# Charging A battery pack with multiple Dynamos

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#### JamesC

##### New Member
Hey,

I was embarking on a new project where i wanted to charge a high capacity battery pack with multiple dynamos.

I only know the basics to electrical concepts so I will be only be able to understand the more basic concepts and terms but I am definitely willing to learn more if required on my own.

So what i do want to know is what type of battery i have to use (Capacity) and what type of dynamos. How can i connect the dynamos(and how many) to the batteries for the best charging option?

I would like the battery to link to a USB hub and USB compatible charging appliances like mobile phones could be connected to it to get charged. So i do believe that it would need a high capacity.

What do i need to know and do to carry this out. Also if more information is needed, please request as well.

Thank you

#### colin55

##### Well-Known Member
A dynamo is a spinning magnet and a coil. It produces AC. You need 4 diodes (called a bridge) between the dynamo and the battery.
The battery can be any size and any voltage.
Connect an ammeter in the positive line between the bridge and the battery.
Spin the dynamo and see what charging current it produces.
If it works, add dozens of similar dynamos in parallel with the first. Get everyone to charge the battery.
If the battery goes flat too quickly, get a bigger battery.

#### Diver300

##### Well-Known Member
If the dynamos are AC, you have to rectify each one separately and then you can parallel the outputs of the rectifiers.

Pedal bicycle dynamos are AC. Car dynamos (if you can find one, I don't think they have been made for 40 years or so) are DC. They have to have cut-outs so that they are disconnected from the battery when they stop, or power flows back from the battery.

#### Nigel Goodwin

##### Super Moderator
Dynamo's are DC - alternators are AC - if a bicycle 'dynamo' produces AC, then it's not a dynamo, it's an alternator.

The OP needs to be more specific in his question.

#### JamesC

##### New Member

Thanks Nigel for clearing up the concept on Alternators and Dynamos. So are bicycle "dynamos" in actual fact alternators or dynamos producing DC?

I do have knowledge on how alternators/dynamos work - concept on electromagnetic induction
and
Some other basic circuitry like resistors, semiconductor diodes (Wheatstone bridge), etc

I will break down what i exactly want to do in this DIY project

1) I would like to use dynamos/alternators (not sure which one to use yet) attached to the wheels of the bicycle to induce an emf

2) The voltage induced need to be used to charge a battery pack (Uncertain on which one to use - Li-Ion, Li-Poly, Alkaline)

3) The battery then needs to discharge to a USB Hub where there are multiple USB ports.

4) Appliances with the standard USB charging interface like mobile phones etc can be plugged into the USB ports to get charged

Its my first time doing something like this so i need help with details of components i will need. I can understand circuit drawings but i do need help in how to set this up and what i specifically need to get.

Thank you so much for your time

#### Diver300

##### Well-Known Member

Thanks Nigel for clearing up the concept on Alternators and Dynamos. So are bicycle "dynamos" in actual fact alternators or dynamos producing DC?

Bicycle "dynamos" are alternators producing AC. Incandescent bulbs don't notice the difference and LED lights (some with supercapacitors for when you stop moving) have bridge rectifiers, often made with Schottky diodes to reduce voltage drop, to rectify the output.

A dynamo, such as what used to be used in cars until the 1970s, had brushes that carried all the generated current. The little motors run as dynamos inside wind-up torches are also DC generators with brushes. They are often fed through a bridge rectifier which stops the motor running when the handle isn't being turned, and it allows the handle to be wound either way.

Cars used to use dynamos because the power had to be DC to work with the battery. The commutator is really a switch that keeps reversing the coil connection, in time with the current reversing in the coils, so that the output current always flow the same way. When semiconductor diodes became cheap, cars changed to alternators.

Wind up torches use motors as dynamos because little motors like that are cheap.

Bicycle "dynamos" can't use cheap motors as they need specially shaped ones, and they don't need a DC output, so they are in fact alternators.

#### JamesC

##### New Member
Any one can help with the above?

#### Nigel Goodwin

##### Super Moderator
Any one can help with the above?

#### ronv

##### Well-Known Member
Here is something that might work. Lets say you get a wheel alternator. It should supply say 3 watts (0.5 amps) at 6 volts. You might need to remove the overvoltage protection to get enough voltage to charge a small sealed lead acid battery of 6 volts and rectify the output. You could then regulate this down to 5 volts to supply a usb charger plug. Does this sound like what you want?

