I’m experiencing an issue with the 7909 DC input plug on my Bluetti EB3A. Unfortunately, this connector is very difficult to find where I live, so I built a DIY charging setup: a car lighter plug connected to a 5.5×2.1 mm DC plug, and then a 5.5×2.1 mm → 7909 adapter to charge my EB3A.
The problem is that my car doesn’t charge the unit using this cable. The EB3A charges perfectly through solar input, so I know the input port itself is functional. However, when I try using this DIY cable—or other similar DC-to-DC setups (such as from 12 V batteries)—the input icon on the EB3A just flashes.
I’ve read that this flashing icon may indicate low input voltage, but I measured 12.8 V (engine off) and 14.2 V (engine running) with a voltmeter, so the voltage appears fine.
While inspecting the wiring, I noticed something that might be relevant:
On the original Bluetti solar cable, the outer layer is negative, and both the inner layer and the center pin of the 7909 plug are positive. (center pin being powered seems to be unique to bluetti cable)
On my DIY setup (5.5×2.1 mm → 7909 adapter), the outer layer is negative, but only the inner layer is positive—the center pin isn’t connected. (Typically a few items that i checked afterwards like laptop chargers with 7909 connectors they didn’t had middle pin powered neither just outer layer and inner layer)
Could this missing connection to the center pin be the reason the EB3A doesn’t charge?
I also tested a 5 V→13 V step-up module connected through Bluetti the solar cable, hoping to feed around 15 W into the MPPT controller, but the input icon still just flashed with no actual charging.
Before I buy a car lighter-to-MC4 adapter and connect it to the Bluetti’s original MC4 To 7909 solar cable, I’d like to confirm if that setup would actually work—or if the issue lies elsewhere (for example, polarity or software protection).
Any insights or guidance would be greatly appreciated! @BLUETTI_CARE
Thank you,
Have you checked continuity with a multimeter? Centre pin to centre pin at each end and the same for the outer.
A D7909 and or a D5521 are both 2 pole only and you are correct, the outer is Neg (-) and the internal pin is Pos (+). As with all electrcal connections, the more adaptors, joiners and cables you use, the higher the resistance. Why not just order the correct cable from Bluetti?
The maximum charge you will get from a 12V vehicle source via a cig plug is <100W.
I am just trying to use what i had before to charge EB3A in emergency situations so 100w is a lot for me actually, actually 7909 has 3; outer layer which is neg inner layer which is + and central pin which is usually doesn’t have pos or neg charge, in bluetti cables it is also pos, Yes i did check polarity and continuity i always do before doing anything. I can’t order from bluetti sadly or even amazon so DYI is my only option.
Hi @Sorenalike, We are very concerned about the issue you raised.
Are you planning to DIY a car charging cable to charge the EB3A yourself?
The original car charging cable for the EB3A uses a DC7909 to cigarette lighter adapter, with a wire gauge of 16AWG.
We do recommend that you use the original cable. @Mandp’s suggestion is very helpful.
Could you please contact us via private message and provide your order information? This will allow us to check if we can find a suitable cable for you.
Thank you for your answer and concern but there is nothing to be concerned about, i got this bluetti from a third party new so i don’t have order number, Yes, i already tried it and i used proper equipment including a wire that is rated for over 10amps or maybe over 20amps there are no problems on that side
the problem is it didn’t work for the reasons that i mentioned most likely @BLUETTI_CARE :(
next i wanna try car lighter plug to MC4 and connect that to bluetti original solar panel cable, car lighter was even fused at 10amps so i do follow all of the safety precautions i also use voltmeter for polarity check to make sure. i just want to know if this would work because previous methods didn’t work while they should have so i am not sure if its just waste of time and money that is all, i mean it is not rocket science its just plugs and wires and such what did i do wrong that it didn’t work while it should have ?
Hi @Sorenalike, We indeed do not recommend customers DIY cables by themselves.
Our car charger cables use the universal standard 7909 connector, not a custom-made one. Under normal circumstances, a regular DC7909 cable will work. We suggest customers purchase finished cables from third-party stores, as DIY attempts may lead to dangers if there are mistakes.
Alternatively, you can tell us your location, and we will check with the warehouse to see if there is any stock available.
Most likely the corresponding cable is an OUTPUT cable meaning its designed to use the Bluetti itself as a power source (not a charging one), that then supplies power to another device via the DC5521 cable. So the 12V cigarette lighter goes INTO the Bluetti and the DC5521 goes into say a CPAP. It’s output, not input.
I live in Iran i don’t believe you guys have any vendors here that is why i bought it from a third party and they don’t have your cable neither sadly :(
What i am trying to do now is to use cigarette lighter plug to MC4 connection and connect that to MC4 connection of solar panel cable that you guys gave us with the bluetti basically like a diagram below
it is a simple easy and safe connection i am using a cable rated for over 20amps so i should be ok, rest is simply polarity check with voltmeter and plug connection its not rocket science
There is literally 0 excuse that this shouldn’t work, i should get my plugs tomorrow to set it up
Ok whatever the issue is my best guess is the middle pin not having power or 5.5x2.1mm to 7909 connector was the issue
Lighter to MC4>MC4>7909 worked like a charm
Thank you everyone for helping me out and being patient with me <3
Hi @Sorenalike, Thank you for the update. We are glad to hear that the issue has been resolved.
If it’s not convenient for you, you can find cables from other brands as replacements, provided they match our plug interface specifications. We hope everything goes smoothly for you moving forward.
