on board DC chargers worthwhile?

So I have been looking around for a while for a way to help pump some juice into my trolling batteries while on the water. I spent a lot of my fishing time last year trolling and have found I enjoy this method of fishing. The terrova got quite a workout using it as my primary propulsion method (in calm water the outboard only gets down to about 3 mph), but after 5 or 6 hours the batteries are done. So the best answer would probably be getting a kicker motor, but that option is off the table for a while and honestly I don’t get enough time on the water right now to justify the cost. I plan on trying hooking my smaller drift sock off the bow eye and seeing how that slows the outboard down (90 hp 4stroke) but again it’s already 14 years old and I don’t want to work it to death either, though I’m probably sending the terrova to an early death too, cheaper than a new 90 hp outboard. Thought about getting solar panels to help charge, but to get one with any meaningfull output would take up a lot of space I don’t have in a 1600 boat either. So I did some studying of a (minnkota) on board DC charger that (is supposed to) use the outboard to help charge your trolling batteries. Pricepoint for a 2 bank is really to my liking and it’s a lot smaller piece of equipment to make room for. But, for those who have any experience with them, is what you get for charge out of it worth it? To get different batteries that gave a noticeably larger reserve capacity the cost is so stinking high I could just buy a second (and third, maybe even a fourth) set of ‘regular’ batteries and just run back to the truck for a swap when needed.

So just looking for feedback if anyone has one, or other suggestions. In addition to extending run time I like the concept in theory for those weekend trips where I won’t have access to an outlet to run my normal on board charger overnight. Cost is a little sensitive I just don’t get out enough to justify dropping a lot of money into this issue, but it is a bit frustrating to be on some fish and then have to head in because batteries are dead. It’s 2 27 series batteries I’m looking to charge, they are located in a compartment just in front of the console (compartment barely fits 2 27 size as it is) boat is a lund pro sport 1600, 80 lb terrova tm and merc 90 hp 4 stroke for an outboard.

Left out, I’m very leary about putting a trolling plate on the outboard, probably unfounded concern but they just seem odd to me.

Scratch that idea, after reading more my outboard doesn’t put out enough amperage to run a 2 bank.

How old are your tm batteries and what size? I would think you should be able to get more run time out of that setup with your size boat.
DT

Sorry, just read all the way through and saw the size.
DT

I don’t have any experience with those chargers, but I did talk to a fella that had one about 9 years ago. His complaint was that he didn’t run his big motor long enough to actually charge the batteries.

That’s all I gots.

Like BK said its going to some down to having to run your big motor enough to charge them. Not sure the output of your 90 but there is a minimum required output from your main motor to even get one to start working. Unless your doing a lot of long running I do not think that is the route to go.

What kind of speeds are you trying to maintain with your electric? A set of good batteries should push a 16′ boat for quite some time depending on what you are trying to run at. 27’s are on the small side.

With a 4 stroke I would fine tune the drift sock / big motor concept. Another way to enhance your battery longevity it to only troll with the wind at your back. Trolling into the wind, waves or current is futile on my 18 foot Tuffy.

I have been using the MK-2-DC on my 2011 90 HP Mercury 4 stroke for a couple of years.
It works real sweet.
I am able to spot lock for a week on Kab every spring with this setup.
We camp on an island and have no access to electricity at all.
It is an intelligent charger in that it determines when the alternator is running at enough output to start charging the Trolling motor batteries.
There is also a wire that runs to the ignition switch to determine when the engine is running.
I have two batteries for my Terrova 80 IPilot Link setup.
I also have a MK-315D.
The two chargers are in the compartment behind the TM battery box on my 1675 Impact.
There I have a standard boat battery terminal connection with the MK-EC-15 extension cables that run to the starting battery in the back of the boat.
A lot of wires but once you get it set up, it is incredible.
I also use the MKR-21 ac power port so the rod locker can remain locked while the ac charger is plugged in.

My alternator has a 50 Amp output
which more than meets the
CHARGING REQUIREMENTS:
In order to achive full output from the DC Alternator Charger, the minimum input from the engine alternator must be as follows:
MK-1-DC: 12 amp minimum
MK-2-DC: 25 amp minimum
MK-3-DC: 35 amp minimum

Yup, according to the service manual for my 2001 I’ve only got a 20A alternator. looks like they put higher output alternators in newer outboards

I have a 50 hp Yamaha tiller and a Powerdrive V2 12-volt. I do a lot of boat-in camping and installed a MinnKota 1-bank charger last spring. It works well for me. The charging indicator light comes on at about 1100 rpm, so any time I’m running from spot to spot, it’s charging the TM battery. But if you’re running the TM for 5-6 hours straight, it probably wouldn’t work for you.

For the mean time I picked up a set of rubber buckets that I’m going to turn into ‘brakes’ to slow the outboard down enough to use if I’m going to be doing a lot of trolling(and if I don’t like it I’m out less than one drift sock in cost), and probably picking up a second set of batteries to keep in the truck as ‘back ups’ in the event I drain the trolling batteries.

One of the thing we would do before I got the Mk-2-dc

hook up a set of jumpers on the hot wire of the starting and TM batteries.
connect the jumper ground wire to the starting battery
start the motor,
connect the ground to the TM batteries.
Drive around for a while to recharge the TM batteries
this will balance the charge levels in the starting and TM batteries
then disconnect the jumpers
drive for 5 more minutes to ensure charge in starting battery

The second iteration of this setup is the Lazarus System
I used on my previous boat where I cut the clamps off the jumpers and hard wired this in. Ran common ground and the hot wire had a 50 amp switch at both ends.
When you want to recharge the TM batteries
start the engine
throw the switches
charge the batteries
throw the switches
run 5 more minutes

The Lazarus system also allows you to throw the switches and “jump” your starting battery if that would get run down ( I play the 200w radio way too much, saves occasional embarrassing situation )

5gal pails work as good as those rubber pails. They also hold more fish going from the boat to the fish cleaning shack. Back in the day, before lake water was toxic, the 5gal pails were great for changing the water in the leech locker. 5gal pails stack nicely and can be used as a storage compartment for a tackle bag so they don’t blow out of the boat while underway. 5gal pails are cheap, they can be found in most road ditches around the state.

Group 29 TM batteries will fit in your location. They are only 1/4 inch longer than 27 group and it will make a difference on how long they last. I don’t know the size of group 31 batteries.