Jon Boat Wiring: Diodes & Transfer Switch

[TimeOutside]

This is a DIY project. I am not an electrician.

I have two LiFePO4 batteries to power the equipment on on a 12 foot plastic jon boat. One battery is 100AH and one is 10AH. The primary use of the 100AH battery is to run the trolling motor. The primary use of the 10AH battery is to run accessories, like a fish finder and lights. The primary reason for the two batteries being on separate circuits is so the trolling motor doesn't introduce electrical noise that could degrade the performance of the fish finder.

For backup purposes, I would like either battery to be able to power both the trolling motor and accessories if ever needed. For this, I'm thinking of using a transfer switch, then a couple of diodes to prevent the battery with a greater charge from dumping into the other battery.

Switch Positions: Normally, the manual transfer switch would be in Position #0, so each battery is on a separate circuit. If the 100AH battery runs out, then moving the switch to Position #2 would allow the 10AH battery to power both the accessories and the trolling motor. If the 10AH battery runs out, then moving the switch to Position #1 would allow the 100AH battery to power both the trolling motor and the accessories.

Will the schematic I've attached work? Is there a better way to achieve my goal?

Thanks,
Andrew

Quick Question: Assuming the schematic is correct, I plan on removing both batteries from the boat when it's not in use, so I don't expect I'll need to worry about isolating the batteries for charging. But it seems to me (if my schematic is correct) that with the diodes the batteries will be isolated regardless of the transfer switch position. So, I could charge one battery, then the other, while they are in the boat. Is this correct?

 

[mainline]

This is a DIY project. I am not an electrician.

I have two LiFePO4 batteries to power the equipment on on a 12 foot plastic jon boat. One battery is 100AH and one is 10AH. The primary use of the 100AH battery is to run the trolling motor. The primary use of the 10AH battery is to run accessories, like a fish finder and lights. The primary reason for the two batteries being on separate circuits is so the trolling motor doesn't introduce electrical noise that could degrade the performance of the fish finder.

For backup purposes, I would like either battery to be able to power both the trolling motor and accessories if ever needed. For this, I'm thinking of using a transfer switch, then a couple of diodes to prevent the battery with a greater charge from dumping into the other battery.

Switch Positions: Normally, the manual transfer switch would be in Position #0, so each battery is on a separate circuit. If the 100AH battery runs out, then moving the switch to Position #2 would allow the 10AH battery to power both the accessories and the trolling motor. If the 10AH battery runs out, then moving the switch to Position #1 would allow the 100AH battery to power both the trolling motor and the accessories.

Will the schematic I've attached work? Is there a better way to achieve my goal?

Thanks,
Andrew

Quick Question: Assuming the schematic is correct, I plan on removing both batteries from the boat when it's not in use, so I don't expect I'll need to worry about isolating the batteries for charging. But it seems to me (if my schematic is correct) that with the diodes the batteries will be isolated regardless of the transfer switch position. So, I could charge one battery, then the other, while they are in the boat. Is this correct?

What sort of current is being drawn ? You may need rather large diodes, also you will get voltage drop across them.

Maybe easier to use a crossover switch or relays.

 

[TimeOutside]

What sort of current is being drawn ? You may need rather large diodes, also you will get voltage drop across them.

Maybe easier to use a crossover switch or relays.

For the trolling motor it will be 35 amps max at 12vdc. The accessories will be less than 10 amps total.

I was wondering how much of a voltage drop would occur across the diodes, but I couldn't find the information. I was crossing my fingers it wouldn't be too much.

Anyway, I just canceled the order for the diodes. It was better to do it before the order was processed. In their place, I just ordered a disconnect switch for each battery. I need something that will surface mount and I couldn't find a crossover that would work for where it needs to be placed.

Thanks for the alert. Please let me know if you notice anything else that it would be better for me to do.

 

[Avo Mk8]

The circuit could be simpler, and diodes not required, if you can common the -ve terminals of the batteries and use it as a common feed. Is that possible, or maybe it's already the case?
Have you identified a marine low voltage DC switch that will perform the function you illustrate, as I haven't

PS Schottky diode (which I think it would be) voltage drop max 0.5V

 

[TimeOutside]

...you can common the -ve terminals...

Doh! I didn't think of that. That indeed makes it simpler for both the schematic and the actual wiring.

