Travel trailer controller issues

Welcome to the forum Redleader,

Assuming you have these items:

https://www.interstatebatteries.com/products/pf-gc2-utl (6 volt @210 AH FLA "golf cart" type battery)
https://cdn.shopify.com/s/files/1/0549/3699/3843/files/R1.2_SolControllerManual_Min.pdf?v=1653600799 (charge controller manual)
https://fsi-sales.com/products/mc-series-50-amp-controller (30 or 50 Amp rated output depending on model)
2x 150 Watt @ 18 volt Vmp @ 8.33 Amps Imp

How you disconnect the battery bank last, and connect it first (then solar panels) is all correct.

Ideally, I would be recommending that you put the two 150 Watt panels in series (assuming Vmp~18 volts) for a Vmp-array~36 volts. Higher voltage/lower current makes wiring much easier and usually runs the MPPT controller "optimally".

Do you have a volt meter/DMM (digital multi-meter) to measure voltages?

If not, you might want to look at an AC+DC Current Clamp DMM... A "good enough" AC/DC clamp meter is around $50 on Amazon:

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

The reason I ask, is it helpful to have a meter that you can use to verify the voltages and currents directly--And not just rely on the charge controller's display.

Measure the voltage and current from the solar array. Double check MPPT controller connections (note that terminals 2 and 3 are both negative...2=solar array negative; 3=battery negative)... With "black on black" lettering, it may be easy to miss.

More or less, you want to see around Varray from 16 to 36 volts (depending on how array is wired and such). And Vbattery to be around 14.75 volts charging (FLA type) or around 13.8 volts float charging (battery full).

Ideally, the controller should hold around 14.6-14.8 volts for 2-6 hours (absorb charging) and drop back to float voltage once charged.

If you have a DC Current Clamp meter, the array will output around 4-8 amps in full sun (per panel) when charging. And can drop to zero Amps when not charging.

The output charging current (battery below 80% State of Charge) will be around:

• 300 Watt array * 0.77 panel+controller derating * 1/14.75 volts = 15.66 amps (cool, clear day, full noon time sun--about best harvest)
• 15.66 amps / 2 = 7.83 amps "typical" day, less than ideal weather current

The controller LEDs do show the "charging state" (boost=max available array power; Absorb=holding 14.75 volts; Float=13.8 full sun). Note that DC loads will affect battery charging and charging voltage. From the manual link:

While it is very possible that your controller is not charging--Some debugging will help you determine what is (or is not) really happening.

Double check configuration (i.e., FLA battery = RED battery type LED) and such is important too.

-Bill

If each 6 volt battery shows around 2.6 volts--They are truly discharged and most likely dead (under 10.5 volts for 12 volt system, under 5.25 volts per battery).

They are probably not good for anything... But if these are the old batteries, I would try to charge them with a 12 volt car charger and get them around 12 volts--And then test the rest of the solar system (most solar charging systems need at least 9.0 volts on the battery bus to boot correctly--for a 12 volt battery bus).

Once you have verified the system is working with the old batteries--Then you can buy new batteries without worrying that they are not being properly charged because of bad wiring/controller/etc...

-Bill