When I converted from Humminbird to Lowrance I made the mistake of wiring into the original wiring point under the console when I networked my two units. The voltage drop from starting the motor caused the bow unit to power off. I did consider a fourth dedicated battery but instead ran a dedicated line from the crank battery straight to the bus bar for my networked graphs and it solved the problem.

The wifey had an orange extension cord she chopped into while trimming hedges so I used the remnants and fished it through the gunnel to the back with a electricians snake.

You didn't say what problem you believe you are solving with the additional battery idea, if any. There is generally no necessity for another battery unless you have so much electronics drain that a large high performance boat battery is insufficient.

Many anglers who think they need a second battery are actually battling voltage drops due to long runs of 16 gauge wire (in the factory harness) that may contain a number of splices that make matters even worse. The most likely improvement would come from adding a dedicated power distribution for the electronics. That should typically use 10 ga. (for power AND Ground) to get to the console where a new fuse block distributes power to each unit. Power for the bow unit comes from that fuse block using 12 ga. wire. The console units usually have power cables that connect directly to the fuse block.

A nice fuse block for this provides 6 circuits with Ground, so can support 6 items that have a power cord. Keep in mind that the cheapie in-line fuse holder is not used when a fuse block is installed. The power feed TO this fuse block would be larger wire, such as 10 ga. and that should have a fuse close to the battery to protect that wire. It's fuse could be 20A if 10ga. wire is used.

https://www.amazon.com/Blue-Sea-Systems-...+sea+fuse+block

Yep...run a dedicated power and ground wire directly from the unit to the battery with a 3 amp in-line fuse. Solved my voltage drop problems as well.

When I converted from Humminbird to Lowrance I made the mistake of wiring into the original wiring point under the console when I networked my two units. The voltage drop from starting the motor caused the bow unit to power off. I did consider a fourth dedicated battery but instead ran a dedicated line from the crank battery straight to the bus bar for my networked graphs and it solved the problem.

The wifey had an orange extension cord she chopped into while trimming hedges so I used the remnants and fished it through the gunnel to the back with a electricians snake.

When I had my new electronics installed in January I had them put another battery in just for the electronics (had 3, now have 4). I have 1 battery for cranking that is inline with my electronics battery so it will charge both batteries when the outboard is running, the other 2 are for the trolling motor only. I would think with 4 batteries already, you should be good and should nt need another battery, are you sure you need 3 batteries for the TM?

Also, the new units with 9+ inch screens draw a lot of power over the day. I run a 10 and 9 (HB's) and a 360 transducer, and I leave them on all day on the water (have LED's also for low light hours that run off same battery), so I would definitely suggest a dedicated battery for electronics, but probably not a 5th battery.

i had the console power issue as well. added a separate circuit box and ran 10ga to the battery and have that dedicated for my electronics - keeps me from having a bunch of wiring going all the way back to the battery.