# Lower total power yield when using more cells

#### thelayman123

Dec 13, 2022
7
I recently got a DIY powerbank powered with 18650's and tested the output with a usb meter with varying numbers of cells. I got 40Wh with 4 cells but only 128Wh with 14 cells of the exact same voltage at the point of insertion, a 32Wh reduction. Aren't parallel circuits supposed to be efficient? No increased heat production noted, same rate of power output used in all tests, same cells too. Or could my meter be the problem?

#### Delta Prime

Jul 29, 2020
1,964
DIY powerbank
Operator error?take a photo of your setup. And if you can draw schematic. Or wire diagram of your setup.
I got 40Wh with 4 cells but only 128Wh with 14 cells of the exact same voltage at the point of insertion
Point of insertion? those 14 cells got bold and infiltrated your command post spreading propaganda.

Or could my meter be the problem
A USB meter? was it calibrated by a third party. Would you like to know how to calculate the cumulative Watt hour of your batteries? That way you can compare the calculations to the reading on your meter. Make, model, link of your USB watt hour meter please.

#### Harald Kapp

##### Moderator
Moderator
Nov 17, 2011
13,719
14 cells of the exact same voltage at the point of insertion
How "exact" was the "same voltage"? Lithium batteries have a flat voltage over a wide range of state of charge (see e.g. here). Which means that cells of the "same" voltage can actually have very different states of charge. To make your experiment repeatable you need to start with truly the same state of charge. One way to achieve this (at least in a good approximation) is to let the cells be charged by the same charger and at the same temperature until the charger stops charging. Use the power bank's built-in charger to fully charge the 4 cells or the 14 cells before starting to discharge and measure the capacity.

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