A question about Brompton dynamos

Sixlegs

Member
My Brompton came with the Shimano dynamo when I bought it second hand with the aim of touring and using it with public transport in my retirement (great plan at the time!). As I will almost never be riding at night, I have considered getting rid of the dynamo to save weight, but wonder how useful it might be for charging my phone once lockdowns allow touring again.

I will only be taking my iPhone, no computers or tablets etc, and mostly camping in out of the way places, so relying on being able to recharge in civilisation is not an option. I also have a decent power bank, which I understand is safer to use as a buffer between dynamo and phone.

My concern is the speed required to actually generate a reasonable amount of power. I am old, with dodgy knees and a loaded bike, so high speed is not going to happen, especially since I installed a 38/28 double chainring to save said knees on my steep local hills.

When I first started researching, I thought the Brimpton’s smaller wheels meant more revolutions = more electricity, however, I have since discovered the dynamo is not a standard model, but one made for the Brompton’s extra narrow axle.

So my question is: does this mean a smaller dynamo which = less power than on a standard axle, thus making higher speeds essential?

Is there anyone out there who has successfully used the Brompton dynamo for this, and if so, what charging system would you recommend? They all seem a bit expensive for experimentation.
 

Ming the Merciless

There is no mercy
Location
Inside my skull
It will be designed to produce similar outputs to other 3W dynamos. So it should work if you want to use it for charging. From flat it takes about 3 hours to fully charge your phone from the Dynamo. Don’t try and charge whilst using the light.
 

berlinonaut

Senior Member
Location
Berlin Germany
I'd disagree to a degree. There are five different dynamo hubs available for the Brompton, the Shimano, Shutter SP8 and SP9, Son Classic, Radial and Semiradial. Of the lot the Shimano is the by far heaviest and the by far lowest end one and the one that produces the highest amount of drag even when turned off. Regarding chargers there is a plenthora of different ones around that massively differ in terms of features, efficiency an price (starting from ~40€ to >200€). While you certainly can charge a phone when cycling relevant charging in most cases needs a speed of relevantly more than 15 kph, the more the better. If you are just around that mark or below it: Forget it. If you stop a lot (like in city traffic): forget it or at least get a charger with a buffer battery as many phones do not like start-stop charging and complain via a message that you have to tap on each time. When you are using your phone while cycling with the display turned on, i.e. for looking at a map you will be able to maintain the battery level or slightly enhance it. Some of the cheaper chargers will happily fry your phone when going downhill.
While charging via hub dynamo has been a relief for a couple of years today there are good power banks available and even huge and high quality ones are cheaper than even a cheap charger. So charging has become mainly a use case for longer unsupported touring (where you cannot charge a power bank at i.e. a campsite) or in case of trouble to gain a minimal level of battery when your phone comes flat unexpectedly.
I've been using a higher quality charger for a couple of years now and in recent years barely made use of it.

However, as you mention you hit the use case "away from civilization" a charger may be useful for your needs. In the German magazine "Fahrradzukunft" there is a series of very serious tests of those chargers that has been going on for more than 10 years now over a series of now eight articles, covering a lot of what's available out there and furthermore gives you an idea what to look for with a charger. You may be able to read those articles using Google translate:

First part
Second part
Third part
Forth part
Fifth part
Sixth part
Seventh part
Eighth part
 

u_i

Well-Known Member
Location
Michigan
I find dynamo with a charger immensely useful in the context of a phone used for navigation. A phone consumes quite a bit of power in such a mode and the dynamo prevents for me the quick discharge. The charger and cable are hidden on the bike, so I do not particularly worry about them when the bike is parked - with all the reservations in the context of a Brompton. The power bank is for me an awkward so solution, to be pursued if you remember about, if you have the bank charged, means to put it somewhere, etc., - with all this, it is normally equivalent for me to do not bother.
 

Tenkaykev

Veteran
Location
Poole
I find dynamo with a charger immensely useful in the context of a phone used for navigation. A phone consumes quite a bit of power in such a mode and the dynamo prevents for me the quick discharge. The charger and cable are hidden on the bike, so I do not particularly worry about them when the bike is parked - with all the reservations in the context of a Brompton. The power bank is for me an awkward so solution, to be pursued if you remember about, if you have the bank charged, means to put it somewhere, etc., - with all this, it is normally equivalent for me to do not bother.
If you are using the phone for navigation then it will use the least battery if you put it into “ Airplane Mode “ I used a navigation app which allowed map downloads so the maps were on the phone. I used a Samsung S7 with screen on permanently and got over 8 hours continuous use. My last phone was a Honor View 20 with even better battery life.
I use a power bank, it holds its charge for weeks, is quite compact and will charge my phone to full from empty twice over. The modern battery banks are reliable and brands such as Anker are tried and trusted.
 

