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Old 07-15-2013
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m25 alternator question

Hi everyone,

Bit by bit I am starting to understand my bigger boat. I have a question about my alternator and how and what they output.

I have a 1986 Catalina 30 TRBS with the m25.

I just put new house batteries in my boat the old ones were in very bad shape. I put in two 232 AH 6 volt on bank 2. On bank one I have a 70hh 12 volt starting battery.I also have a clipper bm-1 battery monitor.

When I installed the 6 volts the battery monitor after doing the load test to calibrate the batteries showed them at 80% and 12.7 volts.(or very close to that) I connected my changer set it to 6 amps and the BM-1 registered the 6 amps change as 6 amps so working thus far.

I then started the motor after disconnecting the charger to se how many amps it would do. the amps wet to about 7 to 9 and the bm-1 told me at that rate it would take 7 hours to get to full charge. Should it not put more in when the battery is at that level? When I switched back to my starting battery it went down to like 2 as it was fully changed so it is seeing resistance.
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Old 07-15-2013
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Re: m25 alternator question

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Originally Posted by SSBN506 View Post
Hi everyone,

Bit by bit I am starting to understand my bigger boat. I have a question about my alternator and how and what they output.

I have a 1986 Catalina 30 TRBS with the m25.

I just put new house batteries in my boat the old ones were in very bad shape. I put in two 232 AH 6 volt on bank 2. On bank one I have a 70hh 12 volt starting battery.I also have a clipper bm-1 battery monitor.

When I installed the 6 volts the battery monitor after doing the load test to calibrate the batteries showed them at 80% and 12.7 volts.(or very close to that) I connected my changer set it to 6 amps and the BM-1 registered the 6 amps change as 6 amps so working thus far.

I then started the motor after disconnecting the charger to se how many amps it would do. the amps wet to about 7 to 9 and the bm-1 told me at that rate it would take 7 hours to get to full charge. Should it not put more in when the battery is at that level? When I switched back to my starting battery it went down to like 2 as it was fully changed so it is seeing resistance.
Two things:

1. A battery monitor is only accurate when it has been properly calibrated. Over time, they become less and less accurate unless re-calibrated.

2. Your new batteries may be nearly fully charged, so both the battery charger and the alternator are putting out about the same low amperage.

New batteries won't deliver their rated capacity until exercised (discharged and re-charged and, sometimes, equalized). I'd suggest you leave them on charge for a few days to make sure they're fully charged, then recalibrate your monitor.

After using the batteries, i.e., discharging them a few times to about 50% SOC and recharging them fully, re-calibrate the monitor again.

While they are 50% discharged or so, measure the voltage output of your alternator (should be 14.4 or above for flooded or AGM batteries) and measure the amperage output as well. That should give you a much better estimate of how well your alternator and regulator are working.

Bill

Last edited by btrayfors; 07-15-2013 at 10:40 AM.
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Re: m25 alternator question

The general rule of thumb is that an alternator won't take the batteries much over 85% capacity. A shore charger will take them to 100%.

This is because the alternator's output voltage is designed such that you can motor for long periods without fear of overheating the batteries, which is good, but the downside is they only reach 85%.

So that's why your charge rate with the alternator was so low - you were already at capacity, as far as the alternator is concerned.

The shore charger is rather smarter, and switches between various modes.
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Re: m25 alternator question

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Originally Posted by btrayfors View Post
Two things:

1. A battery monitor is only accurate when it has been properly calibrated. Over time, they become less and less accurate unless re-calibrated.

2. Your new batteries may be nearly fully charged, so both the battery charger and the alternator are putting out about the same low amperage.

New batteries won't deliver their rated capacity until exercised (discharged and re-charged and, sometimes, equalized). I'd suggest you leave them on charge for a few days to make sure they're fully charged, then recalibrate your monitor.

After using the batteries, i.e., discharging them a few times to about 50% SOC and recharging them fully, re-calibrate the monitor again.

While they are 50% discharged or so, measure the voltage output of your alternator (should be 14.4 or above for flooded or AGM batteries) and measure the amperage output as well. That should give you a much better estimate of how well your alternator and regulator are working.