Have you identified a marine low voltage DC switch that will perform the function you illustrate...

No. However, I'm okay with a non-marine switch. The area where the switch is located is protected from water, spray and otherwise. Also, I didn't show it in the schematic but the 100AH battery has a breaker, all the accessories are all fused, as is the trolling motor. I'm thinking this should protect me from the consequences of any shorts. Am I correct?

Schottky diode (which I think it would be) voltage drop max 0.5V

I'm glad I cancelled the diodes. 0.5V is more than I care to loose. The trolling motor is the sole power source for driving the boat. No outboard. I'll be using an aftermarket propeller to increase the speed of the boat. I'd have to test it after installation, which I'm not going to do, but I suspect 0.5V would negate the gain I get from the better prop.

Thanks again,
Andrew

 

[Avo Mk8]

I'm okay with a non-marine switch. The area where the switch is located is protected from water, spray and otherwise.

It would be desirable to get a switch specifically rated for low voltage high current DC, as marine products typically are, rather than a switch designed for 240V AC, because I believe the contacts and action are designed differently. However if you are always going to be operating the switch 'off load', that may not matter!

Also, I didn't show it in the schematic but the 100AH battery has a breaker, all the accessories are all fused, as is the trolling motor. I'm thinking this should protect me from the consequences of any shorts. Am I correct?

Yes if you're saying the 100Ah battery has a circuit breaker is series to protect the wiring, that seems fine.

Below a sketch (if I dare call it that!) of how you might wire it with a two pole three position centre off switch, assuming making the battery -ve's common is valid, and won't adversely affect EM interference from motor to fish finder! You might find a EM filter would help that.
Otherwise, if you can't common the -ve's, put the same switch arrangement in the -ve side of the batteries as well. You would need a 4 pole version of the switch, but that shouldn't be a problem to find.




PS, Taking 35A from a 10Ah LiFePO4 battery won't do it much good long term, but presumably this is more an 'emergency' provision than for regular use

 

[TimeOutside]

...if you can't common the -ve's, put the same switch arrangement in the -ve side of the batteries as well. You would need a 4 pole version of the switch, but that shouldn't be a problem to find.

So, I've perused the Garmin (fish finder) material and users' comments. A common -ve is not recommended. Am I back to my original drawing for the switch assignments (plus using a disconnect switch on each battery to prevent the good battery from charging the dead battery when in Position 2)? Will that work as well as a 4 pole switch - which I'm not understanding. It seems to me that I'd need a 2 pole 4 position switch, where:
• Position 1: 100AH to motor | 10AH to accessories
• Position 0: nothing
• Position 2: 100AH to motor and accessories
• Position 3: 10AH to motor and accessories
What am I missing with being able to use a 4 pole switch?

Again, thank you for all the help with understanding and implementing this.

 

[Avo Mk8]

So, I've perused the Garmin (fish finder) material and users' comments. A common -ve is not recommended. Am I back to my original drawing for the switch assignments?

Not quite. I'm suggesting that the battery terminals go only to the switch, and you arrange the contacts to take the appropriate battery polarity to the appropriate load. You won't need diodes if you do it this way.

I've drawn a two pole 3 position switch. Since it seems you need to switch the -ve's as well, you would need a 4 pole 3 position switch (as I've drawn but with switches added for the -ve terminals as well)

You can implement this with a 4 pole 2 way centre off switch, or a centre off cam switch with 8 sets of contacts. Are you happy to work out the logic of this?

As long as you route all battery connections only to the switch, and arrange that to feed each appropriate load wire for the switch position, you'll be fine.

 

[TimeOutside]

You can implement this with a 4 pole 2 way centre off switch, or a centre off cam switch with 8 sets of contacts. Are you happy to work out the logic of this?

Sorry, I'm just not getting it. Would you please send me pictures or links to the two types of switches you are describing? I'm not visualizing how it can be done with just 8 sets of contacts and the battery connections only going to the switch.

Thanks,
Andrew

 

[Avo Mk8]

Sorry, I'm just not getting it. Would you please send me pictures or links to the two types of switches you are describing? I'm not visualizing how it can be done with just 8 sets of contacts and the battery connections only going to the switch.