u_i

Well-Known Member
Location
Michigan
If you are using the phone for navigation then it will use the least battery if you put it into “ Airplane Mode “ I used a navigation app which allowed map downloads so the maps were on the phone. I used a Samsung S7 with screen on permanently and got over 8 hours continuous use. My last phone was a Honor View 20 with even better battery life.
I use a power bank, it holds its charge for weeks, is quite compact and will charge my phone to full from empty twice over. The modern battery banks are reliable and brands such as Anker are tried and trusted.
I normally have downloaded maps for regions or countries and I even have whole globe on one of the apps as a fall-back - the last I looked 13GB or so. However most extra power consumption is from the screen being always on and the intense GPS. I also use voice directions there. My phone would run out of power in 3h or so without external power. The dynamo lets me run in practice forever - if I drain too much I just start switching off the screen on straight stretches. I have 700+ apps and 2 SIMs on the phone and the 'Airplane Mode' is the opposite of what I want to use the phone for. Anker and other dedicated banks is what I quit using several years ago - now exclusively 18650 in variety of uses including soldering iron, drill and devices running at different supply voltages in travel. I do use power banks when riding loan bikes, but do not want to bother on my own bike.
 

berlinonaut

Senior Member
Location
Berlin Germany
I normally have downloaded maps for regions or countries and I even have whole globe on one of the apps as a fall-back
You don't seem to trust your and your phone's navigation abilities too much - it's one thing to have a fallback, another to assume to navigate so badly wrong to need maps of the whole globe to compensate for it. :laugh: But honestly: Files on the phone do not consume energy.

However most extra power consumption is from the screen being always on and the intense GPS. I also use voice directions there. My phone would run out of power in 3h or so without external power. (..) I have 700+ apps and 2 SIMs on the phone and the 'Airplane Mode' is the opposite of what I want to use the phone for.
To be fair: To gain longer running times the easiest step ist to reduce energy consumption. Probably most people do not miss oder even prefer not to be "always on and fully connected" during a bike tour for recreational purposes. When you are in out in the woods you phone may consume much more energy than in urban areas just to keep the connection and if you are running a million apps constantly that will take its toll. The more with two SIM cards in parallel. As does the display. But each to his needs. Maybe a small atomic power plant on the rack may help? ^_^ The opposite strategy would i.e. be using Komoot for navigation with the screen turned off and using navigation instructions over audio/headset. That saves considerable amounts of battery.
In my case, suffering from the n+1 syndrome, I do own a bunch of bikes, most with a hub dynamo and some without. Therefor I do own a bunch of power banks of different capacity levels and use them in any combination that may seem appropriate. especially on the Brompton no issue to store them in the front bag while connection to a phone mounted on the bars. As most of my hub dynamos are SON ones I don't have the charger fixed on one bike but switch it between bikes, having added SON-style connectors to it and twin/piggyback-connectors on the dynamo side. As @rualexander says charging a flat phone (let alone other devices) considerably by pedaling takes a lot of time but certainly there are situations where it helps or where there is no alternative to it. Just that in recent years for me it has become more of a second line of defense than being the primary solution that it used to be a couple of years ago. In most cases I do not use constant navigation anyway but look on the map/phone from time to time to check the route and cycling happlily in between w/o electronic support in most cases.
 
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Ming the Merciless

There is no mercy
Location
Inside my skull
3 hours?! You must be cycling at some speed!
It would take me about 3 days touring to fully charge my phone by dynamo.
Not really that is averaging about 13-14 mph when touring. Maybe your phone has a mega battery.
 

berlinonaut

Senior Member
Location
Berlin Germany
It will be designed to produce similar outputs to other 3W dynamos. So it should work if you want to use it for charging. From flat it takes about 3 hours to fully charge your phone from the Dynamo. Don’t try and charge whilst using the light.
3 hours?! You must be cycling at some speed!
It would take me about 3 days touring to fully charge my phone by dynamo.
Not really that is averaging about 13-14 mph when touring. Maybe your phone has a mega battery.
13-14mph average in touringmode is pretty decent I'd guess and many if not most people will be below that mark. Additionaly: Your usecase may be charging a phone that may be turned off while the OP wants to charge a phone with it's screen permanently turned on an with GPS and cellular running permantently as well. Given the statement of the OP...
My concern is the speed required to actually generate a reasonable amount of power. I am old, with dodgy knees and a loaded bike, so high speed is not going to happen, especially since I installed a 38/28 double chainring to save said knees on my steep local hills.
... I'd guess that his average speed may be way lower, having in mind that the standard chainring on a Brompton is 50t and the lower factory option is 44t while he is running 38/28t. Assumably he is typically below the speed where relevant charging starts to happen.
 
