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Old 12-02-2021, 09:12 AM   #1
Junior Member
Join Date: Dec 2021
Posts: 2
Question BIM 225 wired backwards?

I have a 2021 Ventana 4369. I recently upgraded my house batteries to lithium. Now I am trying to swap the current Precision Circuits BIM 225 with the equivalent LI-BIM 225.

Problem is the standard lead acid BIM battery terminals in the coach now are labeled Batt A and Batt B. Do these non-descript names mean it does not matter which post is connected to chassis vs coach? The reason I ask is because the LI-BIM battery posts on the one I am installing are specifically labeled Chasis and Coach. In the case of the LI-BIM it makes a difference and reversing them would result in improper charging.

When I look at this wiring diagram…

It specifically shows the chassis on Batt A and the coach on Batt B. When I measure the voltage on each of the battery lugs on my coach, the voltage on Batt A matches the coach and the voltage on Batt B matches the chassis. This is reversed according to the schematic.

I am a newb when it comes to RVs but am a big time DIYer with all sorts of things. I am hoping the more experienced on this site can help me sort it out and get it right. My wife and I just retired and are full timing it to see the country. Since this is our home I figured I should err on the side of caution and ask here before I mess it up.

Any help is greatly appreciated.

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Old 12-02-2021, 10:36 AM   #2
Senior Member
Join Date: Apr 2020
Posts: 297
The Precision Circuits BIM 160/225 is often wired backwards. Thor even shows it wired backwards on their circuit diagrams. Judge, a smart guy on the Thor forum clued me in to this.

Some say it doesn't matter, but my new Thor Axis was not charging from the chassis alternator correctly. I swapped the connections and now it charges fine.

So, connect the chassis to the side with the ignition connection which is Batt A. See the attached PC data sheet which shows it wired correctly.

If you want to read Judge's conclusions on this, see the attached .doc file.

Attached Files
File Type: pdf Precision Circuits BIM 160-Rev7-1.pdf (711.5 KB, 12 views)
File Type: doc BIM 160 Wiring.doc (14.0 KB, 9 views)
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Old 12-02-2021, 11:08 AM   #3
Newmar Owners Club
Join Date: Feb 2018
Posts: 79
I had the same issue with my 225 BIM when I upgraded to Lithium's. Could not figure out why my chassis batteries were discharging when plugged into shore power. Used multi meter to figure out which wire went to chassis. Sure, enough the BIM wires were reversed. Wired it correctly and the problem when away. Amazing! LOL
2010 Campinn Teardrop
2014 30' Airstream Flying Cloud
2019 Ventana 3717
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Old 12-03-2021, 07:09 AM   #4
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Join Date: Dec 2021
Posts: 2

For those doing a lithium upgrade and replacing their BIM with an LI-BIM, make sure you confirm the battery connections are correct (assuming the wires are not labeled) by confirming the voltages on each battery post. Your lithium coach batteries should be in the 13.5v range when fully charged. Your chassis LA/SLA should be about 12.5v. Thanks to the comments above I have a feeling many manufacturers just attach them without regards to chassis vs coach. With LI-BIM it will make a difference as experienced by Beach Livin.

I will replace mine shortly and post the results.

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