Refresher on wiring electric motor

[talbot]

I know electric motor wiring been discussed earlier on the forum, but I’m a rookie, and need a recap. Sorry for the long message, but if I reveal all my ignorance at once, it might speed things up.

On a DSII, I am connecting a 30amp motor to a 12v AGM battery, located just forward of the mast. I intend to run the hot wire through a 50-amp breaker on the underside of the cuddy top, and from there via a watertight thru-hull into the portside seat back, aft to a waterproof receptacle near the transom. I’m using AWG 8 wire (largest the receptacle will take without an adapter), and calculate a 4% voltage loss over the 24’ circuit. How am I doing so far?

Now, what about connecting a solar trickle charger? Never used one. Because the battery will not be very accessible, I would like to follow the same plug & play strategy as with the motor. I would like a receptacle on the main bulkhead so at the end of the day I can lock the waterproof hatch, prop the solar panel facing south, and plug it in.

So I imagine two sets of wires to each battery terminal: Large appliance hot and ground wires to the motor receptacle at the stern and smaller charging wires from the solar panel receptacle on the bulkhead. Is that correct? My question is, where are the diodes for the charger that keep the current from going into, rather than out of, the panel? Are they at the panel or somewhere else? Would I be foiling the system by connecting the panel leads to an accessory plug? Assuming all this works, when my panel is not connected to the 12v receptacle on the bulkhead, would that receptacle then be available as a power source?

There may be a charge controller in between the bulkhead and the battery. West Marine says that their small panels don’t need one, but I’d hate to cook my battery. Speaking of cooking, I’m assuming the wires from the solar panel go right to the battery terminals, and don’t have anything to do with the breaker or any intermediate bus. Is that right?

I think that’s the sum of my ignorance. Thanks for your patience. Any tips on completing this project would be appreciated.

[GreenLake]

I would not base the wire size on the receptacle. I didn't. I'm running AWG 4, all the way to the back, then connect a short length of smaller stuff just so I can connect a standard trolling motor plug.

According to one of the handy voltage drop calculators, going to AWG4 reduces the losses to about 1/3. I think that's an important figure to minimize, because you'll pick up connection losses for your fuse and receptacle.

I've done dry-runs with the additional wire added to the leads from the trolling motor before mounting a receptacle. That part works, and using AWG4, while a tad bit heavier than I'd like, seems to not have any appreciable effect on the motor.

I don't see why you can't use your idea with a receptacle in the bulkhead for your charger. I would have expected that the solar panels would be used with some kind of regulator, which would then contain the necessary diodes. But what do I know - I've not done this yet. (I do know, from reading about this, that some panels come with internal diodes and others don't). The diodes act line one-way valves, so they don't care whether you add a quick-connect in your system.

And, I'd agree, you connect the charger directly to the battery. However, I would add another (low amp) fuse into the charging circuit, and close to the battery, so that any accidental short doesn't discharge the battery. (Violently at that). A small inline fuse might just be the thing.

[hectoretc]

My handy little iPhone app tells me that resistance of #8 copper wire is .628 ohms per 1000 ft. which maths out to be .013 ohms for 21 ft. Current multiplied times resistance equals voltage, so at 40 amp max draw (worst case) you’re looking at a .52v drop across the wire or the 4% you were talking about (.52v/12.6v). That also suggests with a nominal 12.6 volt battery, you’re getting about 12 volts delivered (at full throttle power) at the motor. We could run this calculation again for various step points in current draw as well but the lower the current, the less the loss will be.
Using GL’s #4 (assuming copper) wire, the resistance is .248 ohms /1000 ft. which comes out to be .005 ohms for 21 ft. Assuming your max 40 amp draw, it works out to a .2v drop (40 A x .005 ohms) across the wiring, for a 1.5% loss (.2 v/12.6v). A clear improvement. In this latter case, you’re getting 12.4 volts at the motor (assuming all things being equal), but as GL also commented, every disconnectable connector adds some additional resistance and potential losses so when you get to the tiny numbers, they can become relevant.
It does however also reinforce the importance of using a deep cycle battery rather than a standard starter battery. The deep cycle will (if memory serves) keep the voltage up until it's nearly empty and suddenly drop off, where a starter battery more consistently drops off over time.
Keeping the voltage up, decreases line losses.
Just some additional information to further confuse…

[talbot]

Thanks. I guess I'll press ahead. I'm a little dismayed at the price jump after AWG 8. All this began when a friend gave us a Minn Kota motor, which would have otherwise cost us about $110. That total cost of this freebie I estimate will be about $500 in battery, cables, connectors, breakers, fuses, chargers, how-to books, etc. I started last season thinking my investment in alternative propulsion would be a new coat of varnish on the oars.

[Alan]

talbot,

I assume you've checked online for wire? I found no. 2 marine grade at www.genuinedealz.com for about 1/4 the price at my local West Marine store ($3.25 a foot vs $15.00). jdoorly also has a link on one of the threads (can't find it at the moment) where he found no. 1/0 wire for quite a bit cheaper than that.

