Energy Density Explained: Why mAh Alone Won’t Tell You If a Power Bank Can Charge Your Bike

Hook: Your power bank’s mAh lied to you — and your e‑bike knows it

If you’ve ever bought a 20,000mAh power bank hoping it would top up your e‑bike, then watched the battery % barely move, you felt the frustration that drives this explainer. The label number (mAh) is useful — but alone it doesn’t tell the whole story. For cross‑device comparisons, especially when you’re mixing small USB devices and high‑voltage systems like e‑bikes, watt‑hours (Wh) and voltage matter far more.

Key takeaway — read this first

Wh trumps mAh when you compare batteries that operate at different voltages. mAh is a count of charge at a specific nominal voltage; Wh is the actual stored energy. If you want to know whether a power bank can meaningfully charge an e‑bike, convert mAh to Wh, account for voltage conversion losses, and compare Wh to the e‑bike pack’s Wh rating.

Why mAh is misleading across devices

Manufacturers often print mAh because it looks big and impressive. But mAh is a unit of charge, not energy. Charge (mAh) multiplied by voltage (V) gives energy (Wh). Two batteries with the same mAh can hold very different energy amounts if their voltages differ.

Quick formula

Use this to convert:

• Wh = (mAh / 1000) × V

Example: a 20,000mAh bank rated at a nominal cell voltage of 3.7V stores about 20 × 3.7 = 74Wh. That’s the real energy available (before conversion losses).

Real e‑bike example: the 36V 375Wh pack

Let’s use a common case in 2026: many affordable e‑bikes use a 36V battery pack around 10.4Ah. That equals 36V × 10.4Ah = 374.4Wh (often rounded to 375Wh). If you compare that to a 20,000mAh power bank (74Wh), the math is obvious: the bank has far less energy.

How many 20,000mAh power banks to fill a 375Wh e‑bike?

We must consider conversion losses. If you try to use multiple USB power banks to charge an e‑bike you’ll need a DC boost converter or a power station with a proper DC output. Assuming very optimistic combined losses of 10–15% (rare in DIY setups) you still need:

• Required energy = 375Wh ÷ 0.9 ≈ 417Wh
• Number of 74Wh banks ≈ 417 ÷ 74 ≈ 5.6 → so 6 banks

In reality, inefficiencies, cable losses, and safety limitations push that number higher. The practical takeaway: a single consumer power bank won’t replace an e‑bike battery.

Conversion losses: where energy disappears

There are several places energy is lost when you convert from a power bank’s cells to your device’s required voltage:

• DC‑DC conversion (boosting 3.7V cell voltage to 5V, 9V, 12V, or 36V). Efficiency typically 85–95% depending on quality.
• USB controller & cables — heat and voltage drop add losses.
• Voltage conversion stage on the device — e‑bikes have their own BMS and tolerance, which can add more losses.

For USB‑PD power banks outputting at 5–20V, manufacturers often list efficiency and output wattage. But stepping all the way to 36V is rare without a dedicated DC output, and it’s where losses mount.

Energy density and why it matters (Wh/kg and Wh/L)

Energy density is energy per unit mass or volume and is usually expressed as Wh/kg or Wh/L. Cell chemistries and packaging determine how much energy fits into a given weight and size.

In consumer products, cells have improved steadily through late 2025 and into early 2026—new cells push higher Wh/kg—but finished packs (power banks, e‑bike batteries) trade some density for robustness, BMS hardware, and protective enclosures. That’s why a compact phone power bank can look impressive in mAh but still store less absolute energy than a heavier e‑bike pack.

mAh vs Wh — one-liner for shoppers

Don’t compare mAh across devices—compare Wh. Convert mAh to Wh using the cell voltage, then account for conversion losses and real output voltage.

Charging math: practical step‑by‑step

When you want to know how many phone charges, or whether a power bank can help an e‑bike, follow these steps:

1. Find the device battery energy in Wh. For phones, if you only have mAh and cell voltage: Wh = (mAh / 1000) × V. Many phone spec sheets show Wh directly.
2. Find the power bank Wh. If only mAh is given, use the power bank cell nominal voltage (commonly 3.6–3.7V) to convert: Wh = (mAh / 1000) × 3.7V.
3. Apply realistic conversion efficiency (phones: 80–90%; e‑bikes with DC charging: 75–85%).
4. Divide energy available by device Wh to get number of full charges: Charges ≈ (PowerBank_Wh × efficiency) ÷ Device_Wh.

Example: phone vs e‑bike

Phone: 4,000mAh at 3.85V → 15.4Wh.

Power bank: 20,000mAh at 3.7V → 74Wh.

Assume 85% conversion efficiency for USB to phone:

• Effective energy = 74 × 0.85 = 62.9Wh
• Number of charges = 62.9 ÷ 15.4 ≈ 4.1 full charges

Compare that to the e‑bike example above: one 20,000mAh bank doesn’t come close.

