours using Watts Law: P=I*Vb (P= Power in Watts, I=the calculated current and Vb = the measured battery voltage)

View attachment 6892
Edit: Added diagram

Wow that is a really great explanation thank you FG!

 

[Steve_S]

I have a Midnite Classic, it comes with a Application when connected to the Shunt & WizbangJr can do what you want, See below. How this is accomplished, the Shunt (Deltec 500A/50mv) is situated in such a manner that the shunt is between the Battery and the SCC & Inverter/Charger, the WizbangJr is an electronic sensor on the Shunt which relays data back to the SCC and shows you the state of charge. This is not very accurate with Lead Acid batteries because you need to test the gravity of the electrolyte, fortunately with Lithium Based batteries that isn't a problem. The Midnite Classics unfortunately do not and cannot communicate with a BMS as such.

Here is a DC Voltage Meter RS485 Modbus with 300A Shunt + USB Converter that also has a PC Application to monitor the energy flow through the circuit: https://peacefair.en.made-in-china....485-Modbus-with-300A-Shunt-USB-Converter.html

 

[Mr.Michael]

I have often wondered how my battery monitor display knows the precise SOC of my batteries; I guess it must make that determination based on voltage.

Yes for lead acid type systems voltage based works, i would not call it precise but a good guess. Since the voltage drops as the SOC goes down the controller can display an approximation. That doesn’t work with lithium cells as those have about the same voltage over most of the SOC range. A 85% full LiFoPo4 doesn’t look much different to the controller than a cell at 25% SOC.

 

[offgriddle]

Yes for lead acid type systems voltage based works, i would not call it precise but a good guess. Since the voltage drops as the SOC goes down the controller can display an approximation. That doesn’t work with lithium cells as those have about the same voltage over most of the SOC range. A 85% full LiFoPo4 doesn’t look much different to the controller than a cell at 25% SOC.

Okay thank you MM. The gentleman who posted a couple of great informative paragraphs above have also shed light upon how battery monitoring occurs and accuracy.