Bill
You reminded me and pointed out the problem. I set up the battery monitor and celebrated it with the battery's at 80% and the manual tells me I have to celebrate at 100%. So I assume that is why my BM-1 is off I think.
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Re: m25 alternator question

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Originally Posted by btrayfors View Post
While they are 50% discharged or so, measure the voltage output of your alternator (should be 14.4 or above for flooded or AGM batteries) and measure the amperage output as well. That should give you a much better estimate of how well your alternator and regulator are working.

Bill
Bill,

That would be very misleading at 50% DOD for that alt if he was looking for 14.4V at 50% DOD.. It will be in bulk and having a tough enough time just getting to 13.6V let alone 14.4V... It should not hit absorption voltage until the bank is around 80% SOC but with voltage drop typical of these factory wiring systems it may not come up to absorption at the batteries until the high 90's when the current really starts to drop off..

The best test to know the regulator voltage set point is when the bank is full. This is when it is "limiting" the voltage and with little current flowing you won't have the voltage drops you will at higher amperages.

Test your regulators set point with the bank full and measure it directly at the B+ terminal on the back of the alt. This will tell you the alternators voltage limiting or absorption voltage set point.

Here's an example of the typical piss poor factory wiring we see on some marine engines and the voltage drops associated with it..
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Re: m25 alternator question

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Originally Posted by SSBN506 View Post
You reminded me and pointed out the problem. I set up the battery monitor and celebrated it with the battery's at 80% and the manual tells me I have to celebrate at 100%. So I assume that is why my BM-1 is off I think.

Yes the batteries must be at 100% SOC before you can calibrate it. You can determine a "good enough" full point by measuring less than 2% of the 20 hour Ah capacity flowing into the bank. With no DC loads on, and you see less than 5A flowing into the bank, you can then do a manual re-set. I find monitors are much more accurate when synched manually and done often. 1-1.5% of the 20 hour rate is an even better "full point" but when out cruising you'll rarely get there.
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Re: m25 alternator question

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Originally Posted by Maine Sail View Post
Bill,

That would be very misleading at 50% DOD for that alt if he was looking for 14.4V at 50% DOD.. It will be in bulk and having a tough enough time just getting to 13.6V let alone 14.4V... It should not hit absorption voltage until the bank is around 80% SOC but with voltage drop typical of these factory wiring systems it may not come up to absorption at the batteries until the high 90's when the current really starts to drop off..

The best test to know the regulator voltage set point is when the bank is full. This is when it is "limiting" the voltage and with little current flowing you won't have the voltage drops you will at higher amperages.

Test your regulators set point with the bank full and measure it directly at the B+ terminal on the back of the alt. This will tell you the alternators voltage limiting or absorption voltage set point.

Here's an example of the typical piss poor factory wiring we see on some marine engines and the voltage drops associated with it..
Voltage Drop From Factory Alternator Wiring - YouTube
This is also good to know. For the most part I charge by solar. I have two 30 watt panels that will put out 2 amps or so for part of the day. My solar regulator will stop at 14.1 ish. What should the battery volts read when it is full or close as I understand volts is not a 100% guarantee. I will test may alternator after I fill the batteries and discharge them a bit. I will use a battery charger when I am at a dock next to top it up and be confident it is toped up.

Last edited by SSBN506; 07-15-2013 at 02:41 PM.
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Re: m25 alternator question

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Originally Posted by MarkSF View Post
The general rule of thumb is that an alternator won't take the batteries much over 85% capacity. A shore charger will take them to 100%.

This is because the alternator's output voltage is designed such that you can motor for long periods without fear of overheating the batteries, which is good, but the downside is they only reach 85%.

So that's why your charge rate with the alternator was so low - you were already at capacity, as far as the alternator is concerned.

The shore charger is rather smarter, and switches between various modes.
This is not the case. Both shore chargers and alternators simply regulate voltage. That is all they do. The battery bank determins how much it will accept at a specific voltage. The battery bank has no idea what is charging it. A dumb regulator will actually charge to full faster than most "smart" chargers because it stays at absorption voltage indefinitely.

The reason the 85% number is used is not because of some magic inside the alts voltage regulator but because we as sailors won't & don't generally run the engine for 10+ hours at a time.

