I set up my GoPro to take 1 shot per minute. First setup is the inverter - on a fresh charge, I plugged in the clock and fired it up in count-up mode.
The last pic looked like...

10 hours and 8 minutes.  Good run - better than expected.

I picked-up a buck converter - Looks like the one out of Bill's buck/boost/buck-boost episode.
Soldered it to the battery adapter.  Set it to 12.1V

I peaked into the basement a few times, and couldn't believe it.  Went to bed to wake-up to it still going.

the last pic, 20 hours and 37 minutes - more than double.  Now you can't believe it, so I recreated the first setup.

Same result

Same battery, same charger, same location. (impressive mess in the bg there)

So i'll locate the buck converter inside the battery holder - it has a nice on/off switch,

and I'll figure out some cord management..

While not a hugely interesting project - the results were unexpected, and it created an actionable result.

Now to make a time lapse film out the all the stills!

Here is some of the internals for the project -

it is, and will always be about asking the right question!

There are better ways to charge a phone - so USB really isn't needed.

The RV has been sold.

Sorry, I almost never document my work, I do have pages of rough notes during the project that get replaced, thrown out etc, but most of it is in my head. What is it you want to know, the basic design of wind and solar hasn't changed in years. Wind is a very low power source but in the right location can provide a continuous 'trickle' charge.

I highly recommend Will Prowse's web site. I also recommend Victron products other than batteries which you should build yourself from prismatic cells. I used Battleborn 100AH 12.8V batteries same size as a car battery but 1/2 the weight and very safe as they were LiFePO4. Of course the downside of being safe is much less energy density.

PLEASE reply yes or no, what you want is to use your ample supply of 18VDC Lithium batteries to power some 12VDC devices of some sort, and also some USB 5.1VDC devices?

Yes -

I want to use the shell of the nimh charging case to  do the above project - as it provides the battery connections - physical and electrical. 

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Still, the goal of the project would be to double the run time of the clock on one battery.

Otherwise i'd just carry another battery, or make a tandem battery holder for the inverter.

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i've heard wall wart too - 

Is your project documented - would like to see some pics!  I live on top of a hill, and would like to try something with wind/solar.

I was going to use the old nimh chargers as a case for whatever is built since it can't charge the lithium batteries.  it will provide a solid connection to the battery.

mean you think you can build your own Lithium charger?

Do you not have a charger or two for your 15 RYOBI Lithium batteries?

LIGHT BULB. Now I remember you did say inverter, so 18VDC to 120VAC. That is not a smart approach for what you need. 

What I now expect is Ryobi Charger->Ryobi Battery->Ryobi (12VDC or 5VDC) Converter.

I have a similar setup with Makita 18VDC batteries and Makita USB Adapters.

DO YOU ALSO NEED 12VDC, OR JUST 5VDC as in USB?

I do NOT see a Ryobi USB Adapter on Amazon.ca, but here is the first listing I see in the USA

https://amz.cx/3KUL so you are in luck.

I never heard of a wall bug, I have always heard wall wart.

Where do you go 18->120->12? Yes that is much less efficient then 18VDC->12VDC. Same for 19.5, the number isn't that important, it's the fact you are going DC to AC to DC. DC to AC is an inverter and anything approaching efficient will cost hundreds to thousands of dollars. My 3,000W inverter was a top brand name and cost a few thousand.

PLEASE reply yes or no, what you want is to use your ample supply of 18VDC Lithium batteries to power some 12VDC devices of some sort, and also some USB 5.1VDC devices?

You MUST use the RYOBI supplied Lithium charger, the charging parameters for Lithium is VERY different than NIMH.

Keep in mind I repowered my 42ft RV with 6 100AH LiFePO4 batteries, the 3,000W inverter and 6 190W solar panels plus the usual accessories like an MPPT charger, shunt, all marine grade fasteners (zinc over copper) and the main battery/inverter cables were 4/0, the biggest you can order. My cost was close to 5 figures, my background includes being an industrial electrician for a couple years. Everything other than the AC, electric heaters, and washer and dryer were Solar powered and that includes the residential fridge, not a propane fridge (fire hazard).

I have all these rechargeable lithium batteries - they are the source in all cases.

I was going to use the old nimh chargers as a case for whatever is built since it can't charge the lithium batteries.  it will provide a solid connection to the battery.

lugs - slang for screw down terminals. 

115v is the line voltage receptacle,  ugh oh, it actually says 120V

This is a wall bug!  Well in the local slang.

What i'm look for is, "What would the gain in run time be if I go straight from 18v to 12v, rather than 18->120->12?"  What about others (dell is 19.5 I think)

another pic of the parts

This also could be me making something more complicated than it has to be