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Topic: Tech Question for Electrical Gurus (Read 599 times) previous topic - next topic

Tech Question for Electrical Gurus

I was sitting sipping my morning cup of tea, and thinking about kevo0000's alternator problem.  I suddenly had a brain fart, and want to run it by the smart people.

Assumption:  A coach has a fully functional standard 3-post diode-based battery isolator.  The coach owner installs a (non DUVAC) alternator with the POS output wire connected to the center post on the isolator.  The alternator does not require a "EXCITE" wire  - with engine running at idle it supplies normal charging voltage to the center post on the isolator.

OK - we know this setup will not supply optimum charging voltage to the START battery bank due to voltage loss across the diodes.

Question: What if a jumper cable (of suitable size) was installed between the isolator center post and the isolator START battery post?  What would be the result?

My conjecture:  The START battery bank would receive full charging voltage.  The COACH battery bank would receive the same (reduced by diode loss) voltage it always does.  The START battery bank would still be protected from being overly discharged by the isolator.

What am I missing?  (It's gotta be a simple flaw in my logic, but I'm not seeing it).
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: Tech Question for Electrical Gurus

Reply #1 –
Chuck, The so-called DUVAC alternators are rewired normal alternators that turn off the alternator output if there is no excite voltage and have a voltage regulator that you can increase the upper limit on voltage to account for the diode losses in old diode based isolators.  Some are set to a fixed higher voltage limit.

Alternators not so modified (most) do not need an excite wire.  Both use a sense wire to measure start battery voltage but many will work without a sense wire with reduced max voltage. The sense wire connection makes the internal voltage regulator increase its output voltage when the battery voltage is low.  As the battery voltage comes up the alternator output voltage comes down. The alternator amp output comes down as well.

Jumping from the common post on a diode isolator to the start battery post would probably work to get the volts and amps from a non DUVAC alternator to the start battery.  The house side would be seeing 0.7 - 1.0 volts less depending in the diode based isolator.  Jumped as you suggest would not protect the start battery from getting discharged by the house battery since you bypassed the protecting diodes and have the start batteries connected to the common post.  Amps will flow the start batteries connected to the common post to the house batteries.

You would be better off using a manual isolator switch, a zero voltage drop (non diode) isolator, a zero voltage drop smart isolator, a voltage sensing relay or a smart battery to battery charger which provides a multi-step charge profile to your house batteries based on their type.  There are other options as well.

If none of these are an option then keep the diode based isolator and the original wiring (no jumpers).  Voltage to the start and house batteries will be lower than optimal which means they will charge slower and without the extra voltage they may not get fully charged.  The slower charge is not so bad.  Not fully charged on most coach battery setups will eventually reduce the maximum capability of the batteries, an irreversible result for most batteries.  Think of it like trying to fill a tire to 120 psi with a compressor that only goes to 110 psi.  No matter how hard it works the tire will never get to 120 psi.

I am pretty sure you will get other perspectives as well.
Roger Engdahl and Susan Green
2001 U320 3610 #5879 (Home2) - 2014 Jeep Cherokee or 2018 F150
Hastings, MN

Re: Tech Question for Electrical Gurus

Reply #2 –
See - I knew I was missing something!  Diodes only "protect" in one direction!  Roger got my mind right.

With the engine off (alternator doing nothing) and my "jumper" in place, if the house battery bank is drawn down below the voltage level of the start battery bank, then current will start flowing from start batteries through the jumper cable to the center post of the isolator and then on to the house batteries.  Exactly like it does when the alternator is powering the center post.  The start batteries will not be protected by the isolator.

With my "jumper" in place, the house batteries would be protected from being discharged by the the start batteries...but that was not my goal.

SO, my idea won't work.  Everybody disregard and go back to your normal activities.  And HAVE A HAPPY NEW YEAR!

Thanks, Roger!
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: Tech Question for Electrical Gurus

Reply #3 –
Quote from: Roger & Susan in Home2 –

Alternators not so modified (most) do not need an excite wire.  Both use a sense wire to measure start battery voltage but many will work without a sense wire with reduced max voltage. The sense wire connection makes the internal voltage regulator increase its output voltage when the battery voltage is low.  As the battery voltage comes up the alternator output voltage comes down. The alternator amp output comes down as well.

