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

If you’re interested in charging electronics while riding, you’re probably already familiar with buffer batteries.

These small intermediary batteries provide a steady flow of power to your devices when your speed drops too low for consistent USB output – such as on steep climbs, or while stopped at traffic lights. This prevents devices from repeatedly gaining and losing charge (like smartphones) or shutting down completely, as can happen with some Garmin eTrex models.

In this article, we’ll take a closer look at buffer batteries, storage capacity, charging times, and dynamo chargers that feature integrated battery systems.

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 typical 3-watt dynamo hub generates around 6 volts at 500 mA. After converting from AC to DC, this translates to roughly 5 V at 500 mA, or 2.5 watts. Over an hour of riding, that delivers about 500 mAh of charge.

For example, charging a 3000 mAh battery from empty would require about 6 hours of riding at 12 km/h on the fastest dynamo USB chargers, or around 17 to 18 km/h with slower dynamo chargers. Accounting for small losses in the charging circuitry, typically around 20%, this extends the total ride time to roughly 7.2 hours to fully charge the battery.

The same calculations apply to phone batteries. An iPhone SE with a ~1500 mAh battery would take about 3 hours, or 3.6 hours including circuitry losses. Larger batteries, like the Samsung S8’s 3000 mAh unit, would need around 7 hours or more.

Some advanced chargers use special capacitors to effectively double the charging power, halving the time needed to charge smartphones. Many of these also include built-in buffer batteries. Notable examples include the Cinq Plug5 Plus and Forumslader V6 USB chargers, which can achieve 4 or 5 watts of output at speeds as low as 17 to 20 km/h.

Pass-Through Batteries

Most buffer batteries will either charge themselves or deliver stored power to your device—but rarely do both at the same time.

Pass-through batteries are designed to send power directly from your dynamo hub to your device while simultaneously storing any surplus energy. In theory, this allows for smoother, more efficient charging while riding.

The reason many manufacturers avoid true pass-through functionality is that it can accelerate battery wear. Even so, most pass-through batteries are still rated for at least 500 full charge cycles (with two half charges counting as one full cycle).

It’s also worth noting that while “pass-through” is often listed on spec sheets, in practice the feature doesn’t always function as expected.

Battery Storage Losses

When energy is stored in a battery, it typically incurs a 15 to 20% storage loss. To minimise this inefficiency, it’s best to charge your devices directly from your USB charger whenever possible, or use a genuine pass-through battery so the power flows straight to your device first.

While it’s reassuring to have stored energy in reserve, those losses can add up over time. Losing the equivalent of an hour of charge on a five-hour ride may not seem dramatic at first—but over days or weeks of riding, it becomes significant.

Lipstick Buffer Batteries

Modern batteries are increasingly equipped with smarter circuitry to manage charging and reduce long-term degradation. But not all batteries are created equal – and not all of them are especially “smart.”

Ultra-distance riders like Jesse Carlsson and Sarah Hammond have been known to use inexpensive “lipstick” power banks to maintain a steady power supply to their Garmin eTrex GPS units. While there’s no guarantee that the cheapest batteries will be “dumb” enough to allow continuous pass-through-style behaviour, the chances are often surprisingly good. And at under $10 each, they can be worth experimenting with.

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

Variable Current Buffer Batteries

If basic lipstick batteries could be considered “dumb,” then variable-current buffer batteries are definitely the smart alternative.

A current-limiting buffer battery is purpose-built for dynamo hub charging, allowing you to select between preset charge rates (for example, 100, 400, or 900mA) depending on your riding speed.

The key advantage is that you can optimise the charge draw at very low speeds (around 6 to 8 km/h) where standard USB charging often struggles. This feature can also be useful when charging from solar panels in low-light conditions, where available current is limited.

At higher speeds, however, these batteries can become a bottleneck. Because they cap the charge rate at around 4.5 watts, they may restrict the output of modern dynamo chargers, many of which are capable of delivering 5 to 7 watts once you’re riding at roughly 20 km/h.

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

Soshine E4S Battery Kit

This buffer battery kit comes recommended by Velogical, a company known for its rim dynamo charging systems.

The E4S kit requires two 18650 Li-ion batteries (sold separately) to function. Velogical specifically recommends Samsung INR 18650-29E cells for optimal performance.

Before use, be sure to pre-charge the batteries with a standard USB charger using a Type-B plug.

Charging Regular Batteries

You can, of course, charge power banks without pass-through functionality.

Large-capacity batteries (5000mAh and above) are excellent for storing energy, but they can be harder to charge from a dynamo hub. Their higher internal resistance means they often require a stronger input current—around 1A—to begin charging efficiently. That’s easy to supply from a wall outlet, but much more challenging for a dynamo hub running at typical touring speeds. As a result, many 5000mAh+ batteries don’t perform particularly well when paired with hub-powered USB chargers.

If you’re riding consistently above 25 km/h, or using one of the more powerful USB chargers, it’s sometimes possible to charge batteries up to 10,000mAh. Even so, I’d generally recommend carrying two 5000mAh batteries instead, as they tend to charge more reliably and offer greater flexibility.

Dynamo Chargers With Integrated Pass-Through Batteries

As dynamo hub charging technology has evolved, more manufacturers are integrating buffer batteries directly into their USB chargers. This reduces cabling complexity and results in a much cleaner, more streamlined charging setup on the bike.

Examples of chargers with integrated buffer batteries include:

• Cinq Plug6 Plus (1100 mAh)

• Forumslader (2100 or 3000 mAh)

• NC-17 Appcon 3000 (3000 mAh)

• Lumi-Con P5 (5000 mAh)

• Busch & Müller USB Werk (300 mAh)

• ZZing (2000 mAh or 2700 mAh)

These integrated systems can be a tidy and practical solution, particularly for riders looking to minimise external battery packs and wiring.

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