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# Recharging a `battery bank Answered

Im looking for a dc motor that will carry a load of three 24v 100a alternators and a suggestion of what size and type of batteries will carry the dc motor. NOTE one of the three alternators purposes is to recharge the batteries powering the motor. Please help with a suggestion of what type of motor to get, the voltage or hp of the motor and what size battiers will be suitable to carry that motor.

Thank you

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This picture reminds me of the the following immortal words, authored by that band, ZZ Top:

"Concrete, concrete and steel. It's like concrete. Hey, baby, what's the deal?"

I am really impressed by this picture, because it looks real. It looks like you have three alternators, securely bolted onto steel frames, that are cast in concrete, and all three are connected mechanically, by belts and pulleys.

It makes me wonder what is connected to the belt, on the right side of the picture. Maybe a big engine, or Lance Armstrong on a stationary bicycle. I dunno. I guess my imagination has to fill that part in.

Also, I am naively guessing this image is really an image of a project you are building. I tried showing it to Google Images, to see if it could guess what it was. Essentially, to check if you just pulled this image from somewhere else on the web, and you were just kind of messing with us, the peoples who regularly read this forum, by posting it here.

But no. I am guessing this is really a picture of your project, and you are sincerely contemplating spinning those alternators up and moving power (perhaps several kilowatts of power?) through them. Perhaps you already have done this, with some other mechanical power source, and now you want to try this with a big DC electric motor, and batteries too.

Where do I begin?

In the mechanical realm, power is the product of angular speed and torque; e.g. (1 radian/second)*(1 newton*meter) = (1 watt).

In the electrical realm, power is the product of voltage and current; e.g. (1 volt)*(1 ampere) = (1 watt).

Also for DC electric motors (or generators), there tends to be a proportional relationship between angular speed and no-load voltage.

In the case of a DC motor, more applied voltage tends to make it spin faster. For example, with 24 volts applied, I kind of expect the motor to spin at twice the speed it would with 12 volts applied. And actually for both of those, I am imagining this is a "no load" speed, or "light load" speed rather than a "heavy load."

So, I think a big part of the trick to choosing a DC electric motor to drive those alternators, is picking one that is sort of matched, to the same speed/voltage ratio as the alternators, or maybe slightly greater than that, since the DC motor is supposed to be supplying power to the alternators.

By the way, I think this relationship between speed and voltage is coming from Faraday's law of induction,

Essentially a product of speed, magnetic field strength, and number of turns.

For a DC motor with permanent magnets, the no-load speed/voltage ratio is, I think, a constant, characteristic of the motor.

In contrast, for an alternator, which has a field winding to produce its magnetic field, the speed/voltage ratio is adjustable. I mean, essentially the ratio is adjustable because the magnetic field is adjustable.

I am guessing the same would be true for a DC motor that used windings for its field, in place of permanent magnets.

Anyway,the reason that might be relevant is because I was imagining this speed/voltage ratio is important, and that you might have more freedom, with a motor for which that ratio can be adjusted.

Or it might be enough, just to have that freedom on the alternators, which they do, since the current to the field winding can be adjusted.

I should probably say a few words about "loading" too. Loading is an expression of how much power a machine must supply, either mechanical or electrical. Maybe it is easiest to describe in terms of the "no load" condition I mentioned previously.

For a motor with "no load," i.e. no mechanical load, it spins at some angular speed, but supplies zero torque, and thus zero mechanical power. E.g. no "belt" is connected to the motor to carry mechanical power out, and it just spins.

For a generator with "no load," i.e. no electrical load, it has some output voltage across its terminals, but supplies zero current, and thus zero electrical power. E.g. no wires, or a so-called "open circuit," connected to its terminals, so there is no electrical path, to carry electrical power out.

In a similar way, we can imagine "light loads" drawing small amounts of power, and "heavy loads" drawing large amounts of power.

I am kind of thinking this is what Downunder35m is describing with language like "just to make it spin" suggesting light mechanical loading, and different sized batteries, palm sized to barn sized, based on how much electrical loading is wanted.

Also, did I mention this project looks kind of crazy. I mean that in a respectful way.

