Power Banks Explained: Series vs. Parallel, mAh vs. Wh, and What Energy Really Means
Three things cause much of the confusion around power banks: how cells are connected in series or parallel, what mAh and Wh measure, and the difference between power and energy. These misunderstandings can lead to the wrong purchase, incorrect estimates of phone charges, or the feeling that a power bank’s advertised capacity is misleading.
This article explains those ideas from an engineering perspective, in straightforward terms.
1. The basics: voltage, current, power, and energy
1.1 Voltage (V)
Think of voltage as the electrical “pressure” that drives current.
Examples include a Li-ion cell’s nominal 3.7 V and a USB port’s 5 V.
1.2 Current (A)
Current is the rate at which electric charge flows through a wire.
For the same resistance, higher current means more heat.
1.3 Power (W)
W = V × A
Power tells you how quickly electrical energy is being used. Examples include 20 W, 30 W, and 65 W fast charging.
1.4 Energy (Wh)
Wh = W × time in hours
Energy describes how much a battery can store. If power is the rate at which a vehicle burns fuel, energy is the amount of fuel in its tank.
2. Li-ion cells and why mAh can be misleading
A representative Li-ion cell might have:
Nominal voltage: 3.6–3.7 V
Capacity: 2,500–3,000 mAh
But mAh is not energy: it does not include voltage. The energy calculation is:
Wh = (mAh × V) / 1000
For example:
10,000 mAh @ 3.7 V = 37 Wh
10,000 mAh @ 7.4 V = 74 Wh
Both are rated at 10,000 mAh, yet one stores twice the energy.
Why do manufacturers still advertise mAh?
Buyers are familiar with mAh, making it easy to recognize and market.
Phones also list mAh, making a direct comparison look natural.
Air-travel battery limits use Wh, while mAh is often the figure printed most prominently on the box.
The key distinction:
mAh is a familiar charge-capacity figure; Wh is the figure to use when comparing stored energy.
3. Connecting cells: series and parallel
3.1 Parallel connection
Voltage stays the same.
Charge capacities in mAh add together.
Example: two 3.7 V, 2,500 mAh cells → 3.7 V, 5,000 mAh → 18.5 Wh in total.
This arrangement is common in inexpensive power banks and produces a larger-looking mAh figure.
3.2 Series connection
Voltages add together.
The mAh capacity stays the same.
Two 3.7 V, 2,500 mAh cells → 7.4 V, 2,500 mAh → also 18.5 Wh.
The total energy is unchanged; only the voltage and charge-capacity combination changes.
3.3 Why do many higher-end power banks use series-connected cells?
A series pack can offer several advantages for USB-C PD fast charging:
• Lower pack current can reduce resistive heating
For a 60 W output, ignoring conversion losses:
A 3.7 V pack requires about 16 A.
A 7.4 V pack requires about 8 A.
Lower current reduces losses in shared current paths of the same resistance, which can help overall efficiency.
• A better starting point for higher PD voltages (9 V, 12 V, 15 V, 20 V)
A higher pack voltage can reduce the conversion ratio required. Depending on the pack and output voltages, the circuit may use buck, boost, or buck-boost conversion, potentially reducing losses compared with stepping up from 3.7 V to 20 V.
• Thermal management and durability
Less heat can help battery life. The actual safety and durability still depend on the cells, protection circuitry, and complete product design.
4. The biggest misconception: “A 20,000 mAh power bank should charge a 5,000 mAh phone four times”
That calculation is not accurate.
Why?
The advertised 20,000 mAh is commonly referenced to a cell voltage such as 3.7 V, rather than the actual series-pack voltage.
Charging involves conversion losses; the 15–30% range discussed here is an estimate, not a fixed value for every device.
You cannot compare the energy of two devices at different voltages using mAh alone.
Use Wh for the calculation
For example:
Power bank: 20,000 mAh at 3.7 V → 74 Wh
Phone: 5,000 mAh at 3.85 V → 19.25 Wh
Assumed overall efficiency: 80%
Number of charges = (74 × 0.8) / 19.25 ≈ 3.07
Under these assumptions, the result is about three full charges, rather than the four suggested by dividing the mAh figures.
5. What should you look for when buying a power bank?
1. Check Wh as well as mAh
Wh expresses the rated stored energy.
2. Consider the pack configuration, including series packs (2S, 3S, etc.)
Higher pack voltage can help fast-charging efficiency, though configuration alone does not establish product quality.
3. Consider PD output of 20–30 W or more
Match the output to your phone and other devices’ supported charging profiles.
4. Pay attention to build quality
Good cells and well-designed electronics help control heat and deliver more usable energy in practice.
6. A quick recap
mAh measures charge capacity, not energy by itself.
Wh is the useful figure for comparing stored energy.
Parallel connections add mAh; series connections add voltage.
Number of charges = power-bank Wh × overall efficiency ÷ phone-battery Wh.
Series packs can help USB-C PD fast charging by reducing pack current and easing voltage conversion; the full design determines the outcome.
Conclusion
Understanding power banks helps you choose with clearer expectations and know what you are buying. Terms such as mAh, Wh, series, and parallel may sound technical, but they become much easier to follow when you keep two quantities in view: voltage and energy.
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