Testing an Aux batt solution...

[kd-]

The fun continues... I think I blew the 125A fuse (F212 in the diagram here: https://www.f150forum.com/f129/powerboost-auxiliary-battery-isolator-569730/) in the BCMC / Battery Junction Box under the hood (behind the battery, next to the fuse box, connected to the battery isolator). Have my new battery all ready to go too!

Anyone ever replace one of these fuses? Haven't been able to find instructions anywhere, if anyone has a resource for a service manual that'd be great too, otherwise I guess I have to start taking stuff apart.

Sponsored

 

[Tired Lizard]

The fun continues... I think I blew the 125A fuse (F212 in the diagram here: https://www.f150forum.com/f129/powerboost-auxiliary-battery-isolator-569730/) in the BCMC / Battery Junction Box under the hood (behind the battery, next to the fuse box, connected to the battery isolator). Have my new battery all ready to go too!

Anyone ever replace one of these fuses? Haven't been able to find instructions anywhere, if anyone has a resource for a service manual that'd be great too, otherwise I guess I have to start taking stuff apart.

Related fuse link https://www.f150gen14.com/forum/thr...-inverter-in-place-of-stock.18901/post-600739

 

[kd-]

Related fuse link https://www.f150gen14.com/forum/thr...-inverter-in-place-of-stock.18901/post-600739

Thanks, in my case (and likely other PowerBoost folks) it's a 275/125/125A, so looks like MU5Z14526B. Add another $35 to the project cost...

 

[Snakebitten]

Do you know how you popped it?

 

[kd-]

Do you know how you popped it?

I'm not exactly sure. If I had to guess - the under-hood AGM voltage was really low at some point perhaps? Or maybe I shorted the positive and negative terminals accidentally on the aux batt side?

When I replace the fuse and re-set up the Aux battery situation (managed to fit my custom 15Ah box and the 7.5Ah factory Aux in there) I'm going to make sure the under-hood AGM is fully charged and the LFP isn't sitting above 14V, then probably hook it all back up under the hood.

Still not 100% sure the fuse is the problem, but I did test with another lead off that battery junction box and was able to get a voltage difference off my multimeter

 

[tsigwing]

I'm not exactly sure. If I had to guess - the under-hood AGM voltage was really low at some point perhaps? Or maybe I shorted the positive and negative terminals accidentally on the aux batt side?

When I replace the fuse and re-set up the Aux battery situation (managed to fit my custom 15Ah box and the 7.5Ah factory Aux in there) I'm going to make sure the under-hood AGM is fully charged and the LFP isn't sitting above 14V, then probably hook it all back up under the hood.

Still not 100% sure the fuse is the problem, but I did test with another lead off that battery junction box and was able to get a voltage difference off my multimeter

The aux battery has its own 125A fuse.

 

[kd-]

The aux battery has its own 125A fuse.

Yup, and I'm surprised that didn't go first. I was able to get the fuses out and confirm it was the issue (in retrospect could have done a continuity test with my meter here too)

I'm going to go with a "temporary" solution of an inline 125A fuse jumped over to the main part of the busbar until I confirm I'm not going to blow it up again...

 

[FriedCheese]

Since this project has fallen way down on my priorities list, I'm going to go ahead and post the Fusion/STEP files for the box design I started working on. It's like 90% there. The last thing I planned to work on was getting the walls to be a bit more rigid without increasing the wall thickness by too much.

There are two plugs for covering the holes the OEM box leaves behind in the floor pan. I printed them in 90A TPU. This reuses the two main bolts holding the OEM battery box in. I also had to make some short jumper cables to connect the positive side cable to the battery terminal.

 

[Davexxxx]

Installed and BMS reset.

And?

Depending on the testing results of tomorrow, I may do the same, except I'll go with Tank's rec for the aux.

https://www.oreillyauto.com/detail/...ort-battery-group-size-btx14ah-agm/ssbn/etx15

 

[UGADawg96]

And?

Depending on the testing results of tomorrow, I may do the same, except I'll go with Tank's rec for the aux.

https://www.oreillyauto.com/detail/...ort-battery-group-size-btx14ah-agm/ssbn/etx15

No issues since replacing the ~4 year old batteries.

 

[kd-]

When I replace the fuse and re-set up the Aux battery situation (managed to fit my custom 15Ah box and the 7.5Ah factory Aux in there) I'm going to make sure the under-hood AGM is fully charged and the LFP isn't sitting above 14V, then probably hook it all back up under the hood.

I'm going to go with a "temporary" solution of an inline 125A fuse jumped over to the main part of the busbar until I confirm I'm not going to blow it up again...

Got the inline fuse wired up and the battery installed (so 15Ah of LFP in parallel with the existing Aux AGM), I mangled the box some more but managed to fit both in the existing battery box location and taped it shut with some flashing tape. I used a JK-BD4A8S4P from JK BMS as the BMS which wasn't cheap, but does have some nifty features like individual cell voltages and active balancing.

And I think I've attained 13v resting nirvana, or at least close enough - I imagine those AGMs really want to pull voltage down to ~12.8V - this is over a day and a half with just a few minutes of Fan Speed 5 charging.

Edit 11/8: After a few days (driving it a bit, those are the big spikes up) it's resting even higher in the low 13.x range. Drop to ~12.9 was me opening the door and keeping it open for a few minutes.

---

One thought I had: with a parallel LFP setup, how do you know when the Aux or under-hood AGM go bad? I imagine it'll be a while but it will eventually fail, I guess if the resting voltage drops too much it's time to replace? Or a cell shorts out and trips the LFP BMS protection or blows a fuse again?

Sponsored