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Old 02-02-2018, 11:29 PM   #1
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Bad Alternator or Bad Batteries?

We have been parked and boondocking, running the generator often.

Just recently, we've gotten low voltage and had some difficulty starting the coach because of it.

We checked the voltage on the chassis batteries (2x 12v) and were seeing just under 10v off of both using a voltmeter.

Obviously, these should be generating 12v's each.

We jumped the house batteries to the chassis batteries and were able to start the coach...but the voltage is still not strong.

Is this pointing to a problem with just two bad 12v batteries? Or are we looking at a bad alternator?

We're hoping not the alternator, as we just had our rebuilt a year or so ago.

Thanks in advance!
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Old 02-03-2018, 12:33 AM   #2
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Sounds like you have a draw somewhere. Check with a digital ammeter and try to figure where the power is going.
Happy Glamping.
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Old 02-03-2018, 01:06 AM   #3
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RE: Unplanned

Quote:
Originally Posted by Unplanned View Post
Sounds like you have a draw somewhere. Check with a digital ammeter and try to figure where the power is going.
Happy Glamping.
Thanks for the answer. Respectfully, how are you making that determination based upon what I said? In other words, why would you go to such a complex issue without first considering whether it may, in fact, be caused by either (a) bad chassis batteries; ours are old; or (b) a bad alternator?
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Old 02-03-2018, 01:43 AM   #4
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Are you sure your house charging system also charges your engine batteries? If so it sounds like there is a problem with that. Check the engine battery voltage after the house has been charging for a couple of hours and you should see charging voltage if you have a working BIRD or similar system.

On the engine alternator it will take a couple of hours of run time to get the batteries back up to full voltage. Not much of a problem if you are going somewhere but idling for a few minutes won't do it. I'd take a drive and see if the voltage came up over a couple of hours. You did not do your batteries any good but you did not kill them by letting them get too low.

Assuming you do not have an engine charging system it depends on how long you were parked. There is a lot of stored power in the DP batteries but there is also a lot of drain in modern chassis with the various controllers built into the system. You might save significant money by extending battery life with a battery minder that runs off the house batteries or a simple charger plugged into your house 120 VAC that tops off the engine batteries when you run the generator.
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Old 02-03-2018, 01:58 AM   #5
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A quick check if charging from the alternator is occurring is to check battery voltage with engine/charging off - then start engine/charging - about 1 volt or more increase should be seen. No change = no charging.
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Old 02-03-2018, 05:21 AM   #6
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Get it started and check battery voltage. If it goes up to 13 volts or more, drive it to an auto parts store or Battery Plus, with a big parking lot, and have them test the batteries.
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Old 02-03-2018, 07:01 AM   #7
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Quote:
Originally Posted by ereadingrv View Post
Thanks for the answer. Respectfully, how are you making that determination based upon what I said? In other words, why would you go to such a complex issue without first considering whether it may, in fact, be caused by either (a) bad chassis batteries; ours are old; or (b) a bad alternator?
Do you have a multimeter? Do the voltage checks. How long g were you boondocked?
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Old 02-03-2018, 07:05 AM   #8
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Quote:
Originally Posted by ereadingrv View Post
We have been parked and boondocking, running the generator often.

Just recently, we've gotten low voltage and had some difficulty starting the coach because of it.

We checked the voltage on the chassis batteries (2x 12v) and were seeing just under 10v off of both using a voltmeter.

Obviously, these should be generating 12v's each.

We jumped the house batteries to the chassis batteries and were able to start the coach...but the voltage is still not strong.

Is this pointing to a problem with just two bad 12v batteries? Or are we looking at a bad alternator?

We're hoping not the alternator, as we just had our rebuilt a year or so ago.

Thanks in advance!
Unless an aftermarket device had been added to your coach, the Diplomats did not charge the chassis batteries from the generator or shore power. Suggest you do as twinboat says and have the batteries checked. Boondocking for only a few days should not be a problem unless the batteries are marginal. You might want to consider one of these:

https://www.rvupgradestore.com/Ultra...ik-l-start.htm

Ultra TRIK-L-START Starting Battery Charger/Maintainer

Bob
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Old 02-03-2018, 08:04 AM   #9
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ereadingrv,

Your statement in post #3 pleasantly surprised me!!! You do seem to know what you are talking about. Based on your response why didn't you check the charging voltages at the Chassis and coach battery's before posting???

This is ALWAYS and has to be thevery FIRST step in diagnosing a discharged battery.

A 12 -V battery that is in service and charged will or should have a voltage of around 12.6-V. When any charge is applied it should read at least 1-V more that the open post voltage reading taken first. Immediately following cranking an engine that voltage will usually be 13.6 or 14.2. Big current drain therefore higher charging voltage.

When plugged into shore power or running the generator the house battery's should be charging. What is that voltage?? The generator charging will be going through your converter just like it's plugged into shore power.

If your engine is running then that charging voltage should be charging the engine battery and the house batteries which is then also going through the converter.

Since you knew you needed those voltage readings then do them and make your diagnosis. If the voltage is not at least 1 volt above the open post voltage reading on your engine cranking battery then your alternator is not charging.

When we start our RV the alternator charging voltage is usually 14.2 - V. My Scan Gauge ($100 investment 3 years ago) instantly displays my alternator charging voltage all the time. I want to know that all the time.

The Trick-L-Charge device is the best $40 you'll spend. Anytime you are running the engine, generator or plugged into shore power you will also be charging the cranking battery.

Few RV's right from the factory will charge the engine cranking battery. That is only accomplished by the engines alternator unless you change it.
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Old 02-03-2018, 10:22 AM   #10
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I'm with TeJay, your original post contains some non-sequiturs.

The main one is if you were boon-docking and running the generator, and you saw 10 V on your chassis batteries, why would you assume your alternator was the culprit? Have you, in addition to running the generator, been starting and running the engine periodically? If not, how would your alternator have provided charge?
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Old 02-03-2018, 11:04 AM   #11
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Your best bet is to fire up the coach engine and then measure the chassis battery voltage. It should be above 13 volts and gradually increasing to 14 volts as the batteries charge. If the voltage does not increase , you have a charging problem . Could be as simple as as loose alternator belt . On some coaches , some items are run off the chassis batteries like the radio & steps. Over a period of time the loads will discharge your chassis batteries. Most coaches charge the chassis batteries when on Genny or shore power . Some do not . With batteries that low , you are not charging them from shore /genny power. You'll have to do some investigating to find out why no charging. I would not run out and buy new batteries just yet.
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Old 02-03-2018, 03:56 PM   #12
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You said you were boon docking so often the generator doesn't charge the chassis battery. Was that your problem. I added a little a device that takes care of it
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Old 02-03-2018, 04:54 PM   #13
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On my last coach, an Imperial I had a phantom draw somewhere that would drain my batteries in a couple days. I replaced the batteries and it still did that. My solution was when parked I would turn off the disconnect switch for the starting batteries and they would still be charged when I switched them back on.
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Old 02-03-2018, 07:12 PM   #14
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For clarification (and this was an accidental error of omission), the coach is started and moved weekly as we travel. So the coach is started and moved, and this problem is relatively recent. Apologies for the omission.
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