AIMS MPPT draining LIFEPO4...

Assuming the charge controller (and other components) are working properly, then NO, adding a blocking diode will not fix anything, and will throw confusion into the charge controller and inaccurate charging voltage

If you don't have one, I would suggest purchasing an AC+DC Current Clamp DMM (digital muli-meter). Something like one of these (or others):

https://www.amazon.com/UNI-T-Digital-Handheld-Resistance-Capacitance/dp/B0188WD1NE (low cost, "good enough" meter)
https://www.amazon.com/Auto-Ranging-Resistance-Klein-Tools-CL800/dp/B019CY4FB4 (mid-priced meter)

You can just clip the current clamp on a wire and see how much current is flowing (and the +/- sign is active in DC mode--Not obvious which direction the current is flowing as just "flipping" the clamp will "flip" the +/- sign.

It would be helpful to know exactly which brand/model of LiFePO4 battery (with BMS or not, etc.). How you know that the battery is being discharged? What is your solar array (number of panels, series/parallel wiring, etc.).

Any other loads (AC Inverter, etc.)?

-Bill

Also with solar charge controllers, always connect the battery bus to controller first, and disconnect the battery bus to controller "last". If you connect solar array first, it can confuse or even damage the charge controller.

-BB

This solar charge controller?:

https://www.aimscorp.net/80-amp-solar-charge-controller-12-24-36-48-vdc-mppt.html

Just to be clear, connect the Charge controller Vbatt  terminals to the Battery Bank first (and battery is somewhere between 10-17 VDC so controller will "configure" for 12 volt bank).

Measure Vpanel input to controller---With the sun up, you should measure around 30-110 VDC--And would be around Vmp-array*0.8 for when everything is operating normally (sun is up, MPPT controller is charging).

More or less, use your volt meter to measure the Vbatt terminal voltage (should be within 0.10 VDC or so of battery bus voltage).

What does you LCD display say (array voltage, array current, battery voltage, battery current)?

Is the AC inverter "ON" for longer periods of time?

Did you connect the Inverter's 12 VDC cables directly to the LiFePO4 battery bank, or to the "LOAD" terminals on the solar charge controller?

Note that a 3,500 Watt inverter can draw a huge amount of current if loaded to 3,500 Watts--For example:

• 3,500 Watts * 1/0.85 AC inverter eff * 1/10.5 volts (loaded/discharged battery bank) = 392 Amps

Or fully discharge your 200 AH battery in about 30 minutes...

I did not find your specific inverter--But it can draw 40-50 Watts @ 12 volt "just turned on" (Tare Current) and zero loads...

48 Watts / 12 volts = 4 amps "Tare current" (guess)... That would discharge your battery in:

• 200 AH / 4 amps = 50 hours (or just over two days if there is no charging from the sun/other charging source)

A 600 Watt array is pretty small for a 3,500 Watt inverter--But if you are careful about your energy usage--It could work. Don't know your location, but a relatively sunny spring day in a sunny location could be 5 hours:

• 600 Watt array * 0.61 DC off grid system eff * 5 hours = 1,830 WH
• 1,830 WH / 12 volt bank = 152 AH of charging in a "sunny" location (typically South West USA).

• The 200 AH battery is rated for 150 AH max discharge (and charge) "Surge?" current...
• 150 Amps * 12 volts * 0.85 AC inverter eff = 1,500 Watt max AC inverter surge current support...

When connecting your AC inverter... It should be connected directly to the battery bus. The LOAD terminals on the solar charge controller are only good for 60 amps maximum (at least my quick search/read).

All of this is to set your "expectations". To better help you debug and understand what you should be seeing. If you can give us some operating voltages and currents (array, battery bus, etc.)... Conditions, Full sun, battery state of charge, etc. 

-Bill

Looks good on the display... One of the major design requirements for a Solar Charge Controller is to prevent "back feeding" the solar array with battery energy at night. Typically, at most, the "vampire loads" of a MPPT solar charge controller is 1 to a few Watts at night:

• I=P/V= 2 watts / 12 volts = 0.17 amps (just a rough estimate)

Your controller would have to be consuming multiple amps of current overnight to measurably affect the battery state of charge... For example, say the controller was "consuming" 5 amps over night for 12 hours:

• 5 amps * 12 hours = 60 AH
• 60 AH * 12 volts = 729 Watt*Hours
• 5 amps * 12 volts = 60 Watts

If the controller was consuming that amount of energy, you would probably feel the heat coming off the controller (60 Watt lamp worth of heat).

You could also measure the night time voltage/current going to the solar array... It can happen that you will see 10's of volts on solar array at night from "controller leakage"--But solar cells are actually just "giant diodes". While not terribly good as "blocking diodes", a solar panel at night will draw very little current--So it should not be the "power sink" in this setup.

Some controllers will display array voltage and current at night--Might be interesting to see what your controller displays (since this is all software controlled... It is possible that the software person "zeros" the values at night to avoid phone calls from customers with questions).

-Bill

ToroPower,

I did send you both a PM (private message) and an email. Please let me know if you have received the email or not (and this is the correct email address).

You probably cannot edit your older posts because of the missing email conformation. I can force that, but I wanted to confirm your email address is correct first.

Also, please start your own thread/discussion about your system. That way your questions do not get confused with the original poster's questions.

Take care,
-Bill