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Forum Index : Solar : Added a winter/cloud cover boost to my solar array
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Gizmo Admin Group  Joined: 05/06/2004 Location: AustraliaPosts: 5116 |
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 |
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phil99 Guru  Joined: 11/02/2018 Location: AustraliaPosts: 2566 |
A very nice setup. Perhaps the new panels provide an opportunity for some redundancy. Connecting the new panels through another charge controller could provide independent charging in the event of a failure anywhere in the original panels - cables - controller setup. Set it to a slightly lower voltage to prevent excessive current going into the batteries when they are almost full. |
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| Solar Mike Guru Joined: 08/02/2015 Location: New ZealandPosts: 1162 |
We had a similar issue with a small 300AH 24v bank of lead carbon 6v batteries, not enough winter charge, so I added a temporary PV array with its own seperate mppt charge controller. The batteries have a max continuous charge rate of 70 amps, this can now be exceeded by a large margin if suddenly we get a bright sunny day. So I cobbled together a 24v bank 200AH bank of Lifepo4 cells (8 cells) wired in parallel to the lead carbon setup. As the end point voltages of both batteries are similar, it makes a great combination. The Lifepo4 cells can take 150 or so amps charge with no issues and absorbing any excessive charge from the additional PV. Besides giving some extra capacity, the Lifepo4 bank really does all the heavy lifting, with the lead carbon bank taking over when the Lifepo4 cells start going flat, a high current vacuum relay switches them out of circuit when they get too low. Cheers Mike |
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| analog8484 Senior Member  Joined: 11/11/2021 Location: United StatesPosts: 144 |
I've wondered how such setup might work. Assuming you have a BMS on the LFP bank, do you see the lead batteries help with surge power that goes beyond the LFP BMS surge limit? Would you mind sharing the vacuum relay you are using? Thanks |
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| Godoh Guru Joined: 26/09/2020 Location: AustraliaPosts: 524 |
Hi Gizmo, looks good. I am wondering why you don't use old GTI inverters and backfeed to charge the batteries. GTI inverters are really cheap second hand. I have two that are connected to two banks of panels. I pay less than $100 for a 5 kw inverter. Much cheaper than MPPT controllers. As far as putting more charge into the batteries than they can take, my experience is that the internal resistance of the batteries tends to limit the current. I have around 10kw of panels, but never see more than 2kw going into the batteries. I have my panels arranged with 4kw of panels facing North, and around 6Kw of panels facing NW. So the mornings the North panels that are on MPPT controllers bring the batteries up and in the afternoons the panels connected to the GTI inverters do most of the work. For the GTI inverters I use voltage sensitive relays to switch them out if the batteries are charged up. So far it works great. The GTI inverters are mainly used to charge the car and run bigger power tools. I do have problems on some very dark cloudy days, but fortunately we don't have too many of them in a row. Pete |
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| Solar Mike Guru Joined: 08/02/2015 Location: New ZealandPosts: 1162 |
Relay: Search on AliExpress for KiloVac EV200, usually around the NZ$40 mark. Yes LFP bank has a DIY BMS, any cell dropping below 2.8v and it turns off the above relay; LFP surge only limited by a ceramic fuse, 250 amps, nothing too fancy here. There was an immediate noticeable improvement on heavy loads, like a 2KW electric jug; the battery volts hardly sag, compared to the Lead Carbon battery on its own. Mike |
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| analog8484 Senior Member Joined: 11/11/2021 Location: United StatesPosts: 144 |
Thanks for the info. Just curious, does your DIY BMS also handle charging protection? |
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| Solar Mike Guru Joined: 08/02/2015 Location: New ZealandPosts: 1162 |
Yes, all cells in the bank are measured every couple of seconds, here is a link to one of the designs Balancer Mike |
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| analog8484 Senior Member Joined: 11/11/2021 Location: United StatesPosts: 144 |
Nice design and very comprehensive. |
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