Thanks,
Andrew

Ok. These were my thoughts. Need checking through.
And to add an edit here - I had not appreciated your desire for different permutations of whar powers what. The following explanation doesn't do that.
But I think a switch with more positions might - I need to give it more thought!!

The first type of switch that came to mind was 4 pole centre off double throw, this sort of thing, but this is not high enough current:

https://cricklewoodelectronics.com/Toggle-Switch-4-Pole-Centre-Off-Double-Throw-4PDT-16A-250V-AC.html?

Each battery would be wired to a pair of contacts in the middle, and the motor and accessories wired to the outer terminals, as illustrated in my diagram.

The more likely choice would be a rotary cam switch, with sets of changeover contacts, such as this:

IMO Stag 100A 4 Pole IP65 Enclosed Changeover Switch - SCO4P100

IMO Stag 100A 4 Pole IP65 Enclosed Changeover Switch - SCO4P100

tewltd.co.uk

Say 100Ah battery -ve to 2&4 batt +ve to 6&8,
10Ah battery -ve to 10&12, +ve to 14&16
Then for 'normal' operation, motor +ve to 1, motor -ve to 5, and accessories +ve to 9 and -ve to 13
And for 'reversed' operation, accessories +ve to 3, acc -ve to 7, and motor +ve to 11, -ve to 15

If the links for UK products don't work for you, please let me know.
I've been trying unsuccessfully to find a marine equivalent of the above changeover switch, but so far no luck. I had been discounting the use of relays because that wouldn't be so efficient!

 

[Avo Mk8]

So, I've perused the Garmin (fish finder) material and users' comments. A common -ve is not recommended. Am I back to my original drawing for the switch assignments (plus using a disconnect switch on each battery to prevent the good battery from charging the dead battery when in Position 2)? Will that work as well as a 4 pole switch - which I'm not understanding. It seems to me that I'd need a 2 pole 4 position switch, where:
• Position 1: 100AH to motor | 10AH to accessories
• Position 0: nothing
• Position 2: 100AH to motor and accessories
• Position 3: 10AH to motor and accessories
What am I missing with being able to use a 4 pole switch?

Again, thank you for all the help with understanding and implementing this.

Sorry when I first read the above post I missed these permutations you want to achieve.
I didn't spot that from your original diagram, and I had thought all you wanted to be able to do was swap the batteries over. As a result the things I've proposed don't meet these requirements!

 

[TimeOutside]

I tried to send an update to my post above, with the picture I attached here, but it appears it didn't post. It's showing the 16 positions like you have shown. I used this same switch (link below) for a project in our camper (that we have since sold). It worked like a charm. It's not marine grade, but it will be in a protected location. And like I mentioned before, there is a DC breaker on the 100AH battery, plus everything is fused. I'm thinking the worst case is the switch fails/shorts and I'd have to drag the battery up to the trolling motor and connect it manually.

Do you think this switch will work?

Amazon.com

 

[Avo Mk8]

I think I have to admit defeat at this point. I hadn't read the spec properly!!
As the permutations go up, so the switch positions go up, and, at least for cam switches, the current capability of the whole switch assembly goes down (heating effect), the price of the switch goes up, and this solution seems to become impractical.
I haven't been able to identify a product that would cover the 4 position scenario with my approach!
Sorry

 

[TimeOutside]

...the current capability of the whole switch assembly goes down (heating effect), the price of the switch goes up, and this solution seems to become impractical.

It surprises me that a switch rated for 27,720 watts (63 amps x 440 volts) AC can't handle 540 watts (45 amps x 12 volts) DC. The wiring seems straightforward. But I don't know what I don't know.

If you can think of something, please let me know.

Thanks again,
Andrew

 

[mainline]

It surprises me that a switch rated for 27,720 watts (63 amps x 440 volts) AC can't handle 540 watts (45 amps x 12 volts) DC. The wiring seems straightforward. But I don't know what I don't know.

If you can think of something, please let me know.

Thanks again,
Andrew

This would do the battery changeover, you could use a pair of relays instead if dpdt aren't easy to get.

Same circuit again for whatever battery you want to run accessories.

You can hide the relays away and have a pair of small water-resistant switches remote from them.

