Charging GPSes and phones on long rides

I often see comments on forums and Facebook discussing the merits of this battery pack or that dynamo setup for keeping GPS devices and phones juiced up on long rides, mostly audaxes, taking 10 hours or longer.  I haven't yet found a standard write-up of what's good and what's not, though, so I thought I'd sketch one out.

Some battery packs of various capacities, along with my trusted devices

Some battery packs of various capacities, along with my trusted devices

I've divided this into charging everything using just battery packs, charging everything using just a dynamo, and charging everything using a combination of both. 

If you're not interested in the detail then go straight to the conclusion for a summary, which basically says that if you can charge from dynamo then take one or two small 3200mAh batteries and keep them topped up during the day, and charge devices at night from the batteries for as long as you need.  Or if you cannot charge from dynamo then you only need one or two small 3200mAh batteries for anything up to about 400km; you only need the biggest and heaviest batteries for 600s and longer.  Although it all depends on what you want to charge and how much power they need.

Take it or leave it — but do so at your own risk, this is just my personal insight, it could be incorrect, you decide (by trying it out and letting me know the results

I'll be updating this article with test data as I collect it (which takes time) — consider this a long-term test and re-visit for updates.  Note that this article was extensively rewritten on 19 April 2016.

The basic charging requirements of cyclists

This article is interested only in charging electronical equipment used on very long bike rides, particularly audaxes, but the principles would be similar for other types of riding.

It would be easy to recommend some huge, 50,000mAh battery as the be-all-and-end-all dog's danglies charging solution.  However, such a battery would weigh a kilogram or more, which is anathema to most cyclists.

Instead, most cyclists want to take just enough weight and size, in terms of batteries or other equipment, to keep everything topped up — less weight and smaller size is good; more weight and bigger size is bad.

Most cyclists only want to charge a few types of equipment:

  • GPS devices, including Garmins and the like
  • Mobile phones
  • USB batteries (when using a dynamo)
  • Perhaps the odd MP3 player

There are inevitably some cyclists with different requirements, but broadly speaking everyone fits into the above needs.

In order to work out how much power is needed overall, first we need to determine how much power each device needs.

By some convoluted jiggery pokery and experiential data, I posit that the following devices have the following power needs:

  • Garmin Edge 500 — 85mAh (1000mAh battery gives 12 hours runtime — thanks, Phil)
  • Garmin Edge 800 — 85mAh (1000mAh battery gives 12 hours runtime)
  • Garmin Edge 1000 — 125mAh (1200mAh battery gives 10 hours runtime)
  • iPhone 5s — 100mAh (1500mAh battery gives 15 hours runtime when used as an MP3 player — I could probably get more life out of it if I switched the radios off)
  • Samsung Galaxy Note 2 — 300mAh (3100mAh battery gives 10 hours estimated runtime — thanks, Richard)

Phones can usually be sidelined from the charging requirement without affecting the ride, i.e. switched off or set to low-power modes, except where they're being used for navigation.  Sometimes prioritising power needs can be the difference between completing a ride or just getting hopelessly lost, it depends on your preferred navigation methods.

If you have a different device and want to determine its power requirement, start by finding the battery size online somewhere, which will be given in mAh (or Ah) and will usually be in the range 1000mAh to 2500mAh.  Then divide that number by the number of hours you typically get from a single full charge and that gives you the current draw.  So a device with a 2400mAh battery that lasts eight hours has a current draw of 300mAh (2400/8 = 300), which would be a bit profligate, shall we sayrolls eyes

Power sources to charge from

There are three basic power sources on the bike, each with its own pros and cons:

  • Batteries, typically 5V 1A USB batteries
  • Dynamo-driven USB sockets
  • Solar panels


These days batteries are cheap and readily available, including many from Amazon.  Capacities vary from 3000mAh to 27,000mAh and above. 

