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Old 13-12-2006, 06:10   #1
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Mixing 'types' of AGM batts??

Hello everyone

I have what is most likely a dumb question, please forgive it in advance.

I have two group 31 AGM deep cycle batts in my house bank. My energy budget is low and they have worked well for the past couple years I have had them and this year of cruising and living aboard full time. One thing though...I have noticed that when I apply a high draw item like my computer or talking on the SSB...the voltage can drop significantly, very quickly. I am given to think this is due to their slower rate of electro-chemistry. No problem with capacity, after the draw is taken off, they equilibrate pretty quickly and the voltage comes back up.

I want to add more capacity to my house bank.

SO....I wonder...aren't there "dual purpose" AGM batts??? Batts that have thinner plates and can deliver charge more quickly?? They cannot deep cycle as many times as the 'deep cycle' ones...but that is not really an issue for me. SO...could I mix these two in order to maintain voltage?????

If I mix the two would the drain then come preferentially from the dual purpose batts, draining them faster and thus ruining them? Then when they are ruined, will they in turn ruin the deep cycle AGMs??

Hopeful for any advice. I know I could simply add more deep cycle batts to the house...but they are the most expensive and I wonder if voltage drop will still be an issue.

Thanks

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John
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Old 13-12-2006, 07:28   #2
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John,

Your two group 31 AGM batteries should be adequate for the loads you mentioned. They should provide about 200AH capacity, assuming you're using them as ONE (combined) house bank as is preferable, not separately.

If you're going to add capacity, you don't need any special battery to provide for the loads you've mentioned. AGMs are more than adequate.

While a marine SSB can draw up to 30A peak, average draw on voice SSB is much lower when transmitting, and only an amp or two when receiving.

Similarly, your (I assume laptop) computer only draws about 5-7 amps maximum. Over time, of course, that can drain a battery easily, but the load itself isn't high and you should not be seeing much voltage drop.

Some voltage drop is to be expected, of course, but AGM batteries tend to "hold in there" longer with relatively higher voltages than do flooded batteries.

In any case, you should check your battery connections very thoroughly to be sure everything is clean and tight. Be ruthless and persistent in this check...it's very important, because it can affect both battery charging and the ability of the batteries to provide sufficient energy for the loads you connect to them.

Also, how are you charging the batteries? Do you have a good smart charger (multi-stage)? Do you equalize them? Recommendations from AGM battery manufacturers differ on the subject of equalization, so be sure to check out what is recommended for your AGMs.

Bill
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Old 13-12-2006, 08:48   #3
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Hi Bill

Thanks for the response, appreciate it. The two AGM batts are less than two yrs old and in good shape. Connections are tight and clean, wire runs of proper gauge. They charge fine via solar panels and the engine alternator, really no problems there. They come up to voltage fine.

I do have them in one bank and, as you wrote, 200Ah is definately enough capacity. The issue really is voltage at night when there is no power generation from the solar panel (and not running the engine). It is not major, but when using the laptop (65W power pack, so draws about 5-6 amps at 12VDC), it can reach the mid 11's after some time and even get to the point where the inverter signals it is not happy (somewhere aroun 11.2 or 11.4VDC).

It seems that these batts are discharging at a rate greater than they are equilibrating charge or regenerating charge.

I have a dual purpose AGM starting batt. When I add this in, the voltage seems to hold better. It could well be that just the added capacity, and that must play some part. I am just curious though, if its greater discharge ...and presumably charge regeneration rate...also plays a part, perhaps a bigger role.

Thanks

John

ps - have the SSB working great now. Made a cleaner path to the topping lift (antenna), with just under 15ft of 14g wire (and a firm connection using a hose clamp), using a 1 inch length of coax stripped to the center conductor (outer braid not going anywhere) from the tuner antenna output and one run of copper foil from the tuner ground to a through hull, and three long lengths of copper foil coming off that through hull running through the bilge...one length tying in two more through hulls. Whew. Being heard loud and clear by boats all over, signal as good as any boat in my area. Can hear boats from Panama to PR to LI NY and off Bermuda. Great to have aboard.
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Old 13-12-2006, 09:07   #4
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Quote:
It is not major, but when using the laptop (65W power pack, so draws about 5-6 amps at 12VDC), it can reach the mid 11's after some time and even get to the point where the inverter signals it is not happy (somewhere aroun 11.2 or 11.4VDC).
If accurate you are correct the inverter isn't happy with 11.2 volts.

