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Old 07-22-2015, 12:42 AM   #1
Brand: Thor Motor Coach
Model: Citation Sprinter 24SR
State: Oregon
Posts: 56
THOR #972
AGM Batteries and Isolator Switch

On my Citation Sprinter 24SR I replaced the stock group 27 wet-cell batteries (they were shot after 4 months) with AGM group 24's . Since then the isolator switch (on my Sprinter 24SR, left side of step-well) has failed twice. Failure means shore power doesn't charge the batteries, engine running doesn't charge them and steps and slide outs won't retract/extend.

Could my use of AGM or a smaller group size be the cause of the switch failures?


Citation Sprinter 24R

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Old 07-22-2015, 10:17 AM   #2
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FW28z's Avatar
Brand: Thor Motor Coach
Model: 2011 Four Winds 28Z
State: Michigan
Posts: 1,273
THOR #531
I am going to go out on a limb and guess it's the AGM batteries that are the issue.

Typically when using AGM batteries, you must have an AGM capable charger as they cannot tolerate the higher overcharge current that standard lead acid batteries can.

Often this is not an issue with the engine battery as there is always a load (engine) when the alternator is charging the battery. But in stand-by situations, where the charger is charging the battery in trickle charge into an overcharge condition, a non-AGM specific charger can damage your expensive batteries.

You are likely to encounter this situation when using AGM for the house battery.

Also, AGM produces slightly different voltages which can also trick the charger.

I would recommend replacing your converter/charger to one that can handle AGM.

If your alternator is setup to charge the house batteries, you would also need to replace the charger on your alternator to an AGM capable charger. You can buy marine engine alternators that do this, but I am not sure if conventional vehicle chargers have this option or not.

And then you would have to also change the vehicle's battery to AGM too, as all batteries on a string need to be the same type for proper charging.

I have never switched to AGM simply because of the extra expense of making sure the chargers all work with AGM, and having to swap out both engine and house batteries.

The only thing that works on a RV is the owner...
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Old 07-22-2015, 10:50 PM   #3
Brand: Thor Motor Coach
Model: Citation Sprinter 24SR
State: Oregon
Posts: 56
THOR #972
Many thanks. I'm going to say your branch is pretty secure. I put a meter on the batteries and measured 5.4 volts. It was parked a week and I left a light on.

Apparently at that low a voltage two things happen. The isolator switch will not energize and no power flows to the batteries when the rig is plugged to shore power. I attached an external charger for a couple hours and was able to activate the switch and am now charging with shore power.

Next step will be to replace the AGM with flooded cell batteries. In researching best choices I came across ads for lithium and briefly gave that consideration until I saw the cost.
oreddie is offline   Reply With Quote

agm, group, isolator

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