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Old 16-10-2009, 18:46   #16
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Not fully charging AGMs periodically will definitely lead to sulfation and, beg to differ with Pelagic, stratification. Even with thin plate pure lead (TPPL) AGMs, a cruising boat should fully charge the batteries every seven to ten days.

Chronically under charging an AGM will shorten its life drastically. But, then again, so will installing the battery in a space where the ambient temperature exceeds 25 degrees C.

Temperature compensation is required for AGMs, period. If you have a warranty claim for these batteries, and you were not using temperature compensated charging sources, your claim is on shaky ground.

Regarding IOTA chargers with no temperature compensation: No amount of PWM magic will provide the feed back to the charger that will reduce the output voltage the nominal 24 mVDC/degree C required for a 12 VDC AGM battery. They may be fine chargers but I would not recommend them for an AGM application.
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Old 16-10-2009, 18:48   #17
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Bill, I will definitely get the IQ4 option. I appreciate your help. I think you've identified a good cost-effective solution. I'll be back on the boat in about six weeks and this will be one of my first projects.

Pelagic,

If I had the room for another battery I would increase my house bank's capacity just to minimize the depth of discharge as you've done. I try to keep my house bank's state of charge between 65% and 85%, or a maximum depth of discharge of about 35%. Based on the attached figure from my battery manufacturer's technical manual you can get an idea on the lifespan of an AGM battery given the depth of discharge. There is no question that deeper discharges will reduce the battery's life.

- Rick

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Old 17-10-2009, 09:01   #18
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Charlie,

If temperature compensation is required for AGM charging, am I somehow damaging my batteries every time I am underway by charging off my main engine's alternator? My house bank and starter battery are charged by the alternator while underway and I know it does not have a fancy 3-stage regulator. It's just your run of the mill 60A marine alternator with internal regulator.

-Rick
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Old 17-10-2009, 10:10   #19
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Thanks Charlie, for setting me straight on the stratification bit about AGM’s

I honestly do remember reading somewhere in all my research about AGM’s not having a stratification problem but I can’t find it now. (driving me crazy!)

Anyway, within 7 days we are normally making water or running aircon so it does gets fully charged up. Also I believe my Victron smart charger has a timed boost to give the batteries a kick every once in a while, when at the marina


In my search, I did find this useful explanation about the subject confirming you are correct.


Thanks again
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Old 17-10-2009, 10:42   #20
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Charlie,

If temperature compensation is required for AGM charging, am I somehow damaging my batteries every time I am underway by charging off my main engine's alternator?
Rick-
You certainly are not doing them any good!

Batteries are rated at 25C (77F). There is a fairly strong temperature dependency between charging voltage and battery temperature. Nominally the charging voltage for AGMs must be reduced approximately 20 mVDC for each one degree C increase from 25C. Conversely, the charging voltage must be increased a like amount for each one degree C decrease from 25C. By the way, there is a strong dependency between battery life and their temperature environment.

A "dumb" alternator regulator will drive the alternator output to put out a constant voltage as long as it "sees" a difference in potential between the alternator output and the battery. It will generally downshift to a "float" voltage when it senses that the battery is fully charged. (There is more to the operation than I have described involving field current limitation, etc., but the description above should suffice.)

For the sake of discussion, let's say your alternator has a typical charge voltage of 14.4 VDC and a float voltage of 13.4 VDC (not uncommon and easy to check with a DMM.)

A quick look shows a the specs for one manufacturer's AGM shows the following:
  1. The absorption voltage at 25C is 14.58 to 14.82 VDC with a nominal of 14.70 VDC. At 50C (122F), a temperature that is not unusual in an engine room or engine compartment in the tropics or sub-tropics, the voltages are: 13.98 to 14.22 VDC with a nominal 14.10.
  2. The float voltage at 25C is 13.50 to 13.74 VDC with a nominal of 13.62 VDC. At 50C, the float voltages are: 13.02 to 13.14 VDC with a nominal of 13.02 (not a typo, that is what the manufacture's table shows.)
So, with an internally regulated alternator without temperature compensation operating with batteries in inadequately ventilated battery boxes in a warm to hot environment, the Owner is chronically undercharging the batteries until the batteries reach 35C (95F) at which time the batteries are being subjected to excessive voltage. The temperature compensation feature ensures that the AGMs are neither over charged or under charged.

No good can come from this and I will not install AGMs on a client's boat unless all the charging sources; solar, wind, prop, alternator, cold fusion, etc. have a temperature compensated output voltage that meets the battery manufacturer's specs.

Finally, AGMs are very expensive and they need to be installed and maintained in accordance with the manufacturer's specs to ensure the maximum life from the batteries and to minimize their life-cycle cost.

Hope this helps.
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Old 17-10-2009, 10:50   #21
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Anyway, within 7 days we are normally making water or running aircon so it does gets fully charged up. Also I believe my Victron smart charger has a timed boost to give the batteries a kick every once in a while, when at the marina.
Pelagic-
Victron does it right! I also recommend that folks that are tied to shore power should turn off their battery charger every couple of weeks and draw about 30% of the Ahr capacity out of the bank and then do a normal recharge. Batteries are meant to be used but they do not like to be abused.
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Old 17-10-2009, 11:06   #22
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Question

CharlieJ,

OP said he has 600 amp bank and his alternator is 60 amp. If that alternator is putting out full amps into that bank, which it won't do most likely only 45 or 50 amps, is it actually going to be able to heat a bank that size up enough to cause problems?

I don't know the answer, I face a similar situation with a 100 amp alternator restricted to 70 amp output by a 3 stage regulator into a 440 amp bank (2 4Ds flooded lead acid). No thermal monitoring. My batteries are not in an area were they are subjected to extra heat beyond ambient and we rarely draw them down below 40%.

