Volts VS. Amp-Hrs

[btrayfors]

Originally Posted by zboss

What I did today was make sure that my shunt was actually reading the voltages correctly. During charge cycle the other day I noticed a big difference between the balmar setting (14.7) and my actual charging showing on the shunt during charging (14.3 to 14.43 during absorption).

So I hooked a voltmeter up to the batts during charge and the shunt matched the voltmeter exactly. Then, I compared the shunt to the voltage coming off the alternator and found the alternator was reading 0.10 volts higher than the shunt. There are about 4 feet between the alternator and the batts. I believe the wire is 4 AWG. We have a Balmar AT 175 amp with a serpentine pulley system.

No problem with the 0.1VDC difference. Unless both meters are calibrated, it could just be metering differences.

But, the AWG4 cable is MUCH too small for a 175 amp alternator. Should be 1/0 at least IMHO, even though it's relatively short. And, there should be a fuse in the alternator positive cable located near the batteries...an ANL or Class-T or MRBF.

Also, there should be a cable from the alternator ground directly to the batteries negative post (or the negative shunt, in your case). This should be the same size as the positive wire, i.e., AWG 1/0 or larger IMHO.

Bill

[Maine Sail]

Originally Posted by zboss

What I did today was make sure that my shunt was actually reading the voltages correctly. During charge cycle the other day I noticed a big difference between the balmar setting (14.7) and my actual charging showing on the shunt during charging (14.3 to 14.43 during absorption).

So I hooked a voltmeter up to the batts during charge and the shunt matched the voltmeter exactly. Then, I compared the shunt to the voltage coming off the alternator and found the alternator was reading 0.10 volts higher than the shunt. There are about 4 feet between the alternator and the batts. I believe the wire is 4 AWG. We have a Balmar AT 175 amp with a serpentine pulley system.

If your regulator is propelry wired with the v-sense wire at the + battery terminal then the .1V is simply compensation for voltage drop and it is doing what it should. If v-sense is not wired correctly and sensing elsewhere then this v-drop may not be representative...

The AT should have a neg wire running direct to the batts or as close to the load side of your shunt as is possible. Both wires should be the same size.

If both wires are less than 4' (wire length not "as the crow flies") then you are at approx 3% v-drop and the reg can compensate for that.

That said an AT-200 (could be an AT-165 but you said 175A so I am figuring you have an AT-200...) alt I would normally wire with larger wire than 4GA as fusing and terminations can also create some v-drop...

EDIT: I can now see than Bill T. said basically the same thing... D'oh....

[skipmac]

Originally Posted by btrayfors

zboss,

I think your shunt is, indeed, of the usual type.....metal....but incorporates additional circuitry to calculate certain parameters, then converts these results to signals sent to the monitor.

A few thoughts which might help:

1. If you have 520AH capacity, then the 20-hour discharge rate is 26 amps. Any average draw more than 26 amps would reduce the effective capacity below 520AH, while any draw below 26 amps would increase the effective capacity above 520AH.

From the remarks you provided, it's clear that your average load is way below 26 amps (since 26 amps x 24 hours would equal 624AH per day). Thus, your effective battery capacity is more than 520AH...maybe as much as 600AH or more.

2. The only effective way to test the real capacity of your batteries is to do a controlled 20-hour test. A sophisticated internal inductance/resistance tester like the Midtronics series ($700 or more) is a quick way and is pretty accurate, but few cruisers have these. Maybe you can find someone nearby who has one and can test the capacity of each of your batteries.

Sulfation can occur if batteries are left below full charge or even if they are left on a float charge 24/7. They need to be bumped up to 14.8VDC or more periodically. And, as MaineSail said, they won't reach full capacity when new until they've been cycled a few dozen times.

I don't agree at all with the fashionable notion these days that voltage is not useful. If you know what you're doing and you know your boat, voltage is a very good way to estimate state of charge, and to guage how well your batteries are doing. I do research on batteries, I install marine power systems professionally, and have over 30 years experience with marine batteries on my own boats. I don't have a battery monitor on my boat, and I don't want one....though I've installed a number of them for clients who've drunk the Cool-Aid and think they're the cat's meow.

What I DO have is a good digital voltmeter, mounted where I can see it from anywhere in the cabin. At a single glance I can get a good estimate of the SOC of my house batteries (six golf-carts), because I know in my head what I should be seeing before I look. I know what to expect in the morning upon waking, when motoring or charging from the diesel genset, when retiring at night, etc. I can see how long it takes to get to 14.8VDC charging, and how long the DC ammeter remains at 80A when running the engine or generator. This is all the info I need.

Don't be seduced by what your monitor is saying. As the retired process engineer said above, use independent tools to measure what's really going on. A good multimeter and a good clamp-on AC/DC ammeter is all that's necessary. You do need to inventory your boat, though, and calculate closely what each piece of equipment draws. Develop your own energy budget on a spreadsheet.

Most promising thing I've seen in all the above discourse is your statement of intent to fit solar panels to your boat. GREAT. That's the only real solution to keeping your batteries fully charged.

Get panels with at least 270-watt total capacity and don't skimp on a good MPPT controller. Forget the wind genny....too much trouble, too much noise, and the solar panels alone, properly installed, will likely do the trick.

Good luck,

Bill

Bill,

As much as it surprises me to say this, I can't totally agree with your recommendations.

Of course you have all the facts completely right but where I disagree is your recommendation for the use of voltage and ammeter as the primary tools for determining charge state.

The problem is that in my experience only a tiny fraction of the boaters out there have anything close to your understanding of batteries and electrical systems. Even fewer would have the knowledge of and your intuitive feel for what the boat systems draw over the day or night and the voltage one would expect to see based on the current load and the implication for the battery state.

