MPPT charge controller and other questions for first time portable solar generator build (wiring diagram included)

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jtysonwilliams asked Mar 10, '21

MPPT charge controller and other questions for first time portable solar generator build (wiring diagram included)

Hi! I'm building a portable solar generator for RV dry camping with my family, and need some help sizing the charge controller, and validating my findings so far. Basically, my needs are to be able to run a couple small fans, charge devices, and intermittently plug in small appliances as needed for long weekends and the occasional weeklong trip.

Currently planned major components:

100Ah LiFePO4 battery (one to start, but potentially expand to two if needed)
SmartSolar 75/15 MPPT
500-700W inverter
100W Solar panel (Potentially want to expand out to 200-400W in the future)
Multi stage trickle charger

Here is my wiring diagram: https://imgur.com/a/MgqJaGU

Questions:

If I start off with only 100W worth of solar (these panels are expensive!), will the 75/15 still charge my battery? Or do certain charge controllers require a minimum solar size to actually charge a battery? I realize I need ~240W to fully charge my 100Ah battery in one day, but I am not going on super long trips to start, and can top off at home with a trickle charger
(See my wiring diagram) - is there any risk in damaging the charge controller by using the trickle charger while the MPPT charge controller is attached? For ease of use, I'd like to be able to leave the battery in place and charge via an SAE port that I'd attach to the same bus bar that the battery / charge controller / inverter are attached to. I wasn't sure if there was any risk of back flow or whatever
Given everything above, does the 75/15 MPPT still sound right, or should I upgrade (100/20) or downgrade (75/10)? It sounds like if I wanted to scale out to a two-battery system, the 100/20 would be better, but given I want to start small, I'm wondering if I should think about replacement when it gets to that point instead of trying to over-future-proof my system

Open to any and all feedback. I was largely inspired by Jim Fisher's setup: https://www.youtube.com/watch?v=offgcMwuTGw&t=300s

Thank you!

MPPT Controllers Generator

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Answer 1 · 2021-03-10T18:53:01Z

seb71 answered · Mar 10, 2021 at 06:53 PM

A single 100W panel? Don't bother with solar charging. Charge the battery at home.

Also try to limit to DC loads and skip the inverter.

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jtysonwilliams commented · Mar 10, 2021 at 08:15 PM

Can you help me with the questions I asked?

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Answer 2 · 2021-03-14T07:23:58Z

Hello, jtysonwilliams. Nice work on the wiring diagram. As it happens, I'm doing similar things with a portable emergency/camping power box also built around a 100AHr LiFePO4 battery (Relion RB100). I'll preface this with the note that I haven't yet installed and tested my SmartSolar 100/30, but I can offer some notes.

Bear in mind that the Victron solar charge controllers start charging at Vbatt+5, per specs. (Once started they operate at Vbatt+1). My battery is currently at 13.14V with 46% capacity remaining, and I therefore wouldn't expect just one of my (2) Renogy Eclipse panels, rated at 17.7 Vmp, to start the controller until Vbatt dropped to 12.7, which would be below 20% capacity for me. I definitely wouldn't want to be restricted to changing only when the battery is nearly empty.

I opted for (2) panels for 200W capacity anyway, so voltage won't be an issue for me in series with the Victron controller. Other panels have different Vmp: for example the HQST 100W operates at 20.6 Vmp. The Victron panels operate at ~19V. Even Renogy's non-Eclipse 100W model is at 18.6 Vmp. So you have single-panel options that would still allow you to charge at any time with just one panel. I connected my two panels with a piano hinge and turned them into a 21" x 41" x 3" (folded) portable unit.

I chose the 100/30 so that I can use up to (4) 100W panels and charge up to 30A/0.3C, and I will soon have a 12V to 28V DC-DC boost converter in reserve to charge at full 30A rate from my alternator should solar be unavailable or should I need to recharge faster before I add more panels. (Victron also makes the Orion Tr Smart DC-DC charger which I considered as well.)

I also decided that the cost difference between e.g. 75/15 and 100/30 wasn't great enough to justify either tossing one unit later or using two. With the 75/15 you're limited to max 220W, and with the 100/20, max 290W which isn't quite ready for (3) 100W panels.

You can certainly connect the two chargers (mains-based and MPPT) as you diagram, if one OR the other will be used at one time. Charge power coming from one won't damage the other. See also this thread: https://community.victronenergy.com/questions/63309/smartsolar-charge-controller-energy-blue-smart-cha.html

[It gets a bit more complex if both are ever charging simultaneously, because you could run into issues with voltage detection in the smart charging circuits (each could see the voltage of the other charger as reason to stop bulk charge), but absent that, both would contribute their amperage to charge the battery.

Also bear in mind that a LiFePO4 battery can handle up to 1C charge rate, though 0.5C or below is usually recommended, i.e., 50A for a 100AHr battery, which you obviously won't hit with 100W solar and a low-power charger.]

I'm planning to experiment with my 100/30 and Blue Smart IP67 12/25 together at some point, but not soon. 4-hour charge time with the 12/25 is a nice option after the ~10-hour cycle of the UPS/charge/MPPT unit I have built into my box now, which charges at max 10A.

Couple of wiring comments: Make sure you have a switch on the inverter so you don't suffer from parasitic load when it's not in use. Re: wire gauge, consider whether you might ever decide to upgrade to a higher-wattage inverter. A 600W unit is rated for 80A draw in a surge, plus you've got the 15A draw on the cigarette socket side. The Blue Sea circuit wizard is a useful tool: http://circuitwizard.bluesea.com/ . Easier to overbuild your wiring now than replace it later, but it's obviously a balance, and if you're building a box like mine, distance is fairly short.

To speak to seb71's comment about trying to stick to DC loads, I'll just observe that as part of my tinkering I picked up a 12V buck-boost converter, popped powerpole connectors on it, got a powerpole distro block and made some 2.1x5.5 and 2.5x5.5 DC cables, and I can run my entire home network off this power box. That includes cable modem, router, switch, Synology NAS box, VoIP box, mini-PC and portable screen, backup hard drive, etc. I have 12V USB chargers built into the box, and even 60W USB-C and MagSafe cigarette-lighter chargers. I'm now considering the value of a 12V chest fridge that would eliminate the ice management and sometimes questionable food safety on a longer camping trip, and also serve as a backup fridge. There's a whole 12V ecosystem out there. I still picked up an inverter for various uses.

If you want a super-quiet fan, I can highly recommend the Noctua units. I used the Noctua NF-A8 PWM with a temp controller for this project and it's amazing. I recommend a switch so you can switch it off completely when not generating enough internal heat (charging or inverter use) to warrant cooling.

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