Trying to use what I've got...

Schrockstudio

Hi, I'm pretty new to the forums, been reading through a lot and learning a lot, thank you all for letting me learn out loud with y'all!

trying to make this post short and concise:

My partner and I are going camping for two weeks with our DIY-RV in August and we want to be able to solar power our mini fridge and a few lights

This is what I have:
1 old 12v 75W panel (I measured and it puts out about 21v and 4.5a in full sun, drops to 19v and .5a in shade/clouds)
1 old 12v 65W(?) panel (puts out about 20v and 3.5a in full sun drops to 18v and .5a in shade/clouds) This one is so old it dosen't seem to have diodes?)
(no charge controller! YET...)
3 pretty cheap inverters, AutoZone 1Kw, PowerBrite 2Kw, Harbor Freight 5Kw
a bunch of dead batteries! (I am learning the hard way about auto store Lead Acid...)
The two dedicated batteries that start the RV(Bus) are new and good.

Power needs: 
are not 100% but I think I can bring them down to between 250W and 500W per day. (currently trying to hack mini freezer to act like a fridge to use less power)

Asking for Opinions on these questions:
Should I bite the bullet and get a 50ah BattleBorn and see if that is enough? 
Then add others in parallel if needed? 
But, none of my inverters are programable... it seems like I need to get a lithium specific inverter in order to get the most use out of lithium battery. 
Also I think I can't use cheap PWM controllers with Lithium
If I need all this lithium specific stuff should I just use what I have and use golf cart batteries, cheap PWMs and call it a day?

Questions I have that I wasn't 100% sure on through searching forum:
Since I have Mix Matched Panels of unknown age it seems I should run multiple controllers in parallel, but couldn't find if PWM were okay for this? If I get another panel is it okay to run 3 or more controllers into one battery bank?
Do I need to add diodes to the wacky panel?

Tentative Plan: (Stop me if I'm going to ruin stuff hah!)
Buy two cheap Morningstar PWM controllers Morningstar SG-4 SunGuard 4.5 Amp 12 Volt Solar Charge Controller
Buy two of these Rolls 6v, Wire in Series. Rolls Surrette S6 GC2 Flooded Deep Cycle Battery 6 Volts 210 Amp-hours (S6 GC2)
Then test and see if everything works well?! use my 1000w first and see if it can handle mini fridge/freezer compressor startup.
I plan to charge the batteries while driving by running one of the inverters directly off of the alternator/staring batteries, and plug in a 110vac cheap battery charger that I already have.(is that okay with the Rolls?)

If Batteries charge up but power runs out, I'll order another battery as it will not be too far off in life of battery.
If batteries don't charge well, I would order a new larger panel and get an MPPT charger (does an MPPT in parallel with PWM act wacky?)
Then maybe learn that still I need to get another battery or two.

BB.

OK, lots of information, but still need to start at the beginning--Your loads and go on from there.

A bit of LED lighting, and a mini-fridge. Assuming the mini-fridge is a standard 230 VAC "bar fridge"... They use something like 300 kWH per year or:

• 300,000 WH per year * 1/365 days per year = 821 WH per day
• If you like AH: 821 WH * 1/0.85 AC inverter eff * 1/12 volt battery bank = 80 AH per day @ 12 volts

LED lighting... Say 4 x 4 Watt bulbs * 5 hours an evening:

• 4 * 4 Watts * 5 hours a night = 80 WH per night

Just looking at that, the refrigerator is 10x the energy usage of the LED lighting, and that is the issue... Refrigerators are very difficult devices to power from off grid power--Not the AC inverters (inverters are cheap), it is the solar panels to harvest the energy and the battery bank to store/release the energy that make things expensive/big and heavy (if Lead Acid batteries. Li Ion batteries are light and small, but terribly expensive themselves).

If you can use a freezer/chest freezer, you can get down towards 250 WH per day--But watch the ambient temperatures, hot sunny climate with lots of sun, in a (generally) poorly insulated bus, hot interior=>refrigerator consuming lots of extra energy.

Your best bet is to measure (and simulate if you want/need to a hot area to measure worst case energy draw) your energy usage:

https://www.amazon.com.au/s?k=energy+meter&ref=nb_sb_noss_1

Your old solar panels--You can probably use them, but not sure it is worth the effort--If you are powering more than some LEDs, cell phone chargers, etc... But without a power budget, you don't know anything yet.

Conservation is really the way to go, especially with an RV. Look for a standard propane refrigerator--Perhaps you can get an older/working unit from an RV wrecking yard??? If you have propane anyway (cooking, hot water), a 1/2 to 1 lb of propane per day and a couple of 20 lb propane bottles will last you your 2 week trip easily. Designing large solar power systems for a few trips a year, gets expensive. If you are planning on going full time in the bus--Then it may be worth a larger "all electric" off grid system (the true electric RV compressor based refrigerators that run from 12/24 volts are not bad--But not cheap either--But more efficient than the typical bar/dorm mini-fridge).

