Connecting a home-brew solar generator to existing solar RV system.

[Defens]

Hi everyone! I have a camping trailer with a home-built solar system on board. Running 2x158w panels to a Bogart PWM solar converter, into 2x 6v 225AH GC2 lead acid batteries. I also have a 1500w Samlex inverter. I monitor the system with the Bogart Trimetric monitor. I have also wired in a plug on the exterior wall to connect a Renogy 100w folding panel on a longish lead to the SCC.

My problem is that even with the portable panel I still get shaded out frequently when camping in the woods of Washington. I just bought a Renogy smart Bluetooth lipo4, 12v 100ah battery. I also have an Epever MPPT controller that I'm going to couple up to the panels and Renogy battery, so I can set the whole system out in the sun to recharge.

My question is, what's the best way to connect my charged, portable LiPO4 battery to the depleted RV system? Should I plug it into the portable solar panel connection, which would then recharge my RV batteries through the solar controller? Directly connect to the RV batteries with jumpers? Something else? I don't really want to (and don't think i need to) buy a $500 DC-to-DC charger for something seemingly this simple!

Any advise is appreciated?

Mike

 

[Bobert]

You will be lucky to get 80% of energy from the lifepo4 battery into the lead acid batteries. You will never be able to fully charge your lead batteries from the lifepo4 you don’t have enough amp hours to do that. If you don’t get them fully charged in a reasonable number of days you will damage the lead acid batteries. If you connect the lifepo4 battery directly to the lead batteries you may have such a large initial amp draw that it might damage the lifepo4 battery even if that didn’t happen and you had unlimited amp hours stored in your lifepo4 battery at 13.3 volts it will take a month of Sundays to charge the lead batteries.
Here is my suggestion. Take your lead batteries somewhere that you can charge them to 100% reduce the loads on them to the smallest amount that you can so that your shaded solar will keep up with it. Then transfer all your loads to the lifepo4 battery when the sun goes down. A 100ah lifepo4 will take 2 days of ideal sun to fully charge. The losses sustained by transferring from the lifepo4 to lead will be staggering by any method I know of. Likely resulting in a 50% or more loss of the solar that you collect from the panel. Both a dc to dc charger and a solar controller require a higher voltage than your battery will produce. It might be possible to use a converter to increase the battery voltage and the use a solar charger to charge the batteries or you could certainly connect a small inverter to the battery and a battery charger to that but in both cases the losses will be extreme.

 

[Defens]

Thanks Bobert - good advice, and you actually made me think of another idea. I could just tap into the 12v circuit right at the breaker panel. Plug the Lifepo4 in there and turn off the master power switch at the 6v RV batteries when I switch over. I'll have to check and see what circuits still have power when I switch off the master, but I'm guessing none. Also need to see what the solar on the roof does, but it should still be feeding the 6v, but not the camper if the master disconnect is off. Which is okay, because I wouldn't mess up the various calculations for battery status on my Trimetric.

When I have decent sun, my existing solar is pretty efficient. I can go from 60% to 90% in about 3 hours of morning sunlight, and be fully recharged by evening. It even does okay in partial shade, but some of the places we camp are deeply shaded. And I hate having to run the generator for 5-6 hours to bring the batteries back up. Hence the desire for a portable solar system.

 

[Bobert]

I wish you the best. I hate running my generator too.

 

[chrisski]

I don’t think converters cost $500. I am a fan of Victron and have a lot of their gear. They have a 12 volt 9, 18, and 30 amp DC charger ($68 to $227)that you could hardwire into your system:

https://www.victronenergy.com/upload/documents/Datasheet-Orion-Tr-DC-DC-converters-isolated-100-250-400W-EN.pdf

You can add a remote on and off through the accessory port. You can adjust the charge voltage of your lithium battery source to what you need the lead acid to be. The only bad thing is fibring a way to shut it off so it does nto drain the battery. You can down load the Victron software to take a look at the settings. It’s pretty easy; there’s only a couple. You could set your converter voltage to somethign above your night time trailer voltage, but below the charge voltage.

To connect the battery to the charger, you could use 50 amp Anderson Plugs. I use those for a similar system. That way, you could take the lithium battery off and charge it however you do, even if its through a SCC with a couple hundred watts of power.


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if you just have 100 watts of portable panels, I’d recommend more portable panels. When I had 4 x GC2 batteries, I had very good luck charging it with 400 watts of panels, made much more power than 600 watts of roof panels. I have had good luck with Lion energy Portable panels fro Walmart. Those are by far the cheapest portable panel I found and they are sturdy enough to be put up and down, probably indefinitely.