AA battery charger from bicycle dynamo?

First of all my knowledge of electronics is very limited, so please bear with me.

I am looking to charge AA batteries from a bicycle dynamo for when I am on a global cycle tour. My dynamo is a Schmidt SON 28 Dynamo hub, which I believe outputs 6 volts. So presumably it would be a good solution to charge 5 batteries (1.2 x 5 = 6V)?

The problem is I have read that the output voltage from a dynamo can greatly fluctuate depending on speed, so my research has led me to believe that I need to use a full wave rectifier. Any suggestions for this?

Secondly I want the charger to stop charging when the batteries are charged and turn on an LED indicating they are charged. Any ideas how to achieve this? Would something like an attiny85 (I have one of these) do the job? Or are there simpler / better solutions?

Thirdly, does anyone have a circuit diagram that may work? I have searched extensively for a circuit diagram, but can’t find anyone else who has done a similar project.

Hi, That dynamo produces AC at 3W, up to 7W if you do 30kph , so you will need to rectify it.

Do you have a DMM?

You will have to rectify the output and filter it to see exactly what voltage you are dealing with.

Tom... :)

Once rectified and filtered, it might be an advantage to look at perhaps using lithium 18650 cells in place of the aa cells. Purely because there are small usb charge modules available on Ebay for maybe $1 . There are available terminals for soldered wiring in place of the usb socket and they have charging and charge finished led indicators. Don't know what your original load is to use AA cells but there are also cheap-as dc dc converters to do almost anything one can think of.

bluejets: Once rectified and filtered, it might be an advantage to look at perhaps using lithium 18650 cells in place of the aa cells. Purely because there are small usb charge modules available on Ebay for maybe $1 . There are available terminals for soldered wiring in place of the usb socket and they have charging and charge finished led indicators. Don't know what your original load is to use AA cells but there are also cheap-as dc dc converters to do almost anything one can think of.

Good idea. google dynamo usb charger Surprising what they make for the Lycra wearing set these days... :)

Tom... :)

bluejets: Once rectified and filtered, it might be an advantage to look at perhaps using lithium 18650 cells in place of the aa cells. Purely because there are small usb charge modules available on Ebay for maybe $1 . There are available terminals for soldered wiring in place of the usb socket and they have charging and charge finished led indicators. Don't know what your original load is to use AA cells but there are also cheap-as dc dc converters to do almost anything one can think of.

No I definately want AA cells, as I have a lot of gear that depends on them (GPS etc) Also AA are available almost anywhere in the world, so perfect for remote places.

Hi,

google dynamo AA charger

https://www.ebay.com/itm/6-Volt-Mini-Battery-Charger-for-Bicycle-Dynamo-Lights-Bottle-or-Hub-Dynamo-/150668573310

Tom... :)

TomGeorge: Hi,

google dynamo AA charger

https://www.ebay.com/itm/6-Volt-Mini-Battery-Charger-for-Bicycle-Dynamo-Lights-Bottle-or-Hub-Dynamo-/150668573310

Tom... :)

believe me I have spent hours googling. I have already seen that, but I asked the seller a question and it does not include a termination circuit.

TomGeorge: Hi,

google dynamo AA charger

https://www.ebay.com/itm/6-Volt-Mini-Battery-Charger-for-Bicycle-Dynamo-Lights-Bottle-or-Hub-Dynamo-/150668573310

Tom... :)

believe me I have spent hours googling. I have already seen that, but I asked the seller a question and it does not include a termination circuit.

There appears to be nothing I can buy on the market. And I am not interested in anything USB. All I want is a AA battery charger that I can hook up directly to the dynamo, complete with a termination circuit and over voltage protection.

tommydog: All I want is a AA battery charger that I can hook up directly to the dynamo, complete with a termination circuit and over voltage protection.

What do you call a termination circuit? The thing is these devices are getting your variable AC from the dynamo and outputting a constant regulated DC voltage, so you are more than halfway there. Tom.. :)

a Schmidt SON 28 Dynamo hub, which I believe outputs 6 volts

That is presumably RMS AC Volts, and even then only at certain cycling speeds and loads.

Before you do anything you should determine the rectified and filtered DC voltage output of the generator at various cycling speeds. A single diode or bridge rectifier, 1000 uF 25V capacitor and a 100 Ohm 1/2 Watt resistor load should suffice for testing.