#### Nigel Goodwin

##### Super Moderator
Here is something that might work. Lets say you get a wheel alternator. It should supply say 3 watts (0.5 amps) at 6 volts. You might need to remove the overvoltage protection to get enough voltage to charge a small sealed lead acid battery of 6 volts and rectify the output. You could then regulate this down to 5 volts to supply a usb charger plug. Does this sound like what you want?

Over-voltage protection?.

What over-voltage protection?.

#### JamesC

##### New Member
@ronv - Yes that is a useful solution.

Ok then let us take it step by step. I think i need to focus more specifically on each pointer. Lets talk about the alternator first.

Since I am new to this field I do not know which alternators and what kind i can use and considering that I need a voltage of 5V to power USB devices, are there many 5V battery options?

If there are 5V batteries, then how can i get a 5V output from a alternator preferably at lower speeds of maybe 5km/h? What type of alternator can i use.

Sorry if i am a bit too vague

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#### alec_t

##### Well-Known Member
Sorry if i am a bit too vague
I think your first step should be to google for information about alternators/dynamos to see what's suitable for your use and is available in your country and in your price range. We will then have something less vague to start with. Let us know how you get on.

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#### tcmtech

##### Banned
There are a few physical issues to deal with first being how long does a average cell phone or other USB charged device take to get a full charge and even if that charge energy comes from a second battery how long and how much energy does that battery take to recharge?

A typical cell phone battery holds about 2 - 3 watt hours of energy but takes about twice that to charge factoring in charging inefficiency's. 6 watt hours off of a 6 volt battery would be one Ah but then charging that battery with a basic bike generator will take at least double that again so you can figure a start to finish energy transfer efficiency will be around 25% at best.

This means that for every single watt hour of energy any device holds you have to produce about 4 watt hours of initial energy. If your bike generator is producing around 3 watt hours of energy you would have to ride non stop for about 4 hours just to put a charge into one cell phone. Add your other 3 devices each taking about the same recharge energy and you would have to ride 16 hours non stop at a fair pace just to get one full charge cycle completed.

human power is horribly inefficient and becomes incredibly time consuming rather fast when any practical level of power is required so unless you are a super cycling athlete with near zero other things to do in your life this endeavor is going to become very old and tiresome very quickly.

#### ronv

##### Well-Known Member
As to the overvoltage protection: Some hub chargers have a 7.5 volt zener to keep them from burning out 6 volt bulbs.

#### JamesC

##### New Member
This means that for every single watt hour of energy any device holds you have to produce about 4 watt hours of initial energy. If your bike generator is producing around 3 watt hours of energy you would have to ride non stop for about 4 hours just to put a charge into one cell phone.

So i was just wondering whether adding a few more alternators in parallel with eachother would reduce the time taken to charge them?

#### Nigel Goodwin

##### Super Moderator
So i was just wondering whether adding a few more alternators in parallel with eachother would reduce the time taken to charge them?

Phones have in-built charging circuits, providing more external power won't affect it.

#### JamesC

##### New Member
Phones have in-built charging circuits, providing more external power won't affect it.

I mean if I used multiple dynamos to charge a separate independent external battery which will then charge the devices like the handphone? Possible?

So What I would like to achieve is using a dynamo to charge a external battery and then this battery feeds the load like handphones or fans etc

In the mean time, let me go research on exactly what specs and battery i would want.

Thanks

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#### Nigel Goodwin

##### Super Moderator
I mean if I used multiple dynamos to charge a separate independent external battery which will then charge the devices like the handphone? Possible?

Of course.

#### alec_t

##### Well-Known Member
Possible? Yes....but so exhausting (see post #13) that you'll be too tired even to use the phone when charged ;-)

#### tcmtech

##### Banned
If you are looking for power I would suggest getting away from the crappy 3 watt bicycle generators and go toward the small engine type generator/alternator units.

From what I am seeing a basic 3 watt bicycle generator sell for around $25 -$40 new where as a small engine generator/alternator sells for around $60 -$120 and put out between 100w - 500+ watts of power depending on size and design.

Do a quick search on eBay for Kobota alternators and you will see what I am referring to. There are two main types 2 wire PM type AC output without regulators and normal 12 DC output types with regulators.

The 2 wire PM type are more like a bicycle dynamo but considerably higher powered. 12 volts at 14 amps is a common working rating but they will put out anywhere from 0 - 12 volts at up to the 14 amps just by what speed you spin them at.

You will still be about dead tired by the time your battery is charged up but at least you wont be at it for 16 hours to get there!

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