Skip to main content ↑ ↓
Topic: Fluctuating Voltage 14.9 to 15.2 on house batteries (Read 1124 times) previous topic - next topic

Fluctuating Voltage 14.9 to 15.2 on house batteries

Just installed new AGM house batteries after removing the old,bulging gel batteries.  Hooked everything up and got good voltage while on shore power.  When the coach was started the voltage for the house batteries starting fluctuating from 14.8-15.2.  Confirmed this with a multimeter.  Checked the chassis batteries and they were charging13.7- confirmed with multimeter.  It seems everything is working ok except the charging of the house batteries while the engine is running.

What could this be?  I am stumped by this one.

Thanks for your help
Hans & Marjet
1995 U300 "Ben" (#4719)
3176B Cat,4060HD,Jake
SKP#139131
Motorcade#17579
2006 Honda Element (towed)

Re: Fluctuating Voltage 14.9 to 15.2 on house batteries

Reply #1 –
Quote from: Hans&Marjet –
What could this be?  I am stumped by this one.

I'll put this thread back on top, as I'm curious, as well.  ^.^d
1993 U-240 "La Villa Grande"..CAT 3116 w/ Pacbrake PRXB...Allison 3060 6-speed..
Previous: 1983 Airstream 310 turbo diesel, 1979 Airstream 280 turbo diesel
                                      Build # 4297
                                      PNW natives
                      Home base:  'Cactus Hug' (Ajo, Arizona)
                        DW Judy & Chet the wonder dog
                        Full-Timers 'Sailing the asphalt sea'

Re: Fluctuating Voltage 14.9 to 15.2 on house batteries

Reply #2 –
Assuming you checked all connections,what inverter/charger do you have,is it set up for the AGM batteries?Were the batteries fully charged when installed?
96 U270 BUILD 4810
85 380SL
Drummonds TN.

Re: Fluctuating Voltage 14.9 to 15.2 on house batteries

Reply #3 –
I would start by checking voltage at the battery isolator.  Check at the center lug (from alternator B+), then at the two outer lugs that go directly to the battery banks. 

Let us know what you find.

And, assuming you do not have solar or other charge source connected to the house bank what could provide an alternate charge source at incorrect/high voltage.

Brett Wolfe
EX: 1993 U240
Moderator, ForeForum 2001-
Moderator Diesel RV Club 2002-
Moderator, FMCA Forum 2009-2020
Chairman FMCA Technical Advisory Committee 2011-2020

Re: Fluctuating Voltage 14.9 to 15.2 on house batteries

Reply #4 –
Cleaned and checked Isolator lugs

Alt post @ 15.2
House post@ 15.1
Chassis post @ 14.2

Checked alternator output 15.7

Bad regulator...? I do not see an adjustment for v-reg

160 amp alt with 3 step 79000 (regulator) ????

Thoughts

Thx All
Hans & Marjet
1995 U300 "Ben" (#4719)
3176B Cat,4060HD,Jake
SKP#139131
Motorcade#17579
2006 Honda Element (towed)

Re: Fluctuating Voltage 14.9 to 15.2 on house batteries

Reply #5 –
Quote from: Hans&Marjet –
Cleaned and checked Isolator lugs

Alt post @ 15.2
House post@ 15.1
Chassis post @ 14.2

Checked alternator output 15.7

Thx All


Couple of issues.

Why the drop between alternator (assume B+ terminal of 15.7) and reading at the center lug of the isolator of 15.2.  That is quite a drop in that large-gauge wire.

Diode-based isolators loose about .7 VDC across the diodes, so you should be seeing that drop between center lug and house battery lug as well as the chassis battery lug.

Since the sense wire is on the chassis battery side (exact location not critical as long as it represents chassis battery voltage) and is reading correct voltage (14.2) I would look very hard at the isolator being at issue.

Again, for troubleshooting, one needs to explain why no voltage drop between isolator center lug and house battery.  As posted above, if solar or any other means of charging the house bank is active, THAT has to be addressed first.
Brett Wolfe
EX: 1993 U240
Moderator, ForeForum 2001-
Moderator Diesel RV Club 2002-
Moderator, FMCA Forum 2009-2020
Chairman FMCA Technical Advisory Committee 2011-2020

Re: Fluctuating Voltage 14.9 to 15.2 on house batteries

Reply #6 –
Brett..I'm abit confused here....is the Alternator output voltage not to high.? was under the impression that the  output should be 14.2V..?

H
Hans & Marjet
1995 U300 "Ben" (#4719)
3176B Cat,4060HD,Jake
SKP#139131
Motorcade#17579
2006 Honda Element (towed)

Re: Fluctuating Voltage 14.9 to 15.2 on house batteries

Reply #7 –
NO, alternator output should be about .7 VDC higher than you want the batteries to see, as there is that .7 VDC drop across the diodes in the diode-based battery isolator.

All those big aluminum fins on the isolator are to dissipate the heat from loosing that .7 VDC.

What you do care about is voltage on the "battery side" of the isolator, and 14.2 at the chassis battery says that part of the system is OK.

You need to find out why house bank is higher-- bad isolator? alternate charging source? other?

