Compare power of different voltages

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Hi all. Still a newbie with electronics but curious on a few things.

I saw a project using power tool batteries that add on USB ports. It converts the 18/24v down to 5v.

I understand a 4ah battery in theory can provide 1amp for 4 hours.

But if I have an 18v 4ah battery (converting down to 5v), and a regular 5v 4000mah (4ah right?) power bank battery, how do they compare with the amount of charge they can provide?

Will the amount of charge be the same?

Thanks.

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You can use Watts to compare the capacity of batteries with different voltages. Watts is Volts times Amps.

So a 18v 4AH battery can deliver 72 Watt-hours, and a 5V 4AH battery can deliver 20 Watt-hours.

But note that this covers only one part of the specification. Actual power available from a battery depends on other things, such as how the AH rating varies with the current drain (4 hours at 1 A will not deliver the same power as 1 hour at 4A, and the difference varies between batteries) and what discharge level the AH rating is based on, which in turn is related to battery type. Also, the need for voltage conversion will introduce losses into the system.

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Hi Jeff, Andrew

This is very true and in practise must be considered.
Most batteries quote an AmpHr (or milliamp hr) capacity. This is usually the capacity available when discharged at a rate of C/20, “C” being the quoted capacity. Increasing this rate will lessen the available capacity. This is sometimes quite drastic and depends a great deal on battery chemistry as Jeff points out. I do note however (as in Jaycar catalog) that there are batteries designed for heavier discharge rates for limited periods. Some batteries have the charge and discharge rate information printed on them or is readily available…

It all boils down to “horses for courses” type of thing. Jeff is correct in that to work out the true capacity you MUST convert to Watts or Watt Hrs and allow for conversion efficiencies.

Regarding power banks quoted usually in mA Hrs (the big numbers look better). This is AT THE BATTERY which is usually a nominal 3.7V. Some (mine does) state this on the device. So my 15000mAhr power bank is a bit over 11000mAhr at 5V.

Below is a graph I got from somewhere for lead acid batteries illustrating this.
image
The dotted line is the minimum recommended voltage for a discharged lead acid cell. 1.8V each so for nominal 12V battery this is 10.8V
Sonnerheim produce similar charts for their Gel batteries as used in golf buggies (Lithium these days).
Cheers Bob

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Hi Andrew,

Jeff and Bob have said pretty much all there is to say, the only thing I have to add is that on good quality step down converter boards, efficiency is given in a curve depending on input voltage and load, which can give you an even better idea of the efficiency you’ll get.

image

Let us know if you have any success or follow up questions :slight_smile:

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Hi James
If conversion efficiencies are unknown or unpublished when advising on this Forum I generally use 85% as a ball park figure. These graphs seem to support that and I think that using 85% would end up very close to the mark. Especially if common sense prevails and a bit of fudge factor or head room is included.
Cheers Bob

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