Pass‑through charging: useful, but watch the caveats

Pass‑through (charging the power bank while it powers a device) is handy for travel and streaming. Recent 2025–2026 improvements in bidirectional USB‑C PD (PPS and PD 3.1) made pass‑through more efficient and safer, but it still has downsides:

• Heat buildup: simultaneous charging and discharging increases cell temperature and long‑term degradation.
• Efficiency hit: you’ll lose more energy in two conversion steps.
• Not all banks support true pass‑through — some will disable output while charging or reduce output.

Actionable advice: use pass‑through sparingly, prefer banks with explicit bidirectional PD ratings, and avoid heavy load pass‑through (like charging an e‑bike) unless the manufacturer specifies support.

When a power bank can charge an e‑bike — the real options

If your goal is to extend e‑bike range, consider these realistic approaches in 2026:

• Portable power stations with DC output (36V/48V) and high Wh (300–1000Wh+). These are purpose‑built and safer for e‑bikes.
• Dedicated e‑bike charger packs — accessories that match pack voltage and communicate with the BMS.
• High‑voltage power banks — a small but growing category by late 2025 that includes DC outputs or adjustable voltage rails for EV/e‑bike use. Check that the bank specifically lists your pack voltage and supports the required charge protocol.

Tip: If you see a power bank marketed for e‑bikes, verify the Wh rating, DC output specs (voltage and current), and safety certifications (UL, CE/EN, UN38.3). If it’s not explicit, it’s probably not suitable.

Regulatory and safety points for 2026 shoppers

By 2026, the market has matured but safety still matters:

• Airlines still regulate lithium batteries: typically ≤100Wh allowed in carry‑on without airline approval; 100–160Wh needs airline approval; >160Wh generally not allowed. Always check your carrier.
• Look for UN38.3 transport testing, UL or equivalent safety marks, and robust BMS. These protect against overcharge, overdischarge, and thermal events.
• Newer PD 3.1 implementations (recently mainstreamed in late 2025) improved bidirectional charging and higher voltage rails — useful for high‑power laptops and future high‑voltage packs.

Energy density trends and future predictions (late 2025 → 2026)

Recent trends through late 2025 and into 2026 are shaping how we choose portable power:

• Cell improvements: incremental gains in Wh/kg continue, but system‑level changes (BMS, thermal management) often determine final pack size.
• High‑voltage consumer outputs: PD 3.1 and EPR‑enabled devices pushed suppliers to offer higher DC output options — expect more power banks with 48V DC or direct e‑bike compatibility in 2026.
• GaN and higher efficiency converters lower conversion losses, narrowing the gap between rated Wh and delivered energy.
• Modular batteries and swappable packs for micromobility are growing in acceptance — expect standardization efforts to make cross‑device use easier.

Checklist: How to pick the right power source

Before you buy, run through this checklist:

• Does the product list Wh? If not, convert from mAh using the cell voltage.
• Does it have the right output voltage (DC 36V/48V) or a proper USB‑PD profile for your device?
• Is the stated output wattage and continuous current enough for your e‑bike’s charger specs?
• Does the manufacturer provide efficiency figures or realistic charge counts for phones and laptops?
• Are safety certifications (UN38.3, UL/CE/EN) present? Is there a reliable warranty and customer support?
• Consider weight and energy density — Wh/kg or Wh/L if available — to gauge portability vs range tradeoffs.

Short case study: From a 20,000mAh bank to a portable power station

Scenario: commuter wants a top‑up on a 36V 375Wh e‑bike for a longer weekend loop. Options:

• Carry multiple 20,000mAh banks — heavy, inefficient, and risky; you’ll need 6+ banks and complex adapters.
• Buy a 500–600Wh portable power station with a DC 36V/48V output — single device, optimized BMS, safe and much easier. Modern stations in 2025–2026 are lighter and more energy dense than before and often accept fast recharging with GaN chargers or solar inputs.

Conclusion: the portable power station is the sensible, reliable option.

Actionable shopping tips

• Always convert mAh to Wh for comparison. If a product hides Wh, ask the seller or avoid it.
• For e‑bikes, prefer devices that list DC output at the correct voltage (36V/48V). Avoid jury‑rigged USB‑to‑DC hacks.
• Factor in efficiency: use 80–90% for USB devices and 70–85% for high‑voltage conversions when estimating real‑world charges.
• Prefer banks with PD 3.1 / PPS and bidirectional support if you want pass‑through or fast recharging of the bank itself.
• Check airline rules before traveling with >100Wh batteries. Plan ahead for shipping or spare packs.

Final verdict

mAh is a useful spec but incomplete. If you want a true apples‑to‑apples comparison, use Wh. Voltage dictates how that stored charge translates into usable energy for your device.

For phones and small devices, a high‑mAh USB‑PD bank can be a great, portable solution. For e‑bikes, you need a matched Wh capacity and voltage — typically a dedicated e‑bike pack or a purpose‑built portable power station. By late 2025 and into 2026, the market is evolving: expect more high‑voltage, high‑Wh portable options, better efficiency thanks to GaN and PD 3.1, and clearer labeling. Until then, do the math.

Call to action

Want help picking the right power solution for your e‑bike or phone? Tell us the specs of your bike battery (voltage and Wh) and what you want to power on the road. We’ll run the numbers and recommend safe, practical options — from compact PD power banks for phones to portable power stations that actually extend your range.

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