I didn't catch this early enough to change it before it got comments.
Not to hammer on you, snake.
I think you are confusing a dump load with a dummy load?
Confusing terms abound in this hobby. Might I suggest we we use dump load from here on?
We had a go with guide wires vs. guy wires awhile back but that got squared away.
It probably does not matter in the context here but future researchers are likely to miss this discussion due to different term for the dump load.
We did use dummy loads for testing transmitters in my RF days so there is such a thing as a dummy load.
The thing we use on battery banks dumps excess power as heat thus a dump load.
Just seemed worth mentioning in the mess of terms related to this hobby lots of folks struggle with understanding them.
Another option for a dump load I have used is broken kiln element nichrome wire from pottery kilns.
They are only useful to the potters in full lengths. You likely only need a few inches so one that has failed is a good source from someones junk.
You would need Ohms Law and or Watts law to calculate the resistance of the dump load. Start with battery volts and maximum input possible and work from that. for calculations on a 12 volt bank I would use 14.4 volts and it always makes sense to oversize the dump load power handling to be sure it can handle all the source can shove into it.
Good luck with it.
Tom