This thread has been a great conversation on charge lines, and towing battery draw. You can tell that there are lots of people with great knowledge experience. I'd like to add a few things to attempt to clear the air.
Towed Vehicle Battery Life:
Most towed vehicle's batteries can go 1-2 days of towing before they need to be recharged by letting the engine run for 5-10 minutes. The caveat is that many of the new vehicles on the market have a high constant current draw on the battery when the key is in the towed position. This is still the case in many vehicles even if you pull fuses or run directly off of the battery with something like our 12V Direct to Battery Kit
Though the car manufacturers are well intentioned when they make a towed vehicle 4 flat towable with new technologies, like the owner of the C-Max described above, there is still current draw.
Why the Towed Battery Charger was Created:
One of the great things about working in the RV industry is that we meet so many talented people from so many different backgrounds. Even though we've been in the industry for 20 years starting with the Brakebuddy, we often learn new lessons from our customers about better ways to do things or new ways of seeing them. We love to grow, so this is one of our favorite parts of working with our customers.
As you've seen from previous posts on this thread, there are people that know the meaning of words like "pigtail", gauge wire, bi-directional isolator, BAT pole, etc. This is awesome, but most people don't know what these terms means or what to do with them. We attempted to create a turn-key product. Just like the TVs we have and use daily, it's nice to know what a PCB is or that they use cloud technology, but the reality is I just need to know that it works. That's the goal of the Towed Battery Charger. To take something that may be complicated and package it as simple as possible.
The magic, if you will, is that we also provide an LED indicator that tells you if the product is working or not. Without it, there is no way to tell if a charge line is even working with peace of mind.
I hope this helps. Thanks for letting us be a part of the conversation! I look forward to your feedback.