Can I hook up a starting and deep cycle battery in parallel?

[fatherfire89]

I have an electric start '93 Johnson 115 hp motor and a minn Kota 55 lb thrust trolling motor and am wondering if I could hook up a starting battery (for starting the 2 stroke) and a deep cycle (for the trolling motor) in parallel without damaging anything. It seems like it would work but I'm not sure how the starter on the 93 Johnson would know to take power from the starting battery as opposed to taking power from the deep cycle. Ultimately I would like the 93 Johnson to charge both the trolling motor battery and the starting battery while I'm on the water so I can stay on the river for several days without any charging of batteries from a house electrical outlet

 

[perchjerker]

it can be done

I had one of these units and it worked quite well.

https://www.fishingminnesota.com/stay-n-charge/

 

[Bob Landry]

It can be done, but you'll need to use something like the Blue Sea Automatic Charging Relay. It connects the batteries for charging while the engine is running and seperates them at other times.
As far as staying out for days, the motor alternator is probably not going to keep up with heavy trolling motor use on a daily basis.

 

[turbotodd]

If both are hooked parallel, you will still have 12v, which is what you want. But if you run them down, you have no backup. They will just be dead. They die the same as one would, but might take a little longer because of the extra reserve. Also, the charging system would take much longer to charge two batteries vs just one; and this is what sometimes gets people into trouble. Conventional wisdom says it should charge both-and it does-but if they're both down, because you have more battery to charge, it takes a lot longer.

I used to just keep a jumper pack or a set of jumper cables (or both) in the boat for situations like that. And be conscious about the batteries and usage.


There is a battery isolator/relay kit just for this deal. Charges both, but starts off of one. The same thing can be accomplished a different way, but using the isolator kit is the best way.

 

[Al U Minium]

Why not install a 2 battery select switch. Then you have the safety of a disconnect switch, the choice of power; battery 1 or battery 2. For charging separately, buy two diodes from Radio shack and install direct into wiring from charging system. The diodes will set you back about $5, the battery switch is worth the cost but buy a Perko or a good brand marine switch.

 

[lovedr79]

i got rid of my starting battery and used the TM battery to start the outboard.

 

[Bob Landry]

Actually using a deep cycle battery for starting is a good practice. it allows you to swap batteries periodically so you get eqhal usuage from both batteries. The diodes , AKA Battery Isolators, are not a good option. You will always have about a volt of voltage across them and your batteries will stay chronically undercharged.

 

[New River Rat]

If both are hooked parallel, you will still have 12v, which is what you want. But if you run them down, you have no backup. They will just be dead. There is a battery isolator/relay kit just for this deal. Charges both, but starts off of one. The same thing can be accomplished a different way, but using the isolator kit is the best way.

Why not install a 2 battery select switch.

Saved me some typing..... :wink: