How to Upgrade a DC Solar Trailer

So You Want to Upgrade Your DC Solar Trailer

DC Solar trailers are a popular and relatively inexpensive way to get solar power to nearly anywhere you need it. The equipment is top notch, from high-quality manufacturers such as SMA and MidNite Solar. And a DC Solar trailer is perfectly capable from a power output perspective, with two SMA Sunny Island 6kW inverters and batteries (even lead acid) that are more than capable of handling that power. However, you'll quickly find that the system is limited by the solar PV panels and MidNite Classic 250 controller.

Limited Solar

Although solar panels have come a long ways from those used on DC Solar trailers, we're still limited by the solar array. Depending on the year produced, the trailer may have anywhere from 2350 to 2650 watts using panels with a width of 39 inches.

DC Solar designed a system with two separate strings of 5 panels in series. These are then wired in parallel inside the MidNite Solar combiner box. Unfortunately, shading has a major effect on solar panel performance, especially in series. Even shading over just one panel will limit the current of the entire string. This is the "bucket effect," and very similar to how a battery's capacity is determined by the cell with the lowest health. Modern power electronics such as EnPhase microinverters or SolarEdge power optimizers mitigate the effect of shading by allowing each PV panel to operate independently. But that adds cost and complexity, something DC Solar Solutions were keen to reduce. The result is that a single panel has an outsized impact on overall production.

Lithium vs Lead Acid Charge Settings

Although lithium iron phosphate and lead acid have very similar full-charge voltages, there are some major differences to be aware of.

• Lead-Acid uses a three-stage charging process: bulk, absorption, and float. Bulk charging applies a constant current until a set voltage is reached. Absorption then holds that voltage while reducing current, and float keeps the battery topped off at a lower voltage. Lead acid also requires a periodic equalization charge, in which the battery is over-charged to de-sulfinate the lead plates.
• Lithium Iron Phosphate (LiFePO4) uses a two-stage process: constant current (CC) and constant voltage (CV). The charger applies a steady current until the voltage reaches its limit, then holds that voltage while the current tapers off. There’s no need for a float stage, and no need for equalization. That said, we do typically set a float voltage about 1 volt below absorb.

MidNite have claimed that they are releasing a lithium-ion charge algorithm for the Classic 250, but we've been able to tune the charger for our batteries using our own voltage points and turning off both equalization and temperature compensation.

However, the MidNite Classic 250 has a maximum output of 55 amps of battery charge (it actually might be 53 amps, but I don't feel like consulting the manual). That's only 3168 watts at 57.6v - aka the battery full charge voltage.

We've done a lot of 325-watt panel installs using Longi or similar 39" width panels, which comes out to 3250 watts of PV. But we recommend a more modern Victron MPPT charge controller when upgrading. This is not only more efficient, it also allows for even more PV by installing MC4 connectors for a third array.

Replacing Solar Panels

IMPORTANT: Make sure to shut off both PV breakers before replacing panels. Be aware that you are working on nearly 250 volts, which can be incredibly dangerous! We highly advise leaving this up to trained professionals!

As I stated previously, DC Solar Solutions used 39-inch width panels on their trailers, so any 39" panel will technically fit. Be advised that you will need to drill holes in your new panels to mount them to the existing brackets. Then simply connect the panels in series. Each rail end has a length of wire with an MC4, making connection straightforward. We offer pre-terminated PV wire if yours is looking weathered (and most are by now).

Replacing the Charge Controller

IMPORTANT: Make sure to shut off both PV breakers and the battery breaker before replacing the charge controller! Be aware that you are working on nearly 250 volts, which can be incredibly dangerous! We highly advise leaving this up to trained professionals!

Replacing the MidNite Classic 250 charge controller is a bit more involved. First, shut off the PV and battery breakers and disconnect the charge cable from the back of the enclosure. This is a single Anderson Power Pole connector on the back left (driver's) side of the enclosure, just above the wiring bundle that comes out the back.

Next, remove the cover to the MidNite Classic using a Phillips screwdriver. Pull the cover back and unplug the comm wire on either the backside of the cover or top side of the PC board. Use a flat screwdriver to loosen the screws holding the battery, PV and ground wires, then loosen the conduit nut. The Classic itself will be fastened with either Phillips or Allen screws, so use the appropriate tool to remove the three screws (two below the PC board and one at the top-middle). You can now remove the Classic and set it aside.

We've found the best place to install Victron MPPT charge controller is just below the MidNite combiner box. Simply use self-tapping screws to mount. You can remove one or more knock-outs at the bottom of the box and run the existing wiring, or run longer wires. We put it upside-down for easy access to the wiring.

If you are adding PV capacity

Remove the plastic cover to the MidNite combiner box. Be aware that the input side of the solar breakers will still have nearly 200 volts! Loosen the screws to the battery breaker and remove the wires, then use a flat screwdriver to pull the tab at the bottom of the breaker. Lift it away from the DIN rail to remove.

We highly recommend replacing the stock 6awg charge cable with 2awg or 4awg when adding PV. The cables are tied with zip ties underneath the master inverter, and can be a pain to fully remove, so you can clip them back as much as possible. The ground wire is run to the shunt to the right of the master inverter when looking from the front, under a metal cover fastened with Allen screws. Simply remove the cover, loosen the shunt bolt with the 6awg battery cable and replace it with #2 or #4.

We use a 125-amp MEGA fuse and holder mounted in the space where the MidNite Classic 250 used to be. Note that our photos show a 125a breaker, which was for mock-up purposes. There's often a pre-drilled hole nearby in the back of the enclosure, which allows you to run your new wire back to the battery. Use an M8 (3/8") ring terminal to connect to your Higher Wire lithium battery.

Even More Solar Power

The MidNite combiner box has room for one more 250-volt 15-amp PV breaker, the output of which can be wired in parallel with the existing PV breakers. By drilling two small holes, you can run MC4 connectors through the enclosure to give you one more string. Then connect 5 more series-connected panels to get to the same operating voltage and viola! You now have 1.5x more solar! And if you need to take the trailer anywhere, you can simply unplug and leave the array. The two rail-mounted arrays will not be affected whatsoever.

Ready to Upgrade?

So what are you waiting for? Contact us today at sales@higherwire.com or 480-442-2007, or simply click this link to explore your upgrade options. When you buy from us, we supply you with everything you need to get the job done right the first time.