And, anytime you have puzzling numbers, spend 20 minutes making sure all connections at the isolator and alternator are CLEAN AND TIGHT. Could save you a lot of time and money.
Brett Wolfe
EX: 1993 U240
Moderator, ForeForum 2001-
Moderator Diesel RV Club 2002-
Moderator, FMCA Forum 2009-2020
Chairman FMCA Technical Advisory Committee 2011-2020

Re: Fluctuating Voltage 14.9 to 15.2 on house batteries

Reply #8 –
    Quote from: Hans&Marjet –
    Brett..I'm abit confused here....is the Alternator output voltage not to high.? was under the impression that the  output should be 14.2V..?
    Hans, adding to Brett's help, if your readings are correct, the start battery side of the isolator is good but there is something wrong with the cable or cable to post connections from the B+ post on the alternator to the alternator post on the isolator, AND the House Battery Side of the isolator appears to be shorted.  Less than stellar Ground cable connections at the alternator and at the Start Battery bank would mess things up too.
    It's always much easier to see and understand these things in person, but here goes anyway:  Just a stream of thoughts that may be of help to you in chasing this down:
    From your Isolator post readings:
    • Alt post @ 15.2
    • House post@ 15.1
    • Chassis post @ 14.2
    • Checked alternator output 15.7
    [/list]
    • start (chassis) battery B+ appears to be at 14.2 and that says The Alternator, the voltage regulator and the sense wire are all functioning properly. 
    • From your readings, the voltage regulator at the alternator is likely set for 14.2 Vdc (it may well not be adjustable or even visible — it may well be inside the alternator). 
    • So, if the B+ voltage at the battery were to drop a bit, to say 14.1 Vdc, the sense wire lets the alternator know this and the alternator increases its output voltage, as much as necessary, to bring that B+ start battery terminal back up to 14.2 Vdc. 
    • Now if the isolator is functioning properly, it will have the expected 0.6 to 0.9 Vdc power diode typical voltage drop across it, due to the need for it to be forward biased, in order for it to pass the DC alternator current to the battery B+ post.  So a reading of 14.8 to 15.1 Vdc alternator output (both at the alternator B+ post and at the isolator middle post i.e. - alternator post) is a good reading to have on a system where the alternator is trying to maintain 14.2 Vdc at the battery.
    • You have 15.2  Vdc at the alternator post on the isolator, so that would seem reasonable if you have 15.2 there AND at the alternator B+ output terminal.  A (+ or -) 0.1 Vdc discrepancy can easily be just a difference in where and how well you have the meter negative lead connected to ground.
    • Also if the isolator diode for the House Battery Bank has no voltage drop across it, it must be shorted and it is passing full alternator voltage to the house batteries.  In order for it to be good, it must always have that 0.60 to 0.90 Vdc drop in voltage,  even if the battery isolation switch is closed.
    • What doesn't make sense is your readings for the two ends of the Alternator cable (15.7 Vdc at the Alternator and 15.2 Vdc at the Isolator end of the cable).  The 0.50 Vdc is way too much voltage drop to have in a fairly short length of heavy wire.  Of course if the meter itself is not accurate (and repeatable) and/or the ground lead connections for the two voltage readings are not at the same potential, that can throw our assumptions off.
    • If the B+ cable on the alternator has good contact with the B+ alternator output post and the cable to post connections on the isolator have just been redone, then the only thing left is a bad (higher than normal resistance) cable.  While not impossible, that is far less probable than a poor connection at either end.
    • Regardless where the voltage is being dropped between the alternator to the battery, the alternator voltage regulator will continue to try to increase the alternator output voltage (until it reaches its maximum attainable output) in trying to keep the start battery voltage at the desired (in this case) 14.2 Vdc.
    • If you had found that the Alternator output and Isolator middle terminal were both at 15.7 Vdc, then the diode to the House battery would have been good (0.60 Vdc drop) and the Start Battery diode (1.5 Vdc drop ) would have been the point of failure.
    • But if you have accurate readings, it looks like it could be two failures (isolator AND cable)
    HTH,
    Neal
    [/list]
    The selected media item is not currently available.
    Neal (& Brenda) Pillsbury
    '02 U320 SPEC, 4200, DGFE, Build #5984
    '04 Gold Wing
    '07 Featherlite 24'
    '14 Jeep Grand Cherokee Summit
    MC #14494
    Exeter, NH & LaBelle FL
    Quality makes the Heart Soar long after Price is Forgotten

    Re: Fluctuating Voltage 14.9 to 15.2 on house batteries

    Reply #9 –
    Does anybody know how the new Cole hershee smart isolator modulate the alternator sense voltage?
    Old Phart Phred, EIEIO
    89 GV ored 36' #3405 300 hp cat 3208 ATAAC side radiator, mountain tamer exhaust brake

    Re: Fluctuating Voltage 14.9 to 15.2 on house batteries

    Reply #10 –
    Hans,

    Do you have a 3-post or 4-post isolator?  On our '93 model, the (still) OEM isolator has 4 posts.  There is a jumper between the two AUX battery posts.  IF you have the same setup, could the jumper have been removed and then reinstalled on the wrong post?  If the jumper was incorrectly attached between the ALT post and the AUX battery post, that would at least explain your high house battery voltage.  Just guessing here...