Get back to us with that information and we can help with a charger.

TomGeorge: What do you call a termination circuit?

Something that will turn off charging when the batteries are fully charged. I don't want to overcharge / blow them up

I am charging 5 AAA (not AA) cells with my bicycle dynamo. Charge termination is determined by temperature (temperature compared to ambient temperature) and also by temperature rise. This requires that the charge current is high enough to make the cells hot. It will probably not work well with AA cells because you only get about 500mA out of the dynamo which will not warm the cells enough. For a good short introduction see this You may try cell voltage as criterion for charge termination

Look for a low voltage comparator circuit. Should be plenty on the web. For 5 AA or AAA nicad/nimh, 7.0v detection should suffice for "full charge" (1.4v per cell) Don't think you are going to have toomuch "overcharge" with that little bike dynamo anyhow.

bluejets: Look for a low voltage comparator circuit. Should be plenty on the web. For 5 AA or AAA nicad/nimh, 7.0v detection should suffice for "full charge" (1.4v per cell).

Can this be done with something like an attiny85 or is it better to use something else? If anyoune knows of any beginners tutorials on this I would be very grateful. My only projects so far with the Arduino are simple LED flashing etc, so this project is quite a large step for me.

Can this be done with something like an attiny85 or is it better to use something else?

Nothing can be expected to work, until you have characterized the generator output.

I studied this a lot for my own charger: A (standard 6V) bicycle generator is a current source (I do really mean a current source, not a voltage source). Current depends on speed and has some upper limit of about 600 mA. This is a good qualification for a battery charger. No worry about voltage, just connect some cells in series and they will be charged. Charge termination by voltage may be good enough. NiMH cells must not be trickle charged for a long time. But if you remove them after full charge you will not face this problem

A (standard 6V) bicycle generator is a current source (I do really mean a current source, not a voltage source)

Nonsense. Electromagnetic generators are voltage sources with internal resistance, so please provide the evidence that you are misinterpreting.

If a current source of 600 mA is applied to a load of 1 Megohm, the voltage across the load is 600,000 V. Is that the sort of behavior you observed?

jremington: Before you do anything you should determine the rectified and filtered DC voltage output of the generator at various cycling speeds. A single diode or bridge rectifier, 1000 uF 25V capacitor and a 100 Ohm 1/2 Watt resistor load should suffice for testing.

Get back to us with that information and we can help with a charger.

Would something like this work hooked up to the dynamo and monitored with a volte metre:

https://www.ebay.com/itm/251890119927?_trksid=p2057872.m2749.l2649&var=550775476166&ssPageName=STRK%3AMEBIDX%3AIT

That is simply a bridge rectifier with a filter capacitor, and will produce a DC voltage about 1.4 times higher than the RMS AC output voltage (minus about 1.5V for the diode bridge voltage drop).

To understand the generator, at [u]various peddling speeds[/u] you should measure the

(1) open circuit AC voltage and

(2) the AC voltage under load.

A suitable load would be a 12 Ohm resistor, which is probably about equal to the winding resistance. A multimeter can be used to measure the AC voltage and the winding resistance, with the wheel not rotating.

For long term charging, the most interesting data points would be the winding resistance and the open circuit AC voltage at a speed where you are comfortable peddling for hours.

With that information we can help you build a charging circuit.

jremington: Nonsense. Electromagnetic generators are voltage sources with internal resistance, so please provide the evidence that you are misinterpreting.

Sorry, only in german: link. It says: "A claw pole generator is a constant current source in first approximation"

If a current source of 600 mA is applied to a load of 1 Megohm, the voltage across the load is 600,000 V. Is that the sort of behavior you observed?

Of course not. If I connect a 1 milliohm resistor to a 5V constant VOLTAGE source I also do not observe a current of 5000A. It may still be a very well regulated voltage source :)

My bicycle charger begins charging 5 cells (6V) at 7km/h. Voltage from the generator goes up to 40V if there is no load. That is why I wrote " No worry about voltage, just connect some cells in series and they will be charged."

Thanks for that link. The figure below, taken from that article shows the output voltage (presumably RMS AC) versus cycling speed relationship for SON generator. This is exactly what I wanted the OP to investigate -- but hopefully do a better job.

According to the figure, below about 10 km/hr, the output voltage is not high enough to charge a 6V battery. But since the author forgot to specify the generator load, the graph is useless.

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