Reddit mentions of Perko 8501DP Battery Selector Switches

You likely have a battery selector switch. You only need 1 battery. The other is a backup. If you run your trolling motor, livewell, stereo, etc you might kill the battery. In that case you would just switch to the other battery.

1. Wire wise - Amazon - look up Anchor Tinned Wire and get a 25' section of 8ga 2 wire red/black at biggest (radio, plus lights, plus accessories ) but 12ga with 2 wires should be fine for small electronics.
   
   
   Example:
   
   https://www.amazon.com/Ancor-121510-Marine-Duplex-Cables/dp/B000NUYBW0/
   
   
2. Lack of a Switch or Disconnect - don't do this. A) Danger Will Robinson ! B) is not approved by coasties/inspection most places. Get a proper battery switch. Put this switch between the battery Positive and the motor positive and your accessories positive. The ground/black from the motor, from the accessories etc can all go right to the negative terminal on the battery. We're only switching the 12+ Positive side.
   
   
   Example:
   
   
   https://www.amazon.com/Perko-8501DP-Marine-Battery-Selector/dp/B00144B6AE
   
   
   
   
   I say these things for the following reasons:
   
   
3. you'll eventually wanna sell the boat and upgrade to something bigger. When I as a buyer see things like accessory wiring going right to the battery instead of a battery switch - I use items like that to lowball the fuck out of the seller. It also says "if they did this janky to save $20, what else did they do janky" ?
   
   
   
4. Safety - you always need a way to 100% disconnect electronics. This can be because of a fire, a mystery battery drain, long term storage etc. We want a disconnect from the battery. Now that said - plugging and unplugging a quick connect/ disconnect versus a proper battery switch is a GREAT way to blow yourself up depending where the gas fumes are, where your gas tank is, etc. It's one more possible thing to corrode as well as ignite. Make sure when you do your charging etc you are careful (connect the tender first THEN plug it into the wall outlet)
   
   
   
5. Long term usage - the more correct you do it now, the more money you save in the long run making small repairs later.
   
   
   
   Wiring Example:
   
   
   https://encrypted-tbn0.gstatic.com/images?q=tbn:ANd9GcRrykWkGL7Y6N6zwAiDNcYPNvgQglOC4aJx0ntB9qn2Iv_INk1TUQ

OK, we are going to try to get one of these. How to I know which one is the correct one?

https://www.amazon.com/Perko-8501DP-Marine-Battery-Selector/dp/B00144B6AE/ref=lp_2598388011_1_1?srs=2598388011&ie=UTF8&qid=1554134911&sr=8-1

I suppose the cables go on the same way?

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Nice, glad to hear she is back up and running! I've had a slow battery drain in my tow vehicle for years. It will kill a battery while sitting over the course of about 3 weeks. After days of fruitless searches trying to find the drain, I eventually just broke down and left it hooked up to a battery tender so that it was always at full charge when I needed it. For my case, a Battery Tender jr. with a cigarette lighter adapter has worked great for me for over 10 years now. If your boat is not protected from the elements, you could also consider the water proof version of the battery tender.

One last thought that has saved me from worrying about any of this on my boat is to install a battery switch, so that you can simply turn off all connections to the battery when you are not using the boat. I currently use this one with two group 27 batteries. You could do something similar if you suspect you have a power drain. You do not need to add the second battery if you don't want, instead you can just get a simple on off switch like this. Though it is nice peace of mind when you are offshore to know you have a backup. Happy boating! Let me know if you have any more questions.