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Modified power management (LiOn instead of NiMH)
I'll add my experience from powering an automatic shifter off a front hub dynamo for the past few years. I’m not recommending you do it this way, may not be safe so do so at your own risk. I should also make it clear that in my case the dynamo has never made it 100% self sufficient so little risk of overcharging the battery. Depending on the shift resistance and type of riding it can do anything between a few hundred and several thousand miles between charges.

The original shifter was powered off a 3 cell lipo (11.1V rated, 12.6V fully charged) from a powerbank. The Arduino directly connected via the VIN pin and the servo with a servo specific step down converter. The battery pack did have a battery protection board which I kept. When I added a front dynamo I connected it via nothing more than a full bridge rectifier.

I shortly after upgraded the circuity and changed the battery to 2 x 18650s ( 7.4V rated, 8.4V fully charged, around 1.5Ah capacity left) savaged from old laptop batteries. Making sure to use a 2 cell battery protection board (under/ over volt protection only) but again with dynamo hub connected directly via only a full bridge rectifier.

On my latest auto shift road bike I am using a high voltage servo rated 8.4V which can be connected directly to the 2 cells. Simplifies things doing away with the regulator, less to go wrong.

I am aware of circuit ideas to boost the output of a dynamo hubs but keeping things simple it brings up an interesting question, what is the optimum battery voltage to charge?

The dynamo I have puts out approximately 1V per mph. A battery won’t start charging until the dynamo voltage is greater than it plus any rectifier voltage drop. Bicycle dynamos have very good current limiting, usually around 300 – 400mA.
Will give a couple of examples, a 12V battery will only charge at a speed over around 13mph but will if going fast enough (> approx. 17 – 18 mph) charge with a max power of around (12 x 0.4) 4.8 watts.
A single 18650 on the other hand will begin charging at about 5mph but with only a max power at (> approx. 10mph) of (4 x 0.4) 1.6W.
Obviously most people will spend more time riding at over 5mph than over 13mph but the power available with a higher voltage battery is far greater if going fast enough. What I’m getting at is there must be an optimum, yet to do the research myself on this one. I’ve settled with 2 cell li ion (7.4V) at the moment for convenience and reckon it's not far off optimum.

I have to add that lithium cells do need to be treated carefully but I’ve yet to have one get angry at me. I don’t use any balance charging with the dynamo and its not been an issue. I do recommend making sure the cells have similar capacity and have been in the same pack together if reusing old. As I do have to charge them occasionally with a proper charger they get checked for balance.
NiMH does seem like a far safer option and don’t see why they won’t charge just as well from a dynamo hub via a simple rectifier.

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RE: Modified power management (LiOn instead of NiMH) - by Ed_R - 10-04-2020, 02:51 PM

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