The svelte Vinsic Tulip 3200mAh USB battery pack — just 80g

The svelte Vinsic Tulip 3200mAh USB battery pack — just 80g

My benchmark battery for both compactness and usability is the Vinsic Tulip 3200mAh battery — and only 80g!  Jackery makes a similar battery, which looks nicer, but is larger and not better.  Although 3200mAh sounds like not that much when compared to some of the very large 20,000mAh and larger units, it's big enough for many needs and an awful lot lighter!

I specifically recommend the Vinsic Tulip because of its smallest size — when packing small then every millimetre makes a difference, and I found the extra 8mm of the Jackery prevented me from using it on my other bike a couple of weeks ago, whereas the Tulip just fitted.  Otherwise, everything else is much the same.

Pros include the fact you know how much energy you have and you can use it whenever you need it, including when stopped, or when other sources are needed for other tasks, such as at night.  Batteries are also capable of delivering full 1A or 2A (depending on the battery and device) charging most of the time, even when stopped or asleep.

The big con is that a high-capacity battery is big and heavy, which is counter to the needs of the cyclist.  Another negative is that once a battery is discharged then that's it, unless you can recharge it somehow.  Both of these cons beg the question about just how big a battery you need, which is covered later.


Technology has moved on and dynamos are better than ever, with the likes of Schmidt building ultra-reliable, although expensive, hubs, and Shutter Precision building competitively-priced, new-tech hubs with longevity yet to be proved (mine's been good for 10,000km so far and counting).

The new-tech Shutter Precision SP-PV8 dynamo hub — they do a disc version, too

The new-tech Shutter Precision SP-PV8 dynamo hub — they do a disc version, too

The advantage of dynamo power is that it's always there, while you're riding. 

The disadvantages are that if you stop then there's no power being developed.  And at night the dynamo is usually needed to run the lights, so charging at the same time reduces the brightness and you can't see as well.  Also, the faster you ride, the more power is available to charge with — more pertinent is that if you don't ride fast then you won't get full charging power.  You may also need an additional USB-charging module, like the Busch & Müller e-Werk, if your lamp doesn't provide this facility built-in.

The excellent Busch & Müller Lumotec IQ2 Luxos U headlamp with USB charging

The excellent Busch & Müller Lumotec IQ2 Luxos U headlamp with USB charging

I use the game-changing Busch & Müller Lumotec IQ2 Luxos U dynamo light, which includes the cables to charge devices over USB, as well as being a terrific light with a very even and wide beam that's friendly to other road users.

Solar power

Solar is an interesting solution, if you're riding in a sunny country.  However, this should really be seen as something for touring-types, and probably unsuitable for audaxians.

Advantages include: power is free, when the sun's out.

Disadvantages include: when the sun's not out then there's no power; it's extra weight; it has to be attached somewhere on the outside of your luggage; and power output is somewhat limited.

Charging strategies 

Keeping devices charged up on long rides is harder than it sounds: the longer the ride, the tireder you become and fine-tuned strategies are nearly impossible to maintain.  What's needed is simplicity.

Let's start by discounting solar as a strategy: it's suitable for touring and camping, but not for any quicker form of cycling.  Nothing more to be said about it … but, FWIW, on a sunny day in the UK you can expect up to 500mAh from a small solar panel.

This leaves battery charging, dynamo charging, and a combination of the two.

A note about charging batteries

It's perhaps useful to mention the process of charging Li-ion batteries at this point.  Li-ion batteries take a good amount of charge very quickly before tapering off; it then takes a long time to saturate the battery with energy.  There's a lot of worryingly fascinating information on all this at the excellent Battery University, including this fancy-pantsy graph, which I've simplified:

A charging graph showing the different charging stages

A charging graph showing the different charging stages

The bit we're interested in is stages 1 and 2: you can clearly see that it takes 90 minutes or so in the graph to provide the first stage, called constant-current charge, which is the first 80-90% of charge.  It then takes a further twice as long again to provide the saturation charge, bringing the battery to 100% of charge.  The third stage is just to indicate that you have to stop charging a Li-ion battery before it explodes on you and most Li-ion batteries have built-in protective circuits to prevent such an occurrence.  Texas Instruments also published an interesting paper on the subject.

