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Old 12-08-2021, 07:55 AM   #1
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Alternator Charging of Auxiliary Batteries

Hi all,
My new to me 2001 Forest River Georgetown alternator charges the starting battery but does not charge the 2 auxiliary batteries. They do charge on shore power. This doesnít make sense that they donít charge while the motor is running. Any advice? Thanks.
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Old 12-08-2021, 08:11 AM   #2
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You might have a couple of different issues that will take some diagnosing to figure out. Others will chime in with the laundry list of items that may or may not be bad in your setup. Start checking the connections from the coach batteries to the charging system and you'll run into the pieces that make it go. Might be something as simple as a disconnected or deteriorated wire, or it might be something bigger. Nothing in that systems is terribly expensive to replace. Finding it will probably take longer than fixing it. If you have someone fix it, you'll save quite a bit if you locate the problem first.
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Old 12-08-2021, 10:16 AM   #3
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The house batteries are supposed to charge from the alternator, so you have a defect somewhere. Most likely the solenoid that cross-connects the house 12v system to the chassis system when the engine is running. Basically when the alternator is producing voltage, that triggers a solenoid to close so that the chassis battery positive connects to the house battery positive terminal and thus charging both systems. Some coaches add a bit of smarts in the form of a "battery control center", but the basics are the same.

One quick check is to determine if the Auxiliary start switch is working - it uses the same solenoid.
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Old 12-08-2021, 10:33 AM   #4
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When and how did you check that? When batteries in an RV age, the battery circuit may take up to 2 minutes to close the solenoid that allows charging via alternator. If you measure it before that time is up, you might not read 13.xx on the house batts. If the batts are too far gone, it may never close.

So first measure the battery voltages, start the engine and wait for the circuit to sense that the starting batt is charged up enough by the alternator to allow closure of the solenoid, then measure the voltages again. If it never reaches that spec'ed voltage, the solenoid never closes. Also, the alternator could be bad not putting out a high enough voltage. Alternator might be putting out enough to charge the batt on a long trip, but too low for the system to allow it to try to charge the house batts. (You would likely know this already if you MUST use the AUX switch to start the RV).

This all depends on the type of control system your RV has.
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Old 12-08-2021, 10:48 AM   #5
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You may also have a diode type battery isolator.
With them, the alternator output is sent to the isolator and split to each battery bank.
They don't have moving parts, but a if diode fails, you won't get charging to that battery.

Pictures of what you have will help.
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Old 12-08-2021, 03:43 PM   #6
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Thanks for your replies. The auxiliary start switch does not work. Will investigate the solenoid first. Thanks again.
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Old 12-08-2021, 03:53 PM   #7
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You should be charging your house batteries with the alternator
Mine has a giant 240 amp alternator because of the 6 gel batteries I have for house batteries.
My old diesel had a 170 amp
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Old 12-15-2021, 09:35 AM   #8
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Hi all,

The fault finding exercise was worthwhile. I found a blown 20 amp fuse on a blue wire coming off the positive terminal of the coach battery. Now the charging system works. However, I don’t know why. Does anybody know what that blue wire is that bypasses the Intelitek control box?
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alternator, batteries, charging

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