DC battery charging question

I’m looking at doing a DC charger off of a 115 Yamaha 4-stroke. If I’ve done my homework right, I may have an issue but need a second opinion.

Looking to charge a main battery + 2 batteries for the 24v trolling motor. As such I would need a 3 bank charger. Per Minnkota(below) I need a minimum of 30 amp from the motor for full output. Per Yamaha 115 manual it gives 25amps in DC which leaves me 5 short of minimum.

Does this mean I can’t run the 3 bank dc charger or does it mean I just won’t achieve full 10amp/bank output because not enough is going in from the alternator? I’m guessing I can do this but will get more towards the 5-7amps/bank.

Per Minnkota manual
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

I believe that the 25 amps would only be at full RPM, everything lower than that would be less which is not going to work even if you had a smaller charger.

I don’t think you will be able to do what you are wanting to do, otherwise it would be a normal setup on boats.

Edit: I guess maybe I am wrong. MinnKota offers a DC Alternator On-Board Charger.

MODEL: MK 3 DC. For use with 12-, 24- and 36-volt systems with 12V/6 cell batteries that are Flooded/Wet Cell, Maintenance Free or Starved Electrolyte (AGM) only.

Minimum Alternator Output:
MK 1 DC is 12 amp min
MK 2 DC is 25 amp min
MK 3 DC is 35 amp min

@311hemi Yep, I’m looking at that DC alternator one. The motor won’t fully power that charger but not sure what that means.

I installed one and called MinnKota with some questions during my installation. The MK tech told me that the charger was designed to limit the charge into each bank to 10 amps, so as not to damage the batts with too much of an amp load. Once the starting batt is fully charged, the current is then directed to your TM batts at 10 amps each, assuming you are generating at least 20 amps off the main motor. I suspect you’ll get a benefit from the charger, but you might have to call MK to be sure of what to expect in terms of performance. My expectations were too high and I am running a larger motor than you – a 250 Yammie. The DC charger does give me longer run times on my 24V system and shorter cycle times at the end of the day. However, it does not keep my TM batts fully or nearly fully charged through the day as I had hoped and expected. Still, I would install this charger again and can’t see being without it considering the benefits received vs. the price of the unit.

I don’t know much about these and don’t have one. Do you really need a three bank? I thought these ran off your main battery but only kicked in when it sees enough voltage, whatever that is 14v or greater(?).

It sounds like you are suggesting hooking one from the main battery and back to the main battery, plus the trolling motor batteries. Wouldn’t the main battery already receive a charge from the motor?

Btw, I do a poor man’s version. I borrow a power inverter from my ice fishing gear, connect the inverter to the main battery and plug in the built in AC charger during long runs.

BrianF wrote:

you might have to call MK to be sure of what to expect in terms of performance.

@Brian Fitzpatrick I took your advice and got a slightly different answer then you.

Essentially there’s no minimum to get output. ex. put half the minimum amps in then get half out.

I’m familiar with these on a friends boat and know it won’t magically get me to full. What it will do is help when making a run between spots. The more help it does during the day is less charging at night. Important to me in Ontario where I don’t have access to power all the time or sparingly on generator.

BrianF wrote:

I installed one and called MinnKota with some questions during my installation. The MK tech told me that the charger was designed to limit the charge into each bank to 10 amps, so as not to damage the batts with too much of an amp load. Once the starting batt is fully charged, the current is then directed to your TM batts at 10 amps each, assuming you are generating at least 20 amps off the main motor. I suspect you’ll get a benefit from the charger, but you might have to call MK to be sure of what to expect in terms of performance. My expectations were too high and I am running a larger motor than you – a 250 Yammie. The DC charger does give me longer run times on my 24V system and shorter cycle times at the end of the day. However, it does not keep my TM batts fully or nearly fully charged through the day as I had hoped and expected. Still, I would install this charger again and can’t see being without it considering the benefits received vs. the price of the unit.

I went with the stealth dc charger and I have gone an entire week without plugging my batteries in to be charged after fishing every day. The run time between spots was enough to put back much of what I use. The lowest my batteries read was 60%. Big fan of the stealth system myself. I researched them all before buying and after multiple phone conversations with Danny at stealth I went with it. No regrets.
We spend a week in lake of the woods in Ontario with no electric hook up. The one guy brings a generator but the last 2 years since I installed the stealth unit I haven’t used it.

Kyhl wrote:

I don’t know much about these and don’t have one. Do you really need a three bank? I thought these ran off your main battery but only kicked in when it sees enough voltage, whatever that is 14v or greater(?).

