"admittedly I upgraded the battery room cabling myself"
That might be a good place to start the autopsy.
Go back and check all your routing and all your connections. check each connection for cleanliness and torque. Do this first, because it needs to be done......then check your batteries for condition and charge.
When this happens on an engine, first steps would be to check for a dead battery and/or a bad ground, but in your case you need to check recent work first.
Keep a very good maintenance log to include actual work done, parts used, part Numbers and costs. Part numbers and suppliers can be kept in the spare parts log.
Or better yet, keep logs any way it makes sense to you.... but do record loads of information that will be usable on down the road.
Hope you will be able to sell your oars shortly.