How many charges from a 20,000 mAh power bank?

Roughly 4 to 4.7 charges of a 3,000 mAh phone, 3 to 3.5 of a 4,000 mAh phone and 2.4 to 2.8 of a 5,000 mAh phone, by cable. The number is lower than 20,000 divided by the phone’s battery, because energy is lost on the way.

Published 30 October 2026 by ONDO.

Why you do not get the full 20,000 mAh

The 20,000 mAh on the label is the capacity of the cells inside, at their nominal voltage of 3.7 V. That is 20,000 mAh × 3.7 V = 74 Wh of energy.

To charge a phone, the power bank first raises that 3.7 V to the 5 V or more that USB uses. At 5 V the same 74 Wh is only 74 Wh ÷ 5 V = 14,800 mAh, before any loss. Then energy is lost as heat in that conversion, in the cable, and in the phone’s own charging circuit, which brings the voltage back down to fill its battery.

Our working figure, until we have measured our own unit, is that 60 to 70 % of the nominal capacity ends up in the phone’s battery by cable. On 20,000 mAh, that is 12,000 to 14,000 mAh.

The arithmetic for three phone sizes

Your phone’s battery size is on its maker’s specification page. A charge here means from empty to full; most people top up from 20 or 30 %, so they get more top-ups than full charges.

Wireless gives fewer charges

Charging through the magnet adds another loss: energy crosses an air gap between two coils, and some of it turns into heat. Expect fewer charges wirelessly than by cable. We will publish the wireless figure we measure rather than guess it here.

What the manufacturer says

For the ONDO 20K, the manufacturer states about three full charges of a typical phone. That is the manufacturer’s claim, and it sits inside the range above. We will test it with a full discharge into real phones and publish the result here and on the product page.

Does 20,000 mAh fit the airline limit

Yes. At 74 Wh it is under the 100 Wh limit for hand luggage; the power banks on planes guide has the full rules.