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Old 03-11-2021, 10:23 PM   #1
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Brand: Thor Motor Coach
State: California
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THOR #10552
Chassis and Coach Battery Voltages

My chassis and coach batteries always show the same voltages, plus or minus 0.1 volts or less. This is true when I'm not on shore / generator / or solar power (discharging), and when I am charging the batteries. I thought that my Victron BMV-712 battery monitor might be the culprit, but I confirmed the voltages using a multimeter, and with the readings on my solar controller (PWM-30). The issue persists when the solar power is disconnected from the batteries.

It appears that the battery isolation is not working correctly. My coach (2018 Thor Windsport 29M) uses a Precision Circuits BIM 160. I don't have any issues charging the batteries via the alternator, converter, or solar controller. My auxiliary switch works correctly as far as I can tell.

If it might be a faulty BIM, can someone tell me specifically how to test it? How can I confirm that my chassis battery is somehow on the same circuit as my coach batteries?

I've read numerous threads and there are a lot of very knowledgeable people contributing. Any help would be appreciated. This problem is driving me crazy.

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Old 03-11-2021, 10:51 PM   #2
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Brand: Thor Motor Coach
Model: 2020 Magnitude SV34
State: Florida
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I learned quite a bit about the BIM160 last year when troubleshooting some electrical issues with my 2020 Magnitude. Turns out Thor had it wired incorrectly. That may or may not be your issue.

First, when on shore or generator power and the chassis battery reaches ~12.6V, the BIM will connect it to the house battery circuit so the Converter can charge the chassis battery. All batteries will read about the same voltage when this occurs.

When the engine is running and the house batteries reach 12.6V, the BIM connects them to the chassis battery so the alternator can start charging the house batteries. All batteries will read about the same voltage when this occurs.

There is a BATT-A and a BATT-B post on the BIM. I found that for the BIM to work properly, the chassis batteries need to be connected to the BATT-A post and the house batteries need to be conncted to the BATT-B post. The wires for reading the chassis battery voltage also needs to be connected to the BATT-A post.

In my case, Thor had the house batteries connected to BATT-A and the chassis batteries connected to BATT-B. This causes all sorts of electrical issues for me... including erroneous voltage readings on the FireFly display for the house and chassis batteries.

There were a few others that had the same problem and when they changed the wiring the way I did, it solved their problems. I spoke with Precision Circuits, the manufacturer of the BIM and they said it should not matter which way the batteries are connected but there is definetly a difference. I don't have a schematic of the BIM internal circuit board but I think the key is the IGN post being next to the BATT-A post and that is why the chassis battery should be connected to the BATT-A post.

I also found other RV manufactures that use the BIM160 and they wire it the way I did. I even found this diagram on one of their websites. Some wiring diagram in old Precision Circuits documentation was very confusing but I see they have now updated it and it looks like the picture I have attached. Thor's wiring diagram shows this reversed for my coach.... which I am convinced is incorrect.

It might be worth checking your BIM wiring. If everything is wired ok, then perhaps the BIM is stucking in a latching mode. Precision Circuits is good to deal with and they may send you a replacement after they troubleshoot with you should you contact them.
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Old 03-11-2021, 11:42 PM   #3
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Thanks for the input. I found the BIM in my coach battery compartment and of course it's very difficult to access, but I'll definitely check the wiring. I have the diagram and saw your previous posts. Maybe...just maybe that's my problem.



As an aside, when checking voltages with my Harbor Freight multimeter, the readings were significantly higher than my Victron voltages. Checked again with another meter...closer. And finally a new meter (HF gives them away with purchases) and much closer. What junk! No wonder Fluke meters cost more.
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Old 03-11-2021, 11:56 PM   #4
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The BIM160 should disconnect the batteries after an hour of charging to prevent overp-charging. When that happens, the voltage should eventually be different between the chassis and house batteries. If not, maybe the BIM is hanging up for some reason.
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Old 03-12-2021, 05:34 PM   #5
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You might want to check and see if your emergency start switch has been turned on?
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Old 03-12-2021, 08:30 PM   #6
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Judge's comment above is correct. I have the BIM 160 on my new Thor Axis and often see very different battery voltages, like 14.0 (chassis engine, running) vs 13.5 (coach with solar controller on float).
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Old 03-12-2021, 09:04 PM   #7
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Thanks...The emergency start switch appears to be working. I found an old BIM test procedure online (2012) from Precision Circuits. I'm assuming it's basically the same for my BIM. It's raining today, so I'll conduct the following tests in the next few days and let you know what I find out. This might be useful to others.

My basic problem is that the Engine (Chassis) and House (Coach) batteries are apparently not isolated properly.

BIM Test procedure:

1 Disconnect Engine battery and check voltage on “Batt A” terminal and ground (middle terminal). Should be 0 volts.

2 While Engine battery disconnected, check Aux Switch to see if engine will start.

3 Reconnect Engine battery (if ok as above) and disconnect House battery. Check “Batt B” terminal and ground.

4 Reconnect House battery. Press Aux switch. Should hear a click.

5 While Aux switch in “on”, check voltage between Batt A and Batt B. Should be less than 0.2 volts.
If not, check for 12 volts between Dash Switch terminal and Ground terminal.
If 12 volts and and relay does not click and does not connect Engine and House batteries, replace.

6 If number 5 okay, release Aux switch and start engine. Turn on house lights. Should hear click within 2 minutes.
Check voltage between Dash terminal and Ground terminal. Should be between 3.5 volts to 6 volts.
If not, voltage between Engine Batt A and ground should be > 13.3 volts.
Voltage between House Batt B and ground should be < 12.6 volts.
Voltage between Ignition terminal and ground should be > 12 volts.

7 Turn off engine. Plug in shore power. Converter should be on. Turn on headlights. Relay should click within 10 minutes.
Voltage between Dash terminal and ground should be between 3.5 volts to 6 volts.
If not, voltage between House Batt B and ground should be > 13.3 volts.
Voltage between Engine Batt B and ground should be < 12.6 volts.
Voltage between Ignition terminal and ground should be < 2 volts.
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Old 03-13-2021, 11:51 PM   #8
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Well, Judge....it was the Precision BIM 160. I disconnected the chassis battery and checked the voltages. THE WIRING FOR THE COACH AND CHASSIS BATTERIES WERE REVERSED!



I had to take out both batteries in order to rewire the BIM. Not fun. I'll contact Thor next week but my rant will probably fall on deaf ears. I'm surprised that Precision said the reversed wiring doesn't make any difference. It definitely does, as you suspected.


My Victron BMV-712 now shows different voltage for the Chassis and Coach batteries. At this time, everything appears to be working correctly. I'll confirm that over the next few days.



Thanks for your help.
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Old 03-14-2021, 12:33 AM   #9
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Glad to see that you solved the mystery.
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