The 24V Question


Last weeks experiment was about setting the batteries and panels up for 24v rather than 12v. That wasn’t he hard bit tho. The issue is that my little inverter and main night light are exclusively 12v and will either cut or burn out if you try to drive them too high. You need a power regulator or buck converter to drop the voltage efficiently, and I don’t have one of those. Sure I could buy one, but that’s hardly the point of all this now, is it? Building from scratch requires some fairly specialised parts I just can’t readily get from salvage.
So Tom got recabled, made MUCH easier with the arrival of some crimpable alligator clips so I can make connections without resorting to cable ties that come loose and short out right when you’re shoving 16A through them. Now, I don’t have a buck converter, but I DO have a device that will take a 18-50v and regulate it down to 12v: Brains.
Brains is a solar charge controller. It sets its target voltage by seeing what the batteries connected are doing. Connect to ~12v and it’ll adjust to that. So I did something a little warranty-voiding and connected the main 24v stack into the solar panel input, then the battery port into Bombadil on its own to give me the 12v reference. The 12v light went into the load port and lit up just fine. The inverter connected to Bombadil directly. That fired up too. Success! It was dark by then, so I left it for a live test the next day.
“The operation was a success, but the patient caught fire...”
Energising my absurdly “so not recommended it’s not even converted in the manual” setup went just great until I ran some load through the inverter. Voltages went bonkers, Bombadil drained like a pisshead after half a dozen pints and Brains started smoking until I killswitched it (i have an honest-to-gods Big Red Button killswitch on order, by until that arrives my killswitch consists of rubber-handled pliers and rubber-soled shoes). My best guess is that while the solar port can take up to 20A, it “accepts” it, rather than “pulling” it. Sort of a pressure arrangement. I haven’t dismantled it to check, but the smoke was probably insulator around the diode that prevents back-drain on that circuit.
24v is going to make things a lot more efficient, not to mention reducing the need for heavy cabling (cables are gauged based on current. Double the voltage, halve the current for the same power, thank you Fr Ohm. Still, until I lay my hands on a better inverter I’m stuck on kiddy-voltage so the batteries got another rewire. It did give me the opportunity to update my work area with some bookshelves salvaged during a late-night explore of the kerbside collection last month...
I don’t have any photos from the experiment, but this is an updated Tom layout, plus a cat enjoying her solar panel cave...



Dave Murray Thought about using usb powered devices? Less of a drain. I have a small power bank and a usb lamp that's there if I have a black out
Manage
Peter Raven Yes and amusingly that was what I was trying to achieve before the Project had a name. The challenge is that USB devices have a very low draw - ~10Wh/day, and you can get plenty of solar power banks online for not a lot of money. There's not a lot of value-add to be made. i had a couple of power banks I was using this to charge, but they've all died one form of death or another. I have a couple of shells on order that I'll slot spare LiPo cells into (topic of a future post). 

The other thing is that USB is historically limited to 5V which severely limits what you can run on it. USB 3.0 PD gives you 20V which is FAR more useful, but then also requires a much more advanced system. What really interests me is seeing how much I can run on straight DC - laptops, routers, NAS, even the monitors on my desk have DC inputs. I have plans for building a power delivery system to run a lot of these straight DC -> DC -> DC, but that's proving non-trivial. Setting up a simple device that can convert from 12-24V to all of the different voltages required is... painful. In the short term going DC -> DC -> AC -> DC is simpler and while it's lossy compatibility cost is a factor. 

Lastly, lead acid batteries can take a beating and used units are cheap (in that people are often happy of you just take them away). Proving that you can breathe new life into dead units has been interesting. It makes for a cheap source of capacity, and conforming to automotive standard opens up a lot of options for cheap accessories and interconnect.

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