It sounds like you are suggesting hooking one from the main battery and back to the main battery, plus the trolling motor batteries. Wouldn’t the main battery already receive a charge from the motor.

kyhl, I don’t believe you need a 3 bank. I have a two bank model myself for a 24 volt trolling system and it works perfect for me. They monitor the cranking battery for proper voltage before charging the rest so with the alternator still hooked to the cranking battery and charging it, once that reaches desired voltage the unit will charge the others. I can literally watch it happen with my stealth because I have voltage displayed on my hds unit and a stealth digital gauge. When the hds says 13 volts the stealth gauge will start to climb.

CaptainMusky wrote:

I don’t believe you need a 3 bank. I have a two bank model myself for a 24 volt trolling system and it works perfect for me. They monitor the cranking battery for proper voltage before charging the rest so with the alternator still hooked to the cranking battery and charging it, once that reaches desired voltage the unit will charge the others.

Did a little more reading and looks like you’re right. I just thought 3 batteries meant I needed a 3 bank. Motor takes care of the starting battery and then a 2 bank takes care of both batteries in a 24v system.

Looks like I don’t have any issue at all. Should be talking to the dealer tonight and making arrangements for payment. Thanks to everyone for their help!

Not sure you are doing this right.
You would need to have two banks for the TM batteries,
but you would not need an additional bank for the
main battery, unless it is not attached to the main motor that you are
using the alternator to run the MK -DC from.

I have been using a MK2-DC to charge my TM batteries for years.
If I recall right, it requires 17 amps to trigger the system to charge the TM batteries.

patk wrote:

Looks like I don’t have any issue at all. Should be talking to the dealer tonight and making arrangements for payment. Thanks to everyone for their help!

Did I miss something? New boat?

Does anyone have any thoughts on whether or not this could harm the main motor charging system? I’ve heard from an unreliable source that the voltage regulator on the motor could burn out from constant load.

Great topic. This may be the answer to my problem for how to power new downriggers on my boat.

I suppose in time it could possibly make a difference but I know guys who have used the stealth units for nearly 10 years and never had problems. Not necessarily a ringing endorsement I guess but it’s the only feedback I’ve heard. Don’t know of anyone personally who had issues.

Ralph Wiggum wrote:

Did I miss something? New boat?

Headed to the bank right now to get a check cut. All came together in about 48 hours. 2013 Alumacraft Dominator w/115 Yammi.

That’s why I had a flurry of questions the past couple days and why I just put an add in the classifieds. I won’t actually take possession until sometime in October but will have to get some pics up then of the new ride.

Not sure whether to thank all of you for the help or curse The quick and great responses helped me make sure this was what I wanted. On the flip side, now I have to go pay for it:) Pretty excited.

regarding the original topic here. I’ve always been a Minnkota guy but will check out those Stealth chargers.

I was always a minnkota guy too, but now I’m a minnkota (2 bank charger), lowrance, motorguide and stealth guy.

I installed my MK2-DC in spring of 2012
It allows me to spot lock on Kab for a week without access to electricity to plug in the AC charger.
It has never been an issue with the charging system.

Btw, I do a poor man’s version. I borrow a power inverter from my ice fishing gear, connect the inverter to the main battery and plug in the built in AC charger during long runs.

You could also use a “Lazarus” system like I had on my previous boat.
I used a set of long high gauge jumper cables as they are cheaper than just buying the wire.
cut the ends off and use the wires.
E.G. for a 12v system.
run the ground from the starting to the TM battery (also know as common ground).
install a high amperage toggle switch at each end of the positive wire.
install between the postives on the main and TM batteries.
in use, allow the main battery to fully charge from alternator,
flip the switches on and charge the TM battery.
flip the switches off and allow the main battery to top off.

This system also provides a jumper system when you play the radio or do something to drain the main battery down

eyekatcher wrote:

I installed my MK2-DC in spring of 2012
It allows me to spot lock on Kab for a week without access to electricity to plug in the AC charger.
It has never been an issue with the charging system.

Eyekatcher or anyone else!! Need some help. Going to instal the mk 2 dc for an upcoming rainy trip. Where do I tie the purple wire into the ignition circuit? Yamaha 115 4 stroke! Thanks to anyone who can help!

Splice it into a wire that is hot only when the key is in the on position.

It’s been a while but I think I tied in the purple wire to the tachometer. I remember it was one of the gauges that was only hot when the key was on.

I mounted the dc charger right in the bow just in front of the driver’s console. Pretty short run for the purple wire.

Sorry, double post