 

[Avo Mk8]

It surprises me that a switch rated for 27,720 watts (63 amps x 440 volts) AC can't handle 540 watts (45 amps x 12 volts) DC. The wiring seems straightforward. But I don't know what I don't know.

I didn't mean to scare you away from ac switches! I think the 'closed contact' part of their lives should be fine. It's just that I understood what happens when the switch contacts open and close is a bit different with ac and dc current. As you have already suggested, when operating your switch it may not be loaded, or at least not very much. And it's not as if it will have to withstand many thousands of operations in the boat! I suspect an AC switch will work for you fine, so please ignore the comment.

Re your post #12, I'm still cogitating how you connect say the "motor + Accessories +ve" combination without connecting them together. Was this done including a diode somewhere?
If you draw where all the wires go to on the right side of the switch, you end up connecting circuits together that need to be separate for position 1 of the switch to work?

Mainline's post of having a switch for each battery sounds interesting!

 

[TimeOutside]

Re your post #12, I'm still cogitating how you connect say the "motor + Accessories +ve" combination without connecting them together.

Doh! you are correct. It won't work. Back to pondering.

 

[brianmoooore]

Switches intended for DC operate their contacts quickly and to a larger separation gap than AC switches to avoid continuous arcs.
I'm old enough to remember light switches being sold that were designed for AC/DC or AC only. The AC/DC ones used an underslung 'over centre' mechanism, where the switch operates suddenly with an audible 'clang', however slowly the rocker is moved, while the AC only used a cam mechanism, similar to that in current switches.

 

[Avo Mk8]

This would do the battery changeover, you could use a pair of relays instead if dpdt aren't easy to get.

Same circuit again for whatever battery you want to run accessories.

You can hide the relays away and have a pair of small water-resistant switches remote from them.




View attachment 122110

that works
But it's the additional combinations of both motor and accessories from either one or the other battery that are described in post #1 that are more difficult to achieve, though I think relays might be the answer to that as well.
I've failed to come up with a purely switched solution that doesn't require diodes otherwise!

 

[TimeOutside]

Thank you everyone for the input.

I'm not sure about using relays. I haven't used one in decades and I'd need more schooling to know just what to get.

I'd like to avoid using diodes. I suspect the voltage drop will negate the extra speed I will gain by purchasing an after-market propeller. Since the trolling motor is the only propulsion for the small boat, I want as much speed as I can get. Any voltage drop is a step in the wrong direction.

Please take a look at the attached. I'm thinking it will work, but I'd like your eyes.

Thoughts?

 

[mainline]

that works
But it's the additional combinations of both motor and accessories from either one or the other battery that are described in post #1 that are more difficult to achieve, though I think relays might be the answer to that as well.
I've failed to come up with a purely switched solution that doesn't require diodes otherwise!

Another relay using the same circuit as described could be used to switch the accessory supply to either battery.

There are plenty of high current relays around and would probably be cheaper than the cost of the switches.

 

[TimeOutside]

that works
But it's the additional combinations of both motor and accessories from either one or the other battery that are described in post #1 that are more difficult to achieve, though I think relays might be the answer to that as well.
I've failed to come up with a purely switched solution that doesn't require diodes otherwise!

Another relay using the same circuit as described could be used to switch the accessory supply to either battery.

There are plenty of high current relays around and would probably be cheaper than the cost of the switches.

If it works safely then that's what I need to know, should I choose to go that route. I already have the switch. It's left over from my camper. That said, using relays would certainly be more attractive and elegant of a solution - and I like elegant. So if I'm going to take the time and trouble to do this then I'll mostly likely go with the relay solution.

But I just got the boat. Right now, I'm only planning and won't immediately be implementing either option. For the moment, I'm going to run the trolling motor directly through the breaker from the 100AH battery. I'll run the accessories directly through the fuses and 10AH battery. Should one battery die then just I'll manually move the leads to the remaining battery. It would be kind of a pain, but doable. If after using the boat for awhile I find out that I do commonly run one of the batteries dry (most likely the accessories battery) then I'll be doing what we've discussed (most likely with relays).

Thanks everyone. I'm always learning something new here and greatly appreciate the help and the time you have spent. If/when the time comes, I'll post back with my chosen relays and remote switches for your review - to assure they are appropriate. More than even convenience; shock, fire, and problem avoidance is my goal here.

Thanks again,
Andrew