Why oh why.......

Why oh why do these things happen????

I just spent a couple of hours this morning working on a relatively new mid to high $$$ six figure custom Down East style boat.

The complaint, chronically low batteries. Now keep in mind this is a power boat with a MASSIVE Cummins and a MASSIVE 200A J mount alternator and external regulator. It also has over 1000Ah's of AGM batteries on board. Sounds good, right....?

So, the batteries were at 12.2V when I got there and I went through the usual trouble shooting, checking how the system was wired and then testing for any parasitic loads. The parasitic load when the battery switch was turned on was 0.12A and when OFF was 0.008A or near perfect.. Because this is a very expensive and well built boat all the wires from the regulator and any other device immediatly disapear into wire loom. The factory wiring work was very good. It is how it was wired where the issue arose.

So we get to the part where we fire up the motor to test alt output. I have a Fluke clamp meter on the + wire to the house bank and a Fluke DVM measuring terminal on the house bank of batteries, 750Ah of AGM's. Within a few seconds of the regulator coming on we have 95A which then quickly falls to 22A of accepted current.....

Why? Because the battery terminal voltage was 13.2V and what the regulator was seeing was 14.4V due to voltage drop in the system and where the voltage was being measured by the voltage regulator.. The regulator was in absorption mode and limiting the voltage to 14.4V yet at the batteries we were only really at 13.2V.

The B+/volt sense wire for the regulator was measuring the back of the alternator. The alt output wire was also far to small for this alt. On top of that the alt B+ / feed wire picked up a bank of engine management relays along the way, pre-heater, common rail system, fuel pumps etc. etc.. This was adding even more voltage drop to the alternator charging wire. The voltage regulator was simply sensing an incorrect voltage. It was measuring the voltage ahead of any voltage drop in the wiring and should have been measuring it after the voltage drop.. The alt output did go straight to the house battery bank, but, due to poor wiring practices, it had loads pulled off it before it got there. A full face palm on this one.

I made some test jumpers and connected the regulators B+ and B- wires direct to the house battery bank. We then fired up the motor and the house bank was now taking 165A of current and was at 14.3V and climbing towards 14.4V. There is a HUGE difference in the current a battery can accept between 13.2V and 14.3V...

Multiple technicians had looked at this boat and found nothing wrong. All one had to do was put a meter on the battery bank and at the regulator to see where the problem was but no one had done so, of if they did this very simple issue still did not ring a bell.

Always measure your voltage at the regulator B+ / B- wire and then at the batteries. Low voltage at the batteries means CHRONIC undercharging no matter how big the battery bank or alternator is. Remember this was a 200A large frame alternator only capable of delivering about 22A at 13.2V yet it went right up to about 165A at 14.3V....

I will go back in a few weeks and run a 2/0 wire for the alt B+ & B- and permanently re-wire the regulator to properly senses the correct spot in the system to eliminate the charging voltage drops..

If you suspect you are not charging properly I would suggest first measuring the voltage directly at the back of the alternator and then directly at the battery posts. These voltages should be within 0.1V of each other...

I know I harp on this a lot but the vast majority of charging issues with alternators are due to voltage drop, low voltage or voltage sensing issues not necessarily what alternator or regulator you have....

Brian D said:

So Maine, are you saying that when you bypassed the alternator directly to the battery bank, you got full output from the alternator? If that is so, then someone (manufacture?) had added devices between the alternator and the house bank in order to leech off the alternator when it was running. Since it was a power boat, that would be practically all the time.

Nothing like adding loads before the power distribution instead of after the power distribution.

Nice find!

Click to expand

No not really. The alt was already direct wired to the bank. The regulator however was sensing the voltage at the back of the alternator.

The alternator output wire was:

A) Undersized for the loads

B) Had loads pulled off it before it got to the battery bank.

By moving the regulator sensing wires to the battery it allowed the regulator to get to the absorption voltage so that the batteries were seeing the target absorption voltage.

At the alt it was probably 15.8V or so but at the battery it was now 14.3V - 14.4V... The new location of voltage sensing allowes the regulator to compensate for the voltage drops.

Prior to fix (sensing voltage at alt):

14.4V At Alternator 13.2V At Battery Bank

After Fix (sensing voltage at battery bank):

15.8V (guesstimate) At alternator 14.4V At Battery Bank

This video shows a very similar situation and may explain it better...

kloudie1 said:

So those of us with plain old "dumb" alternator/regulators would be best to concentrate on low volts drop from back of alternator to batteries.. (no "in-line" automotive ammeter, all terminal connections clean and tight and big wire gauge)

Click to expand

Yep...

Brian D said:

That is what I was saying, Maine.

Click to expand

I was confused when you said "by-passed the alternator". I moved the regulator sense wires to the house bank.. Sorry for that..

Brian D said:

Loads were placed between the alternator and the battery bank. Hence (one of) the reason for the low voltage at the batteries.

Click to expand

These additional loads only compounded on top of the already too small alt output wire...

Brian D said:

Is it possible the sensing wire would have detected the correct voltage had there been no loads in between?

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There still would have been voltage drop. You can't run a 200A alt, sensed at the alt, through 4GA wire and wind up with no voltage drop between the back of the alt and the battery bank.