To say that another way: I am not totally sure what you're doing here, but I wish you success with it.

I'm not sure what you're trying to do but I'm pretty sure it's not going to work, I'm sure you've been told this already.
To drive 3 alternators that size you are going to need 7200 watts plus loses so around 10000 watts or 10KW. If you can find a large electric forklift you could get the motor and battery pack out of it.
I knew a guy who bought a big battery hooked it up to an inverter that runs a battery charger to charge the battery. That didn't work either.
Save your money but some solar panels, you can pick them up quite cheap second hand.

It could work though.....
He only wants one alternator to charge the batteries for the motor.
He did not say he needs to draw the full power of the alternators.
A 50W DC motor should make it all run nicely.
So the batteries and motor are only needed for short periods of time to keep the motor running.
Like when one of the family members has to get off their dedicated treadmill to grab something to eat or go to the toilet.
You know: To keep the momentum up so the other family members won't have to run faster to keep the airconditioner running from the alternators....
But then again....
If they wouldn't be all running so fast they wouldn't need the airconditoner.... ;)

I figured 8.5kW but yeah: Douptfull on the successrate...

Do I understand this correctly?
You use one of the alternators to charge the battery the motors needs to run? - How is that supposed to work?

24V @100A means 7200W under full load.
No offense but if it is already impossible from your wall outlet with mains power, how big you said is your battery shed?

To just make them spin any motor will do.
To make them charge your battery while the motor is running you need to overcome the power the motor need plus enough to charge the battery.

How much power the motor needs depends on how much the alternators have to provide.
So you can use anything from a 10Ah SLA battery with a 10W DC motor to a 10kW motor with frequency converter and barn sized battery shed...

Lets do some back-envelope-math:
3x alternator
Each alternator has a power of 24V * 100A = 2.4kW
Your alternators have a total power output of about 3 * 2.4kW = 7.2kW
Lets assume your loss in the alternators, mechanics and motor are about 15%.
This leaves us a power input at the motor of 7.2kW / 85% * 100% = 8.47kW
So: Your motor needs to have around 8.5-9kW input power electrical.
Depending on your alternator specifications, they need a specific rpm to have the best efficiency normally. Your motor needs to match that: Pick a motor which pulls 8.5-9kW at the voltage needed to have the correct rpm for the alternators.
Lets say, you find a motor with 24V to drive your 3 alternators at the perfect speed. Thats about 350 Ampere! According to http://www.luminoindustries.com/lt-pvc-&-xlpe-pow... that results in a cable with somewhere in the area of 270mm^2 Copper or about 19mm diameter! Wow...
Ok... So much to the motor.

Any battery will do as long as it can deliver 350 Ampere at the given voltage... You didnt mention how long the motor needs to run on this battery. So i cannot tell you really the capacity needed. Voltage and controller of the battery has to fir the voltage and power of the motor which has to fit the speed needed by the alternators.
But again: Back-envelope-math:
You pull about 8.5kW from the battery.
If you want to supply the motor for 1h that means you need 1h * 8.5kW = 8.5kWh of energy in the battery at the voltage you need. Remember: A partially depleted battery will lower voltage and you will not have the correct voltage at the motor and therefore not the right speed.
Normally i go with a factor of 5-10 if there is no controller (Boost and PWM) involved.
So lets make this for both assumptions:
5 * 8.5kWh = 42.5kWh
10 * 8.5kWh = 85kWh
Lets say your motor runs directly on the battery at 24V for 1h
Capacity of your battery would be Energy / Voltage =
42kWh / 24V = 1.77kAh or 1770Ah
85kWh / 24V = 3.54kAh or 3540Ah

A lead acid battery has about 35–40 Wh/kg. That would result in somewhere between 45kg to 100kg of lead-acid batteries.

If we go with LiPo (245 to 430 W/Kg) that would result in 4kg to 15kg of LiPo batteries.

Honestly: If you have to ask such questions in a forum like here, you better make a step back, realize that energies and powers involved are frightening and you call a proper electrician to build you this thing... And he knows more on how to calculate this shit...