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Old 02-07-2020, 03:15 PM   #1
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Paranoid Battery Question

My motor home is setting in a garage for the winter. When I checked the battery levels last week my chassis batteries were at 3.x volts. These are 2, 12v batteries in parallel. They are 4 years old. So at least one, possibility both of them are bad. Iíve decided to replace them both.

The problem is that I parked the MH a little too close to the wall and so itís going to be a pain to physically do this, at least for my old carcass.

I canít start the RV and move it on dead batteries, but I have a switch on my dash which allows me to start the RV with the house batteries. This would let me reposition the rig and do this chore myself.

The house batteries are new AGMs and there are six of them in series parallel. My concern is that when I throw that switch I put them in parallel with two badly damaged batteries.

Will it hurt them?

I just spent a lot on these babies and would rather hire a skinny guy to do the grunt work than damage them. What are your thoughts?

Thanks,
óRod
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Old 02-07-2020, 03:51 PM   #2
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I'd go ahead and try the AUX start. Push and hold for maybe a minute. Then while holding try to start the genny. The low/bad chassis batterues may sink too much current for the AUX start to be effective.
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Old 02-07-2020, 03:54 PM   #3
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That's what the button is for.
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Old 02-07-2020, 07:40 PM   #4
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Using the AUX start switch to crank the engine isn't going to hurt the house batteries. Your alternator will give you the juice to move the coach a few feet.
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Old 02-07-2020, 08:27 PM   #5
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Batteries that are that far discharged have a high internal resistance, so if you connect a charged battery to them they'll just more or less slowly come up in voltage, current draw will be minimal. If they're any good they'll start pulling appreciable current once the engine starts and the alternator supplies charging voltage. Batteries drawn down really far may take minutes or sometimes hours before they "wake up" and start charging. One that still has a bit of voltage on it will probably wake up right away but it will take a good long charge to fully come back, to the extent they can. Sounds like you're just going to swap them out anyway but you'll be OK using the boost switch.

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Old 02-08-2020, 10:32 AM   #6
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Before you put new batteries in I would figure out what drained them and maybe figure out a way to use a solar maintainer or some form of trickle charging.
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Old 02-08-2020, 04:24 PM   #7
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Quote:
Originally Posted by Mark_K5LXP View Post
Batteries that are that far discharged have a high internal resistance, so if you connect a charged battery to them they'll just more or less slowly come up in voltage, current draw will be minimal. If they're any good they'll start pulling appreciable current once the engine starts and the alternator supplies charging voltage. Batteries drawn down really far may take minutes or sometimes hours before they "wake up" and start charging. One that still has a bit of voltage on it will probably wake up right away but it will take a good long charge to fully come back, to the extent they can. Sounds like you're just going to swap them out anyway but you'll be OK using the boost switch.

Mark B.
Albuquerque, NM
Just hold the switch down for 1 minute before turning on the key to start.
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Old 02-08-2020, 04:27 PM   #8
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Using the aux start button is really no different than hooking jumper cables from one car to another to give a jump start. They'll only stay connected for the length of time that you're actually holding the button, so no worry about causing harm or draining the good batteries.
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Old 02-10-2020, 01:13 PM   #9
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Quote:
Originally Posted by Mark_K5LXP View Post
Batteries that are that far discharged have a high internal resistance, so if you connect a charged battery to them they'll just more or less slowly come up in voltage, current draw will be minimal. If they're any good they'll start pulling appreciable current once the engine starts and the alternator supplies charging voltage. Batteries drawn down really far may take minutes or sometimes hours before they "wake up" and start charging. One that still has a bit of voltage on it will probably wake up right away but it will take a good long charge to fully come back, to the extent they can. Sounds like you're just going to swap them out anyway but you'll be OK using the boost switch.

Mark B.
Albuquerque, NM

Mark,

Thanks for the advice. I havenít bought replacements yet. I assumed (and we all know that means) that these were so far gone they were not recoverable.

Part of the reason for my assumption is that I store the RV plugged in to a 50 amp service. So they are charging in trickle mode by the inverter, as are the house batteries. Iíll move the RV, take them out, and check them without a load. Maybe one is still ok.

Thanks,
Rod
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Old 02-10-2020, 01:19 PM   #10
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Before you put new batteries in I would figure out what drained them and maybe figure out a way to use a solar maintainer or some form of trickle charging.


Keymastr,

Weíre on the same page. Since they arenít the house batteries, Iím not sure what it was. The key was in the ignition but I think it was fully in the off position. Still, a key in the acc position is the most likely culprit.

The unit is stored indoors and plugged in. So the batteries should have charged. Once I have good batteries in, Iíll be checking carefully.

Thanks,

Rod
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Old 02-12-2020, 09:59 AM   #11
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I have seen ignitions that will connect a small current drain when the key is inserted so possibly that might be the issue.
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Old 02-13-2020, 12:41 AM   #12
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Originally Posted by cyclist922 View Post
Mark,

Thanks for the advice. I havenít bought replacements yet. I assumed (and we all know that means) that these were so far gone they were not recoverable.

Part of the reason for my assumption is that I store the RV plugged in to a 50 amp service. So they are charging in trickle mode by the inverter, as are the house batteries. Iíll move the RV, take them out, and check them without a load. Maybe one is still ok.

Thanks,
Rod
Chassis batteries (often) do NOT charge on shoreline, and can charge (ON some, on shoreline) if the ESS Switch is a TOGGLE vs a MOM (Momentary); to determine your particular coach, you need to check BATTERY VOLTAGE with No shoreline vs w/ shoreline plugged in/ ON. Luck to ya
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Old 02-14-2020, 05:08 AM   #13
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If your chassis batteries donít charge, when you are plugged into shore power, you can install a small charging circuit called Amp-L-Start. They work great. It siphons 15 amps from the house side, when the house side is charging on shore power, to charge your chassis batteries.
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Old 02-14-2020, 06:02 AM   #14
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When I store mine I always open the battery disconnect so it will have voltage

However I don't leave the coach plugged in since changing to AGM house batteries

After you get going you need to see if the chassis batteries are actually seeing the charger current

Your BIM or whatever you have might have failed if you don't have a bad battery
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