diy solar

diy solar

chaining (as in DC out -> MPPT in) of Bluetti or equivalent

Kae

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Apr 8, 2020
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my solar use case is dual purpose: saving some electric bill day-to-day (< 500W load), and having access to off-grid power in an emergency

urban / renter setting - it means portable generators, and flexible solar panels on my balcony or non-permanently affixed to the roof

why would I want to do such a thing as chain self-contained solar generators like the Bluetti? (By chaining I mean solar input goes to one unit, it feeds another, and all loads are connected to the 2nd unit.)
  • in my region, Bluetti 1500Wh is the best value, but I need more storage
    • cost per Wh of Bluetti 1500Wh is less than Bluetti 2400Wh (and 2x 1500Wh gives me 3000Wh vs 2400Wh)
    • Point Zero products are not available, otherwise I would go with Titan
  • versatility - in a power outage I can bring multiple units where needed inside or outside the house, lend one to a neighbor, etc.
  • durability - if one unit dies, I can get by with the remaining one
  • safety - more cooling, less batteries in one container. And 1500Wh units are reasonable weight for an old person to carry up and down stairs.
  • I happen to have two panels, but hooking one to each solar generator isn't attractive
    (for daily use I'd need to worry about balancing power use between them)
  • regarding battery cycling: on typical days I don't expect to go below 2/3 capacity of both battery banks combined
so yes, I can plug the A/C brick into the first unit to feed the second. Generates a huge amount of heat and 15%+ power loss... basically negates the MPPT benefit.

so I'd love to power the 2nd unit from 12V on the first (leaving its AC inverter off). Something with >90% efficiency. And I realize it's greatly complicated by the MPPT controller, after having read countless threads by people wanting to drive MPPT from their auto's 12V.

I tried a cheap DC/DC "constant current" boost converter at various settings, but no go. https://www.droking.com/dc-power-su...-Regulator-DC-12V-24V-36V-48V-Adapter-Charger

Next I plan to try a few higher-priced converters: 1) https://www.droking.com/600W-Consta...80V-DC-Step-Up-Electrical-Converter-Regulator, 2) https://www.droking.com/900W-Power-...wer-Supply-with-LED-Display-Voltmeter-Ammeter (The 2nd doesn't meet my efficiency needs.)

open to advice and criticism, but understand that I already own two Bluetti units
 
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I've just revisited this chaining project now that the days are getting shorter.

The Drok 600W converter did the trick. The efficiency is like 98% at my operating voltage and it doesn't need a fan.

Unfortunately the noisy fan of the Bluetti kicks in when you draw more than 50W DC. So I've got the converter set at 18.6V, which settles nicely just under 50W.
 
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