    In photo, isolator posts (left to right):  AUX(house) - ALT - AUX(extra) - MAIN (start)
    1993 U280 SE 40' WTBI, Build: 4359
    C8.3 300hp, 6-Speed, Exhaust Brake
    960 watts on the roof (6 x 160)
    Sorento (or BOLT) on a Kar Kaddy SS
    "Surely, no age has been more fraught with insecurity than our own present time."

    Re: Fluctuating Voltage 14.9 to 15.2 on house batteries

    Reply #11 –
    Chuck....the jumper wire is OK..

    Update:Not good

    We still have a problem of overcharging. After cleaning the Isolator cables and connections we see the following results.

    Alt V = 15.7
    Isolator Alt V = 15.2
    Isolator House V = 14.8
    House V resting 12.8
    House V running 15.2...?????? not good

    Chassis running V = 13.9-14.0


    Lost as to what it could be....

    One question, why is the voltage coming from alt @ 15.7.....seems to high ??

    Hans


    Hans & Marjet
    1995 U300 "Ben" (#4719)
    3176B Cat,4060HD,Jake
    SKP#139131
    Motorcade#17579
    2006 Honda Element (towed)

    Re: Fluctuating Voltage 14.9 to 15.2 on house batteries

    Reply #12 –
    Hans,
    The alternator voltage is NOT your issue.  With a diode-based battery isolator, alternator output needs to be .7 VDC higher than you want at the batteries to account for power turned in to HEAT in the isolator.

    House battery "running" is the problem. Could be a direct connection from isolator alternator to house lug. That would suggest an isolator replacement.

    Secondary issue is still the .5 volt drop between alternator B+ and alternator lug of the isolator. That is NOT the cause of high house bank voltage, but that is way too much drop in that large-gauge wire.
    Brett Wolfe
    EX: 1993 U240
    Moderator, ForeForum 2001-
    Moderator Diesel RV Club 2002-
    Moderator, FMCA Forum 2009-2020
    Chairman FMCA Technical Advisory Committee 2011-2020

    Re: Fluctuating Voltage 14.9 to 15.2 on house batteries

    Reply #13 –
    Brett....

    Sorry I had a typo

    Alt V = 15.7
    Iso Alt V = 15.5
    Iso House V = 14.8
    House V running 15.2

    Hans

    Quote from: wolfe10 –
    Hans,
    The alternator voltage is NOT your issue.  With a diode-based battery isolator, alternator output needs to be .7 VDC higher than you want at the batteries to account for power turned in to HEAT in the isolator.

    House battery "running" is the problem. Could be a direct connection from isolator alternator to house lug. That would suggest an isolator replacement.

    Secondary issue is still the .5 volt drop between alternator B+ and alternator lug of the isolator. That is NOT the cause of high house bank voltage, but that is way too much drop in that large-gauge wire.
    Hans & Marjet
    1995 U300 "Ben" (#4719)
    3176B Cat,4060HD,Jake
    SKP#139131
    Motorcade#17579
    2006 Honda Element (towed)

    Re: Fluctuating Voltage 14.9 to 15.2 on house batteries

    Reply #14 –
    I also have these cutout switches on the house negative side..

    Hans & Marjet
    1995 U300 "Ben" (#4719)
    3176B Cat,4060HD,Jake
    SKP#139131
    Motorcade#17579
    2006 Honda Element (towed)

    Re: Fluctuating Voltage 14.9 to 15.2 on house batteries

    Reply #15 –
    Quote from: Hans&Marjet –
    I also have these cutout switches on the house negative side..



    So, check voltage from one lug of the disconnect to the other. Said another way, see if there is significant resistance/voltage drop across the disconnect switches.
    Brett Wolfe
    EX: 1993 U240
    Moderator, ForeForum 2001-
    Moderator Diesel RV Club 2002-
    Moderator, FMCA Forum 2009-2020
    Chairman FMCA Technical Advisory Committee 2011-2020

    Re: Fluctuating Voltage 14.9 to 15.2 on house batteries

    Reply #16 –
    Update; Problem fixed....Bad Isolator on the house side...Installed 240amp unit a few cable mods but its done.

    Brett,Chuck,Neal....and all,as always Thank You for the help it's truly appreciated 

    Hans

    Hans & Marjet
    1995 U300 "Ben" (#4719)
    3176B Cat,4060HD,Jake
    SKP#139131
    Motorcade#17579
    2006 Honda Element (towed)
     

    Re: Fluctuating Voltage 14.9 to 15.2 on house batteries

    Reply #17 –
    Great news Hans!  Thanks for the update and closing the loop.
    See you down the road.
    Neal
    The selected media item is not currently available.
    Neal (& Brenda) Pillsbury
    '02 U320 SPEC, 4200, DGFE, Build #5984
    '04 Gold Wing
    '07 Featherlite 24'
    '14 Jeep Grand Cherokee Summit
    MC #14494
    Exeter, NH & LaBelle FL
    Quality makes the Heart Soar long after Price is Forgotten