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Tenkaykev

Veteran
Location
Poole
I had Komoot rides downloaded to phone, phone in "Airplane Mode " ( GPS is "passive, it just receives a satellite signal and overlays it on the map ) it doesn't use much power at all.
My test a couple of years ago:
Samsung Galaxy S7, IP 68 rated so waterproof. go for test ride using gps with maps updating on screen and screen permanently on and easily readable in daylight saw the battery last about 8 hours. With a small power
bank plugged in and charging the phone as I ride ( this works, I tried it ) 24 hours continuous should be attainable.
The S7 was spare phone I had after upgrading to a Honor View 20 with a much bigger screen but excellent battery life.
I've since moved to an iPhone 12 Mini so the Honor View 20 has now become the spare and I may well run the test again to satisfy my curiosity.
 

u_i

Well-Known Member
Location
Michigan
3 hours?! You must be cycling at some speed!
It would take me about 3 days touring to fully charge my phone by dynamo.
Well, the charging time is not hard to estimate. Let us take 4000mAh=4Ah battery as an example. At the nominal 3.6V, the energy inside is 4Ah*3.6V=14.4Wh. At 3W of the dynamo and 80% charging efficiency, this yields 14.4Wh/(3W*.8)=6h. That's an estimate with the issues being of the charger limiting the current, load that the charger represents to the dynamo, speed, lights on, etc. My practical experience is that when I use the navigation I am close to even, maybe draining a bit, but at the level that can be remedied. If the lights are on, charging from the dynamo still helps - the drain is faster, but usually good enough to get me to my destination.

GPS uses power in the context. The GPS data are constantly cross-checked against other location info to improve accuracy.

You don't seem to trust your and your phone's navigation abilities too much - it's one thing to have a fallback, another to assume to navigate so badly wrong to need maps of the whole globe to compensate for it. :laugh: But honestly: Files on the phone do not consume energy.

To be fair: To gain longer running times the easiest step ist to reduce energy consumption. Probably most people do not miss oder even prefer not to be "always on and fully connected" during a bike tour for recreational purposes. When you are in out in the woods you phone may consume much more energy than in urban areas just to keep the connection and if you are running a million apps constantly that will take its toll. The more with two SIM cards in parallel. As does the display. But each to his needs. Maybe a small atomic power plant on the rack may help? ^_^
I have been talking about some of the 'been there, done that', though not all. The beauty of the GPS and phone, in combination with the bike, is that you can be thrown into a region that you have never been to, culture and language group you have never encountered and within half an hour of getting to the base you can get out and move around as you if you lived there. The connectivity is, though, essential there and, if you are not careful enough, you can even lose your life. Yes, I will err there on staying away from the 'Airplane Mode'. As I normally travel in the context of work and must be connected to be accessible, there is even not much discuss for me.

The opposite strategy would i.e. be using Komoot for navigation with the screen turned off and using navigation instructions over audio/headset. That saves considerable amounts of battery.
Different people have different strategies. For bike navigation, I rely mostly on OsmAnd as it has right combination of bike tailored features and works basically anywhere. For maps of the world I use another, actually a truck oriented app, where one keystroke updates the whole world. For easy areas I may use Google Maps. For OsmAnd I end up downloading maps region by region, with all the details that are available. However, at times you find that you only have a mobile connection and bad in addition.

In my case, suffering from the n+1 syndrome, I do own a bunch of bikes, most with a hub dynamo and some without. Therefor I do own a bunch of power banks of different capacity levels and use them in any combination that may seem appropriate. especially on the Brompton no issue to store them in the front bag while connection to a phone mounted on the bars. As most of my hub dynamos are SON ones I don't have the charger fixed on one bike but switch it between bikes, having added SON-style connectors to it and twin/piggyback-connectors on the dynamo side. As @rualexander says charging a flat phone (let alone other devices) considerably by pedaling takes a lot of time but certainly there are situations where it helps or where there is no alternative to it. Just that in recent years for me it has become more of a second line of defense than being the primary solution that it used to be a couple of years ago. In most cases I do not use constant navigation anyway but look on the map/phone from time to time to check the route and cycling happlily in between w/o electronic support in most cases.
I have SON on one bike and I put Shimano on all other. For loan bikes, I carry a 18650 headlamp + rear. I find all the dedicated power banks a complete waste as they have a narrow purpose and became outdated or lose capacity and get discarded. Cells can be replaced one by one and can be used for multiple purposes.
 
Well, the charging time is not hard to estimate. Let us take 4000mAh=4Ah battery as an example. At the nominal 3.6V, the energy inside is 4Ah*3.6V=14.4Wh. At 3W of the dynamo and 80% charging efficiency, this yields 14.4Wh/(3W*.8)=6h......


Thanks, but I can do the sums.
But the sums don't ride a loaded touring bike in varying terrain, often involving riding uphill for several hours a day where the speed doesn't get much above 7-10kph.
 

shingwell

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