If you own a power bank, you must’ve noticed how they don’t deliver their claimed capacity. Theoretically, a 10,000mAh power bank should be able to fully charge a 5,000mAh phone twice, but the last time this actually happened was never. Why is that?
Why Your Power Bank Doesn’t Deliver Its Capacity
No, the manufacturer didn’t lie to you; not technically, at least. The power bank you bought does indeed contain lithium batteries of the advertised capacity, but unfortunately, not all of this capacity is translated into usable capacity, aka “rated capacity.” There are a lot of factors responsible for this.
Voltage Conversion Efficiency
Power banks usually output 5V, but the lithium battery inside them naturally stores energy at a lower voltage (usually 3.7V). Power banks use what’s called a boost converter to convert this energy from the battery to the required 5V. This converter isn’t fully efficient, so some energy is lost during the conversion process, which you experience as lower capacity.
Reserve Buffers Reduce Usable Capacity
Smartphones and power banks both reserve a portion of their batteries as a buffer in order to prevent overcharging and over-discharging. This helps avoid permanent damage to the battery and extends its lifespan. However, since smartphone batteries are non-removable and optimized for daily use, the buffer they require is relatively small.
Power banks need more provisions because they function independently and store more energy. In the worst case, a complete discharge can result in your power bank becoming unresponsive and not accepting charge.
Weaker Thermal Management
Heat is bad news for any battery, which is why smartphones use all sorts of ways to reduce the heat generated during charging and discharging.
Power banks do this too, but their thermal management hardware is often not as good as smartphones, especially on cheaper models. This leads to more energy being lost, reducing the usable capacity.
Battery Degradation
Unlike smartphones, power bank companies don’t come up with new models each year, and so many units sit in storage facilities for years before being sold.
By that point, the battery inside the power bank had already gone through a not-insignificant amount of wear, resulting in reduced capacity.
Of course, manufacturers try their best to preserve battery health, but even if you do everything right, lithium batteries inevitably degrade over time.
How to Find a Good Power Bank
Manufacturers rarely mention the rated capacity of their power banks due to market competition, but it’s usually a little over 60% of the advertised capacity. For instance, a 20,000 mAh power bank will deliver roughly 12,000 mAh of usable capacity. That’s enough to charge your 5,000mAh phone nearly two and a half times.
A more accurate way to gauge rated capacity is to check if the marketing material mentions the number of charges for specific phone models, then do the math yourself. As long as the rated capacity is above 60%, you’re not getting ripped off. The higher this number, the better.
For instance, the Ugreen Nexode 20,000mAh power bank claims it can charge the Galaxy S24 Ultra three times. The phone has a 5,000mAh cell, so the rated capacity of the power bank would be around 75%, which is impressive.
Of course, nothing beats reading product reviews and checking real-life performance. Try to avoid no-name brands and buy from trusted brands only, since the former sometimes skimp on essential protection mechanisms and tend to use cheaper materials that affect durability.
I also recommend sticking to wired power banks instead of magnetic ones. That’s because wireless charging is inefficient and produces more heat, so you’ll lose even more energy during charging. And on top of that, magnetic power banks are also more expensive. The convenience they offer is simply not worth it.
