If you’re keen on charging electronics, you’re probably already aware of the option to incorporate dynamo hub cache batteries (or buffer batteries) into your dynamo system. As long as your cache battery has some charge, it can provide continuous power to your devices when your speed drops too low for USB charging, such as when climbing a steep hill or when stopping at traffic lights.
Cache batteries will also help you stay off-the-grid for a significant amount of time provided you ride enough, you aren’t a heavy electronics user and you have a little bit of power on reserve.
The best dynamo hub cache batteries have a couple of key characteristics which we’ll cover in this resource.
Most batteries will either charge up via USB OR send their stored power to your device; rarely will they do both simultaneously. But, there is a special battery feature that makes both possible with some battery models.
Pass-through is the most important requirement of dynamo hub cache batteries. With this feature, the battery can actually send a charge to a device with no loss in efficiency. If your speed drops too low, the battery storage will pick up the difference and keep a constant charge to your device. When your device is full, the battery will be able to charge itself back up.
The reason many battery manufacturers don’t allow pass-through is that it can deteriorate batteries a bit quicker than otherwise. That said, you can expect a minimum of 500 total charge/discharge cycles (empty to full) with most batteries. And yes, two half charges equate to one full cycle.
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 using the pass-through feature of a 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!
Variable Current Dynamo Hub Cache Batteries
Some batteries are available with a variable current. What does this all mean? Is it important?
Well, current increases with your speed. At really low speeds, a dynamo hub will make 100mA current for example, but at high speeds, it can make greater than 1A current. With ‘current limiting’ batteries, you’re able to switch between set charging currents (eg. 100, 450 or 900mA).
The main advantage is that you can charge your battery with a constant current (eg. 100mA) at really low speeds (eg. 8km/h). However, the charging rate at 8km/h is approximately 5x slower than if you are traveling at 20km/h, so you’re not getting all that much power at those speeds anyway.
At higher speeds, the current limiter puts a ceiling on how many watts can be produced (eg. 3 watts @ 900mA). If you are likely to ride at speeds exceeding 25km/h+ then you’re better off without any current limit.
How Long Will A Battery Take To Charge?
It’s not too hard to determine how fast a battery can charge, but we will need to make a few assumptions first.
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 (the USB 2.0 charging standard), which for an hour of riding is 500mAh. Charging a 5000mAh battery will, therefore, equate to 10 hours of riding. However, there are also small losses in the charging circuitry, perhaps around 20% of the total generated power. Adding the 20% extra ride time (10 hours + (10 hours x 0.2)) and we’re looking at 12 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 5S 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 almost twice as much power as discussed. There is a downside though: additional dynamo hub resistance. Check out the Forumslader V5 and Dynamo Harvester Plus USB chargers which can achieve 5 watts (1A) power at 20km/h.
The battery size that you choose will depend on what devices you’re planning on charging, as well as how far you’re cycling per day. With most USB chargers you’ll be able to yield between 2500-3000mAh per 100km. To give you an idea of battery sizes in devices, a GoPro battery is 1220mAh, a Kindle battery is 1400mAh and smartphones tend to be between 1500-3000mAh.
Larger capacity batteries can be problematic to charge as they have more internal resistance in the battery cells. This requires a higher current (eg. 1A) to overcome the resistance. That’s no worries for a wall outlet, but can be hard to achieve with a dynamo hub at normal touring speeds. This can be a reason why some batteries won’t charge from a dynamo hub. So in summary, go smaller where you can, because the battery will likely be better able to match the inflow of current.
If you’re not planning on using battery storage, you can get away with as little as ~2000mAh in your cache battery. Remember that you’ll need more capacity if you’re likely to be dipping below 15km/h often.
Pre-Charging Batteries and Long-Term Storage
I fill up my cache battery from a wall outlet when I can, allowing me to use more power per day than I generate. By knowing roughly how big the batteries are inside my devices, I can ration my power (if required) so that my battery is almost depleted by the next wall outlet.
Many battery manufacturers recommend storing batteries with a 75% charge. This is because the chemicals inside are less active when left unused. To extend a battery’s life, it’s a good idea to discharge and recharge it at least once every 3-6 months too.
Cache Battery Recommendations
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Cinq5 Smart Power Pack II / 3000mAh / 85g – US $130
Powermonkey Explorer 2 / 6000mAh / 286g – Amazon Special US $99