All About Buffer Batteries With Pass-Through Charging For Dynamo Hub Systems

If you’re keen on charging electronics while you ride, you’re probably already aware of buffer batteries.

These special batteries provide continuous power to your devices during times when your speed is too low for USB charging, for example, when cycling on hilly terrain, or when stopped at traffic lights. This prevents your devices from receiving then losing charge (eg. smartphones) or switching themselves off entirely (Garmin eTrex models).

We’ll be taking a closer look at buffer batteries, battery storage, charging times and some dynamo chargers with in-built batteries in this article.

dynamo hub cache batteries
USB chargers are best used in conjunction with buffer batteries in order to supply a constant charge to your devices.

Pass-Through Batteries

Most batteries will either charge OR send their stored power to your device; rarely will they do both simultaneously.

Pass-through batteries are able to deliver power from your hub straight to your device, while also taking a charge if there’s any power leftover.

The reason many battery manufacturers don’t allow pass-through is that it can deteriorate batteries quickly, although you can still expect a minimum of 500 total charge/discharge cycles with most pass-through batteries (two half charges equate to one full cycle).

It’s worth noting that many manufacturers include “pass-through” as a feature on the battery spec sheet, but in my experience, this is rarely the case.

Battery Storage Losses

When power is stored in a battery, it experiences a storage loss of 15-20%. In order to minimise this loss, it is best to try to charge your electronics either straight from the USB charger or by using a pass-through battery. It’s nice to have that power on tap, but when it means you lose 1-hour of power per 5-hour ride… it adds up.

Lipstick Buffer Batteries

Batteries are getting smarter circuitry in order to reduce long-term deterioration. But not all batteries are smart!

Jesse Carlsson and Sarah Hammond use the cheapest lipstick batteries they can find to keep a constant stream of power running to their Garmin eTrex GPS units. It’s not guaranteed that the cheapest batteries will be ‘dumb’ enough to allow pass-through, but the odds are good and given they’re less than $10 – it’s worth giving them a go.

Check out Jesse and Sarah’s video on how to waterproof these cheap buffer batteries in their video HERE.

Variable Current Buffer Batteries

hub dynamo cache batteries
The Cinq Smart Power Pack II.

If lipstick batteries are dumb, variable current buffer batteries are smart!

A current limiting buffer battery is designed for dynamo hub charging, allowing you to switch between set charging currents (eg. 100, 400 or 900mA) for different riding speeds.

The main advantage is that you can optimise your charging current so that your battery can draw a charge at really low speeds (eg. 6-8km/h). This kind of feature is also handy for charging with solar panels in low-light conditions.

If you’re riding at higher speeds, however, these batteries are less useful because they limit your charging potential to 4.5-watts (many modern dynamo chargers will charge at between 5 and 7-watts once you hit 20kph).

You can see the power graphs for different dynamo USB chargers HERE.

Soshine E4S Battery Kit

This buffer battery kit has been recommended by Velogical, who make rim dynamo charging systems.

The E4S requires 2x 18650 Li-ion batteries (sold separately) to complete the kit. Velogical recommends Samsung INR 18650-29E batteries.

Make sure to pre-charge the buffer batteries using a regular USB charger with a type-B plug.

How Long Will A Battery Take To Charge?

To understand this section well, you may need to learn about volts, amps and watts HERE.

A 3-watt dynamo hub will put out an average of around 6-volts at 500mA. Once the power has been converted from AC to DC the output will translate to roughly 5V and 500mA (or 2.5 watts), which for an hour of riding is 500mAh. Charging a 3000mAh battery from scratch will, therefore, equate to 6 hours of riding at 12km/h with the most powerful USB chargers, and 20km/h with other USB chargers.

However, there are also small losses in the charging circuitry, perhaps around 20% of the total generated power. Adding the 20% extra ride time (6 hours + (6 hours x 0.2)) and we’re looking at 7.2 hours of riding to fill the battery from scratch.

You can also use the above assumptions when determining how long it will take to fill a phone battery. My Apple iPhone SE has a ~1500mAh battery, so at 500mA per hour, it should take 3 hours. Adding in the circuitry losses and I’m looking at 3.6 hours of riding to fill it up. Other modern smartphones like the Samsung S8 will have 3000mAh batteries, resulting in 7 hour + charge times.

Having said all that, there are products which use special capacitors to achieve twice the power, charging smartphones in literally half the time. Many of them also come with build-in buffer batteries. Check out the Cinq Plug5 Plus and Forumslader V5 USB chargers which will achieve 4-5 watts power at just 17-20km/h.

Charging Regular Batteries

You can, of course, charge batteries without pass-through too!

Large capacity batteries (5000mAh+) are great for storing power but can sometimes be problematic to charge as they have more internal resistance in the battery cells. In order to overcome the resistance, bigger batteries require a higher input current (eg. 1A) which is easy for a wall outlet but harder for a dynamo hub at touring speeds. This is why many 5000mAh+ batteries don’t charge particularly well from dynamo hubs.

That said, if you’re cycling at speeds over 25km/h, or you use one of the powerful USB chargers, you can sometimes charge batteries up to 10000mAh – but I’d still recommend 2x 5000mAh batteries instead.

Dynamo Hub Chargers With In-Built Pass-Through Batteries

dynamo usb charger
The Cinq Plug5 Plus has an integrated buffer battery.

As dynamo hub charging technology matures, we’re finding manufacturers fitting buffer batteries straight into their chargers. This simplifies the cabling, in particular, making for very neat charging solutions!

You can find integrated buffer batteries in the:
Cinq Plug5 Plus (1100mAh)
Forumslader Ahead (2100mAh)
NC-17 Appcon 3000 (3000mAh)
Lumi-Con P5 (5000mAh)
Busch & Muller USB Werk (300mAh)
ZZing (2000 or 2700mAh)

My full list of dynamo chargers is HERE.

Buffer Battery Recommendations

Soshine E4S 18650 x2 Kit / 5700mAh
Cinq Smart Power Pack II / 3000mAh / 85g – €99 on Cinq
Lipstick Battery (I haven’t tested this battery, but it should do the trick) / 2600mAh / $9.99 on Amazon

Igaro also keeps a list of batteries with pass-through (click the link for the latest):
Blitzwolf – BW-P3, BW-P5 (probably other models in this range)
Miller – ML202 V4
RavPower – Turbo+ RP-PB043
Thrunite – C2
Tomo – M2, M4 (probably other models in this range)
Blackweb – 20,000mAh (Walmart, Adsa)

Check Out Some Of My Other Resources On Dynamo Systems, Dynamo Lights and USB Chargers

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