3 solar designs which do you recommend

Hi all,

I am new to this forum and have spent the better part 2020 reading and watch Youtube videos on RV mods. I have focused on solar and Lithium batteries. Getting ready to break free from the home office.

I am planning 400-800 watt of solar power. Starting with existing lead acid batteries and later moving to lithium Iron of some sort. Not sure if I will stick with 12 Volts system or switch to a 24 Volts.

My background. I have spent the career in cybersecurity dating back to 1995. I have an electrical engineering degree, but have not practiced since graduation.

I have designed 3 configurations using 2 panel types with a combination of series and parallel options. I am looking for feedback on these designs. I have put together questions to kick it off. See attached PDF with diagrams.

• Are the 3 designs sound in and of themselves? I know the 3x195w PV input power roughly converts to 39A charge controller output current. This is probably a bit close for the 40A charge controller. But I am sure I saw this CC can handle 520W of solar for 12V charge. What are your thoughts?
• I understand the reasons for series vs parallel configurations and what it does to voltage and currents and therefore the wiring requirements. Also understand the benefits as it related to shading or panel reliability. Is there anything else I should consider?
• Are the fuses and circuit breakers spec’d appropriately?
• I believe I have the wire gauges spec’s appropriately looking for input.
• Given the differences in the lead acid battery types, any considerations I need to be aware of WRT one discharging the others.
• Read that many RVers manually disconnect solar or converter when inverter is used. I definitely don’t want to be manually disconnecting various components. The system should manage itself with some initial oversight and tuning.

Series or parallel on the panels.....if you have any shading, go parallel. Series will have smaller wire size requirements. Total solar input needs to be as large as you can fit/afford if you want even fair charging in less than ideal conditions.

24v is better in several ways including more power through the charger (similar amps/heating for 12 and 24). Heat in the controller is a limiting factor.

Santan Solar sells used panels. It's quite a bargain. I bought 2x 235watts, chose rectangle size for portable ground mount, as they are less likely to catch the wind.
I had EPEVER 20A. Chose to go with Victron 100v 50A. A bit pricey and feature rich. Not much effort for DIY sales, lots of training for Pros. Features for Lead acid charging are: Adaptive absorption time adjusts for state of discharge & Programmable Tail Current can early exit absorption time. SCC Blue Tooth can network with the Smart Shunt and deliver data and history to your cell phone.
IMHO, you don't have enough battery for that inverter. Peukerts Law for LA, IMHO you may only get half the AH under heavy loads.
Also, I thought batteries should be matched, same type and age.
Series vs Parallel. Wiring Watts lost = Current * Current * Ohms. So for same wire: 2 parallel will maximize loss, series is 1/4 the loss. 200w seems to be the break point for going series. Shading is not such a big problem for series as most panels have Bypass Diodes. Shading will drop the voltage output, so one string of series-parallel may be loafing.

Output amps occasionally over MPPT SCC spec a little shouldn't be a problem; it just limits output.

Single series PV string is better than parallel for partial shading, only loses output from whatever diode-bypassed portion of panel has one or more cells shaded. So long as diodes and their thermal path can handle the current (some panels can't take partial shading when getting full sun). And so long as all panels are oriented the same.

The 3s configuration is over 50V, getting to the range where it could be a shock hazard. Ground the panel frames so in the event of a fault the frame can't have voltage on it.

Parallel panels of different orientations is a way to fit more panels on a given charge controller. For instance four panels 2s2p, with one string of 2s aimed at 9:00 AM sun and one at 3:00 PM sun. That is 90 degree angle, so area presented is 0.7x what it would be with all four flat. This makes lower power peak, but more broad for more hours of production.

12A fuse is too small for 9.87 A Isc. Should be at least 1.56x panel current (1.25x for times of extra reflected light times 1.25x for headroom to avoid nuisance trips.) Whatever fuse size shown on panel label which is likely 20A. 15A would work.

"3 in parallel" you show panels wired in series. 35A fuse is not enough for 3 in parallel.
Instead, you should use 3 fuses, 15 to 20A each, one per parallel panel.
You can get an MC4 "Y" connector. Should be 8 awg or at least 10 awg given nominally 30A. Per NEC, 10 awg supposed to be 30A max so 8 awg is better considering the 1.56x multiple we use in design.