From 40 years of boating and uncountable requests from some pretty experienced boaters for help with their electrical systems I think there's an awful lot of people for whom even basic 12V is a black art, closely akin to magic. For those that are unwilling or unable to decipher the details of electricity a battery monitor, while not a perfect tool, would give them much more accurate and usable information than they could derive from voltage reading.

[btrayfors]

Skipmac,

Well, I've gotta agree with your partial disagreement :-)

Understand that I wasn't recommending that everyone go with the voltage/amperage measurements to the exclusion of a battery monitor. Rather, I was pointing out that a battery monitor isn't absolutely necessary. Moreover, lots of boaters are confused by their readings, because battery monitors are sometimes inaccurate, especially over time as they become more and more out of calibration and as the batteries age and lose capacity.

I guess the bottom line is that it always pays to learn something about the electrical systems on your boat....at least enough to be able to tell when things are going awry. A good current example: the loss of power (two alternators and a separate generator) aboard the abandoned 42' catamaran on its maiden voyage in the north Atlantic just last week. Clearly, the crew -- including the "professionals" -- didn't have the knowledge, experience, or tools available to troubleshoot and correct these problems in a brand new catamaran.

For those with minimal knowledge I can only repeat this:

1. Electrical systems aboard are not black magic. Learn as much as you can about them, by inspection and by reading; and

2. Carry a good voltmeter and clamp-on AC/DC ammeter aboard and learn how to use them to double-check the battery monitor readings, to check actual amperage draw, and to do basic trouble-shooting.

Bill

[zboss]

Sorry, I have an AT 165... also the wire is 2 AWG, not 4 and the total "round trip" distance is 5 feet.

To your point, I am trying to learn as much as possible.

Question: does the speed at which an alternator spins (i.e. 1500 RPM on the tach vs. 2000 on the tach) change the voltage or just the upper amp output? I am already seeing 150ish amps during bulk @ 1500 RPM.

Maybe I am running the engine too slow to charge properly?

[btrayfors]

Originally Posted by zboss

Sorry, I have an AT 165... also the wire is 2 AWG, not 4 and the total "round trip" distance is 5 feet.

To your point, I am trying to learn as much as possible.

Question: does the speed at which an alternator spins (i.e. 1500 RPM on the tach vs. 2000 on the tach) change the voltage or just the upper amp output? I am already seeing 150ish amps during bulk @ 1500 RPM.

Maybe I am running the engine too slow to charge properly?

AWG2 has an ampacity rating in engine spaces of 178 amps, so it is allowable. I'd still feel more comfortable with larger wire, say 1/0.

You didn't say anything about the ground wire from the alternator....does it go directly to the batteries or neg bus?

What about fusing?

The RPM affects maximum amperage output. There are graphics on the Balmar website showing maximum output of their alternators at specific RPM.

If you're seeing 150A output in bulk stage, then the alternator is turning fast enough to provide good charging current. A full charge is determined by the charging amperage over time. The batteries themselves will limit the amperage they'll accept at varying stages of charge, no matter the size of the charging source, so long as the voltage is held at the proper level.

Example: If at a given state-of-charge (SOC) your batteries are accepting 100 amps @ 14.8 volts with your setup, they'd accept exactly the same amperage @ 14.8 volts even with an alternator twice or five times the capacity of yours.

Bill

[zboss]

Originally Posted by btrayfors

AWG2 has an ampacity rating in engine spaces of 178 amps, so it is allowable. I'd still feel more comfortable with larger wire, say 1/0.

You didn't say anything about the ground wire from the alternator....does it go directly to the batteries or neg bus?

What about fusing?

Bill

200 AMP fuse

The wiring appears to go directly to the battery but I need to check tomorrow when I have more light available, it may go to the shunt, which then sends the current to the batts.

Also, I did notice that my monitor had the peukert value set at 1.27 vs. 1.25 (Trojan t-145)... how much a difference that would make in what I am seeing on the battery monitor?

[mitiempo]

Originally Posted by zboss

Also, I did notice that my monitor had the peukert value set at 1.27 vs. 1.25 (Trojan t-145)... how much a difference that would make in what I am seeing on the battery monitor?

Some but not very much difference.

As I posted earlier a battery monitor will accumulate errors over multiple discharge/charge cycles. These errors are eliminated 2 ways. One is for you to reset the meter when you know the batteries are fully charged. This should be done whenever the bank is known to be full. The other is typically when the charge current is less than 2% of bank size - at this point most monitors consider the bank full and reset. Because you're only charging method when away from the dock is alternator charging you will not be charging the bank fully. Each time you discharge and then recharge with the engine the error will increase. The monitor will not reset as the current from the alt will not drop to less than 2% of bank size and you cannot manually reset it until the batteries are fully charged.

This could well be the reason for the errors you are getting.

[zboss]

Originally Posted by mitiempo

Some but not very much difference.

As I posted earlier a battery monitor will accumulate errors over multiple discharge/charge cycles. These errors are eliminated 2 ways. One is for you to reset the meter when you know the batteries are fully charged. This should be done whenever the bank is known to be full. The other is typically when the charge current is less than 2% of bank size - at this point most monitors consider the bank full and reset. Because you're only charging method when away from the dock is alternator charging you will not be charging the bank fully. Each time you discharge and then recharge with the engine the error will increase. The monitor will not reset as the current from the alt will not drop to less than 2% of bank size and you cannot manually reset it until the batteries are fully charged.

This could well be the reason for the errors you are getting.

We have been motoring down the east coast ICW most of the way. This is our first extended stay away from the dock and without moving for a while. In most cases, we motor at least 8 hours and the monitor does reset. We plan on leaving Tuesday to continue south so we will see what happens.