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Old 06-15-2018, 01:57 AM   #1
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THOR #11959
Coach batteries not charging from engine alternator

I don't have a Thor and I ask for help a few days ago in the wrong place (modifications and updates) and I still got some helpful responses! Thank you guys. I'm now starting to understand the problem.

The RV is a 25' Dynamax Isata (2003) which is a pretty high end RV. It has a Intellitec BD0 coach battery disconnect that seem to work fine and a "emergency" start button and isolator relay that also works fine. It doesn't seem to have a BIRD or a second chassis battery disconnect solenoid. I found the Isata wiring diagram and have attached that and a photo of the solenoids. They are mounted on a plastic base which I think is just a fuse/junction box not a BIRD.

Since dry camping more than one night in a row is rare for us, I don't think it's worth adding a complete BIRD system. I'm not sure it is even possible.

I did discover that locking down the "emergency start" button, while the engine is on, connects both batteries and starts charging the coach battery from the engine. I'm wondering if occasionally doing that would cause trouble elsewhere. The isolator relay has no label but i know they come in "intermittent" and "continuous" flavors. From it's intended use, its probably intermittent. Changing to the continuous wouldn't be difficult.

I'd appreciate any suggestion or comments
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Old 06-15-2018, 02:53 AM   #2
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THOR #10965
From my recent experience with this problem, try and track down any fuses between the relay / solenoid and batteries. I had to replace the solenoid (it is rated continuous) and then it behaved like yours (hold the button down and it charged). Found some a blown fuse and everything started working normally.
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Old 06-15-2018, 03:26 AM   #3
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THOR #11959
Thanks Bogie,

I did check all the fuses after reading your posts and all ok. I think everything is operating as designed, but there is no BIRD (bi-directional isolator relay delay) circuit on this RV. The circuit diagram and a pretty thorough search confirm that. If I want to occasionally charge the coach batteries from the engine, I'll need to install a BIRD system or find another way.

Latching the "emergency start" button with a broken pencil does the trick, but I not sure if there are other consequences.
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Old 06-15-2018, 10:59 AM   #4
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THOR #1765
It seems that at least some of the new Class Cís do not have a BIRD or BCC to recharge the chassis battery from the engine alternator, so it is not surprising that a 2003 coach was built that way. The only issue I can see with using the emergency switch as a solenoid is heat. The emergency switch and wiring may not be designed for sustained use and could heat up and cause a fire. I would carefully check the emergency switch wiring gauge and check and see if the wiring or switch gets hot. If you have a generator, it probably would be safer (but not as convenient) to use the emergency switch to start the generator and charge the house batteries from there.
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Old 06-15-2018, 03:00 PM   #5
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THOR #11959
Test results and my conclusions

I discharged the the coach batteries sightly (12.4v) overnight in order to test charging via the emergency start switch. I idled the engine to warm up a bit then locked down the emergency start switch. The voltage of the coach batteries slowly started to rise to 13.3v after 15 minutes. No heat detected in the isolator relay or surrounding wires. The relay's control wires are likely 18 gauge and felt cool. After turning the engine off, and releasing the emergency start switch, I connected the shore power and the coach batteries began charging at 13.7v. Since the the converter is only only a foot or 2 from the coach batteries, and the engine alternator is probably 30 wire feet away, the voltage difference (13.3 vs 13.7) seems normal.

The RV does have a generator, and I have used that to charge the coach batteries while driving. It good to know I have a more efficient alternative. If something bad happens, I'll report back.
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Old 06-15-2018, 07:20 PM   #6
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THOR #1765
Thatís a lot of amps going thru 18 gauge for a long period of time. The wires from the generator/shore power or alternator to the batteries are like 4 gauge. Thatís why you are getting some of that drop off. Sounds like it is good for an emergency, but I would watch it carefully over a longer period of time.
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Old 06-16-2018, 12:28 AM   #7
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THOR #11959
Sorry, I wasn't clear. The wires through the emergency start switch to the relay's coil are 18 gauge. Only a few mA is likely needed to activate the relay's coil which then connects the working contacts. The wires to the working contacts are huge since they carry the engine starting current. A modest charge current back through those should be OK.
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Old 06-16-2018, 01:14 AM   #8
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Right. That is way my Trombetta and BIRD are wired.
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Old 06-16-2018, 03:27 AM   #9
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I'm Very impressed with the clean wiring job!
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