There is about 15 + feet of 4 GA wire in the alt circuit with a couple of suspect terminations. Even if we remove the other loads prior to the current getting to the battery, one of which was the combiner to the starting battery, we would still have about 1.2V of drop at 165A.. Or with 14.4V at the alt we would still only have 13.2V at the battery at 165A. This is why it will be critical to use 2/0 wire, to minimize the drop, and to voltage sense at the battery to correct for any voltage drop even with the 2/0 wire.

Any voltage drop in a charging circuit can be very, very punishing to the charging performance. Keep in mind that even a "minimal" 3% volt drop at 14.4V results in the batteries seeing less than 14.0V.... This is very detrimental to charging performance. Lower voltage means less charging current can be accepted by the batteries....

kjretlaw said:

So for those of us with 55-100 amp alt is 4 AGW sufficient or would it be best to use 2 AGW in all cases. Also would the same go for the battery chargers, too small a wire resulting in voltage drop. 30 amp charger 3' 6 AGW is this not heavy enough.

Appreciate all the information it a great help to give all of a better understanding of our systems.

Click to expand

The biggest problem with most stock alts is in the alts ground path back to the batteries.. Engine blocks, paint, rusty metals and multiple poor conducting metals can lead to tremendous voltage drops. Often times the positive cable is only part of the problem.

The remedy for most stock alts is a MUCH larger neg wire fed directly to the main neg battery cable or direct to the bank.

The jumper from the alt to the starter post should also be larger. And if the alt is routed directly to the house bank then the wire distance need to be calculated for the least voltage drop that is reasonable. The distance of the wire run round trip and the amperage of the alternator will dictate how big of a wire you will need in order to get good charging with a machine/self sensed alt.

jibes138 said:

What about the Motorola 51A alternator that was OE on your Universal M25? What was the output voltage with the original regulator? I ask because I sense that it was in the 13V range. Do you recommend replacing this alternator or just replacing the regulator?

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Actually some of these regulators are actually set to 15.0V to help compensate for the voltage drops they knew they had in the systems.

The only way to know your regulators set point is to:

*Fully charge the battery bank to 100%

*Start the motor and let run at fast idle for 10+ minutes with all DC loads OFF

*Measure the voltage at B+ & B- at the back of the alternator

You should have 2A to perhaps 4A flowing, depending upon bank size, so this should not impact the alternator voltage reading.


Now discharge the bank to 50% SOC and do the same:

*What is the voltage at the back of the alternator?

*What is the voltage at the battery banks + & - terminals?

Ideally when you get to absorption voltage, your alts target voltage, there should be no more than 0.1V difference between the back of the alt and the battery bank. Ideally, in a perfect word, you would not see much more than a 0.2V difference even at full output/bulk.

As the batteries near full your target/absorption voltage will eventually get reached but low voltages along the way only serve to greatly extend the time it takes to get to full/absorption voltage and can essentially turn your alternator into a trickle charger........

gettinthere said:

I have a 100 amp alt and the feed to the battery is 1/0 cable
The balmar ars 5 is sensing the alt, not the batt bank.
Numbers on the victron seem to be good. I can get the I number down to 2% of battery bank capacity.

Is it never OK to sense at the alternator?

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If you are feeding the alt directly to the house bank then it makes much more sense to sense the voltage at the bank as well.

Next time the batteries are taking some good bulk current measure the voltage at the back of the alt and then go measure it at the bank... You may be surprised, even with 1/0 cable...

The only time you'd need to self sense the alternator is when it's output can be switched to a different bank, like if the alt output goes through a 1/2/BOTH switch. Even then, I prefer to sense the battery switches "C" terminal as it gets you that much closer to the banks.

Also get the regulators neg wire as close to the bank as well, usually right at or near the batt monitor shunt. The reg will compensate for voltage drop, if it can see it, but it is easier if it can more accurately sense so this means regulator red/B+ and black/B- go to the battery.

Remember, a VR can not sense & correct for voltage drop at the alternator. It can only compensate for voltage drop when sensing at the destination...

One of the main reasons I use the MC-614, almost exclusively, is because it has a dedicated voltage sense wire. It is also mandatory and the regulator will not operate it if you don't connect it..

This sense wire carries zero current so gives very accurate voltage sensing to the regulator. The ARS-5 is a great regulator but the red and black wires carry more current because it lacks a dedicated volt sense circuit. Still, if fed directly to the bank the ARS-5 does a fine job of voltage sensing.

If your alt circuit runs through a 1/2/BOTH switch it is unwise to sense the house bank..

Follow me on this:

*Alt output goes through 1/2/BOTH

*House is bank 1 and regulator is voltage sensing bank 1

*All is fine when Bank #1 is selected!

*You now move the battery switch to position #2

*All is no longer fine!!!:cussing:

*Alternator output current now goes to bank #2 but regulator sees no change in voltage at bank #1 and drives starter battery voltage over 17V trying to boost the voltage at bank #1, which it can't do....!!!!


This is why for the best high output alternator performance the alternator is fed to the house bank and the voltage regulator senses voltage at that bank...

See this link:

Alternators & Voltage Sensing - Why It's Important