It sounds like they are discharging far deeper sooner than they should. It would be nice to measure the charging voltages and examine the regulation since improper charging can prematurly cause loss of lifespan with any batteries.

For a laptop consider getting a car DC adapter plug and don't use the inverter. You can cut the power demand in almost half by not using an inverter. Inverters are very inefficent and using an AC inverter means you are converting DC to AC then the computer is converting AC back to DC. At the very best you have a 40% loss of power to heat maybe more.
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Old 13-12-2006, 09:30   #5
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I agree with Paul that the batteries are discharging much faster than they should, which indicates to me that there has been a loss of capacity over the past couple of years, either through sulfation of the plates or other physical problem internal to the battery (like shorting, AGM matte breakdown, etc.). These batteries should deliver a lot more power.

Yes, check the charging to be sure they're really getting a full charge. And, what about equalization (cf my earlier remarks)?

I also agree with Paul that it's a good idea to run your laptop from a power supply designed for it (like those from Lind Electronics) which will work down to about 10-11 volts or so and will save some energy. This is because most inverters, while quite efficient at higher loads -- say, over 30% of their rated output where they may be 85 to 95% efficient -- are much less efficient when used at low loads or with, e.g., electric motors.

Still, even if you ran your laptop overnight for 8 hours and were pulling 10 amps (125 watts) from your house bank, that would be a total of 80AH which could be expected to draw your 200 AH AGM bank from full charge to about 60% charge. You should not be seeing 11 volts.

Bottom line: spend a lot of time trying to figure it out, equalizing the batteries, etc. OR get some added capacity. My hunch is the AGMs are not in such good shape, and may need to be replaced. If so, while you're at it, build a larger bank -- possibly with T-105 golfcarts if you have the room. That is, unless you are wedded to AGMs and wanna spend the extra bucks :-)

Bill
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Old 13-12-2006, 09:45   #6
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battery internal resistance

Your problem is descriptive of a battery bank having a higher internal resistance than desirable for those case sizes. Good quality AGM batteries fully charged and at full capacity (not synonymous) exhibit 3 milli-Ohms or less internal resistance per 100 Amp-hour rating at a 20 hour rate and near 70 deg F.

Poor quality brands do not have low internal resistance and are not suitable for good inverter performance. What brand do you have and what, exactly, is the charge acceptance voltage achieved during recharge?

Regardless of the marketing "poop" that you might read about how you might be able to recharge your battery at 13.9V the battery will NOT recover lost capacity on a cycle-by-cycle history of use without being subjected to at least 14.4V at 70 deg F. If you are as little as 50 milli-volts below this value your batteries will not demonstrate adequately low internal resistance over several cycles of charge/discharge. This fact becomes especially important when not having the benefit of achieving a full charge before beginning new discharge cycle.

Paul it is not true that inverters don't "like" 11 Volts or so. To be accurate most 12 V inverters today are designed to operate with terminal voltages down to 10 or 10.5 Volts at rated load. To be sure they will not be as efficient as they would with higher terminal voltage yet let us examine the real impact of that concept; Good inverters today are 80 % efficient (perhaps more for "modified sine-wave, perhaps much less for true sine-wave) at rated load with terminal voltages on the order of 12.0 Volts. With relatively small loads such as computers the power loss is not worth concern (from a system design philosophy) when considering the inconvenience of having a "mixed" 12 V and 120 V source solution, like making someone search for and buy components that allow "12V" computing.

Today a good battery bank and inverter and charge source(s) installation should be designed so that one can leave the inverter on all of the time and merely turn on and off loads at your convenience. This has been true now for over 15 years. Yes, there are some inverters (like the Prosine) that eat up current all of the time (like over 5-7 Amps, depending) yet there are others which do not.

Even 30 years ago the market was much the same in that 80% of most cruisers would be well served with a 1500 Watt inverter and a 400 Amp-hour battery bank and a 100 Amp (or better) alternator with a 3-step regulator (solar, and wind are merely "gravy"). Fifteen years ago one could buy a 1200 Watt Heart inverter that works today well with computers, audio/video systems, etc. that would "draw" ony 50 mA at an "idle". Today's inverter designers are not so parsimonious with their internal "housekeeping" power dissipation and, therefore, the idle losses are about 10 times that much yet still tolerable.
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Old 13-12-2006, 10:54   #7
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Guys - Now that you have me thinking more critically about it ....I think you have hit on the problem. Thanks.

