Inverter/Charger and Solar Charge Controller - Current Concern - Trailer

[seprintz]

I've built a 24 volt battery system in a travel trailer before, but I never got around to installing the solar.

So... current concern (pun intended).

My inverter/charger is 3000 watts, capable of charging the 24v battery at 120 amps.
If I add a solar charge controller that's capable of 50 amps, 1200 watts of solar, 24 volt battery, do I need to size my battery cables to handle 170 amps? What would you do in this situation, just disconnect the solar when plugged into shore power?

Side question, with a separate solar and shore power charger, will they detect the voltage coming from the other and not charge? This would be a concern if I'm wanting to charge from solar and my generator at the same time.

 

[sollap]

Your inverter/charger and solar CC can charge the battery at the same time. They won't be aware of each other. The wire between the CC and the battery only needs to be rated for 50A since that's the max the CC can put out. The inverter/charger should have 150A wiring, to cover the draw from the 3000W inverter. Make sure each wire is appropriately fused at the battery to protect them from the battery's current in the event of a short circuit.

 

[FilterGuy]

do I need to size my battery cables to handle 170 amps?

Yes, Any part of the circuit that has to carry the full 170 will need to be sized and fused for that or you need to reduce it in some way.
Most of the AIO inverters have a configuration to limit the total charge current so you could take care of the problem by limiting the charge current from the inverter.

A 3000W 24V inverter should already have battery cables capable of handling ~175A.
The fusing and wiring for 170A of charge current should be 170 x 1,25 = 213A.
If you set your inverter to only allow 90A you will be fine if the battery is already fused for 175A.

What size (Ah) is the battery bank?

 

[FilterGuy]

The inverter/charger should have 150A wiring, to cover the draw from the 3000W

My calculation is different.
* If we assume a 90% efficiency, the inverter can draw 3000W/.9 = 3333W
* Worst case is when the battery is low and the inverter is at full output so the worst case current is 3333W/24=138.9W
* You need to fuse for 25% greater than the max continuous current so the fuse needs to be 173.6A (call it 175A) and the wire must be sized to 175A

Note:
In the real world, there probably won't be a full 1200W from the panels. If we assume the panels put out 85% of their rating they would only produce 1200 x .85 = 1020W or 1020W/24V= 42.5A. Since it is a trailer, the panels are probably flat and will probably only produce at about 75% of the rating. This works out to be ~38A.

Note2: The other thing to consider is how fast the batteries should be charged. If we assume an aggressive .5C charging, the 24V battery capacity for 170A of charge current would need to be 340Ah. ( I like to keep the charging lower than .5C.)

 

[seprintz]

FilterGuy, thanks for addressing all that. In one way or another you addressed all the concerns I had listed, and other thoughts that I have had, but hadn't gotten around to in the original post, because I didn't want to detract from my primary concerns/thoughts.

When I do my math, I tend to err. on the side of caution. So, while I understand that I'll never see 1200 watts from my panels in the real world, if I build the system to handle it, I know I'll be safe.

I've got 16 CALB SE180 cells, wired in 8s2p, so conservatively I've got 360 AH @ 24V.

I was concerned regarding overall charging amps if the solar is running well and the Inverter/Charger is running at full blast, thanks for addressing that. My charger does have the ability to limit the charging rate, unfortunately it isn't a programmable setting, but rather a knob on the top of the inverter that doesn't allow for me to set it precisely. It's good to see, that by your math, even if everything is charging at 100% I'm slightly below 0.5C.

It's nice to review all of this, when I do final assembly I'll be sure to have appropriately sized battery cables, and be fused for at least 175 amps.

 

[seprintz]

QQ:
1.) Do y'all use anything as a shutoff between your solar panels and the charge controller?
I've looked at 1 combiner, and think I'd like to use them for this purpose, any recommendations?
2.) What are we using these days between the solar charge controller and the battery? Will's schematics include a DC breaker, but I've heard that those cheap thermal breakers aren't reliable.

TIA

 

[RoadTurtle]

I used a cheap non polarized breaker to kill the pos line on two separate solar strings. You want to have a shutoff of solar so you can bring batteries online before solar panels are brought on line.

 

[time2roll]

Assuming the inverter/charger can be adjusted to reduce the charging amps... I would bring it down well below the rated combined charge rate for the battery. Even set to the minimum is fine. Plenty O time to charge when plugged in, no rush.