Upgrading from a 40a to 100a Charge Controller

jchizuk

I'm finally real-world-testing my solar setup on my off-grid bus, and I'm suddenly realizing the solar "kit" I purchased has a woefully undersized charge controller. I have a 1200 watt array on the roof (six 200 watt 24v panels) charging a lithium battery bank via a 40a MPPT charge controller (I'm seeing 560 watts as a max output which made me run the numbers and realize this is an issue). I'm planning on upgrading to a 100a MPPT charge controller about as soon as possible. 

When I was doing my bus conversion I ran the wires from the roof to the charge controller through the walls and the access isn't great (one of the very few things that I didn't think to leave accessible, sadly). My concern is obviously wire size here. I have 10AWG on the roof to the controller and 10AWG between the controller and batteries (less than a foot away). Upgrading the wire from the controller to the batteries is not an issue and seems necessary (I'm getting mixed answers via Google but am thinking 4AWG would be safe), but the roof is my more pressing concern. Are those 10AWG cables safe as they are between the panels and a 100a charge controller? Obviously they've been working thus far, but realizing the charge controller is completely wrong for this setup has me second-guessing everything in this "kit" I purchased. (Last time I'll make that mistake). 

Any thoughts (or better ways to go about this) are greatly appreciated. 

BB.

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

Photowhit

Usually once you get close to 1000 watt array, you should consider moving to a higher system voltage, assuming you are at 12 volt now...

jchizuk

Bill, 

Thanks very much for all of this incredibly helpful information. 

Yes, we do travel to hot and cold weather, but we spend far more time in hot weather, perhaps 70-80 degrees on rough average?

I have a 12v 618ah lithium battery bank. I have a maximum wattage of 1200 and physically do not have room to add more panels, I highly doubt I will ever upgrade the panel setup. 

These are my panels: https://richsolar.com/products/200-watt-24-volt-solar-panel

They are wired like the diagram shown in this listing (second image): https://richsolar.com/products/1200-watt-24-volt-solar-kit-with-40a-mppt-controller All of these are 10AWG cables and the roof is about 17 feet long. Like shown in the picture I have just one negative and positive cable running from the solar array to the charge controller, those are 20 foot cables. 

I'm considering this charge controller (I don't think I have room for some of the larger, more affordable 100a charge controllers but would very much prefer a cheaper option if one exists): https://www.victronenergy.com/solar-charge-controllers/smartsolar-mppt-ve.can

My current charge controller is less than a foot from the batteries so no issue there. Based on that chart I can splurge for the 3AWG cables to be safe. 

You've said a 100a charge controller might be overkill (assuming that still stands with this additional info). How much smaller can I go? I was under the impression that with a 12v battery and 100a current (W = V x A) I could see a max of 1200 watts output with that setup. I suspect I'm missing a bit of information there.

I don't want to spend $1k on a new charge controller if I can get something smaller to work, but unit size and function are paramount for my system. 

I think that should answer all your questions. Thanks again for such thorough help. 

BB.

You are very welcome.

Can you confirm which panels you have?

There are two types of 200 Watt panels... One with Vmp~20.4 volts and a second with Vmp~37.6 volts (under standard test conditions).

https://richsolar.com/products/1200-watt-24-volt-solar-kit-with-40a-mppt-controller

200 Watt Solar Panel Specifications

• Maximum Power(Pmax): 200W
• Maximum Power Voltage(Vmp): 20.4V
• Maximum Power Current(Imp): 9.80A
• Open Circuit Voltage(Voc): 24.3V
• Short Circuit Current(Isc): 10.2A
• Weight: 26.5 lbs
• Dimensions: 58.7 x 26.8 x 1.4 in
• Warranty: 25 years output warranty

https://richsolar.com/products/200-watt-24-volt-solar-panel

Specifications

• Maximum Power(Pmax): 200W
• Maximum Power Voltage(Vmp): 37.6V
• Maximum Power Current(Imp): 5.36A
• Open Circuit Voltage(Voc): 45.4V
• Short Circuit Current(Isc): 5.83A
• Maximum System Voltage(Vmax): 1000VDC
• Temperature Range: -40°C ~ 90°C
• Max Series Fuse Rating: 15A
• Weight: 26.5 lb
• Dimensions: 58.7 x 26.8 x 1.4 in

For the MPPT charge controller that came in your kit--You should have the first panels (Vmp~20.4 volts) with three panels in series and two parallel strings.

If yo had the second panels (Vmp~37.6 volts), three panels in series is too high of input voltage for your 40 amp MPPT charge controller (100 VDC max input voltage).

-Bill

jchizuk

Bill, 

I was mistaken with my link in my last comment to the panels - I do in fact have the first type of panels that you listed above, with Vmp~20.4 volts. Typically have approx. 60 volts coming into the charge controller. 

It seems as though this means I was sent 12v panels instead of 24v as promised and managed to never notice.... Unbelieveable. Really kicking myself for not doing my due diligence over here. 

Now I'm wondering if that changes anything in regards to upgrading the charge controller? 

jchizuk

Photowhit said:

Usually once you get close to 1000 watt array, you should consider moving to a higher system voltage, assuming you are at 12 volt now...

I do have a 12v system now, and this is definitely on my radar as something to do (and something I wished I had done in the first place), but I'm currently on the road in my rig and trying to stick with the easiest possible fix for the time being.