I am pretty sure you will get other perspectives as well.

I respectfully disagree with your characterization on what a sense wire is and does.

In my humble opinion, a sense wire is similar to the test lead for a volt meter.  No current flows through the sense wire so there is no voltage drop along its length.  This way the alternator's regulator knows what the voltage is at the load and can keep that voltage where it needs to be, by increasing the output voltage to allow for voltage drop along the charging lines.
1992 Foretravel Grand Villa
U225 SBID Build No. 4134
1986 Rockwood Driftwood
1968 S.I.A.T.A. Spring
1962 Studebaker Lark
1986 Honda VF700C
1983 Honda VF750C
Charlie, the Dog was broken out of jail 24 Oct 2023
N1RPN
AA1OH (H)e who must be obeyed.

Re: Tech Question for Electrical Gurus

Reply #4 –
Quote from: Chuck & Jeannie –
See - I knew I was missing something!  Diodes only "protect" in one direction!  Roger got my mind right.

Thanks, Roger!

But a properly installed BIRD, a voltage sensing bidirectional relay disconnect will protect your batteries with a no diode loss relay and a voltage sensing circuit.  At the expense of moving parts.
1992 Foretravel Grand Villa
U225 SBID Build No. 4134
1986 Rockwood Driftwood
1968 S.I.A.T.A. Spring
1962 Studebaker Lark
1986 Honda VF700C
1983 Honda VF750C
Charlie, the Dog was broken out of jail 24 Oct 2023
N1RPN
AA1OH (H)e who must be obeyed.

Re: Tech Question for Electrical Gurus

Reply #5 –
Quote from: Old Toolmaker –
But a properly installed BIRD, a voltage sensing bidirectional relay disconnect will protect your batteries with a no diode loss relay and a voltage sensing circuit.  At the expense of moving parts.
Completely agree...and...this subject has been discussed here ad infinitum, so no need to beat a dead horse.  I was just looking for a way to help our Brit friend find a easy solution to his conundrum.

But this wasn't it.
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: Tech Question for Electrical Gurus

Reply #6 –
As I understand this topic the sense wire is there to let the alternator see what the voltage is at the battery and adjust its voltage accordingly, I think that is what I said. The voltage at the start battery may be lower due to voltage loss in the wiring between the alternator and the start batteries or more likely because you just started your engine.  With a LN DUVAC alternator with an adjustable output voltage the voltage loss across the isolator and along the wiring is compensated for by adjusting the output voltage.  Sized properly voltage loss in the wiring between the alternator and the start battery should be less 1% or about 0.13 volt.  Much less than the diode loss in an OEM isolator.

My Delco Remy 40si alternator will charge at (up to) 14 volts with no sense wire and up to 14.5 if a sense wire is attached depending on the sense voltage.  As battery voltage comes up, alternator charge voltage drops.
Roger Engdahl and Susan Green
2001 U320 3610 #5879 (Home2) - 2014 Jeep Cherokee or 2018 F150
Hastings, MN

Re: Tech Question for Electrical Gurus

Reply #7 –
Oh I cannot say nothing so I am going to jump in here.

In reply 3 is the statement: "No current flows through the sense wire so there is no voltage drop along its length."
 This statement is not true, there is a very small current (a few milliamps) in the sense wire and an insignificant voltage drop along its length. I measured this current flow so I could calculate how many ohms of resistance I needed in the sense wire to raise the output voltage of the alternator by 0.2 volts. The alternator in my Foretravel is charging at 14.4 volts while the 3 step regulator is set to 14.2 volts.

Another item that is not easy to understand is that all alternators with a built in regulator have a sense circuit (in the regulator), so they have a sense wire which is not obvious with most (non DUVAC) alternators. The sense wire in non-DUVAC alternators is connected (inside the alternator) to the charging post which holds the heavy wire for charging the battery. It could be argued that the heavy charging wire is also the sense wire.

I hope this helps!

 



Wyatt
96 U320 40 WTFE, build 4943
84 Toyota Supra towd
2015 Jeep Wrangler towd
Victoria, BC, Canada