I am building an everlasting solar power station and need help.

I have a bank of 10 NiFe Edison cells rated for output of 12.4V @ 90 amps for up to 4 hours. The recommended charge voltage is 18V and the recommended balancing charge is 20V once per year. The lowest voltage the cells output is 10V.

I want to charge the cell bank via a solar array. I can get a charge controller to output 18V, but I'm concerned because the batteries will charge daily and appliances will sometimes be running. The appliances are designed for 12V - 14V use.

I have been looking for a variable voltage converter that can accommodate the 10-20V input range and output a constant 13V DC. I would connect it between the battery and appliance circuit. A combination buck/buckboost that can output 450 watts would be ideal. I have been unable to find a device that can do this and output more than 150 watts. I'm told these devices cannot be connected in parallel to increase wattage.

A second complexity is that I see the buck/buckboost devices are inefficient at low draw. My usual draw will be only 20 to 40 watts.

Am I going about this the right way? I'm starting to wonder if I might need to wire each LED light, fan, and appliance with a constant current circuit, and then run a second DC line for high power devices and connect it to a relay so it won't operate if the charge is happening. It's a lot more work than a variable voltage converter, but I seem to have hit a wall. Any help is GREATLY appreciated!

 

A second set of batteries would solve that problem and more. Any reason you want to avoid that common solution?

 

Good question. Here are the big reasons I'm trying to avoid that.

A second bank of Edison cells costs $3K.
The cells have a discharge rate of about 20% per month, so twice the batteries is twice the wasted power.
Each bank weighs 600lbs. As the power station is for an RV, the extra weight is quite limiting.

 

Get a 5V power supply that will accept the 10-20V input and then feed it to a 5V in 12-15V out (adjustable) boost supply. You might need some caps in between to filter out the ripple.

 

Maybe you could use a buck converter only, with the output voltage set for the maximum that your appliances can tolerate, 14V. Then when the battery is charging, the converter does its stuff and limits the output voltage. But when the solar array isn't producing anything, the battery is always at less voltage than the converter wants, so it doesn't function. Current flows through the inductor but nothing else happens. See the basic diagram for a buck converter on Wikipedia:
http://en.wikipedia.org/wiki/Buck_converter

One question is, if you have an appliance that draws a lot of d.c. current via a large inductor, will you get a colossal voltage surge when that appliance turns off? You'd need to plan for that situation.

And is it feasible to build/buy a converter capable of delivering 450 watts at all? That is a pretty enormous current, close to 40 amps.

 

Thank you both.

John, the point you raised about the voltage surge got me to think more about redesign. I think the best solution is to run two circuits, one for low power devices @12V and another that's tied to a 110V inverter and enabled with a switch. I'll use the 12 volt circuit for lighting and fans alone, reducing my 12V circuit to a peak of 6-10 amps. The other circuit will be turned on when high current devices are needed.

Here is the buck I'm looking at right now. Any thoughts? Does anyone know of a similar device that's industrial grade?

http://cgi.ebay.ca/0-8-28V-150W-buc...143?pt=LH_DefaultDomain_0&hash=item3f084560cf