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Has anyone successfully added a 3rd battery to the dual AGM stock setup? The van has dual alternators onboard as well.

I want to avoid buying another large battery. adding another 70Ahr battery would be plenty to support my planned setup. (LEDs, water pump, USB charging, and webasto heater)

is this possible? I imagine it should be, but I'm not well versed enough on the stock wiring setup. Can I just hook it up I'm series with appropriate gauged wire and be good to go?

I'm Ok with having house and crank battery be on the same setup. I'd rather have a jumper on hand than install a separate house battery system.

please resist the "you need a separate house battery" thread hijack
 

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I believe only two or three people have done this before, Here is the first one.

 

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2021 W2X High Roof Cargo T350, 148", EcoB, AWD, Avalanche Grey
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Have you figured out what programming changes, if any, are necessary for your smart charging system to recognize that a 3rd battery has been added? Seems it could be important for the battery management system in the transit to "know" the battery capacity was changed so it manage the charging for the correct SOC.
 

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IIRC there was a post where someone upgraded from a single battery to dual battery configuration that shows the Forscan settings for battery type/size/qty, It seem that there would not be a setting for 3 batteries, at least that would be intended for the Transit based on the documentation in 4.5.5 - 4.5.7 et.al. of the BEMM

Note that the installation discussed in the posting that @Michael Ophus linked is for a pre-2020 Transit which has a more conventional charging system than the 2020+ Transits have (which based on your dual alternator comment indicates is what you have) BTW - it is helpful to add the basic info on your van in your signature

Personally, in this case, I would not go with an off label installation that Ford specifically notes will impede the operation of the charging system as designed.
 

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2020 High-Extended AWD EcoBoost Cargo with windows
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Has anyone successfully added a 3rd battery to the dual AGM stock setup? The van has dual alternators onboard as well.

I want to avoid buying another large battery. adding another 70Ahr battery would be plenty to support my planned setup. (LEDs, water pump, USB charging, and webasto heater)

is this possible? I imagine it should be, but I'm not well versed enough on the stock wiring setup. Can I just hook it up I'm series with appropriate gauged wire and be good to go?

I'm Ok with having house and crank battery be on the same setup. I'd rather have a jumper on hand than install a separate house battery system.

please resist the "you need a separate house battery" thread hijack
Seems like there was also someone considering replacing the stock batteries with lithium batteries as well. Not sure that's a great idea... but it might show up in a search.

While the 2020+ has a shunt that tracks, I don't think you'd do any harm by just chaining off it. Not particularly different than any other load so long as you go with another AGM - same/similar voltage profiles.
 
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