Need to start somewhere, so lets just pick a nice round number. 1,000 WH per day, may run a small chest freezer converted to fridge (maybe). And use "cheap" (at least in the USA) 6 volts @ ~200 AH "golf cart" batteries--Not a bad place to start when learning how to build/run a smaller off grid power system. You can always convert to Li Ion or other batteries a few years later once you have worked out the details. For the battery bank, a conservative design is 2 days of storage, 50% maximum discharge (full time off grid usage). You could go with 1 day and 50% discharge, but lets start with a conservative design first and see how it all fits/plays together.

Generally, a 1,000 WH per day system is (500 WH per day minimal system) will power an off grid cabin nicely (LED lighting, radio, cell charger, 12/24 volt water pump, tablet/small laptop computer, and perhaps a small fan for air circulation).

• 1,000 WH per day * 1/0.85 AC inverter eff * 2 days storage * 1/0.50 max discharge * 1/12 volt battery bank = 392 AH @ 12 volt battery bank

Or, 2x 6 volt @ 200 AH golf cart batteries in series * 2 parallel strings for a 12 volt @ 400 Battery bank (4x "golf cart" batteries total).

Next, we need to define the solar array. Two different calculations. One based on the rate of charge of the battery bank (larger battery bank, needs more solar panels for proper charging). The second is based on where you will be, mounting (flat or tilted), and your daily loads. You want your array to meet "both" needs (charging and daily loads).

First, rate of charge. 5% can work for a weekend/summer system. 10%+ is better for full time off grid. With RVs, you frequently have to "go smaller" because of area/weight restrictions. But if you are going to be near full time off grid/traveling during "winter", etc., more solar panels is really helpful--Unless you plan on using a backup genset a fair amount of time (which is not bad when trying to stay small/light/and cost effective for a few trips a year).

• 400 AH * 14.5 volt charging * 1/0.77 panel and controller deratings * 0.05 rate of charge = 377 Watt array minimum
• 400 AH * 14.5 volt charging * 1/0.77 panel and controller deratings * 0.10 rate of charge = 753 Watt array nominal
• 400 AH * 14.5 volt charging * 1/0.77 panel and controller deratings * 0.13 rate of charge = 979 Watt array "typical" cost effective maximum

And there is sizing the array to meet your needs. Say flat mount to roof, and start with Melborn (farthest from the equator (less sun):
http://www.solarelectricityhandbook.com/solar-irradiance.html

Melbourne
Average Solar Insolation figures

Measured in kWh/m2/day onto a horizontal surface:

Jan	Feb	Mar	Apr	May	Jun
6.36	5.83	4.51	3.23	2.23	1.78
Jul	Aug	Sep	Oct	Nov	Dec
1.94	2.59	3.55	4.72	5.74	6.22

In general, lots of sun in summer, not much in winter (especially with array mounted flat, vs tilted installation). And you said looking at August for travel... Not great sun, and a tilted array will give you more sun:

Melbourne
Average Solar Insolation figures

Measured in kWh/m2/day onto a solar panel set at a 37° angle from vertical:
(Optimal winter settings)

Jan	Feb	Mar	Apr	May	Jun
5.02	5.20	4.81	4.25	3.48	3.06
Jul	Aug	Sep	Oct	Nov	Dec
3.17	3.55	3.98	4.43	4.69	4.79

Lets do "worst case" harvest (flat roof mount, August) (remember, these are long-term average hours of sun per day--Bad weather, etc. can easily cut this amount of sun):

• 1,000 WH per day * 1/0.52 off grid AC system eff * 1/2.59 hours of sun per day = 743 Watt array "break even" for August

So, for this "mythical" system, I would be suggesting 743 Watt array minimum, and probably 979 Watt array minimum if you want to use mostly solar and a minimum amount of genset runtime (or conserve energy during "dark times").

At this point, a 140 Watts total of old panels--Not even worth going through the issue of mounting/connecting a solar PWM charge controller (although, might not be bad to add to the bus' vehicle batteries--Keep them nice and charged).

A ~979 Watt array, you are looking at 3-5 "large format panels" (~200 to 350 Watt per panel).

Next question was the AC inverter... Large inverters waste power (2 kW and above, you might be looking at 40 Watts just turning on your inverter). For running a typical AC refrigerator, you probably are looking at a ~1,200 to 1,500 Watt AC inverter (has to be large enough to start the compressor).

Anyway, I will stop here. Made lots of (probably invalid) guesses. As you can see, there are lots of choices and trade offs to be made. The math is not "handed down from God", but just starting points to help guide your choices/discussions.

-Bill