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Joined: 05/06/2004
Location: AustraliaPosts: 5193
| Posted: 05:06am 21 Jun 2025 |
Hiya.
I live off grid, solar and battery with a windmill to help on windy nights. In summer or on sunny winter days it works fine, the batteries complete a charge cycle usually by lunch time. But there are times when there just isnt enough solar to give the batteries a good charge, like heavy overcast rain days in summer or cloudy days in winter. Even on those days the system breaks even, enough energy coming in to keep everything working, but the batteries cope it hard after a couple nights. Still, its never gone down, never had a blackout.
Easy, add more panels. Problem is my charge controller is limited to 60 amps, and while it will current limit to a point, those little switching mosfets will be carrying whatever current the panel can provide in the few milliseconds they are switched on. I dont need a bigger charge controller, my ideal charging current is 49 amps into the batteries during boost.
I had 20 190watt panels on the roof of my shed. At 50 volts that's about 76 amps on a good day. In summer I tend to switch a few panels out of circuit, be nice to the charge controller. The controller does complain if its got to much coming in, plus it gets hot, even with some extra fans fitted.
To provide some extra amps on overcast days I added another 8 panels to my 20. Picked up the same size and wattage panel, for free, just got to know the right person and second hand solar panels are almost given away here in Australia. On the shed roof.
So that gives me 5320 watts potential, or 106 amps. Thats just to much for the long life on my controller. As these extra panels are really only needed for overcast days, I built a light sensitive switch to only switch these extra panels into circuit when there is cloud cover.
Its uses a LDR mounted on a stick up near the panels, fed into a opamp, with a level and hysteresis adjustment pots. The output drives a SSR. In my solar junction box in the middle row you can see the 48v - 12v power supply ( used for cooling fans on the charge controller, etc ), the SSR and the opamp in a box. I wont show the circuit board, its a real prototyping mess.
A bit of adjustment while some clouds were going over, and it now works great. When its overcast the extra panels give about 6-8 extra amps into the charge controller. A couple days ago it was overcast all day and for the first time my system completed a full charge cycle under those conditions.
As a backup in case the light switch doesn't shut off under full sun ( eg, a bird poops on the sensor ), the extra 8 panels are fed via a 10 amp circuit breaker, this should pop in full sun as the panels can deliver 30 amps.
Glenn
The best time to plant a tree was twenty years ago, the second best time is right now.
JAQ