A question about Brompton dynamos

[berlinonaut]

To those who have done this: do you use anything (charging device/circuit) between the dynamo and the phone/battery pack?

Obviously yes, these are the "chargers" we have been talking about in this thread.

What happens when the battery voltage of thd device is greater than the dynamo output voltage... does the dynamo then become a motor, and the battery attempt to drive it? (Just a diode would stop this, but that would drop 0.6 volts which could make a big difference to the charging efficiency, USB is 5V, dynamos are nominally 6V.)

- If the dynamo output is too low nothing happens.
- If the dynamo output is ok charging happens and the charger maintains a certain and safe current and wattage to load the phone or powerpack, amount depending on the dynamo output (which depends from your speed) and the losses in the charger.
- If the dynamo output is too high better chargers limit the current/amount of electric power while some of the cheapest ones may simply fry your phone.

One of the issues is that many phones do not like the idea of charging being constantly switched on and off (like it happens in city traffic) and bombard you with a pop up message on the phone's screen each time that you have to tap "ok" to start charging. Which is annoying and means that you have to have your phone within reach and you have to be able to see the display to allow reliable charging.
To avoid a lot of the niggles you could charge a powerbank, acting as a buffer battery, that then charges your phone. Two troubles with this approach: For one losses in dynamo, charger and powerbank aggregate, so you will end up with way less charging power in comparison to charging the phone directly. Secondly only a fraction of powerbanks is able to be charged itself and charge a device (like your phone) at the same time. Some do exist but they are hard to find. So in most cases you have to charge the powerbank first and then only later are able to charge your phone. As charging a bigger power bank via dynamo is close to impossible a small powerbank will do for this purpose. Some of the better chargers (like i.e. the Forumslader) do offer an included buffer battery as standard or optional and with some this buffer battery may even be "precharged" at home.

Maybe have a look at the articles that linked to earlier in this thread to get an idea what is possible, what is not, what constraints and issues there are and what the advantages and disadvantages of the different chargers are for different usage scenarios. What you better should not do is to connect your dynamo hub directly to the power-input of your phone w/o a charger in between. This will not end up well.

 

[shingwell]

Ah ok thank you, rather naively I didn't realise this is a sorted problem with commercial products etc. I am an electronics engineer so I was thinking in terms of cutting wires and wiring things up, not buying something ready made to the job!

I actually do the opposite: I run dynamo lights from a rechargable USB battery pack on one of my bikes, works for the good makes of light (mostly German) but sometimes not for the cheap makes (mostly Chinese).

 

[berlinonaut]

I didn't realise this is a sorted problem with commercial products

Yup, for more than ten years already. Still there are new features, developments and products. The issue is probably more complex than it seems on first look, else the price and quality of the products would not differ so vastly.

I am an electronics engineer so I was thinking in terms of cutting wires and wiring things up, not buying something ready made to the job!

In this case I'd assume this is for you: http://www.forumslader.de/automatiklader/ - readiliy developed high quality charger (one of the best one can buy) with the opportunity to build it yourself but you have to be able to deal with a soldering iron. Complete variants are also available, but for an electorinics engineer there should be no need for that. Here the Englisch translation of the building manual: http://www.forumslader.de/fileadmin...in/English_Assembly_ForumsladerV5_kompakt.pdf

 

[u_i]

Ah ok thank you, rather naively I didn't realise this is a sorted problem with commercial products etc. I am an electronics engineer so I was thinking in terms of cutting wires and wiring things up, not buying something ready made to the job!

As I have built a system myself that seamlessly integrates dynamo power with backup pack charging, light operation and other power needs on the bike, I can assure that developing such a system on one's own makes absolutely no economic sense. It has been fun and maybe my system is the most efficient there is, it also cost me a year or more of solid evenings and plenty of components for trials. In this context, paying $200 for a market-proven charger is actually quite cheap . As lights consume now less power than when I started, the systems do not need be maximally efficient to be good enough.

 

[Sixlegs]

As I have built a system myself that seamlessly integrates dynamo power with backup pack charging, light operation and other power needs on the bike, I can assure that developing such a system on one's own makes absolutely no economic sense. It has been fun and maybe my system is the most efficient there is, it also cost me a year or more of solid evenings and plenty of components for trials. In this context, paying $200 for a market-proven charger is actually quite cheap . As lights consume now less power than when I started, the systems do not need be maximally efficient to be good enough.

My thanks to everyone who has replied to the question. A lot of useful information gained. It seems average speed is the crucial part of this equation, and as I have not yet been on a loaded tour, I don't know what that will be. I very much doubt it will be above 10mph. I will be doing a few short trips to begin with once the lockdowns allow which should give me an idea of what to expect. Think I will start off with just the power banks before making a decision on dynamo charging.

I am happy to use the phone in airplane mode, just checking the maps occasionally, also use paper maps and compass. The phone would be more for finding quiet cycle routes and traffic-free trails and camp sites. I assume it is going to be a lot more difficult to charge in cafes and pubs during this year at least, with so much takeaway only service and so many closing down completely.

 

[rualexander]

Don't dismiss the usefulness of solar charging, if the weather is reasonably decent, even in the UK.
Some of the modern panels do a pretty good job.

 

[berlinonaut]

Don't dismiss the usefulness of solar charging, if the weather is reasonably decent, even in the UK.

Rain charging would be good, especially in the UK.

 

[shingwell]

Rain proofing is an issue with USB connections. On my big bike with USB-powered lights I have a little top bar bag which contains all the USB connectors etc as well as the battery pack. But space for a spare bag is a bit more problematic on the B. But I have got a bag that fits within the M bars...hmmm....

 

[berlinonaut]

It seems average speed is the crucial part of this equation, and as I have not yet been on a loaded tour, I don't know what that will be. I very much doubt it will be above 10mph.(...) Think I will start off with just the power banks before making a decision on dynamo charging.

Possibly for your needs the Igaro D2 may be worth a look once/in case you decide for a loader. It is a UK product (so no import hassles) and it seems to be one of the best in terms of relevant power at lower speeds, judging from the latest in the series of articles linked earlier. The price is however in the higher regions and it offers USB-C only which is clearly an advantage in terms of future-proofness but may require an adaptor to use it with your current phone. Alternatively the Forumslader with buffer battery ( also mentioned earlier) may be a valid alternative - overall one of the best chargers in my opinion and with a long track record of happy users. The Pedalcell does not make use of the hub dynamo but as a consequence profits from the smaller Brompton wheels that spin faster than normal 28" wheels, so possibly also a possible way to go (though even more expensive).

 

[rogerzilla]

BIke dynamos are actually magnetos, and try to push a constant current, letting the voltage vary. The rated power of 3W at 6V is reached at quite a low speed (the hefty Shimano hub works at REALLY low speed and may have more poles than necessary for a 16" wheel), and the voltage goes on increasing if you go faster than that point. With hub dynamos the voltage can get quite out of hand because they don't saturate (self-limit) as much as bottle dynamos. All modern headlights have circuitry to dump this excess power and keep the voltage at no more than 6V. However, other devices can take more than 3W if the hub supplies it. 6W is certainly possible with the right load; it used to be common to run two 3W halogen lamps in series from a nominally 3W hub. You needed to be going briskly to get both lamps at full brightness.