How likely is it that I could overheat them? They never feel warm or seem to be gassing much when being charged.
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Old 17-10-2009, 11:16   #23
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How likely is it that I could overheat them? They never feel warm or seem to be gassing much when being charged.
jdoe71-
It is not the overheating caused by the electrochemical reaction that I am talking about...it is the functional dependence between the battery's internal temperature and the charging voltage seen at the battery terminals.

Does this clear it up?
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Old 17-10-2009, 12:05   #24
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CharlieJ,
Yes, I get that now. Thanks for the info, some good stuff to know here.
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Old 18-10-2009, 09:43   #25
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So, with an internally regulated alternator without temperature compensation operating with batteries in inadequately ventilated battery boxes in a warm to hot environment, the Owner is chronically undercharging the batteries until the batteries reach 35C (95F) at which time the batteries are being subjected to excessive voltage. The temperature compensation feature ensures that the AGMs are neither over charged or under charged.
Charlie, thanks for the detailed explanation. Since we cruise only in south Florida from December to May and our batteries are in the engine room (not in a battery box) I believe the amount of time charging from the alternator with the battery temperature under 35C is likely limited. Our alternator is charging the house bank and start battery through a diode-type battery isolator. Although I haven't measured ours, the voltage drop across a diode is typically 0.6 to 0.7 volt. If that is the case, it seems unlikely the batteries are subjected to excessive voltage from a 14.4V alternator through the isolator. Undercharging is more likely.

-Rick
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Old 26-10-2009, 23:45   #26
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Rick, regardles the charger you pick, read its manual carefully, since some does the rated high bulk charge just for a while, or a small percentage of the charging stage, not the entire cycle. This alone will end your shortening charging time project ....
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Old 27-11-2009, 18:09   #27
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Thanks everyone for your comments. I've given this enough thought and I'll be back on the boat in two weeks so here's my plan. I have ordered an Iota DLS90/IQ4 (thanks for the suggestion Bill). I will use the Iota for bulk charging while away from shore power and when at the dock I'll use the temperature-compensated Xantrex TrueCharge 40+. Later I'll look into a high-output alternator and external temperature-compensated three-stage regulator.

To install the Iota 90, I will replace the existing 15 or 20A AC circuit that powers the Xantrex TC40 with a 30A circuit which I will run into a 2-gang box next to both chargers. One side of the box will have a 20A commercial-grade GFCI duplex receptacle for the Iota charger. In the other side of the box I will install a 20A SPDT switch (On-Off-On). I will connect the hard-wired Xantrex TC40 to one side of the switch and the GFCI to the other. With this configuration, only one charger can be on at a time and I always have a backup. For months away from shore power, I suppose I could even use the Iota for bulk charging then switch to the Xantrex for absorption and float to occasionally get close to a full charge. I'm looking forward to the shorter genset runs while we are on the hook.

- Rick
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Old 28-11-2009, 09:09   #28
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As another poster already stated, no matter what you do or buy or spend it won't speed up the charging of your battery bank to any significant degree. Again, the reason is because you are taking your bank down to 75%, and most modern 'smart' chargers begin throttling back the charge current around 80% to prevent overcharging.

Ironically, probably the only way to really speed up charging is to go 'low-tech', and spend _less_ money, not more, buy buying an inexpensive big stupid charger and manually charging the bank for an hour or so at full output until the battery bank is full. With that method, though, one needs to be very attentive to the charging so that the battery bank is not overcharged.
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Old 28-11-2009, 16:53   #29
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Not Sure,

I disagree. While I'm only using 20 to 25% of the Ah capacity of my bank in 24 hours, in practice I may be discharging the bank to about 65% state of charge many days. That's because I don't attempt to fully charge while at anchor.

What I am after is faster bulk charging during multi-stage charging. While at anchor I rarely spend much time charging beyond the bulk phase because the absorption and float stages take considerable time to restore that last 15 to 20% of battery capacity. I have a battery monitor so I can see when bulk charging ends and absorption begins. So, a 90A charger should give me close to 90A during bulk charging, which means I can restore 90Ah in less than half the time required by my current 40A charger. Again, I'm not trying to get back to 100% state of charge just 80% to 85%.

- Rick
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Old 29-11-2009, 04:49   #30
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Not Sure,

I disagree. While I'm only using 20 to 25% of the Ah capacity of my bank in 24 hours, in practice I may be discharging the bank to about 65% state of charge many days. That's because I don't attempt to fully charge while at anchor.

What I am after is faster bulk charging during multi-stage charging. While at anchor I rarely spend much time charging beyond the bulk phase because the absorption and float stages take considerable time to restore that last 15 to 20% of battery capacity. I have a battery monitor so I can see when bulk charging ends and absorption begins. So, a 90A charger should give me close to 90A during bulk charging, which means I can restore 90Ah in less than half the time required by my current 40A charger. Again, I'm not trying to get back to 100% state of charge just 80% to 85%.

- Rick
I have exactly the same at anchor charging requirement as you Rick but at 24v. I want the minimum generator run time per day.
I have no agm battery experience.
This is my experience....
I have a 6kw 220v generator.
I have 650 amps wet cell fork truck batteries and I charge them at anchor as fast as i can, up to absorbtion and then stop, so this is from about 60% to 85 - 90%. Once a week I then charge them fully.
I have an 80 amp Victron and it is not big enough. I needed 150 amps because that what my wet cells can take before they reach the absorbtion voltage of 28.4.
The problem with only having 80 - 90 amps of charger is that the bulk charge phase will be alot longer as if you put in as much as the batteries will accept. I am sure your batteries will accept 150 amps in bulk mode.

I agree with the other comments about charging from 80% to 100% as this is the absorbtion phase and more amps will not be accepted.
John
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