Just watched a video on it to charge a Bluetti and/or Battery using an extra solar panel through this.
The B300s already has an input plus the cable to plug in extra solar panels DIRECTLY into the battery. You just have make sure the panels are within the 60 Voc input spec for the battery.
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I’ve been using all my spare panels after I set up some new arrays, plugged directly into 3 B300s batteries. I used a clamp ammeter to see that the batteries are indeed pulling power from the panels even while being charge through the AC500 head with it’s 2 arrays.
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It would seem more logical to spend that money on solar panels that you CAN plug in directly, unless you have money to burn, which if that’s the case I could use another AC500 head to start running 240V 
D050S is mainly used for PV charging of the B300S from solar panels via the adapter input of B300S.
So are you saying this is to add an ADDITIONAL 500 watt solar panel to the battery in the lower plug AFTER you already have a 500 watt panel connected directly to the battery in the upper plug with the supplied cable (whatever it’s called), for a total of 1000 watts solar?
Ok, that makes sense now.
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I was looking at those befacial 550 watt panels, but you still can’t get the 500 watt limit out of them because the battery input limit is 10 amps, and the panel voltage is 42V, so only 420 watts or less in the real world, likely in the 380 watt range.
The DO50S also allows additional solar input on the AC200 and AC200P models as well as allowing a single solar panel to charge when the solar output is less than 35 watts.
There’s not a whole lot of information about it on the website, or even in the downloadable manual. They could likely sell more if they eloborated on the description.
You can seen in various youtube commemts people needing more information about Bluetti products, that they can’t get from the website.
The B300 has two input ports, a DC and an adapter input port.
The DC port can directly connect to the solar panel, but the maximum input is only 200W.
The adapter input port is mainly used for adapter charging, the maximum input power is 500W, but this port needs D050S if you want to connect solar panel.