2s2p with 12.4A output should have 20A fuse, 15A too small for 1.56x multiple

Trailer DC loads should connect to battery after a fuse/breaker.

Fuses are only needed with 3 or more panels in parallel, because one shorted panel could get backfed with excess current from the others. But a disconnect switch/breaker can be convenient.

"4 awg solar mc4"
The inverter cables would not be MC4, and MC4 isn't available in 4 awg.
2000W inverter at 10V to 15V will draw up to 200A continuous. 250A breaker (or fuse) is appropriate.
4 awg is small, should be at least 2/0. Keep as short as possible.

6 awg and 50A breaker is good for 40A charge controller.

Paralleling one 12V and two series 6V batteries does boost total Ah, won't necessarily divide current equally.
Should be OK if batteries of same age and same chemistry. There are several different kinds of lead-acid battery.

Shore power & inverter, transfer switch.
Shore power would have neutral and ground bonded (hopefully wired correctly to the neutral pin of outlet.)
Does inverter have neutral and ground bonded? Does it permit you to bond?
Bonding neutral and ground on the inverter side of the transfer switch (and having it bond neutral) is one way to accomplish this.
Another is a relay to do the bonding when shore power absent. That is common for inverters that have internal transfer switch.
Another would be get an outlet like shore power outlet on pedestal, and wire neutral to ground, nothing else. When not connected to shore power, plug the shore power cord into this "dead" outlet.

Wow! Thanks all the great info and the quick response.

@DThames I live in BC Canada so idea conditions are rear. July-Sept maybe. This config is to get me started and I will likely expand. My plans allow for 800W. CC is capable of 1050W with 24V batteries. Still debating 12 vs 24.

@Pappion based on your experience the victron charge controller is worth the cost?

I know the battery capacity is not enough, but I has plans for 5Kw of lithium down the line. Don’t plan for replace the inverter. My assumption is if I manage the AC loads to well below 2000 watts, it should be fine. The periodic hair dryer and microwave use to defrost meet is expected. Laptops for remote work etc. Where possible, I will power from DC first.

WRT to your series vs Parallel comments, I did not understand the wire loss calculation. Can you clarify? It seems to me you are saying when you use 2 parallel wires, it is worse from loss perspective. This doesn’t make sense to me. 2xwire, means more copper, same effect as larger gauge wire.

@Hedges Thanks for you input, especially for the fuses and the 1.56x rule of thumb. I was using 1.25x. The Solar MC4 wire reference is simply cut and paste oversights. I would only use Solar wire between panels and charge controller or maybe only to junction box on roof.

Shower Power:I have not selected an inverter yet, if you have a recommendation that would be great. I was not aware that RVs have AC neutral and ground connect together, if that is what you mean by bonded. I will have to think about how to deal with that. Essentially, what I want is to ensure the batteries ->inverter does not power the converter/charger, but I can still use the AC distro and circuit breakers from the converter. I also have a portable generator which currently connects to shower power plug. I do plan to wire it into the cabinet eventually.

You have a battery monitor- if it is a Victron smart (BMV712) then definitely get the Victron Smart Solar Charge controller. They can pass some info (battery temps and voltage) to each other

Also if you haven’t purchased the inverter yet look at Victron Multiplus 2000 then ditch the converter (the Multiplus is an inverter charger). (Plus the Victron stuff you set all the voltage- so it works for lead or lithium or other ex

Victron is more expensive but when it is all setup with the GX monitors it gives you all the needed info of what is happening to your system all in one place.

Spend the time to plan your future system - so you don’t have to rebuy stuff down the road (that you didn’t plan to rebuy).

Yes, I think Victron is worth the extra money. I like seeing graphs and history on my smart phone. They also network with each other, and allows multiple Smart SCC to work together. Smart SCC can network with remote Smart Sense or Smart Shunt (with voltage sense).
I played with it for a few days and really liked it. Batteries that came with my TT aren't up to the load. I'm waiting on two SOK 206AH batteries.

WRT: Series vs Parallel, I was talking panels, and should have added that word in to be clear..