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I have a 10000mAh battery pack for charging phones/tablet/whatever but I also have some cordless tool batteries which can be used in an inverter to power a usb charger similar to the power bank. The cordless tool batteries only go up to 5Ah (I think you can get larger ones but they’re not very common) and that on it’s own is far larger than the power bank as well as having a lower capacity. I assume it is to do with the batteries being 18v rather than 5 or whatever the power bank is?

Edin Fifić
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Frostywood
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    Unclear what you're comparing to what. Just multiply voltage and capacity to get WattHours and compare that: 18 V * 5 Ahr = 90 Whr. Also realize that almost all 10000 mAh USB power banks are **NOT** 5 V * 10 Ahr = 50 Whr. In reality the battery inside is 10 Ah at 3.7 V (it used a converter to make 5 V) so the **real** capacitance is 3.7 V * 10 Ahr = 37 Ahr and you loose around 20 % of that due to converting 3.7 V to 5 V. (so you end up with around 30 Whr) – Bimpelrekkie Apr 20 '21 at 11:31
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    Bimpelrekkie is right on the money, also keep in mind that you might be comparing different battery chemistries, for which energy density can be quite different. And finally, you can make a big USB power bank "as big & heavy as you want" because it goes in a backpack, but for a cordless drill you are somewhat more limited in size/weight. – Vladimir Cravero Apr 20 '21 at 11:50

2 Answers2

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A graphical representation may help.

enter image description here

Figure 1. The energy capacity of a battery is the product of the battery voltage and the mAh rating.

Here it's clear that the 18 V, 5 Ah battery has the higher capacity.

The Wh (watt-hour) rating is a better number to use when comparing battery capacities.

Transistor
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Your assumption is correct. The powerbank has higher Amp-hour rating but significantly lower voltage because it usually has a single 3.7V cell vs. at least 3 cells for a power tool. In your particular case, the 18V battery likely has 5 cells, so even if the powerbank has the Ah rating of 10,000mAh (10Ah), and the power tool battery of 5Ah, which is 2 times more Ah for the powerbank, the tool battery has 5 times the voltage, so it actually has more energy in watt-hours (Wh). Powerbank: 10Ah × 3,7V = 37Wh Power tool: 5Ah × 18.5V = 92.5Wh So, both the Ah rating AND the voltage of the battery need to be considered, in other words you need to use the ENERGY capacity in Wh for a proper comparison between batteries.

Edin Fifić
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