Calculating power bank needs

Hi all

Hoping for some help working out what size power bank I would need for a range of 300-400km audaxes in coming weeks.

These will take 12-20 hours, including time in darkness and with minimal opportunity to recharge.

Tips and experience much appreciated! I’ve never really had to think about this and don’t know where to start. Would like to minimise weight and clutter in baggage as much as possible, for obvious reasons.

Will be running:

  • Exposure Trace front/rear as “be seen” lights (all ride - USB-A)
  • Lezyne mega drive “to see” light - USB-A
  • Varia RTL515 and UT-800 - USB-A
  • iPhone - USB-A to lightning
  • Shokz for poscasts when lonely - USB-A
  • some battery left over for mental reassurance!

I would expect everything to need at least one charge. I’ve got a 26k mAh Anker power bank with three USB-A ports, but this feels like massive heavy overkill?

I have an Anker battery with 10k capacity, and I find that plenty. I can charge my iPhone more than twice, for example. It only has two USB plugs, though.

What I would do is figure out which devices need to be recharged across the day.


  • Newer, in particular larger iPhones have rather long battery life, especially once you put them in battery saver mode. Do you really need to recharge it or can you make do with a single charge.
  • There are battery packs for iPhones, too. They attach magnetically and at least Apple’s is relatively small and unobtrusive.


  • I reckon you will need to recharge your Varia. Mine lasts 5–7 hours on a single charge, I think, and you want a radar running for the entire ride.
  • Which of your lights actually needs recharging?
  • You might fare better with replacement batteries/lights. I have a CatEye front light with a replaceable battery, for example. Other front lights feature external battery packs.
  • Rear lights are tiny, and you could just pack a spare instead. I would definitely pack a spare if I were you.

Other things you may need to charge

  • What about your head unit, will that run for 20 hours?

I have an iPhone 8. This is going to depend on phone signal and whether you put your phone in aeroplane mode or leave it with data turned on. If you leave it on as usual, then you may need to recharge your iPhone twice during a 400km event. Thus that’s 3,600mAh used.

The other stuff I don’t know as I run a dynamo front and AAA rear, which don’t need any external charging during audax events.

What are you running as “to see” lights? If it’s a UK audax you’ll be in unlit lanes and definitely want “to see” lights not just “be seen”.

All of the things should have information about what their internal battery capacity is (in mAh, most likely). For example, google says the Exposure lights Tracer is 850 mAh. Find that for all of them, think how often it will need charging, and add it all up. The total should be under the capacity of your power bank.

As said above, you might not need to charge everything. Your headphones for example - how long do you expect to listen to something, and could you live without it if they run out? Your big front light - how much riding do you expect to do in darkness, and how long does the light last? (But I’d make sure you have a lot of extra time available, in case tjings go wrong). As said above, do you really need to use your phone, or could you put it i to battery saver mode and leave in your pocket? How long does it last on a normal day?

I do a lot of Audax (Randonneur) riding and my suggestion would be to do a test at home to simulate your ride to see what you need.

Set your alarm for 3 AM and turn on the lights and electronics. Go back to bed.
Wake up a half hour after dawn and turn off your headlight. Plug it into a battery.
Periodically plug devices in during the day as needed.
When dusk approaches, turn the headlight back on. You get what I’m saying?

Personally, I’ve never once needed to recharge a phone (use airplane mode to save power) or wireless headset on a long ride, but I don’t use them except the 2 to 5 am yawning hours.

Also, be aware that most electronics do not allow you to charge the device while it’s running. Lights in particular can and will overheat when you’re doing this which will cause failure. Many Rando (Audax) riders who rely on Varia radars actually carry two of them so one can be used and the other charged. Likewise with battery based lights. Regarding the “be seen” lights, just mount three of them on the bike and use them sequentially.

After many years of managing batteries, I finally splurged and got a dynamo set up like @GoLongThenGoHome and have zero regrets about the purchase.


Super useful answers - thank you! I feel like I know enough now to plan properly