Upgrading from a 40a to 100a Charge Controller

Welcome to the forum jchizuk,

With MPPT (maximum power point tracking) solar charge controllers--You do have a lot more options for wiring (with the higher costs of MPPT type charge controllers).

An MPPT controller is a digital switching buck mode power supply internally (computer controlled). This means it can take high voltage/low current from the solar array and efficiently (around 95% or so) "down convert" to the low voltage/high current needed to charge the battery bank and run your loads.

For the MPPT controller--We have the Solar Panel input, and the Battery Bus output. And we need to look at these to separately (from the wiring point of view).

One of the simple power equations is Power = Voltage * Current

We will need more details--But the wiring from your solar array with a "moderately capable" MPPT charge controller has the input voltage limit of ~140 to 150 VDC (typical). And given this is mobile applicaiton, I will assume that sometimes you will be in the desert (hot days) and sometimes in the mountains (cold/sub freezing weather).

Solar panels have ratings for "room temperature" operation. For example you will see Vmp (voltage maximum power) at 38 volts, and Voc (voltage open circuit) at 48 volts. For example:

https://www.solar-electric.com/solarland-slp270-24c1d2-270-watt-24-volt-solar-panel.html

SLP270-24C1D2 Specifications:

• Maximum power (Pmax): 270 Watts
• Nominal voltage: 24 Volts DC
• Voltage at Pmax (Vmp): 37.7 Volts DC
• Current at Pmax (Imp): 7.16 Amps DC
• Open-circuit voltage (Voc): 44.86 Volts DC
• Short-circuit current (Isc): 7.6 Amps DC

With solar panels, Voc and Vmp rise in cold temperatures and fall in hot temperatures and we need to know where you will be (min/max temperature) and also your battery bus voltage (12/24/etc. VDC) so that your array will work with your MPPT controller.

If you can supply links to your proposed Solar Charge controller and your Solar Panels you have--Then we can answer specifically for your needs. (Note that "24 Volt panels" can have Vmp from 24 volts to 38 volts... Details matter here).

Also, can you tell us more about your present wiring... Do you have 2x 10 AWG cables (+/-) from roof to charge controller--Or do you have several sets of 2x 10 AWG cables from roof to charge controller? And what is the distance of the wiring run?

I hope your solar charge controller is relatively close to the battery bank--Keep the cable run from charge controller to battery bus short and heavy (heavy wires so the cables do not overheat from current, and short to keep voltage drop low).

Wiring wise, we need to know the maximum Wattage of your solar array (are you going to add more panels in the future?) and the voltage of your battery bank (I assume 12 volts).

The current output of the solar charge controller is limited by the internal electronics (safely and reliably) to it rating (100 Amps in this case?).

The maximum current from controller to battery would be:

• 100 Amps limited by charge controller (some controllers have programmable output limit)
• 1,200 Watt Array * 1/11.0 volts min battery bus voltage * 1.25 NEC solar derating = 136 Amp max current from array to battery in very cold weather
• 1,200 Watts * 0.77 * 0.77 solar+panel derating * 14.5 volts charging = 64 Amps typical max current you will see in most "very good" solar conditions

A 100 Amp MPPT controller may be a bit overkill--But it will work fine.

Here is a handy (simplified NEC/National Electric Code) chart for AWG wire size vs insulation type vs current capacity:

https://lugsdirect.com/WireCurrentAmpacitiesNEC-Table-301-16.htm

4 AWG cable with 90C rated insulation is rated for 95 Amps... So it is "close" (the "real" calcuations include conduit type/size/wire fill/max ambient temperature/etc.).

You can also refer the the manual for your MPPT controller--It should tell you what is required for their controller wiring.

-Bill