It is simple battery acceptance rates & voltage that determines the length of time it takes to get to "full" not the alt or shore charger. Shore chargers just have the luxury of more time to do what they do than an alternator on a sailboat. Run the engine long enough and you'll eventually get to .5% acceptance or less. An alt will most definitely get your banks full, if you give it the time. The only time an alt won't get a bank full is if the voltage regulator is set too low, like 13.6V or if the system has massive voltage drop problems. This of course has nothing to do with an alternators capability just that it is set wrong or the wiring is insufficient. A properly installed 100A alternator will charge as fast as a 100A shore charger if both have the same voltage set points.. (except when that alt gets hot it will run at 75-80A and the shore charger will often shut down on over temp if run at max for long enough)

We now have a LiFePO4 bank (Lithium Iron Phosphate) and just had to wire in an alternator regulator CUT-OFF switch because these batteries don't have the acceptance issues that LA batteries do... It is an ODD feeling to have to turn off your alternator because the bank went from 70% SOC to 100% in an hour...... Using that same exact alternator on my LA bank, in that that same scenario, took roughly 10+ hours to get to full from 70% SOC.......

Our alt can charge the LiFePO4 bank to full from 70% in about 1 hour. It took 10+/- hours with a lead acid bank using the same alternator... No changes other than programing the regulator for a lower charge voltage for the LiFePO4 bank... Lower voltages normally mean slower charging but not so much with the newer battery technologies....
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Last edited by Maine Sail; 07-15-2013 at 02:54 PM.
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Re: m25 alternator question

I think I was actually trying to say pretty much what you just did, except that you explain it better. I am fully aware that an alternator simply outputs a voltage, but my point is that a shore charger may output a higher absorption voltage BECAUSE it will switch to trickle when absorption is done. An alternator will not switch, so there has to be a compromise in the absorption voltage, so as to not cause excessive electrolyte consumption over the long term.

You didn't mention the thermal compensation in most alternators. Mine will start out at 14.4V, but once the engine has run for a while and the engine compartment has warmed up, the output will drop to 13.6V or so due to the alternator's internal compensation reacting to the higher temperature. So I'm never going to reach 100%, and 85% is a good assumption. Suits me, as I hate topping off the battery electrolyte.
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Re: m25 alternator question

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Originally Posted by Maine Sail View Post
Bill,

That would be very misleading at 50% DOD for that alt if he was looking for 14.4V at 50% DOD.. It will be in bulk and having a tough enough time just getting to 13.6V let alone 14.4V... It should not hit absorption voltage until the bank is around 80% SOC but with voltage drop typical of these factory wiring systems it may not come up to absorption at the batteries until the high 90's when the current really starts to drop off....................
Yes, I agree it could be misleading. I should not have been so cryptic. Thanks for catching that.

At 50% discharge one would expect to see the alternator putting out near maximum current while it's still cool and IF the voltage output is high enough. Later on, nearing 70-80% you would expect to see a maximum of absorption voltage....14.4VDC or greater for flooded batteries.

However, there is an important interaction between charging capacity, battery absorption capacity, and the time to reach absorption voltage.

Take, for example, these results from real world laboratory testing by Concorde/Lifeline, using a new AGM 4D battery discharged to 50% SOC and charged with three different size chargers. The tests continued for 4 hours and voltage and amperage readings were taken each minute.

1. 400A capacity charger: Voltage reached 14.4VDC in under 2 minutes while the charger was delivering 265 amps.....@ about 53% SOC.

2. 105A capacity charger: Voltage reached 14.4VDC in 24 minutes while delivering 105A.....@ about 70% SOC.

3. 52A capacity charger: Voltage reached 14.4VDC in 66 minutes while delivering 52A......@ about 77% SOC.

The 14.4VDC charging voltage was continued until the test ended at 240 minutes.

These three charging capacities were chosen to represent:

1. virtually unlimited charging capability (400A with a 210AH battery bank);

2. a 50% rate of battery rated capacity (105A with a 210AH battery bank); and

3. a 25% rate of battery rated capacity (52A with a 210AH battery bank).

Point is, absorption voltage maximum varies directly with the charging capacity. Most boats don't have more than 10-25% of rated AH charging capacity.

Bill

Last edited by btrayfors; 07-15-2013 at 06:17 PM.
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