Charging a battery with an alternator with electric motor

Thread Starter

Tarver4107

Joined Jul 6, 2017
4
Here's the idea, I have a 10 ft fishing boat powered by a 12 volt trolling motor. I want to hook an electric motor that will turn the alternator to charge the batteries. I'm not sure how this stuff works 100% and I haven't seen anyone ask this question. If I have a 12 volt motor turning a 24 volt alternator, it should send 12 volts to the battery charging it right? I'm trying to learn how to do this and I'm building this from scratch. Can it be done by this design? If it can, can you give me some tips and pointers on how to achieve what I'm trying to accomplish? I'd like to be able to run the motor at the same time but by my southern math, a 12 volt trolling motor and a 12 volt electric motor turning a 24 volt alternator would only move the boat and not charge the battery until the trolling motor is switched off correct? Be gentle. I have an idea and if I'm dead wrong, explain to me why it won't work. Thanks.
 

profbuxton

Joined Feb 21, 2014
419
Where does the alternator come into the picture?. Is it on the boat or are you intending to mount it somewhere else?
What sort of electric motor are you going to use to run the alternator and where is it supplied from.
Why not just use a normal 12v battery charger and connect it to the mains?
I hope you're not thinking of having the12 battery in the boat running the alternator and trolling motor and expecting the alternator to charge the 12 v battery. Ain't gonna happen! Even if the trolling motor is off you cannot run the alternator off the battery and expect it to charge its own battery. Have you ever tried lifting yourself up by your own bootstraps?
 

Thread Starter

Tarver4107

Joined Jul 6, 2017
4
Where does the alternator come into the picture?. Is it on the boat or are you intending to mount it somewhere else?
What sort of electric motor are you going to use to run the alternator and where is it supplied from.
Why not just use a normal 12v battery charger and connect it to the mains?
I hope you're not thinking of having the12 battery in the boat running the alternator and trolling motor and expecting the alternator to charge the 12 v battery. Ain't gonna happen! Even if the trolling motor is off you cannot run the alternator off the battery and expect it to charge its own battery. Have you ever tried lifting yourself up by your own bootstraps?
The alternator is going to be on the boat and it'll be being turned by the little electric motor. Both the alternator and motor will be mounted on a plate that I'll put near the battery. Basically I want to charge the battery with using power from itself. I figure if I used less power to turn the alternator than it puts out, it'll send the excess juice, energy not used in the small motor, back to the battery and charging it slowly.
 

Thread Starter

Tarver4107

Joined Jul 6, 2017
4
If we are talking about only one battery then that's perpetual motion which ain't gonna work.:rolleyes:
Ok so if one battery won't work, what do I need to make something work? 2 batteries and an alternator or 2 alternators powered by the same motor and one battery?
 

Alec_t

Joined Sep 17, 2013
11,875
If you are concerned about you boat battery going dead, simply carry a spare ready-charged battery. That will be far more efficient and a lot cheaper.
The concept of using a battery to power a motor to drive an alternator to charge the battery is very old but simply doesn't work, because of all the mechanical and electro-mechanical inefficiencies involved. More energy is taken from the battery than can be put back in.
 

Thread Starter

Tarver4107

Joined Jul 6, 2017
4
If you are concerned about you boat battery going dead, simply carry a spare ready-charged battery. That will be far more efficient and a lot cheaper.
The concept of using a battery to power a motor to drive an alternator to charge the battery is very old but simply doesn't work, because of all the mechanical and electro-mechanical inefficiencies involved. More energy is taken from the battery than can be put back in.
That's what I was looking for. If it's not worth the time and money then I'm going to scrap the idea. A buddy said use a weed eater engine to power the alternator. Which is cool but if I'm going to have to do that, why don't I just turn the weed eater into a gas powered trolling motor? Lol. He had no response except let's do it. Thanks for your help!
 

MrAl

Joined Jun 17, 2014
7,959
The alternator is going to be on the boat and it'll be being turned by the little electric motor. Both the alternator and motor will be mounted on a plate that I'll put near the battery. Basically I want to charge the battery with using power from itself. I figure if I used less power to turn the alternator than it puts out, it'll send the excess juice, energy not used in the small motor, back to the battery and charging it slowly.

Hi,

The battery will discharge more than it will charge, so the net result is the battery discharges.

That's because everything has an efficiency rating. If the motor was 70 percent and the generator 70 percent then the overall efficiency is about 50 percent. That means to charge at 1 amp you need to draw 2 amps from the battery approximately. That means the battery discharges.

To charge a battery you need an energy source that is not getting it's energy from the SAME battery. This can come from many places like the wall socket, or gas motor, or even hand crank.
 

crutschow

Joined Mar 14, 2008
26,355
If you really want to charge the battery then how about a small AC generator, such as this, with a battery charger.
It basically has your weed eater motor connected to a generator.
 

cork_ie

Joined Oct 8, 2011
428
Save yourself a million hours of grief and just get a battery charger or make one with an old transformer and a metal cased 25A full wave rectifier that you can buy on ebay for 1$
 

Egzb

Joined Oct 14, 2017
2
But you can use a battery to power a motor that turns an alternator to charge the battery that turns the motor that turns the alternator that charges the battery. Its called renewable energy
 

be80be

Joined Jul 5, 2008
2,051
Can't get something for nothing it takes power to make power anyway it goes.
Power has to come from somewhere.
Sun you could say is free but it's still power problem with it is it takes lots of room or longer time to charge.
Motor turning alternator takes power your adding a load to motor means your going to drain the battery faster.
To much load and your dead in the water. Gas you run out same thing.
Put a sail on the boat and let the battery charge free power as long as the boat is moving.
 

Reloadron

Joined Jan 15, 2015
5,781
But you can use a battery to power a motor that turns an alternator to charge the battery that turns the motor that turns the alternator that charges the battery. Its called renewable energy
OK, while doing this place a hand on the motor, do you feel heat? Now place a hand on the alternator, do you feel heat? Do the bearings in the motor and alternator have any friction on the bearing surfaces? Got any answers to those questions?

Renewable Energy, depending on how you choose to define it can be energy from a source that is not depleted when used, such as wind or solar power. So a solar panel array is renewable energy or a wind turbine is renewable energy. In your example there is loss, it is not a loop that will run indefinitely. Your example is not renewable energy and it would never sustain itself. Believe me as I tried it a long time ago,matter of fact I tried it in 1960.

Ron
 

dendad

Joined Feb 20, 2016
3,720
I used to think those that asked for this sort of perpetual motion were just in it for a laugh and could no way be serious. Until my Dad thought it would be a good idea to put a wind generator on a Prius so the battery could charge from the wind caused while driving along at speed.
Oh well, it is easy to forget most people have not a lot of technical knowledge these days.
But it is surprising the number of posts I've seen that have folks arguing for perpetual motion. I wish we had it!
Then again, if we did have unlimited free power we would all most likely die of the heat build up with massive amounts of power being spontaneously generated from nothing so I guess we must be grateful that it is impossible.
 
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Reloadron

Joined Jan 15, 2015
5,781
Until my Dad thought it would be a good idea to put a wind generator on a Prius so the battery could charge from the wind caused while driving along at speed.
That's a good example and sounds good. So if I do this how much additional energy will I need driving along at say 40 MPH ( 64.3738 Kilometers per hour) to overcome the drag created by my added fan turbine? After I invest in a wind turbine for my backyard how long will it take before I am showing a return for my investment? :)

Ron
 
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