this post was submitted on 29 May 2024
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I have a 100 W rigid solar panel including a charge controller that I currently only use for camping to charge batteries (also useful in an emergency at home). It strikes me as a waste that I could be generating more clean energy with equipment that I already have, but I don't have anything in mind to use this energy for.

Obviously I could try to tie it into my home to run more of my household on solar, or buy more/bigger batteries to charge, but with 100 W of generation, it's probably not worth it without a significantly increased investment.

I tried searching around online, and I found plenty of discussion for what to do with a whole house that generates excess capacity (mainly sell to the grid), but nothing really on what to do with small scale DC generation.

Anyone here have thoughts?

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[–] [email protected] 10 points 3 weeks ago* (last edited 3 weeks ago) (2 children)

I had a spare 50W panel and charge controller that I used similarly for camping. Had more or less the same thought as you: I should use this year round.

Ended up building a wooden box with a couple of old car batteries inside along with the controller. Kept it outside and ran a cable inside carrying 12v (make sure you put an inline fuse after the battery in case it short circuits along the way). Used the 12v from that and some old car chargers to set up a charging station for all my devices.

The car chargers were all random plug styles since they were from a box of stuff I've accumulated over the years, so I cut those off and spliced on USB-A female ports so they could charge any USB device. I eventually ran a second pair of 12v wires so I could run a small 100w inverter from the batteries.

Worked great, but the car batteries crapped out after about 3 years (in all fairness, they were junk to start with).

If I had to do it over again, I'd do it similarly but use 12v USB-C power-delivery adapters instead so I could charge bigger stuff like my laptop without having to use an inverter.

[–] [email protected] 4 points 3 weeks ago (1 children)

3 years seems pretty good for using a car battery outside of it's preferred use case. I guess that depends on how good/bad you were about deep cycling it.

Currently, the batteries I'm using are my power tool batteries, which are 18V so they charge through a dedicated (12V) charger, and I have a little USB A/C and low powered inverter that uses them. I probably wouldn't necessarily want to put my lithium batteries through every day cycling, though.

I've thought about generating hydrogen with it to use for experiments and such, but idk if I have the space for that.

[–] [email protected] 1 points 3 weeks ago* (last edited 3 weeks ago)

3 years seems pretty good for using a car battery outside of it's preferred use case.

Better than that :) They were junk car batteries when I got them, and I still got 3 years out of them. But yeah, they didn't get deep cycled much or at all since there were 2 in parallel and the loads I had on them were very small (5-6 500 mAh cell phone chargers and occasionally some LED rope lights).

When I eventually hooked an inverter up to charge my laptop or run a lamp during power outages, they started to show their age with the extra current draw. I think that's what finally did them in.

Yeah, lithium batteries wouldn't like that kind of daily cycle without some kind of charge/discharge limiter to keep them in the 30-70% range. That's basically what hybrid and EV battery managers do to prolong their useful lifespan. I think LiFePO4 lithium batteries would tolerate that better (they're the ones typically used in e-bikes), but they're not cheap. I've also found it difficult / expensive to find solar chargers for them (to be fair, mine is 48v 20AH so finding any aftermarket charger for it has been a challenge lol).

[–] [email protected] 4 points 3 weeks ago (1 children)

Awesome effort! If not done already, you may want to look into venting that outside box. I believe there are examples of the out gassing of enclosed lead-acid batteries causing corrosion of nearby metals. Hilarity ensued.

[–] [email protected] 3 points 3 weeks ago

It doesn't exist anymore, but yeah, good call. I never replaced it once the batteries finally gave out, and I moved less than a year later.

It did have vents on the sides for airflow, but that was more of a consideration for heat build up and keeping the controller from cooking in the summer. It was also less a "box" and more like a small doghouse (including shingles lol). I atually gave it to a neighbor after I was done with it; they cut a hole in the door and used it as a cat house.

[–] [email protected] 2 points 3 weeks ago (1 children)

I'm no expert, but I'd imagine figuring out a (safe) way to use it with your water heater would be the low-hanging fruit, in terms of least-complexity for the amount of grid consumption you'd potentially save.

[–] [email protected] 3 points 3 weeks ago

That's an interesting idea. A water heater is a really underutilized battery that most households have. I suppose you could hook it up to a thermostat with a set point a couple degrees higher than the mains (or gas) powered thermostat.

A quick search says in my location with a 100W panel, I'll generate 400Wh as my daily average (1.44 MJ). With a 150 L tank, that gives you about 2.25 K increase in temp for a day.

That's not nothing.

[–] [email protected] 1 points 3 weeks ago
[–] [email protected] -2 points 3 weeks ago

Get a DC to AC converter, plug your computer into it, and mine Monero.