I obtained the batts from a dedicated batt store, not a marine supplier. They are good, but may not be the best...they were well priced.

In addition, it is doubtful that they are in a state of full charge when I use them. I use the computer mainly at night and first thing in the morning. First thing in the morning is when the batts are obviously at their lowest, since nightime use is heavier than day time use (lights, cell phone etc).

And....I spend long long periods of time at anchor or on a mooring and do not run the engine. The batts are thus charged by the single 125W solar panel I have (figure less than 7 amps for 6.5 hrs and you get less than 40Ah/day...but I should not be using all that much). And true enough, cruising...there are days even in FL that are cloudy and you don't get the power you would like.

Insofar as my energy needs are really very small, this has worked quite well for the years I have owned the boat. Only recently have I been using the computer more without the benefit of shore power. And ...thus...the issue.

Paul...I think the suggestion of getting a DC-DC power source is a good one. The iGo from Radio Shack might be a good choice..what do you think? I would not want to get one dedicated for this computer, since I go through a new computer every couple of years and do not want to keep buying a new DC power adapter.

The inverter I have is a low cost WM SeaVolt 700W one. Modified sine wave. Obviously, not high quality or high power. It has done the job though given my small needs.

At present, I am just trying to improve or optimize, not replace anything, it does serve me in most respects. I am a believer in conservation of energy being easier and less costly than generation. SO....the DC power supply idea is good.

Still to think about....adding more batt capacity (and yes, I know...I DO need more generation capacity...but that is a different thread and I am undecided as to whether I will add more solar or get a generator or genset).

So..given the above....would it be more prudent to add another deep cycle AGM or could I go dual purpose (I am still interested in this question...even though it seem academic at this point...just interested).

Thanks! Looking forward to your input...this is helpful.

Best

John
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Old 13-12-2006, 11:05   #8
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Chasing your tail;

Sorry, you're situation is similar to those of many who abandoned gel-cel and AGM batteries many years ago. The problem is that you must first create a reliable charge source that is capable of, usually once per day, of getting the batteries to 14.4V, even if it means running the engine and then letting solar take over. Otherwise you will one day discover that your AGM batteries will no longer charge accept and you will blame them as being "no good, never were any good" and go back to flooded batteries with their attendant disadvantages.

I have brought many such batteries (that were degraded by situations such as yours) back "from the dead" after getting them to charge accept using constant-current high voltage (as much as 20V) sources and then applying a normal 3-step charge regimen. You will have trouble, I predict, until you establish a good charge regimen.
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Old 13-12-2006, 12:12   #9
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I certainly agree with Rick that ensuring a good charging regimen is top priority to keep your batteries healthy. If they are consistently undercharged, their capacity will diminish over time, sometimes rather quickly.

Also agree with the thrust of Rick's comments about modern inverters in some ways being a step backwards. However, the "best-of-breed" modern inverters aren't that bad.

My 17-year old Heart 1200 watt modified sine wave inverter, removed from my boat this year, indeed has a very low amperage draw at idle. I just measured it at around 150 milliamps. It worked very well for many years, and is still in great condition, with a built-in 45A "smart" charger. However, it is very noisy from an RFI perspective: creates substantial interference with my radios. It also doesn't have a "equalization mode"...just a variable voltage control which tops out at about 14.4 volts -- not enough for equalization.

My new top-of-the line Victron Multi-plus pure sine wave inverter draws 10 watts at idle (800 milliamps). This isn't anywhere near the 5-7 amps of the Prosine Rick mentioned, but it's a heck of a lot more than the old Heart.

Further, the Victron's efficiency approaches 94% (at best). And, its output is a pure sine wave, compatible with ANY 115 VAC appliance. The old Heart's modified sine wave output worked well with most loads, but there were a few which it could not handle (mostly, small charging devices for cellphones, computers, etc.).

Whether or not an inverter should be run full time just because it can be done without draining the batteries has to be an individual decision, and is very much related to how the inverter is wired into the boat's AC circuits. In some configurations with automatic changeover switches, the inverter could drain your batteries flat if the shore power went out and there were no low voltage sensors or other protections.

Call me chicken, but I don't use the inverter unless I need it. And, I have a typewritten checklist next to its switch to be followed before turning it on.

