Monitoring in and out power

[0truck0]

I already have the system built, but wanting to add a monitor. What is the best way to go to monitor SOC of lithium with two systems, can I have one device for this? and sometimes I might connect the two systems together too in the event the fridge batt is too low, or if charging from truck alternator I can connect them together also (via positives).

I have two solar systems on the trailer. 200 watts through an mppt into one 200 amp hour battery. Times two, so 400x400 total. I can connect the batteries via a circuit breaker, and I did originally have all 400 watts feeding one 200 a/hr batt via the two mppt controllers, but when I added the second battery the real intent was to split the system into fridge power and everything else power.

Now I want to monitor the SOC. My old lead acid batts were easy, just look at voltage, but not so with lithium.

* the system. Chins 200 amp hour lithium. 2x100 watt glass mono panels in series (24v) through Epever Tracer 30A mppt controller. Two identical systems in the same trailer. The tow vehicle is connected to the one fridge battery through a circuit breaker that I just leave off almost always and that flows through a Renogy 20A 12V DC-DC Dual Battery Charger. The two batteries can be connected together via circuit breaker from positive post to positive post. The fridge runs straight off the terminals, but the other batt powers everything through the mppt controller "lightbulb" output.

 

[sunshine_eggo]

I already have the system built, but wanting to add a monitor. What is the best way to go to monitor SOC of lithium with two systems, can I have one device for this? and sometimes I might connect the two systems together too in the event the fridge batt is too low, or if charging from truck alternator I can connect them together also (via positives).

Very good chance you trigger short circuit protection if you do this when one battery is high SoC and the other battery is low SoC.

I have two solar systems on the trailer. 200 watts through an mppt into one 200 amp hour battery. Times two, so 400x400 total. I can connect the batteries via a circuit breaker, and I did originally have all 400 watts feeding one 200 a/hr batt via the two mppt controllers, but when I added the second battery the real intent was to split the system into fridge power and everything else power.

Now I want to monitor the SOC. My old lead acid batts were easy, just look at voltage,

Many people think this is the case, but they're wrong.

but not so with lithium.

No less accurate than lead-acid and arguably more accurate.

If you have two separate batteries, you need two separate monitors. The Aili monitor is cheap, and you would get SoC for both batteries.

Another potential is the smartshunt. It will report SoC for the primary battery and will report voltage for the secondary provided they both share a common ground.

 

[0truck0]

thanks.
so, the two batteries will both deplete on consecutive cloudy days without charging, though I suspect the fridge will be more depleted. Never will the fridge batt be depleted and the other batt be 100%. What level of different charge would be an issue?

I dont understand quite your comment, "not the case".
Lead acid depletes and lose charge, lithium will hover around the same voltage then drop off more quick near the end. Voltage shows SOC well enough with LA (assuming no load at observation time). I can't ust look at the lithium voltage and get an idea of anything.

 

[sunshine_eggo]

thanks.
so, the two batteries will both deplete on consecutive cloudy days without charging, though I suspect the fridge will be more depleted. Never will the fridge batt be depleted and the other batt be 100%. What level of different charge would be an issue?

It depends on your batteries and wiring. My personal rule of thumb is 0.2V. This should limit instantaneous surge to 250A.

I dont understand quite your comment, "not the case".
Lead acid depletes and lose charge, lithium will hover around the same voltage then drop off more quick near the end. Voltage shows SOC well enough with LA (assuming no load FOR MANY HOURS at observation time).

Corrected. 10 hours is a minimum and 20-24 hours is where it actually gets accurate. If you want to take a quick reading by unloading it for a few minutes, you'll get a conservative, but likely very wrong value by 10-20%.

I can't ust look at the lithium voltage and get an idea of anything.

IR is lower, so there is less voltage drop. There is a range between 40 and 70% where the voltage change is so little, it's pretty meaningless, but snapshot voltages under light load are accurate enough. If you want a fairly accurate reading, unload for 5 minutes and compare to chart - will be more accurate than lead.