The truth is that it takes longer to saturate the final 10-20% of a Li-ion battery, such as those found in almost all electronic devices, than it does the first 80-90%, so it can pay dividends to stop before devices are fully charged and use that charge more efficiently charging something else.

What's really interesting is that Li-ion batteries hate to be fully charged, so avoiding a saturation charge prolongs the life of the battery.  It's also relatively quick to do the initial constant-current charge — less than five hours for a 3200mAh Vinsic battery at 0.5A average current, which gives 80% or 2500mAh charge.

It's worth bearing that in mind.

Charging from batteries

Many cyclists don't have access to dynamo power, so their only real option is to use battery power.  This means calculating how much power they'll need and carrying at least enough stored power to recharge their chosen devices for the duration of the ride.

I usually ride with a Garmin Edge 1000 and iPhone 5s.  If I start the ride with both fully charged then I will need to charge the Garmin after ten hours and the iPhone after 15 hours:

  • for a 14-hour ride, i.e. a 200km brevet, then I need to charge the Garmin once and that's it — the smallest-capacity battery would suffice, since I'd need a top-up of about 1200mAh;
  • for a 20-hour ride, 300km brevet, I'd need to charge the Garmin and the phone once each, again a single 3200mAh battery would do;
  • for a 28-hour, 400km brevet, I'd need to charge the Garmin twice and the phone once — I would need two batteries for this (one-and-a-bit, actually), since I'd need 2×1200 + 1500 = 3700mAh, but if I let the phone die then I could get away with just one battery.
  • for a 40-hour, 600km brevet I'd need to charge the Garmin three times and the phone twice:  3×1200 + 2×1500 = 6600mAh, which is near the limit of two batteries, although I could let the phone die and then I'd need just one-and-a-bit.

You can work out your needs in this way with simple maths. However, I would always carry an extra 3200mAh battery, just in case circumstances on the ride end up consuming more juice than expected, leaving you short near the end!

Charging from the dynamo

In principle you should be able to charge your devices forever from the dynamo and never worry about power.  However, the truth is not quite so simple.  On paper the Shutter Precision SP-PV8 can provide 1A of charging current at 5V, which is 5W of power, but this is only achievable at very high speeds of 50kph or more!  The truth is that power output is related to speed in a non-linear fashion, as discussed in this excellent Cycling UK (formerly CTC) article comparing and assessing dynamos:

  • 2W @ 10kph
  • 3W @ 15kph
  • 3.5W @ 20kph
  • 4W @ 25kph
  • 5W @ 50kph

Coupled with that limitation, cyclists rarely maintain a constant speed for long, so only a proportion of the charging is done at any one power output.  As an exercise I looked at the "time spent at or above speed x" graph for one of my rides in RideWithGPS, and by considering the percentage of time during the entire ride, including stops, and totalling the proportional power output against time using the above values, I would've been averaging about 3.13W of power at 5V, which is about 625mA; given charging inefficiencies, this would be closer to 500mA or 0.5A — just half the claimed current.

On the basis of 0.5A, it should take:

  • 2 hours to charge my Garmin 800 or Phil's Garmin 500 (1000mAh battery)
  • 2.5 hours to charge my Garmin 1000 (1200mAh battery)
  • 3 hours to charge my iPhone (1500mAh)
  • 6 hours to charge Richard's profligate Galaxy Note 2

From experience, 2, 2.5, and 3 hours are about what I experience for a decent, if not quite saturated, charge of each of my own devices from the dynamo+light when the light is off.

Another way of looking at it would be "can my dynamo power my device all the time?"  During daylight conditions we can expect around 0.5A of current at 5V, but during the night this drops to perhaps less than half that.  Richard might have trouble keeping his Galaxy Note powered up during long winter nights, but otherwise he should be okay.  All other scenarios, including powering both my Garmin 1000 and my iPhone should be tenable with judicious swapping of cables during the day.

