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Help! 2 Stupid Newbie questions

boxerboy2

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Joined
Sep 8, 2020
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Thank you to everyone here, this forum has it all!

I am building a solar system for my race bike trailer. Load consists of 4 LED lights, a 12v fan, and a 3,000w inverter that powers mostly a few battery chargers (phone, suit, lap timers, and occasionally a cordless drill charger, pretty light load all around. If needed I can hook up to my genset for something larger.
Right now I have 2 x 90amp hour flooded batteries and am thinking of using 320 watt Canadian Solar panel. Specifications are attached.

My two questions are:
1) I want to be able to theoretically charge my two batteries in one solar day so will one panel be sufficient, or do I need two?
and
2) I am totally confused as to which controller I need to use.

I dont want to cheep out on junky stuff but I dont need to be living in my trailer for the rest of my life either.

Thanking you in advance for your advice!
BB2
 

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2 x 90Ah 12V batteries is roughly 2.16kWh if you were to fully discharge them. A single 320W panel would need approx 7 hours of 100% production just to generate that much power so just from that the answer is going to be a 'no'. There is also the issue of lead acid's long slow march to fully charged. You can get to about 60% quickly but after that even with elevated absorption voltage current tails off so it takes a fair amount of time to get to fully charged so even if you had that 7 hours of full production from a single 320W panel it wouldn't get you there.

If you can do two panels that would be better but you start to run into other problems. That's potentially 53A to the batteries, or about 0.3C charge rate. That's doable but you are riding the batteries hard at that rate.

Finally, you probably won't actually get 320W out of a single panel at all, and certainly not for 7 hours as the sun moves across the sky. 320W is an artificial specification. If you are lucky your panel will show you another rating, not just the STC rating, the NOTC rating. That's much more like what you'll get in the real world.

If you can do lifepo4 batteries that would be better too, but maybe outside of the budget. lifepo4 batteries don't have that long slow march from 60% to fully charged and, depending on the battery, accept a higher charging rate too.

Is this all doom and gloom? No, but you may have to live with it taking more than a day of good sun to bring your batteries back to fully charged if you do fully discharge them. Normally you wouldn't actually fully discharge a lead acid battery to prolong its life.
 
Last edited:
TL;DR
1) One will do.
2) see last line of message

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Panel performance is dictated by solar availability. Use link #5 in my sig to determine solar availability for your area to include direction and tilt of panels.

Take that number, and multiple it by panel wattage to get your total Wh/day. A common "average" number is 5 hours, thus your proposed panel could produce 1600Wh/day, which includes ALL charging available from dawn to dusk.

Thus, you could supply 100W of charging for 16 hours, 200W for 8 hours, etc. If your drill is 12V/6Ah, then it will use 72Wh of your total available.

2 * 90Ah * 12V = 2160Wh, but you should never use more than 1/2 to keep the batteries healthy. Your panel should be able to charge them in a day.

Worth investing in a 320W/12V = 26.7 or 30A MPPT charge controller.

26.7A is about 15% of the "C" rating of the battery and is about as high as you want to go.

 
Not sure its a good idea to use a 3000 watt inverter with 180 amp hours of batteries. Some might say 440 amp hours would be good, or 600 amp hours. My 2000 watt inverter will have a 350 amp fuse at 12 volts, and 2/0 wiring, and 460 AH of deep cycle flooded lead acid batteries, and I think I am pushing it.

Those large amperage loads need to have proper wiring. If you can give up those large amperage loads and scale back the inverter, 180 amp hours of batteries will be doable. You mentioned using a GENSET for larger. I'm not sure how much you will need to charge you batteries, and the place to start would be the sticker. Perhaps its not really 3000 watts. A 3000 watt device is more than can be run on the typical 15 amp plug, and a the washing machine plug at 20 amps can deliver around 2400 watts. Perhaps you don't need 3000 watts to charge the drill.

Will Prose, who runs this site, has a book for sale on Amazon that will help greatly. It is even available for free download if you have Kindle Unlimited.
 
My mistake - the inverter I am using is a 2000 watt unit. I will get Will's book, I am sure it will be worth every penny!
 
My mistake - the inverter I am using is a 2000 watt unit. I will get Will's book, I am sure it will be worth every penny!

Also, with a 12 volt system and a 2000 watt inverter, really think you'll need 2/0 wiring, at least that's what my owners manual said, and coincidentally, my calculations off a DC loss calculator. At Home Depot the wire was cheaper, but very hard to manipulate, and there was some 2/0 welding cable available, but at about 3 times the price. I've read plenty of reviews off Amazon where inverters "did not work" for things like "wires smelt like they were burning" and they used much to skinny wires, sometimes 6 gauge.
 
Last question - given snoobler's comments (thank you!) I am going with one panel and am wondering if a 40amp controller is better in this situation vs the 30amp? Again thank you to everyone!
 
IMHO, no. The extra amperage would allow for expansion, BUT you're already over half, so it's impractical to add additional capacity. If you think you might want to double your panels in the future, a 50-60A SCC would make sense.
 
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