How to convert solar car trickle charger to usb/battery charger?

I would like to know how I could convert one of these solar car battery trickle chargers to a charger that could charge ideally AA and/or AAA batteries but have a female usb port to charge usb powered devices. The point of storing the power in portable batteries would be that it would then double up to charge those batteries if I ever needed to charge them. I do want to spend as little as possible and buy as few parts to do this as possible. I understand that larger batteries would be able to store more power but it would then mean it would not charge AA or AAA batteries and make the whole thing a lot bulkier and expensive.

I have a choice of solar trickle chargers being 12V/12W, 12V/4W, 12V/2.4W and 12V/1.5W. They come with crocodile clips and a cigarette lighter plugin. I also have an unrelated usb charger (with female usb port) that can plug into a cars cigarette lighter so Im not sure if it would be possible to attach the usb section to the solar panels cigarette lighter plugin. I also have a usb charger that plugs into the mains. I know almost nothing about circuits. The general point is to be able to make the charger as multi-purpose as possible, like being able to charge batteries or usb powered devices but without making it awkward to use or un-portable. I have looked at the related instructables but im unsure how you could change the voltage to make it safe for the batteries/usb port etc.


Get a buck converter, basically a stepdown transformer. Cost is about $2 bucks. Since these 12v panels can put out about 23v @ 1.5W. On a nice average day will produce about 19v steady, you can use the buck converter to drop the voltage down to a more usable voltage, which will increase your amperage. The current on the model I have is only 100mA (really about 125mA) but my USB charger's output is 1000mA (1A). I have all kinds of voltage but no amps. I can connect the charger but get no output. Hence the need for a buck converter. You're going to need a multimeter as well, as you step down the voltage (you only need 5v to charge your phone, but the charger needs 12v), the amps will go up slightly and you don't want to exceed the rated current for your charger, I have found about 300-400mA is plenty. Here is a video which explains how this works:

I think that your USB charger, the one that plugs into a car's cigarette lighter,  that gizmo is a good choice for the power converter at the "core" of this, your, I don't know what you want to call it, your miniature solar power plant.

You said you know "almost nothing" about circuits, but consider the following thought experiment:  

Just plug all this junk into your car

What would happen if you just took two of those gizmos,  the solar panel for trickle charging a car battery, and the USB-charger-that-plugs-into-car-cigarette-lighter, and just plugged both of those into your car?

Well, presumably that would work.  The trickle charger would charge (put energy into) the car's battery, and the USB-charger could be used to recharge smaller batteries, specifically those expecting the 5 VDC supply provided by a USB port.

You did not mention if you owned a gizmo that can charge AA or AAA (guessing NiMH chemistry) from a USB port.  If you had one of those, that would be very handy, because then you could plug it into the USB-charger-that-plugs-into-car-cigarette-lighter, and presumably that would work for charging your AA or AAA NiMH batteries.

So you know, you just plug all this junk into your car, and that's something that seems like it should work.  But of course you're not going to like that answer, because of the inherent bulkiness and expense of having a car attached to it.

A smaller, cheaper substitute for your car's electrical system

The solution to this is to substitute something smaller than your car, for the role of storing energy from the solar panel.  For this I suggest a small, sealed, 12V lead-acid battery, or if that is still too bulky and expensive for you, then perhaps an electrolytic capacitor, with a maximum voltage rating of around 25V.  

There are some trade offs, regarding battery versus capacitor, the main one being it will be hard to find (at the time of this writing) a capacitor whose stored energy (at 10 to 20 VDC) is even a small fraction of the energy that can be stored in a lead-acid battery of the same volume and weight.

Practically what that means is a capacitor will give you a few seconds of stored energy (power*time product) compared to the lead-acid battery which can give you minutes or hours of stored energy.

The downside to using a lead-acid battery is that component can be broken, erm.. be killed, die, easily, and the easiest way to do that is by discharging it deeply too much; i.e. consistently taking out more energy out than you put back in.  In contrast, electrolytic capacitors are unaffected by discharging them deeply (or completely), and thus almost impossible to kill.

Energy accounting

Back to this claim that you "know almost nothing about circuits".  If true, that makes it kind of harder for me, since it means there is more I have to explain.  Anyway, sort of one of the fundamental things you should know about, is the idea of conservation of energy, the fact that energy cannot be created or destroyed, merely transferred from one place to another.

The usual analogy is to think of your battery like a checking account, with energy being analogous to money.  You can't write checks for money you don't have, and similarly you cannot draw energy from a battery that is dead, or "spent", you know, completely discharged already.

So how do you keep track of electrical energy, to make sure you're not spending energy you don't have?  Well, the answer to that lies in the ability to calculate, or measure, the amount of energy, or rather power (the time derivative of energy dE/dt), your devices are using.  The short answer is that P = V*I, power is the product of voltage and current, and energy is the time integral of power, E= integral (P*dt).

You know, the definition of electric power,

But I might be wasting my time with talk like that. 

It might be easier to explain just by building it.  Reply if you don't "see it", what I'm describing, and I'll draw you a picture (wiring diagram) of it.

Back to what I was writing before about battery versus capacitor:  the capacitor version will be instructive.  I mean with that version it will be pretty much impossible to "spend" stored energy you don't have.  However the capacitor version will probably "brown out" every time the sun goes behind a cloud, and I'm not sure if your little USB-AA-battery-charger will like being jerked around like that.  It would be annoying if it were one of those things that just sort of shut themselves off (or reset, or go to sleep) every time the
power goes off.

Smart Battery?

A battery that comes with some kind of built in monitoring/protection gizmos, i.e. the kind intended to protect the battery from the user, that might be a good idea here.  For example lithium-ion battery packs typically come with some such protection circuit built in, but I am trying to think how to charge a battery like that from a circa 20V (open circuit) solar panel...  Perhaps someone has already built such a charger, or charger + li-ion battery combo.  That might be worth looking for, if it exists.

In contrast, your typical cheap, sealed lead-acid battery, is cheap, comes with no built-in protection circuits, and is easy to kill through deep discharge.  The plusses for sealed lead-acid being that this kind of battery is inexpensive, and it is well matched to your circa 20V (open circuit), nominally 12V, called, sold as "12V" solar-trickle-charger-panel