A combined charging strategy

By far the most effective strategy is to combine battery power with the dynamo, in order to harvest spare energy during the day for use whenever it's needed, but particularly during the night. 

Using just a single 3200mAh battery as a cache means that during a decent length of daytime riding it should be possible in my case to charge that battery (6 hours' charging), plus supply a decent charge to both my Garmin 1000 (2.5 hours) and my iPhone (3 hours) — that's about 12 hours' charging time, easily achievable for half the year, and more than enough to keep everything running constantly.  In fact, in summer I should be able to charge two 3200mAh batteries and charge both devices at least once between dawn and dusk, providing an additional level of insurance against loss of power.

The advantage of this is that the battery can be used to charge devices during the night, when charging directly from the dynamo compromises the amount of light available to ride by.  Also, battery charging provides 1A of charge current constantly, irrespective of how fast we're moving, so a quick recharge of 90 minutes or so is possible for all devices.  Finally, I listen to music on my iPhone, from my back pocket, so I can charge the iPhone from the battery in my back pocket while still listening to music.  Cool cool

Empirical data and real-world use cases 

It's important to confirm the very-simplified maths and assumptions I've used above with real experimental data.  If you have anything you can share to enhance this section, please email me.

Me, my Garmin and my iPhone 

If I try to keep both my Garmin and my iPhone charged then a single battery is good up to 300km and two batteries are good up to 600km if I let the iPhone die.

For PBP, which is about 60 hours' riding or 90 hours elapsed then 20,000mAh of battery would be required to keep my two devices topped up.

Alternatively, everything could be kept topped up with the dynamo.  Better still, a Vinsic as a cache battery to get me through the night, all charged from the dynamo.

Note that I do notice a distinct drop in battery life when I use the map screen on my Garmin 800, and I presume the same would be the case with my 1000.  This is undoubtedly due to the processor-intensive process of scrolling the map, pixel by pixel, as I ride across the landscape — yesterday the 800 died after about eight hours of riding, about two-thirds of what I was expecting!  This must be accounted for if you're going to use these numbers.

Keeping a smartphone charged, when used for GPS duties

There's no denying that a GPS designed for the job is much more efficient, energy-wise, than a general-purpose pocket computer — a smartphone — seconded to the task.  GPS devices are designed to be weatherproof, easy to use while riding, easy to attach to the bike, compatible with lots of sensors, and reasonably energy efficient.  Phones aren't any of those things (although weatherproofing is getting better, Apple phones excepted).

When a phone is used as a GPS then its energy requirements are reasonably severe.  The obvious culprit is the screen, which is often huge, far larger than actually required to navigate on an audax, and the brightness must be set to high to use on a sunny day, representing a massive energy draw.  Also, the navigation and tracking apps are often inefficient in the way they work, draining the battery even quicker, often requiring a data connection, which means keeping the phone connected to the network, and in hard-to-reach places that causes its own power issues.

Going by Richard's estimates for his Galaxy Note 2 he used about 9000mAh for 32 hours riding and 6000mAh for 19 hours riding, which approximates to about 3000mAh for 10 hours riding, or 300mAh per hour, i.e. 300mA current draw — that's nearly three times what my Garmin 1000 draws.  In fact it's more than all three of my devices put together! Note these are only estimates based on Richard's observations.

Simply put, by my estimate then Richard's 12,000mAh battery pack should last for about 40 hours, plus whatever charge was in the phone at the start — I think that is too close to marginal for a 600km ride and I would want to pack more juice, just to be sure.

When you consider that I would need only two little Vinsic batteries for an entire 600 to keep my Garmin 1000 going with plenty to spare (i.e. it's not marginal for me), or just one Vinsic to keep my Garmin 800 going (although it would get marginal towards the end!), then you can see why there's still very good reason to use a tool designed for the job.

If Richard had a dynamo from which he could keep his phone charged then he should be able to keep it topped up, so long as he kept moving and charged either the Note or a battery all day long, but it could be a close-run thing in winter!

