How to store the electric energy produced by solar panels properly?

Question

I am building some solar panels from 6" cells (4W, 0.5V, 8A). At the moment I have two 1x2 panels and one 2x2 panel. So there are 8 cells in all panels and the added voltage is up to 4V.

Now I want to store the energy somewhere. I have a laptop with the input of 19V DC 2.1A (40W) and I'm also planning to buy a 12V 12Ah battery. As far as I know if I will provide voltage that is too high/low for the laptop/battery, it may cause some problems or even damage, so I need to control the voltage. But what about the current? What happens if I provide a current that is too high/low for the battery/laptop (since there are many models out there, I'm asking about usual situations if there are such)?

So I need to control the voltage and I assume that I also need to control the current. Since I'm on a low budget and have lots of time, I decided to build my own voltage/current controler circuits (one for the laptop and one for the battery). When building a controller for my laptop, it's pretty clear (?) that I have to provide 19V DC 2.1A. But what about the 12V battery? The only parameters given on the battery is 12V and 12Ah (in the catalogue of the shop that i'm going to buy it in), so what input current should I provide for that battery?

I want the circuits to be as simple as possible, with less components, since my knowledge in electronics is very narrow (for now). I would appreciate if you would give me advice, links to information or even suggest your own circuits for this application.

Answer 1

So, you have panels wired all in series to give up to 8A at 4V.

The first stage will be a boost converter to give you up to 8/3 amps at 12V. You can buy these, which is generally cheaper and much easier than trying to build one. Note that for 8A it'll probably require a fan, and should itself be kept out of the sun to keep cool.

Let's have a look at some references: http://www.powerstream.com/SLA.htm and http://batteryuniversity.com/learn/article/charging_the_lead_acid_battery

Key facts to take from there are:

• the voltage must be slightly more than 12V to charge the battery, but not too high
• total charge duration should be 12-16h

That implies that a 12amp-hour battery should not be charged with more than 1A. That presents a problem for the charger design as you have (in ideal conditions) too much power available from the cells. So you want a boost converter that's current limited to 1A on the output, preferably designed for battery charging.

Answer 2

You need maximum power point tracking (MPPT) boost converter for this. This is most common effective solution for your task.

Somewhat less effective solution is to charge Pb/NiCd/NiMH battery those charge voltage is near to nominal output voltage of solar cell. (4V Pb battry is ok for you)

Conventional (non-MPPT) boost converter could deliver bad performance, becuase it is not aware of solar cell characteristics. Conventional converters assume power source can deliver enough current for normal operation, but is they try to drain more current, than solar cell produce, it will drastically drop in voltage, thus give much less power than it can.

Of course you can use non-MPPT converter after Pb/NiCd/NiMH battery as in second paragraph, but it should be aware not to overdiscarge buffer battery.

It is not recommended to power laptop from solar cells without intermediate accumulator, because solar power output is very unstable.