12v TM batteries — parallel wiring or switch?

I run a 12v minn-kota bowmount on my 16 ft tiller boat. Going to do a few 3-day camping trips this summer and I’m looking at extended my run time as I use the TM a lot, especially for spot-lock and auto pilot to free up my hands while trolling. I don’t have the budget to go to a 24v motor, but I’m planning to add a second battery up in the bow.

My question: Is there any performance advantage to be gained by wiring the 2 batteries in parallel -vs- wiring both batteries to a switch and just changing to the 2nd battery after the first is drained?

If so, I would consider getting two new deep cycles at once, since I know that’s best practice when wiring in parallel. (current deep cycle is 1 year old).

The deeper you discharge them the faster they wear out. I think they’d last longer in parallel.

If you’re getting by fine for a day on one battery now, I’d look into a small generator instead of a second battery for longer trips.

They weigh about the same as a second battery, can be left at camp, provide years of service, have “unlimited” charging capabilities (ie longer trips wouldn’t matter) etc.

If you do go with a second battery in parallel, it’s best to have identical batteries, including the age (meaning buy two new batteries).

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Thanks for the thoughts.

The generator would be great but my trips are primarily on basswood and saganaga in the Bwca, so no generators allowed.

I also need a new starting battery, so I am leaning toward taking my current trolling motor battery (interstate grp 27 deep cycle) and moving that to the stern to use as my starting battery (just a 25hp yammie so plenty of cranking power), and buying 2 new grp 31 deep cycles to run in parallel for the TM. (The outboard also has a pull start so running it down low isn’t a disaster)

If the extra weight is not a issue I would definitely run them in parallel while on your trips. I run a single battery on my 12v most of the summer but if I’m camping or fishing somewhere I know I will be running it hard I throw my second battery in and wire them in parallel.

Have you considered an onboard alternator charger? I use one and do several 5-6 night remote camping trips every summer. Whether it helps or not will depend on the output of your alternator and how many hours you run the outboard vs. the trolling motor. The single-bank Minn-Kota is only $110, and it weighs a lot less than a group 31 battery.
https://minnkotamotors.johnsonoutdoors.com/battery-chargers/board-alternator?id=13891

A second battery in parallel will definitely give you better performance.

dbright wrote:

If the extra weight is not a issue I would definitely run them in parallel while on your trips. I run a single battery on my 12v most of the summer but if I’m camping or fishing somewhere I know I will be running it hard I throw my second battery in and wire them in parallel.

If you had to guess, how much would you say this extends your run time? Sounds like a pretty similar setup to mine.

@vnpfan it’s not letting me quote your post for some reason.

I do like this idea. One question: How much running around do you have to do to make this effective, in your estimation? Because the other factor here is whether I want to carry and burn extra gasoline for this purpose? In some cases I’m running 30 miles round trip to get out to my campsite and back to the dock, but that’s before and after heavy TM use so not going to help charge the batteries mid-trip.

I will also have to dig in and try to find the manual for my outboard. The MK manual says it would require the outboard’s alternator to put out at least 12amps for the MK alternator to work. I know I have some kind of an engine alternator because I can see the voltage reading on my Garmin boost when I’m running wide open, but I have no idea what it’s actual output is.

I mainly fish Namakan and Rainy, which is pretty big water, and I do a fair amount of running the outboard from spot to spot. Alumacraft Classic 165 with a Yamaha 50 tiller. Main battery discharges are from Humminbird 898, Helix 8 and Piranha sonars, plus occasional livewell pump to keep bait alive. When running from spot to spot, as soon as the main battery is fully charged, the alternator charger light goes on, showing it’s charging the TM battery. If I’m using the TM a lot, by the end of a 5-day trip the TM battery gets low. There’s really no way to put any numbers on it, but it works for me. Every trip is different, everyone’s usage is different, everyone’s equipment is different. Just an option to consider. Costs less and weighs less than most deep cycle batteries.

Appreciate the feedback, thanks Ron.

I ran 12 volt batteries in parallel for my trolling motor back in the day and it really worked well.
If you use batteries of the same size it’ll double the run time vs one battery.