Running a just Garmin Edge 500 (switching the phone off)

Phil mentioned on Facebook that he used two 3200mAh USB batteries to keep his Garmin 500 charged for the duration of London-Edinburgh-London.  He said he got nearly three full charges from each battery and the Garmin ran for about 12 hours between charges.  Phil saved energy by switching the 500 off when he was stopped at controls. 

For an average moving speed of something like 20kph on a 1400km ride, the Garmin was running for just 70 hours — he would've needed about six full discharges of his Garmin to give 72 hours runtime, and he had nearly seven full charges with him, including the one in the Garmin's battery itself, which was fully charged on the start line.

Phil's quite a quick rider, so he actually rode for less than 70 hours, otherwise I consider this use case a bit marginal and I would take a third battery just in case.

Interestingly, Phil switched his phone off between controls, so it didn't require a charge at all for the duration of the event, the same on Paris-Brest-Paris — this can be an important strategy on very long rides, making sure you make best use of the energy available and save it not squander it where you can.

Charging on the bike

Almost as soon as I posted this article I was asked how to charge on the bike — most specifically how to charge in the rain, but it makes sense to answer the whole question.  This is more about the mechanics rather than the chemistry of charging on the bike.

To charge while on the bike does take a bit of forethought and planning, and a few extra items purchased, but it pays in the long run, especially as you step up to longer rides with more unknowns.  I've been fine-tuning my approach to charging on the bike for several years and I'm still making tweaks and adjustments.

The most important point is that there are no singularly correct answers to this, it's all about what works for you.

The physicals of charging

Charging from the dynamo is typically easy: the power comes from the front wheel via an e-Werk, or from the headlight, and you just need a short cable to connect that to your GPS.  With most new charging systems, a waterproof-cable system is supplied, which means it's all there, ready to use, plug it in and start pedalling.

If you want to charge your phone, unless it's mounted on your handlebars then you're going to need to do something different.  I tend to charge my iPhone from a battery while it's in my back pocket so I can continue listening to it, and I use a very short cable to connect the two.  FWIW, I also pack the iPhone into a waterproof case, as it's not known for its aqua-resilience.

Charging batteries from the dynamo needs a bit more thought.  I usually have a top-tube bag on my bike, just behind the steerer, so I can put my battery in that and run the micro-USB cable to it easily enough for a few hours' charging (all the current batteries charge from micro-USB).  It makes getting to the Jelly Babies a bit harder, but I tend to not eat that much on the bike these days.  The top-tube bag is also good for charging from the battery: just swap the cable around and it's good to reach forwards to whatever's mounted on the handlebars to give it a top-up. 

For my Garmin Edge 1000, which has its USB socket on the bottom, I use a 30cm wire from StarTech with an elbow at the micro-USB end so that the cable points backwards, which is also really useful when charging the battery, as it all fits together better, more compactly, in the top-tube bag. 

For the Garmin Edge 800, which is mini-USB and has its charging port on the back face up against the handlebars, when using the Garmin OutFront mount at least, I use a short cable with a down-facing elbow, again from StarTech, but unfortunately not available in 30cm.

Café charging 

I find on short rides I don't need to charge from the dynamo at all, one 3200mAh battery is enough.  My approach to charging is then to plug the battery into the GPS at any decent-length café stop or control — 15-20 minutes isn't enough to fully charge it, but I usually just need a top-up to get me to the end and 15-20 is long enough for that.

If I'm on a longer ride then I use the café stop/control as an excuse to at least plug it in, even if I then leave it plugged in until the next control when I can unplug it all.  With confidence and a bit of forward planning, it's possible to do all the plugging in and unplugging while riding, although it requires confidence and good bike-handling skills, so it's definitely not for everyone.

Charging in the rain 

I got asked the question on Facebook about how to charge in the rain.  Note that generally charging in a bit of mizzle should be fine, we're talking about proper rain here.