[hectoretc]

Thanks. I guess I'll press ahead. I'm a little dismayed at the price jump after AWG 8. All this began when a friend gave us a Minn Kota motor, which would have otherwise cost us about $110. That total cost of this freebie I estimate will be about $500 in battery, cables, connectors, breakers, fuses, chargers, how-to books, etc. I started last season thinking my investment in alternative propulsion would be a new coat of varnish on the oars.

It's all about balance (like I know what that is). The cost (and weight) of batteries, and wiring vs. the return you want or need. In my case, 90% of my motoring will be in and out from the dock when the wind is blowing the wrong way. For that, even against a nasty headwind, if it takes me 15-20 minutes at full throttle I'll have more to worry about than my battery capacity, so the difference between #8 and #4 wire will be meaningless to me. If you think you may actually need 4-5 hours of motor capacity to get in from the middle of the ocean to port, depending on your situation, it would be well to plan for the worst plus 10%, but I'd be tempted at add a 2nd battery (or even a very small gas generator) before I'd worry about 2-3% efficiency in wiring.
The solar charger is probably going to be your biggest hit for minimal return, but you do need to get the battery charged somehow. Cost effectiveness I guess is what I'm suggesting should be your guide. Is it worth the couple hundred bucks for a solar charger, or more cost effective to be able to just pull the battery easily and bring it home with you?

[Alan]

talbot,

One other thought: I assume also that you've test-run this motor with a temporary hookup and it's powerful enough for your needs? I was pretty disappointed with an 80-pound, 24-volt trolling motor, and jdoorly was less than thrilled with his (50 pounds, as I recall).

[GreenLake]

If you want an electric outboard with real performance you'd need to get a Torqeedo. By all accounts they work well, and if you get the one with its own battery pack, it's light and self-contained. Problem there is the price.

Trolling motors aren't designed for being used as main propulsion and it shows. The advantage they have is price.

That said, I can get my DS to about 3kn using a 40lbs trolling motor. It's great for getting in and out of docks, and it gets me through stretches of calm. I also use it to maneuver through short sections of canal or under bridges where sailing just isn't an option.

In all these situations, the power is adequate, and especially when motoring through calm spots, I enjoy the silence.

A 30lbs motor would be less powerful, but because the power to speed curve is not linear, it might not matter that much in terms of maximum speed in a calm.

The big limitation of any trolling motor is lack of power to push against the wind, and their inability to efficiently motor for long distances. They are just not the right type of engine for "self-rescue" type use - a DS with a trolling motor will have to cope under sail with adverse weather.

(PS: even paying for a new trolling motor, I don't think I've come near the $500 for my set-up. But I charge the batteries on land).

[GreenLake]

... every disconnectable connector adds some additional resistance and potential losses so when you get to the tiny numbers, they can become relevant.
It does however also reinforce the importance of using a deep cycle battery rather than a standard starter battery. The deep cycle will (if memory serves) keep the voltage up until it's nearly empty and suddenly drop off, where a starter battery more consistently drops off over time.
Keeping the voltage up, decreases line losses.
Just some additional information to further confuse…

The issue is really with any type of connection - the all can have some (very small) resistance, how much would depend on the details and whether the connection got properly established and / or perhaps degrades over time.

About the difference in battery types. I hadn't heard or read about any difference in voltage vs. discharge level between these types. The primary difference is in how many deep discharges a battery can handle in a lifetime. Starter batteries are designed for shallow discharges and will degrade rapidly if discharged deeply.

Deep cycle or marine batteries should be good for discharge down to 50% of rated capacity. An 80Ah battery would give you 40A for an hour. A trolling motor draws roughly 1A per lb of thrust, so I can run my 40lbs motor for about 1 hour, giving me a theoretical range of approx. 3nm.

At half speed, I can go farther. Two effects combine: the resistance of the hull is a bit less at lower speeds and the battery is willing to give more power if discharged at a lower rate. (In fact the 80Ah nominal capacity assumes a specific discharge rate - probably over 10 hours, so that I should actually see less than 1h of 40A - I usually assume I have about 40min, or 2nm).

[talbot]

Thanks to all for the advice. Yes, we used the 30lb motor this fall, and it worked fine, even into a moderate wind. The motor's previous owner had put about 10' of AWG 14 (!) on it. He apparently had not considered voltage loss. I'm glad we only had to motor in short stretches last season. AWG 14 is about the gauge of Minn Kota primary wire; they obviously expect the motor to be close the the battery.

[hectoretc]

About the difference in battery types. I hadn't heard or read about any difference in voltage vs. discharge level between these types. The primary difference is in how many deep discharges a battery can handle in a lifetime. Starter batteries are designed for shallow discharges and will degrade rapidly if discharged deeply.

I'm not sure where I thought I'd heard that a Deep Cycle holds it's voltage up for the duration, but having done some checking I see that it doesn't seem to be the case, so I take that one back. Thanks GL

[Alan]

hectoretc,

Maybe that applies to lithium-ion batteries, at least if my assorted cordless tools are any guide. The nicads the tools came with lose their charge noticeably (slower drills and saws, dimmer flashlight), whereas the li-ion "upgrade" batteries just shut off without any indication that they're getting tired.