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Had alternator tested

Since I had the alternator out while changing the thermostat/orifaced disc, I decided to bring it down to Pacific Auto Electric and have it bench tested.  It was attached by pulley to an electric motor that ran at 1750 RPM.  It showed 13.8v (tester said it should have been over 14) and when put under load produced 130 amps.  The load seems to slow down the electric motor a little, so it was interesting to  see how much the alternator's magnets affected it.  If it were driven by the Cummins at 2000+ RPMs the amperage would have increased, possibly to its original 160 potential.

I would be interested in some thoughts on this.

2000 U320 mid entry  #5688
2006 Jeep Liberty


USMCR retired
SDFD retired
FEMA US&R TF8

Re: Had alternator tested

Reply #1 –
Wouldn't worry so much about the amps, but 13.8V seems kinda low.  By the time you run it through the isolator, you'll lose another 1/2 volt or so, and your start bank will only see a little over 13V.  Not enough to fully charge the batteries.  Need to see around 14.8V at the alternator (+) output.
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
"Nature abhors a vacuum"

Re: Had alternator tested

Reply #2 –
I was just talking to Keith at MoT about that very issue yesterday.  He said that at the alternator it needs to be 15v or so because of loss through the isolater etc. 
Rick & Rhonda
2003 U320 4220  Build #6199
Was
91 36' GV 300 Caterpillar, 92 40' U280 300 Cummins, 97 36' U295 300 Cummins, 2002
U320 450 Cummins
(Guess we're hooked)

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Re: Had alternator tested

Reply #3 –
Your alternator may or may not run at engine rpm depending on the pulley sizes, testing shop may or may not have been running it at proper rpm. Voltage might output is probably adjustable. Good news is it's pumping out many amps. You just don't really know how many.
Old Phart Phred, EIEIO
89 GV ored 36' #3405 300 hp cat 3208 ATAAC side radiator, mountain tamer exhaust brake

Re: Had alternator tested

Reply #5 –
Dedicated factory testing unit as shown in Chuck's reply would be reliable standard. Just hooking it up to an electric motor not so much.
Old Phart Phred, EIEIO
89 GV ored 36' #3405 300 hp cat 3208 ATAAC side radiator, mountain tamer exhaust brake

Re: Had alternator tested

Reply #6 –
Quote from: Johnstons –
I was just talking to Keith at MoT about that very issue yesterday.  He said that at the alternator it needs to be 15v or so because of loss through the isolater etc. 

Agree, BUT, on the test bench, likely the sense terminal was connected to the B+.  In that case 13.8-14 is the correct answer.

Said another way, it is 13.8-14 at the chassis battery side of the battery isolator as communicated to the alternator by the sense 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: Had alternator tested

Reply #7 –
Quote from: wolfe10 –
Agree, BUT, on the test bench, likely the sense terminal was connected to the B+.  In that case 13.8-14 is the correct answer.
What if (as I'm guessing) the sense terminal was connected to nothing?  Going by Sven's description, not a very sophisticated test setup.
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
"Nature abhors a vacuum"

Re: Had alternator tested

Reply #8 –
My rebuilt LN is set to output 13.6 volts to both batteries through a  auto combiner.

As Brett posted a higher voltage over a long drive in warm weather can be overcharging the batteries.

The house batteries in a BTMS controlled charging circuit in cold weather can adjust the voltage to over 14.

In hot weather closer to 13 volts.

As the batteries are expensive I took Brett's recommendation and left the output at 13.6.

An isolater would require more voltage because of the losses, but then at least one battery bank would be even more overcharged in hot weather? 

Factory used gel house and optima red tops and a isolator new for a reason.  Less susceptible to the over charging and if under charged they do not lose capacity and cannot and do not need equalizing.

Replacing the power consuming isolator that can  allow over and under charging of a bank with the auto combiner has finally eliminated my occasional battery draw down issues that can cause permanent battery damage.

Two medium  solar panels feed my coach so it's charge maintenance is automatic.

What a difference the changes have made versus the constant issues the separate house and engine battery setup with an isolator and only the house side charging since 1984

No new coach uses an isolator to my knowledge.  All use combiners.

"Riding and rejoicing"
Bob
1997 U320 40' Mid entry, build 5132,  wtbi ce27, 4th owner
2007 Solara convertible
2 prodeco tech outlaw ss electric bikes

1095 watts solar
08 Ls 460 and a sc430
2000 Ford F-250 superduty 4x4

Re: Had alternator tested

Reply #9 –
Quote from: Caflashbob –
An isolater would require more voltage because of the losses, but then at least one battery bank would be even more overcharged in hot weather? 


No, should no happen.  The voltage drop across the diode "feeding" the house battery has the same voltage drop as the diode "feeding" the chassis battery. So voltage on the battery side of the isolator should be the same, or very very close.
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: Had alternator tested

Reply #10 –
Sven, if you have the same regulator as I have, the "three step", I would check the voltage output (at the start battery or Duvac terminal) when installed on your engine with the engine running. The "three step" regulator can be set to one of three voltages: 13.8v, 14.2v, or 14.6v. Based on your reported 13.8v and 130 amps, I believe your alternator is in good condition, but your regulator may be set too low.
I would not be concerned about not reading 160 amp output, because the battery charge state, the alternator temperature, and the RPM of the alternator all have to be just right to obtain maximum alternator output.
I would be concerned about the 13.8v reading, because if you have Gel batteries, the alternator output voltage (measured at start batteries) should be 14.2v; however, with flooded batteries the alternator output should be 14.6v. If the alternator output is only 13.8v, your batteries will never reach a full charged state when driving.
Wyatt
96 U320 40 WTFE, build 4943
84 Toyota Supra towd
2015 Jeep Wrangler towd
Victoria, BC, Canada

Re: Had alternator tested

Reply #11 –
Correction to last post:
If you spend most of your time in the southern states where it is hot, you may be better with your regulator set to 13.8 volts, than to the 14.2 volts that I use, because I spend half my time in Canada, and half in southern states only in the winter.
Wyatt
96 U320 40 WTFE, build 4943
84 Toyota Supra towd
2015 Jeep Wrangler towd
Victoria, BC, Canada

Re: Had alternator tested

Reply #12 –
On the three step voltage regulator does anyone know what  Foretravel set the voltage output new as measured at the batteries?

The long reported life of both oem battery banks posted here many times shows their initial setting worked well WITH the exact batteries and isolator type and alternator  installed new.

Any changes may require testing over long term and/or resetting of the regulator.

Not worth the time to test over years and monitor the actual amp hour losses from less perfect voltage setting to me.

The optimas and the mk gels seem to be very robust in their abilities to handle over and undercharging and temp swings.

"Riding and rejoicing"
Bob
1997 U320 40' Mid entry, build 5132,  wtbi ce27, 4th owner
2007 Solara convertible
2 prodeco tech outlaw ss electric bikes

1095 watts solar
08 Ls 460 and a sc430
2000 Ford F-250 superduty 4x4

Re: Had alternator tested

Reply #13 –
Thanks for all the replies.  With no consensus, I'm going with Brett's response, partly because it won't cost me any money, but also his expertise.  On my next trip, I will check the battery voltage after it has been on the road (alternator) for a while.
2000 U320 mid entry  #5688
2006 Jeep Liberty


USMCR retired
SDFD retired
FEMA US&R TF8

Re: Had alternator tested

Reply #14 –
Sven,

Please let us know what you find.

And, if not "as expected", let us know readings at each terminal of the battery isolator with engine at 1,200 or so RPM.
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