# When charging mobile decices using an external battery is DC > USB more efficient than AC > USB?

I bought a "jumpstart" battery that has a built in power inverter to support both AC and DC ports. I have it for a project but also want to use it for charging my mobile devices occasionally.

I do have an AC > USB adapter, but I'm wondering if it would be more energy efficient (read: save battery life) if I were to get a DC > USB adapter. My logic being that these small electronic devices use DC power and so there's no / less conversion when going from DC to DC.

Summary:

• As you suggest, the Jumpstart_DC -> converter -> USB_5V is more likely to be the most efficient method, but this is not certain and depends on some extra points which need to be clarified. See below.

• There are too many unknowns at present to be certain but

• I guesstimate DC-USB is likely to be in the 85% - 90% range BUT could be as low as 40% done poorly.
• I guesstimate AC-USB to be in the < 70% efficient range but could be 70% +.
• To obtain a good comparison the following needs to be known.

• What is the JumpStart battery voltage?
• What is the JumpStart DC output voltage?
• Is DC out a direct battery connection or is there an internal SMPS (switching converter)?
• If a SMPS is used for DC out is the voltage selectable, and what are the voltage available?.
• Any available information on efficiencies, ratings etc.
• Knowing Jumpstart battery technology/chemistry would be useful.
• Direct measurement may proide superior results to any technical guestimating

• For a known AC load measure power in. If direct measurement not possible do run time tests with a known load.

• For a known DC load do as above.

Lonnnng version:

Assumptions:

Update:

Mauvis says:

The phone is an iPhone 4gs.

So - appears to be 12V lead acid battery with direct from battery 12V DC output. So comverter can be highly effiint if SMPS.

Converter shown may or many not be SMPS - should be.
Output is 500 mA.

• (1) I assume that the use of an AC -> USB or DC-> USB converter means that either is powered from an output on the "jumpstart" device. ie either

Jumpstart_AC -> converter -> USB_5V
or
Jumpstart_DC -> converter -> USB_5V

(2) I assume that the JumpStart uses a 12 volt lead acid battery.
Alternatives: Could use LiIon or NimH or ...

(3) I assume that the Jumpstart DC output uses direct connection to its battery at a nominal 12 VDC. (Alternatives: could use an internal DC-DC converter and output voltage could be selectable).

Re " ...and depends on some extra points which need to be clarified ..."

You need to know or have an adequate guesstimate of:

• The Jumpstart DC output voltage & how it is provided

How the proposed DC-DC converter converts to USB-5V and it's efficiency

Jumpstart battery to AC efficiency and converter AC-USB_5V efficiency.

DC -> USB_5V.

Depending on how everything is done efficincies DC-> USB could be in the 40% to 85% range. Near the higher end is likely but a better knowledge of the overall arrangements is needed for certainty.

• If the Jumpstart DC output is a direct connection from its internal battery at say 12VDC and if the DC-USB converter uses a SMSP (SMPS = swith mode power supply = switching regulator - here probably a buck converter) then overall efficiency should be good.

i Jumpstart to DCout is effectively 100% efficient and the converter should be able to be 75% - 85% efficient. Higher efficiency is possible but not likely end to end.

ii If as in i above but the DC-USB converter uses a linear regulator then efficiency will typically be 5/12 =~ 40%

iii If as in i above but the Jumpstart battery and DC out is say 7.4V nominal LiIon then with a SMPS DC-USB is still probably about 75% - 85% but linear efficiency is now say 5/7 =~70% (varies with battery voltage)

iv If the Jumpstart uses a SMPS from its battery to its DC output (certainly not unknown) then that will add another 70-80% efficincy in the chain BUT if using a linear regulator the DC level may be set close to 5V so the linear regulator will be very efficient.
So in this case overall efficiency = say
SMPS: 0.8 x 0.8 =! 65% battery to USB. And
Linear: 0.8 x 0.8 =! 70% overall

AC -> USB_5V

This is a lot more certain in methods used.
AC out is an internal SMSP - probably around 70%-80% end to end. AC-USB almost certainly rectifies the AC and uses a SMSP at 80%-90% overall. An iron cored transformer using 60 Hz may be used and in this case may provide an excellent solution if well designed at say 90% efficiency.

SO:
AC-USB with smps converter ~=(0.7-0.8) x (0.8 - 0.9) =~ 55% - 75% range. AC-USB with iron core transformer and careful design = (0.7 - 0.8) x 0.9 ~= 65% - 75% range.

So for comparison:

DC-USB

Could be as good as 85% - 90% with best case arrangements.
Could be a low as 40% worst case :-(

AC-USB

Using SMSP adaptor probably 55% - 75% range.
Using iron core supply = 65% - 75%.