The latest Garmin Edge 1000 models are all, apparently, waterproof enough to be charged in the rain, but they come with a warning that you must dry the USB socket out afterwards to prevent corrosion — which would prevent electrical conductivity, somewhat removing the ability to charge!  Earlier models, I seem to recall, suffered from poor charging in when the socket got wet, so charging in proper rain was a no-no.

To get around this, I use the café-charging technique, but I move the Garmin to my saddlebag, where I charge from the battery.  By doing this at every control, it should be enough to keep the Garmin juiced up.  In extreme conditions, I could leave the Garmin in the saddlebag and navigate using the routesheet for an hour or more to give it a decent charge — but you need to have the routesheet with you to do this, which I always do, but many riders rely solely on technology, and even I wouldn't do that!

Charging the battery from the dynamo in the rain is more straightforward: typically a top-tube bag is waterproof enough to run the cable into there and charge it in everything but a monsoon.  The charged battery can then be used to charge everything else in the saddlebag as required.

When things go wrong 

Things can and do go wrong — be prepared!

I was on The Flatlands 600km event in 2013 when the USB cable I was using failed — it seems that one of the wires in the cable snapped through repeated use.  I managed to borrow a cable off another rider, but you can never rely on this.  Now I carry two of everything: a long 30cm USB cable for charging normally, plus a short 10cm cable in case the long one fails — I can resort to café charging to get me to the end.

On Paris-Brest-Paris I did lose some of my recorded activity, because I hadn't managed to keep on top of charging the batteries.  I now prefer two or three small batteries to one large one, because it's more obvious when I'm tired as to how much charge I have and where.  A single, monolithic battery with just four charge-indicator LEDs is much harder to gauge than three small batteries with four LEDs each.

Sometimes catastrophic things happen: the next time I rode The Flatlands 600 route, this time as The Flatliner permanent event, my Garmin 800 packed up after about 340km.  I had a choice: to reset the Garmin and lose all my recorded track, or to ride to the end by routesheet.  In the end I couldn't reset it and I did lose the track, but I also did get to the end by routesheet.  Two points arise:

  • always carry the paper routesheet and be prepared to use it, it won't fail in the middle of nowhere
  • record your activity to the SD card, because it's recoverable from there even if the Garmin dies

I have had similar crashes on the Garmin since then, but I've retained most of the activity on the SD card, and my Garmin 1000 has never let me down in this way.


In summary, this report considers charging using just batteries, just dynamo, or a combination of both, the latter being the most reliable and workable for rides longer than 20 hours.

For the most common setup of a Garmin and a phone, using just battery power then a single 3200mAh battery should be good for 300km and three 3200mAh batteries (or instead, a single 10,000mAh battery) should be good for 600km, assuming that you start the ride with everything fully charged.  For LEL or PBP you would have to carry at least half a kilo of battery, unless you kept your phone switched off. 

In contrast, using a dynamo to recharge the devices, there are definite moments in time when keeping two or more devices charged that 'collisions' occur, in that you can only charge one or the other not both, and you don't want to charge while the lights are on at night.

The combined dynamo+battery approach is the most flexible strategy: a single 3200mAh battery can be used indefinitely in combination with recharging both the battery and the individual devices from the dynamo, at least for two devices — and I would suggest that for the sake of 80g then taking two 3200mAh batteries provides a degree of risk-management as it enables more energy to be captured and stored during the days, providing more flexibility, particularly through the nights.

Generally I find that multiple small batteries work better for me than one large one, but you'd have to suck it and see for yourself to find what works for you.  Once you're having to carry more than 10,000mAh then a single large battery is noticeably lighter than the equivalent multiple, smaller ones.

At the end of the day you want to take just enough battery power with you to finish the ride, a little bit extra to give you wiggle room, and no more than that, because every battery increases the weight, takes up room in the bag, and makes it harder to keep track of which one's fully charged and which one needs a charge (the LEDs on the batteries help, but it's more to think about when you're tired).

Nick Wilkinson

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