Can CTEK D250SE # CT99FR Be Wired To 12V Power In 7-way Trailer Connector?

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Question:

I have a 2019 Ford F-150 with max towing package and smartalternator. I would like to charge a 100Ah LiFePO4 battery in my small slide-in camper from the 7-pin trailer harness while driving. My truck has an auxiliary 12V line in the 7-pin harness labeled as trailer tow battery chargeinch in the manual and it has a 25A factory installed fuse. I would like the most charge possible while driving. 1. I am wondering if the CTEK D250SE 20A dc-dc charger will be able to provide a full 20 amps of charging or close while connected to the 7-pin harness on this particular truck? CTEK tech support thought it would, but I value your expertise and opinion as well. 2. Or would I be better off going with the smaller Redarc BCDC1212S 12amp charger and be confident I can get at least the full 12amps through the factory 7-pin wiring? I also have a separate 20A solar MPPT charging system that I plan to keep even though both chargers mentioned above have solar inputs. Both Redarc and CTEK didnt think that would be a problem. If you have a different opinion please weigh in. My thinking is that my battery can be receiving 20A of solar charging and an additional 12-20 amps from the dc-dc charger while driving. If you could at least answer questions 1 and 2 I would appreciate it. Thanks for all you do to help everyone! Eric

asked by: Eric F

1

Helpful Expert Reply:

Hey Eric,

I wish your 7-way could output that much amperage for any amount of time, but it just cannot. To use a DC to DC charger like the Ctek D250SE # CT99FR you are going to have to connect it to the start battery of your 2019 Ford F-150. Two reasons, one is the 7-way power output is tiny wire from the factory as its designed for intermittent loads, it is not going to be able to sustain a DC to DC charger, the other is newer Fords like yours do a lot of odd stuff with the 7-way like needing a trailer connected to send power back to the 7-way a lot of the times. While its more work to run the charger to the start battery, its the only way its going to work reliably.

There is zero problem keeping your MPPT charge controller and using the DC to DC charger. Both of them will monitor the battery to make sure its being fed the voltage it needs to charge/stay charged with no issues overcharging.

What kind of trailer are you towing? Do you get out boondocking?

expert reply by: Samuel C

1

Eric F.

2/20/2026

Hi Samuel, Thanks for your great reply to my question! After I posted my question to the etrailer site I took apart the 7-pin trailer plug on my F-150 and found that it only has a 16 AWG wire supplying the 12V+ ("trailer tow battery charge" as listed in my manual) as well as finding a factory schematic that confirms it. I couldn't believe that it has a factory installed 25A fuse for that circuit! From my years of electrical work that seems like a bad choice, or a dangerous oversight! Regardless, you are 100% correct about not being able to successfully use a dc-dc charger plugged into the 7-pin, at least not for what we want to do. I have done my homework and now have a plan for installing a Redarc BCDC1225D and related overlanding installation kit E89ZR and relay kit 331-RK1260 from etrailer. We have a custom built aluminum slide-in pop up camper (similar to Four Wheel Campers) that we use for boondocking and I want to keep the battery charged up both via solar and as we drive, so the only reliable solution is a dedicated line from the vehicle battery as you recommended. Thanks again for your reply, I'm sure it will be helpful to a lot of people. Cheers! Eric

Reply

Samuel C.

2/20/2026

@EricF Yeah its pretty wild how the auto manufacturers use such small gauge wire for these circuits and put larger fuses than the wire can normally support. I've been told by some industry folks that it has to do with the circuit only being used intermittently, but seems really silly to me. Glad you got a solution figured out! If there's anything else you need, just let me know!

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Can CTEK D250SE # CT99FR Be Wired To 12V Power In 7-way Trailer Connector?

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