Bill
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Old 13-12-2006, 13:17   #10
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Thanks guys, all good advice.

I think the smart chargers I have for both the solar controller and the alternator are fine. They do get voltage up to 14.4. I do not think that is the major issue. The more significant issue is that the batts more than likely do not get fully charged each and every day. I do go through a full charge at least once a month, usually when I am off the boat for a day or more. The batts like that.

It would be very nice to have a much better charger, much larger batt bank and much greater charging capability. Funds do no permit at this time.

Thus...looking for a short term, low cost solution that will improve things until I can implement a full system upgrade. If I got the iGo DC charger and another AGM batt, that would be a total of around $250. That is about the budget right now for this.

I think the iGo is a good idea and makes sense. So that is a definate.

Decision point is another batt. I began this working on the assumption that if I am indeed not charging the batts to full every day, then increasing the house Ah capacity would thus mathematically lower the depth of discharge the bank is incuring. True or not true? Would this make a difference in the voltage drop and/or the long term service of the current batts?

Appreciate the advice, please help with these final points...it is about all I can do right now...unless I do nothing, use the batts for another year or two and switch to 6V ones or upgrade then.

best

John
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Old 13-12-2006, 13:44   #11
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$250 would get you another solar panel...
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Old 13-12-2006, 13:55   #12
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Well...yes. But...I have no problem with running anything on the boat all day long with the sun shining using my current panel. If I add another solar panel, it would be a min 85W ($450), but more likely another 125 ($600+).

The problem is more at night and in the early morning when I have no solar. And, do not wish to run the boat's engine.

Of course the other solution is just not to surf the net so much
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Old 13-12-2006, 14:16   #13
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John,

The Radio Shack iGo is rated at only 15watts continuous output. It isn't nearly enough to power your computer. Yes, it could perhaps charge the battery if the laptop isn't in use, but I'm sure you already have a 115V power brick to do that.

I can sympathize with your situation in not wanting to spend a lot right now. However, I can't envision any scenario for solving your problem which is going to be adequate and within the budget you specify. This is partly because the cost of good, marine AGM batteries is so high, but also because you really need to pay attention to the charging capacity. By the way, 14.4 volts isn't enough to equalize the batteries.

I can think of one FREE thing you might do which could help somewhat. As I'm in the midst of testing a number of "desulphators" with different battery setups, if you send me your address I'd be happy to send you one of these devices for free. They are small, and only need to be connected across the terminals of your battery. They emit a pulse which is tailored to cause sulfate crystals to disintegrate and return into solution.

I can only send you one if you promise to provide me with regular reports on your battery condition and performance. It doesn't have to be "scientific" at this point, just a gross measurement of whether or not your batteries improve in their ability to reach higher capacity than they now have. It will take several weeks before you notice any improvement, but you well might. Others have reported restoration to approx. 80% of original capacity, and I've noticed some improvement with old gelled golf-cart batteries so far.

You can send me an email at: bill at wdsg dot com

This sure ain't the "final solution", but it might help in the short run.

Bill
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Old 13-12-2006, 15:41   #14
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Hi Bill

Thanks for the advice, I appreciate it. And yes, I would like to take you up on your kind offer. It will be good to be in touch as well. I spent 20 yrs sailing the ches, miss it. Just slipped the lines for good last April.

expect an email from sv invictus

For now, I will use the computer less and make sure the batts are charged more fully. I do think they are in good shape.

I looked at the iGo in Radio shack today. The auto/air 6500 or 85 says it puts out 70W at 12VDC continuously. My computer uses a lower power AMD sempron proc and draws 3.42A at 19VDC...which should be 5.4 at 12V. The brick is 65W....so....I did think the iGo would work. I have heard from friends that it does...but...you never know. At least with RS, I can return it.

But also...what do you think about adding another AGM to the house bank? Would that not lessen the % discharge on an overall basis?

Thanks for all

John
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Old 13-12-2006, 18:06   #15
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John I would add another battery to the bank, the lower the % discharge on an overall basis the better, I would also try and bring up the system to a really full charge at least once every two weeks or so either using the engine or something. That helps alot.

I added a 'Nanopulser' desulfator to my bank about a month ago but can't really tell if it's doing anything right now as we are plugged in at a marina for the winter. I will be able to run some good tests begining in the spring when we head back out as to what effect the unit has had on bot SG and holding voltage as I do have good history.
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